Computational detangling chalcogen elements substitutions associated ESDPT mechanism for oxazolinyl-substituted hydroxyfluorene derivatives

In view of the distinguished photochemical and photobiological characteristics of oxazolinyl-substituted hydroxyfluorene and its derivatives, herein, we mainly focus on probing into excited state behaviors of the novel 9,9-dimethyl-3,6-dihydroxy-2,7-bis(4,5-dihydro-4,4-dimethyl-2-oxazolyl) fluorene (Oxa-OH) derivatives. In light of the significant effects resulting from substituting oxygen elements, three Oxa-OH derivatives (i.e., Oxa-OO, Oxa-SS and Oxa-SeSe fluorophores) are considered in this work. For these three different fluorophores, we detangle the effects of atomic electronegativity and charge recombination related to oxygen elements in excited state double proton transfer (ESDPT) processes. Because of the low potential energy barriers, we confirm the ESDPT happens by the sequential type. Based on heterosubstituted Oxa-OS and Oxa-OSe compounds, we further verify the chalcogen atomic-electronegativity-regulated stepwise ESDPT mechanism. We sincerely wish our work could provide a theoretical reference for proving this novel mechanism of ESDPT experimentally.

Barrierless reactions of C2 Criegee intermediates with H2SO4 and their implication to oligomers and new particle formation

The formation of oligomeric hydrogen peroxide triggered by Criegee intermediate maybe contributes significantly to the formation and growth of secondary organic aerosol (SOA). However, to date, the reactivity of C2 Criegee intermediates (CH3CHOO) in areas contaminated with acidic gas remains poorly understood. Herein, high-level quantum chemical calculations and Born-Oppenheimer molecular dynamics (BOMD) simulations are used to explore the reaction of CH3CHOO and H2SO4 both in the gas phase and at the air-water interface. In the gas phase, the addition reaction of CH3CHOO with H2SO4 to generate CH3HC(OOH)OSO3H (HPES) is near-barrierless, regardless of the presence of water molecules. BOMD simulations show that the reaction at the air-water interface is even faster than that in the gas phase. Further calculations reveal that the HPES has a tendency to aggregate with sulfuric acids, ammonias, and water molecules to form stable clusters, meanwhile the oligomerization reaction of CH3CHOO with HPES in the gas phase is both thermochemically and kinetically favored. Also, it is noted that the interfacial HPES- ion can attract H2SO4, NH3, (COOH)2 and HNO3 for particle formation from the gas phase to the water surface. Thus, the results of this work not only elucidate the high atmospheric reactivity of C2 Criegee intermediates in polluted regions, but also deepen our understanding of the formation process of atmospheric SOA induced by Criegee intermediates.

Atropisomerism of diflunisal unveiled by rotational spectroscopy and quantum chemical calculations

The most stable conformer of laser-ablated diflunisal has been isolated in a supersonic expansion and experimentally detected through high-resolution chirped-pulse rotational spectroscopy. State-of-the-art chemical calculations allowed to understand the nature of the strong stabilization of the detected conformer and its atropisomer among a total of sixteen theoretically predicted conformers and confirmed the presence of a resonance assisted hydrogen bond (RAHB) between the hydroxyl hydrogen atom and the carbonyl oxygen atom of the carboxylic acid group. The comparison of the experimental data from this work and the information found in the literature about the molecule in condensed phases corroborates the existence of these two atropisomers and is contextualized within the complexation arrangement of diflunisal with relevant proteins.

Quantum chemical treatment, electronic energy in various solvents, spectroscopic, molecular docking and dynamic simulation studies of 2-amino-N-(2-chloro-6-methylphenyl)thiazole-5-carboxamide: A core of anticancer drug

The titled molecule 2-Amino-N-(2-chloro-6-methylphenyl)thiazole-5-carboxamide (ANMC) is a core of anticancer drug dasatinib (leukemia). Its derivatives exhibited bioactivity against breast cancer. Experimentally, the titled compound was described using NMR (1H NMR and 13C NMR), FTIR and UV-visible spectroscopy. The results were compared with the theoretical predictions, showing good agreement such as theoretical NH vibrations showed symmetric stretching and asymmetric stretching at 3429 and 3440 cm-1 respectively, λmax values appear at 305 nm for experimental and 307.75 nm for theoretical observations in acetone medium. Hirshfeld surface analysis well described the secondary internal and external interactions obtained like dnorm and di ranges -1.8551 to 1.4590 and 0.0918 to 2.6756 respectively. Comparing UV-visible spectra obtained in various solvents with the calculated TD-DFT results revealed minimal solvent effects. Molecular electrostatic potential (MEP) map and Fukui functions were employed, which indicated reactive sites of the molecule and the obtained order of nucleophilic reactivity was C16 > C2 > C8 > Cl1 > C22 > C21. The bioactivity profile probability of ANMC was theoretically explored by calculation of electrophilicity index and drug-likeness. Molecular docking of the ANMC molecule was performed with ten receptors to obtain the best ligand-protein interaction and the minimum binding energy obtained was -8.0 kcal/mol. Biomolecular stability of ANMC was investigated by Molecular Dynamic Simulation (MDS). And also the analysis of free energies showed strong interactions between the ligand and the protein.

Observation of reversible conformational interconversion accompanied by 3p internal conversions in Rydberg-excited N,N-dimethylethylamine

Conformational dynamics has been well observed in the 3s Rydberg state of amines, whereas its observation in higher-energy, non-equilibrium 3p Rydberg states is very rare, especially for a reversible conformational transition that could compete with other non-adiabatic transitions. Herein, we report the observation of a reversible conformational interconversion phenomenon in the 3p Rydberg excited-state dynamics of N,N-dimethylethylamine (DMEA). Upon electronic excitation, a forward and backward interconversion between the initially prepared 3p_l and 3p_h conformers accompanied by 3p internal conversions occurs, resulting in a 3p_l/3p_h equilibrium ratio of 61 %/39 % within ∼1.5 ps. The ensuing parallel internal conversions from the 3p_l to 3s_l and 3p_h to 3s_h deposit about 1.80 eV of vibrational energy into the 3s state, enabling a fast conformational interconversion between the 3s_h and 3s_l conformers to proceed within ∼2.0 ps. The final 3s_l/3s_h equilibrium ratio was determined to be 76 %/24 %. This work presents a real-time observation of the entire conformational interconversion process initiating from the higher-energy 3p states and finally reaching an equilibrium on the lower-energy 3s state.

Computing Accurate & Reliable Rovibrational Spectral Data for Aluminum-Bearing Molecules

The difficulty of quantum chemically computing vibrational, rotational, and rovibrational reference data via quartic force fields (QFFs) for molecules containing aluminum appears to be alleviated herein using a hybrid approach based upon CCSD(T)-F12b/cc-pCVTZ further corrected for conventional CCSD(T) scalar relativity within the harmonic terms and simple CCSD(T)-F12b/cc-pVTZ for the cubic and quartic terms: the F12-TcCR+TZ QFF. Aluminum containing molecules are theorized to participate in significant chemical processes in both the Earth's upper atmosphere as well as within circumstellar and interstellar media. However, experimental data for the identification of these molecules are limited, showcasing the potential for quantum chemistry to contribute significant amounts of spectral reference data. Unfortunately, current methods for the computation of rovibrational spectral data have been shown previously to exhibit large errors for aluminum-containing molecules. In this work, ten different methods are benchmarked to determine a method to produce experimentally-accurate rovibrational data for theorized aluminum species. Of the benchmarked methods, the explicitly correlated, hybrid F12-TcCR+TZ QFF consistently produces the most accurate results compared to both gas-phase and Ar-matrix experimental data. This method combines the accuracy of the composite F12-TcCR energies along with the numerical stability of non-composite anharmonic terms where the non-rigid nature of aluminum bonding can be sufficiently treated.

Stable, aromatic, and electrophilic azepinium ions: Design using quantum chemical methods

Cyclic nitrenium ions containing five-membered and six-membered rings are available, however, the seven-membered cyclic nitrenium ions (azepinium ions) are rare. The chemistry of these species is related to their stability originating from the aromaticity due to 6π electrons. Very few theoretical and experimental studies have been conducted on the azepinium ions. Related clozapine and olanzapine cations (diazepinium ions) were observed during drug metabolism studies. In this work, quantum chemical analysis has been carried out to estimate the stability, aromaticity, and electrophilicity of several derivatives of azepinium ions. A few of the designed azepinium ions carry ΔES-T values in the range of 50 kcal/mol favoring singlet state; π donating groups at the 2nd position increase the singlet-triplet energy differences. Most of the substituents reduce the NICS(1) values compared to the parent system. Ring fusion with heterocyclic five-membered rings generally increases the aromaticity and the stability of the azepinium ion ring systems. The electrophilicity parameters estimated in terms of HIA, FIA, and ω values indicate that it is possible to fine-tune the chemical properties of azepinium ions with appropriate modulation.

Chemical Bond Overlap Descriptors From Multiconfiguration Wavefunctions

The chemical bond is a fundamental concept in chemistry, and various models and descriptors have evolved since the advent of quantum mechanics. This study extends the overlap density and its topological descriptors (OP/TOP) to multiconfigurational wavefunctions. We discuss a comparative analysis of OP/TOP descriptors using CASSCF and DCD-CAS(2) wavefunctions for a diverse range of molecular systems, including X-O bonds in X-OH (XH, Li, Na, H2B, H3C, H2N, HO, F) and Li-X' (XF, Cl, and Br). Results show that OP/TOP aligns with bonding models like the quantum theory of atoms in molecules (QTAIM) and local vibrational modes theory, revealing insights such as overlap densities shifting towards the more electronegative atom in polar bonds. The Li-F dissociation profile using OP/TOP descriptors demonstrated sensitivity to ionic/neutral inversion during Li-F dissociation, highlighting their potential for elucidating complex bond phenomena and offering new avenues for understanding multiconfigurational chemical bond dynamics.

Infrared Spectroscopy of Ethanethiol Monomers and Dimers at MP2 Level: Characterizing the Dimer Formation and Hydrogen Bond

Ethanethiol, also known as ethyl mercaptan, is an organosulfur compound that appears as a colorless liquid with a distinctive odor. It has been detected in the interstellar medium, and its self-association has been the subject of a few known experimental studies, where the SH vibrational mode was used. However, unlike the analogous ethanol dimer, the ethanethiol dimer has not been thoroughly explored theoretically. In this study, ethanethiol and dimers were investigated using the MP2 method with various basis sets to determine the properties and stability of these structures. For the monomer, both trans and gauche structures were computed, with the gauche conformer being more stable, consistent with the available data in the literature. Local mode decomposition analysis of monomers showed that the CH2 rocking mode, associated with the CSH bending, is present only for the gauche isomer aligning with the experimental assignments. Furthermore, eight stable dimer configurations were identified and categorized into three groups: trans-trans, gauche-gauche, and trans-gauche isomers. Among these, the trans-gauche isomer was found to be the most stable. Dispersion is the dominant term for the ethanethiol dimer.

Direct mass spectrometry analysis of exhaled human breath in real-time

The molecular composition of exhaled human breath can reflect various physiological and pathological conditions. Considerable progress has been achieved over the past decade in real-time analysis of exhaled human breath using direct mass spectrometry methods, including selected ion flow tube mass spectrometry, proton transfer reaction mass spectrometry, extractive electrospray ionization mass spectrometry, secondary electrospray ionization mass spectrometry, acetone-assisted negative photoionization mass spectrometry, atmospheric pressure photoionization mass spectrometry, and low-pressure photoionization mass spectrometry. Here, recent developments in direct mass spectrometry analysis of exhaled human breath are reviewed with regard to analytical performance (chemical sensitivity, selectivity, quantitative capabilities) and applications of the developed methods in disease diagnosis, targeted molecular detection, and real-time metabolic monitoring.

Two Alkaline Metal Borates: from Layer to Layer-Pillared Framework

Two borates, Na2K[B5O8(OH)2]·H2O (1) and Na4K2[{B5O8}2{B3O5}{BO2(OH)2}]·2H2O (2) have been designed and made under solvothermal conditions. Compound 1 exhibits a 2D fluctuant layer based on the [B5O10(OH)2]7- clusters, containing two types of 9-membered ring (MR) channels and showing a four-connected sql topology net. By modifying the reactants and reaction temperature, compound 2 was obtained from compound 1. The addition of boron-containing components not only increased their concentration but also enriched the B-O fragment while higher temperatures promoted greater condensation of the anionic clusters. Compound 2 crystallizes in the acentric space group Pc and is composed of two types of [B5O11]7-clusters, [B3O7]5- clusters and [BO2(OH)2]3- tetrahedra. The two kinds of [B5O11]7- clusters form a 2D layer similar to 1, which is further connected by the chain built from the [B3O7]5- clusters and decorated by [BO2(OH)2]3- groups to produce a 3D layer-pillared framework. Notably, compound 2 is the first layer-pillared borate containing three types of B-O clusters and six types of channels. Second harmonic generation (SHG) measurements reveal that compound 2 displays a moderate SHG response of about 1.1 times that of KH2PO4 (KDP).

Thermogravimetric analysis of rice husk and low-density polyethylene co-pyrolysis: kinetic and thermodynamic parameters

The purpose of this study is to examine how co-pyrolysis of low-density polyethylene (LDPE) and rice husk is impacted by LDPE. It also looks into the physicochemical characteristics, thermal behavior, and kinetic parameters of these materials. To understand the thermal behavior through TGA, rice husk and LDPE blends in the ratios of LDPE: RH (50:50), LDPE: RH (25:75), and LDPE: RH (75:25) were prepared and tested. These tests were carried out in the presence of a nitrogen environment at a flow rate of 20 ml/min with a different heating rate of 10 to 40 °C/min from 30 to 600 °C.In this paper, activation energy (Ea) was measured using the integral method (coats and Redfern) and two distinct iso-conversional approaches are Flynn wall Ozawa (FWO) and Kissinger Akahira Sunose (KAS). According to this study, the Ea values during co-pyrolysis varied with the conversion points, demonstrating the complex nature of the materials that resulted from the process. Moreover, it can be said that the assessment of low-density polyethylene in conjunction with rice husk led to noteworthy changes in thermos kinetic behaviors. In the meantime, the calculated average activation energy is, in turn, 110-117, 101-102, and 102-107 kJ/mol. In this study, we analyze the thermodynamic parameters, including enthalpy, Gibbs free energy, and entropy, and also pyrolysis performance index was thoroughly explored to understand the co-pyrolysis process of rice husk and plastic waste. To develop efficient reactors for continuous operation regardless of feedstock composition, it was necessary to establish the significance of blending biomass with plastics in terms of augmented carbon conversion, volatiles, and reaction rate.

Gauge and unitary transformations in multipolar quantum optics

Multipolar quantum optics deals with the interaction of light with matter as a many-body bound system of charged particles where the coupling to electromagnetic fields is in terms of the multipolar electric polarization and magnetization. We describe two transformations applied to the conventional non-relativistic formalism, namely a gauge transformation applied directly to the fields at the Lagrangian stage and a unitary transformation applied to the old Hamiltonian. We show how such transformations lead to the same Power-Zienau-Woolley (PZW) formulation of the quantum electrodynamics (QED) of an overall electrically neutral many-body bound system of charges, including the internal motion as well as the gross dynamics of the centre of mass. Besides highlighting the utility of the multipolar formalism as a reliable and convenient platform in dealing with optical processes in atomic and molecular physics, it is shown how the analysis can also lead to the identification of the Röntgen effect arising from the gross motion of an electric dipole moment in a magnetic field and the Aharonov-Casher effect due to the motion of a magnetic dipole moment in an electric field. The importance of the two effects is pointed out in both experimental and theoretical contexts.This article is part of the theme issue 'The quantum theory of light'.

Short-time collective dynamics of an ionic liquid: A computer simulation study with non-polarizable and polarizable models, and ab initio molecular dynamics

Molecular dynamics (MD) simulation is used to study the intermolecular dynamics in the THz frequency range of the ionic liquid 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide, [C2C1im][FSI]. Non-polarizable and polarizable models for classical MD simulation are compared using as quality criteria ab initio molecular dynamics (AIMD) and experimental data from far-infrared (FIR) spectroscopy and previously published data of inelastic x-ray scattering (IXS). According to data from IXS spectroscopy, incorporating polarization in the classical MD simulation has relatively little effect on the dispersion curve (excitation frequency vs wavevector) for longitudinal acoustic modes. When the AIMD simulation is used as a reference, the polarizable model leads the time correlation functions of velocity, mass, and charge currents to relax abnormally quickly. The charge current spectra from the AIMD simulation and the non-polarizable model agree with the experimental FIR spectrum, while the polarizable model gives an excessively broad band. When compared to the non-polarizable model, the polarizable model does improve the calculation of transport coefficients (diffusion coefficient, viscosity, and conductivity); however, it yields overdamped short-time collective dynamics.

Assessment of electron-proton correlation functionals for vibrational spectra of shared-proton systems by constrained nuclear-electronic orbital density functional theory

Proton transfer plays a crucial role in various chemical and biological processes. A major theoretical challenge in simulating proton transfer arises from the quantum nature of the proton. The constrained nuclear-electronic orbital (CNEO) framework was recently developed to efficiently and accurately account for nuclear quantum effects, particularly quantum nuclear delocalization effects, in quantum chemistry calculations and molecular dynamics simulations. In this paper, we systematically investigate challenging proton transfer modes in a series of shared-proton systems using CNEO density functional theory (CNEO-DFT), focusing on evaluating existing electron-proton correlation functionals. Our results show that CNEO-DFT accurately describes proton transfer vibrational modes and significantly outperforms conventional DFT. The inclusion of the epc17-2 electron-proton correlation functional in CNEO-DFT produces similar performance to that without electron-proton correlations, while the epc17-1 functional yields less accurate results, comparable with conventional DFT. These findings hold true for both asymmetrical and symmetrical shared-proton systems. Therefore, until a more accurate electron-proton correlation functional is developed, we currently recommend performing vibrational spectrum calculations using CNEO-DFT without electron-proton correlation functionals.

Electronic Spectroscopy of the Halogenated Criegee Intermediate, ClCHOO: Experiment and Theory

A chlorine-substituted Criegee intermediate, ClCHOO, is photolytically generated using a diiodo precursor, detected by VUV photoionization at 118 nm, and spectroscopically characterized via ultraviolet-visible (UV-vis)-induced depletion of m/z = 80 under jet cooled conditions. UV-vis excitation resonant with a π* ← π transition yields a significant ground state depletion, indicating a strong electronic transition and rapid photodissociation. The broad absorption spectrum peaks at 350 nm and is attributed to contributions from both syn (∼70%) and anti (∼30%) conformers of ClCHOO based on spectral simulations using a nuclear ensemble method. Electronic structure theory shows significant differences in the vertical excitation energies of the two conformers (330 and 371 nm, respectively) as well as their relative stabilities in the ground and excited electronic states associated with the π* ← π transition. Natural bond orbital analysis reveals significant and nonintuitive nonbonding contributions to the relative stabilities of the syn and anti conformers.

Vibronic Interactions in the Photoelectron Spectra of CAl3Ge-: A Theoretical Study

This study probes the vibronic interactions in the photoelectron spectra of CAl3Ge-, exploring its six excited electronic states through an approach that combines the ab initio electronic structure calculations and the quantum nuclear dynamics. Central to this investigation is utilizing a model diabatic Hamiltonian, which allows for the exact evaluation of Hamiltonian parameters and fitting potential energy cuts (PECs). Notably, the analysis of these PECs uncovers pronounced nonadiabatic effects within the photoelectron spectra, emphasized by the presence of multiple conical intersections. The investigation of nuclear dynamics, with and without the same spatial symmetry coupling, is achieved by employing time-dependent (TD) and time-independent (TI) quantum mechanical methods. These nuclear dynamical studies effectively simulated all the photoelectron spectral bands. Eventually, the theoretical findings conformed well with available experimental observations, highlighting the nonadiabatic effects among the closely situated spectral bands.

DFT-based calculation of vibrational sum frequency generation spectral features of crystalline β-sheets in silk: Polarization and azimuth angle dependences

Sum frequency generation (SFG) necessitates both noncentrosymmetry and coherence over multiple length scales. These requirements make vibrational SFG spectroscopy capable of probing structural information of noncentrosymmetric organic crystals interspersed in polymeric matrices and their three-dimensional spatial distributions within the matrices without spectral interferences from the amorphous components. However, this analysis is not as straightforward as simple vibrational spectroscopy or scattering experiments; it requires knowing the molecular hyperpolarizability of SFG-active vibrational modes and their interplay within the coherence length. This study demonstrates how density function theory (DFT) calculations can be used to construct the molecular hyperpolarizability of a model system and combine it with the SFG theory to predict the polarization and azimuth angle dependences of SFG intensities. A model system with short peptide chains mimicking β-sheet domains in Bombyx mori silk was chosen. SFG signals of the amide-I, II, III, and A bands and one of the CH deformation modes were simulated and compared with the experimental results and the predictions from the group theory. The SFG features of amide-I and A bands of antiparallel β-sheet could be explained with DFT-based theoretical calculations. Although vibrational coupling with neighboring groups breaks the symmetry of the D2 point group, the group theory approach and DFT calculations gave similar results for the amide-I mode. The DFT calculation results for amide-II did not match with experimental data, which suggested vibrational coupling within a larger crystalline domain may dominate the SFG spectral features of these modes. This methodology can be applied to the structural analysis of other biopolymers.

Data science shows that entropy correlates with accelerated zeolite crystallization in Monte Carlo simulations

We have performed a data science study of Monte Carlo (MC) simulation trajectories to understand factors that can accelerate the formation of zeolite nanoporous crystals, a process that can take days or even weeks. In previous work, MC simulations predicted and experiments confirmed that using a secondary organic structure-directing agent (OSDA) accelerates the crystallization of all-silica LTA zeolite, with experiments finding a three-fold speedup [Bores et al., Phys. Chem. Chem. Phys. 24, 142-148 (2022)]. However, it remains unclear what physical factors cause the speed-up. Here, we apply data science to analyze the simulation trajectories to discover what drives accelerated zeolite crystallization in MC simulations going from a one-OSDA synthesis (1OSDA) to a two-OSDA version (2OSDA). We encoded simulation snapshots using the smooth overlap of atomic positions approach, which represents all two- and three-body correlations within a given cutoff distance. Principal component analyses failed to discriminate datasets of structures from 1OSDA and 2OSDA simulations, while the Support Vector Machine (SVM) approach succeeded at classifying such structures with an area-under-curve (AUC) score of 0.99 (where AUC = 1 is a perfect classification) with all three-body correlations and as high as 0.94 with only two-body correlations. SVM decision functions reveal relatively broad/narrow histograms for 1OSDA/2OSDA datasets, suggesting that the two simulations differ strongly in information heterogeneity. Informed by these results, we performed pair (2-body) entropy calculations during crystallization, resulting in entropy differences that semi-quantitatively account for the speedup observed in the previous MC simulations. We conclude that altering synthesis conditions in ways that substantially change the entropy of labile silica networks may accelerate zeolite crystallization, and we discuss possible approaches for achieving such acceleration.

The V30 benchmark set for anharmonic vibrational frequencies of molecular dimers

Intermolecular vibrations are extremely challenging to describe but are the most crucial part for determining entropy and hence free energies and enable, for instance, the distinction between different crystal-packing arrangements of the same molecule via THz spectroscopy. Herein, we introduce a benchmark dataset-V30-containing 30 small molecular dimers with intermolecular interactions ranging from exclusively van der Waals dispersion to systems with hydrogen bonds. All the calculations are performed with the gold standard of quantum chemistry CCSD(T). We discuss vibrational frequencies obtained via different models starting with the harmonic approximation over independent Morse oscillators up to second-order vibrational perturbation theory (VPT2), which allows a proper anharmonic treatment including coupling of vibrational modes. However, large amplitude motions present in many low-frequency intermolecular modes are problematic for VPT2. In analogy to the often used treatment for internal rotations, we replace such problematic modes by a simple one-dimensional hindered rotor model. We compare selected dimers to the available experimental data or high-level calculations of potential energy surfaces and show that VPT2 in combination with hindered rotors can yield a very good description of fundamental frequencies for the discussed subset of dimers involving small and semi-rigid molecules.

Systematic ab initio calculation of spectroscopic constants for A-reduced rotational effective Hamiltonians of asymmetric top molecules using normal ordering of cylindrical angular momentum operators

This research paper presents a new fundamental approach for evaluating accurate ab initio quartic, sextic, and octic centrifugal distortion parameters of A-reduced rotational effective Hamiltonians of asymmetric top molecules. In this framework, the original Watson Hamiltonian, expanded up to sextic terms of kinetic and potential energies, is subjected to a series of vibrational and rotational operator unitary transformations, leading to reduced Watson effective Hamiltonians for the equilibrium configuration, ground state, and weakly perturbed vibrationally excited states. The proposed scheme is based on a numerical-analytic implementation of the sixth-order Van Vleck operator perturbation theory with the systematic normal ordering of vibrational rising and lowering operators (a†, a) and cylindrical angular momentum operators (Jz, J+, J-). The efficiency of the developed theoretical model is demonstrated by the juxtaposition of predicted centrifugal distortion parameters for several three to eight atomic molecules, including H2S, CH2O, C2H4, CH2D2, CH2F2, CH2Cl2, and B2H6, using the coupled-cluster single double triple/quadruple-ζ level of quantum chemistry. In comparison with the values derived using the customary analytic expressions, the calculated quartic and sextic parameters may improve by an order of magnitude in the fourth and sixth orders, respectively, reaching an accuracy of about 1%. Predicted octic constants can serve as an excellent starting point for fitting to experimental spectra.

Enhanced Circularly Polarized Luminescence of Urea-Bridged Dimers of Axially Chiral BODIPY-Carbohydrate Hybrids

Herein, we report the synthesis of novel dimeric urea-bridged BODIPY-carbohydrate conjugates, which display circularly polarized luminescence (CPL). The dimers are composed of diastereomerically pure, axially chiral (P or M) BODIPY monomers containing a pendant glucose (d- or l-) unit. The latter was intended to add chirality, biocompatibility, and enhanced water solubility and facilitate the chromatographic resolution of the intermediate atropisomers. The dimerization process was based on the ureation reaction of azidomethyl BODIPYs. The rigorous structural assignment was possible by X-ray diffraction analysis of one of the BODIPY atropisomers.

Anion-Cation-Anion Ion Triplet Characterization by Computation and Photoelectron Spectroscopy

Ion triplets of the chloride salts of two commonly used weakly coordinating cations are reported (i.e., Cl-·NMe4+Cl- (1-) and Cl-·PPh4+Cl- (2-)). Negative ion photoelectron spectra at 20 K afford vertical and adiabatic detachment energies of 5.18 and 5.00 eV (1-) and 5.03 and 4.70 eV (2-), respectively. These results are well reproduced by coupled cluster calculations with single, double, and perturbative triple excitations (CCSD(T)) whereas M06-2X is systematically too small by ∼0.3 eV (i.e., 7 kcal mol-1). The structures of both 1- and 2- have five or six C-H···Cl- interactions that stabilize these cluster anions by 32 (1-) and 27 (2-) kcal mol-1 as given by their chloride dissociation enthalpies. These values drop to 7.4 and 3.8 kcal mol-1 in dichloromethane based up conductor-like polarizable continuum model calculations and suggest that X-·M+X- ion triplets with a weakly coordinating cation maybe the reactive form of salts under some conditions.

The Manifestations of "l-Doubling" in Gas-Phase Rotational Dynamics

The "l-Doubling" phenomenon emanates from the coupling between molecular rotations and perpendicular vibrations (bending modes) in polyatomic molecules. This elusive phenomenon has been largely discarded in laser-induced molecular alignment. Here we explore and unveil the ramifications of l-Doubling on the coherent rotational dynamics of linear triatomic molecules at ambient temperatures and above. The observed l-Doubling dynamics may be wrongly considered as collisional decay throughout the first few hundreds of picoseconds past excitation, highlighting the importance of correct assimilation of l-Doubling in current research of dissipative rotational dynamics and in coherent rotational dynamics in general.

Experimental and Theoretical Investigation of the Reaction of C2H with Formaldehyde (CH2O) at Very Low Temperatures and Application to Astrochemical Models

Rate coefficients for the reaction of C2H with CH2O were measured for the first time over the temperature range of 37-603 K, with the C2H radicals produced by pulsed laser photolysis and detected by CH radical chemiluminescence following their reaction with O2. The low temperature measurements (≤93 K) relevant to the interstellar medium were made within a Laval nozzle gas expansion, while higher temperature measurements (≥308 K) were made within a temperature controlled reaction cell. The rate coefficients display a negative temperature dependence below 300 K, reaching (1.3 ± 0.2) × 10-10 cm3 molecule-1 s-1 at 37 K, while only a slight positive temperature dependence is observed at higher temperatures above 300 K. Ab initio calculations of the potential energy surface (PES) were combined with rate theory calculations using the MESMER master-equation program in order to predict rate coefficients and branching ratios. The three lowest energy entrance channels on the PES all proceed via the initial formation of a weakly bound prereaction complex, bound by ∼5 kJ mol-1, followed by either a submerged barrier on the route to the H-abstraction products (C2H2 + CHO), or emerged barriers on the routes to the C- or O-addition species. MESMER calculations indicated that over the temperature range investigated (10-600 K) the two addition channels were uncompetitive, accounting for less 0.3% of the total product yield even at 600 K. The PES containing only the H-abstraction product channel was fit to the experimentally determined rate coefficients, with only a minor adjustment to the height of the submerged barrier (from -2.6 to -5.9 kJ mol-1) required. Using this new submerged barrier height, and including the subsequent dissociation of the CHO product into CO + H in the PES, rate coefficients and branching ratios were calculated over a wide range of temperatures and pressures and these used to recommend best-fit modified Arrhenius expressions for use in astrochemical modeling. Inclusion of the new rate coefficients and branching ratios in a UMIST chemical model of an outflow from an asymptotic giant branch (AGB) star yielded no significant changes in the abundances of the reactants or the products of the reaction, however, removal of the C-addition channel currently in the UMIST Rate22 database did result in a significant reduction in the abundance of propynal (HCCCHO).

Chlorine Substitution Effect on the S1 Relaxation Dynamics of Chlorobenzene and Chlorophenols

The S1 state relaxation dynamics of chlorobenzene (CB), 3-chlorophenol (3-CP), 3-CP·H2O, and 2-chlorophenol·H2O (2-CP·H2O) have been investigated by means of picosecond time-resolved pump-probe spectroscopy in a state-specific manner. For CB, the S1 state relaxes via the S1-S0 internal conversion in the low internal energy region (<2000 cm-1), whereas the direct C-Cl bond dissociation channel mediated by the upper-lying repulsive πσCCl* state is opened to give the rather sharp increase of the S1 relaxation rate in the high internal energy region (>2000 cm-1). A similar dynamic feature has been observed for 3-CP in terms of the lifetime behavior with an increase in the S1 internal energy, suggesting that the H atom tunneling dissociation reaction from OH might contribute less compared to the internal conversion, although it is not clear at the present time whether or not the sharp increase of the S1 relaxation rate in the high internal energy region of 3-CP (>1500 cm-1) is entirely due to that of the internal conversion. The fact that the internal conversion is facilitated by the Cl substitution implies that the energetic location of the S1/S0 conical intersection should have been strongly influenced by chlorine substitution on the aromatic ring. The approximate energetic location of the saddle point of the S1(ππ*)/πσCCl* conical intersection along the seam coordinate for CB or 3-CP could be inferred from the energy-dependent S1 lifetime measurements. It is discussed in comparison with the dynamic role of the S1(ππ*)/πσCCl* conical intersection, which is strongly influenced by the O-H···Cl intramolecular hydrogen bond in the rather complicated yet ultrafast S1 relaxation dynamics of the cis-2-CP. The S1 lifetimes of 3-CP·H2O and 2-CP·H2O reveal the importance of the conformational structures, especially in terms of the intramolecular hydrogen bonding.

State-to-State Time-Dependent Quantum Dynamics Studies of the Si(3P) + OH(X 2Π) → OSi(X 1Σg+) + H(2S) Reaction Based on a New HOSi(X2A') Potential Energy Surface

Quantum and quasi-classical dynamics calculations were conducted for the reaction of Si with OH on the latest potential energy surface (PES), which is obtained by fitting tens of thousands of ab initio energy points by using the many-body expansion formula. To obtain an accurate PES, all energy points calculated with aug-cc-pVQZ and aug-cc-pV5Z basis sets were extrapolated to the complete basis set limit. The accuracy of our new PES was verified by comparing the topographic characteristics and contour maps of potential energy with other works. In addition, the anharmonic vibrational frequencies of HOSi and HSiO based on the present ab initio and PES by means of quantum dynamics methods were calculated. Dynamics information such as reaction probability, integral cross sections (ICS), product distribution, and rate constants was obtained on the new HOSi(X2A') PES. The dynamic information calculated using the quasi-classical trajectory method and time-dependent wave packet method is generally in good agreement, except for the vibrational state-resolved ICSs of product. The calculated differential cross section and capture time reveal that the reaction is primarily governed by the complex formation mechanism.

Correlation between Noncovalent Bond Strength and Spectroscopic Perturbations within the Lewis Base

Me2CO was allowed to interact with 20 different Lewis acids so as to engage in various sorts of noncovalent interactions, encompassing hydrogen, halogen, chalcogen, pnictogen, and tetrel bonds. Density functional theory computations evaluated the interaction energy of each dyad, which was compared with spectroscopic, geometric, AIM, and energy decomposition elements so as to elucidate any correlations. The red shift of the C═O stretching frequency, and the changes in the nuclear magnetic resonance shielding of the O and C atoms of acetone, are closely correlated with the interaction energy so can be used to estimate the latter from experimental measurements. The standard AIM measures at the bond critical point, ρ, ∇2ρ, and V also correlate with the energy, albeit not as well as the spectroscopic parameters. The σ-hole depth on the Lewis acid is not well correlated with the energetics, due in part to the fact that electrostatics in general are not an accurate metric of bond strength.

Exploring the Influence of Approximations for Simulating Valence Excited X-ray Spectra

First-principles simulations of excited-state X-ray spectra are becoming increasingly important to interpret the wealth of electronic and geometric information contained within femtosecond X-ray absorption spectra recorded at X-ray Free Electron Lasers (X-FELs). However, because the transition dipole matrix elements must be calculated between two excited states (i.e., the valence excited state and the final core excited state arising from the initial valence excited state) of very different energies, this can be challenging and time-consuming to compute. Herein using two molecules, protonated formaldimine and cyclobutanone, we assess the ability of n-electron valence-state perturbation theory (NEVPT2), equation-of-motion coupled-cluster theory (EOM-CCSD), linear-response time-dependent density functional theory (LR-TDDFT) and the maximum overlap method (MOM) to describe excited state X-ray spectra. Our study focuses in particular on the behavior of these methods away from the Franck-Condon geometry and in the vicinity of important topological features of excited-state potential energy surfaces, namely, conical intersections. We demonstrate that the primary feature of excited-state X-ray spectra is associated with the core electron filling the hole created by the initial valence excitation, a process that all of the methods can capture. Higher energy states are generally weaker, but importantly much more sensitive to the nature of the reference electronic wave function. As molecular structures evolve away from the Franck-Condon geometry, changes in the spectral shape closely follow the underlying valence excitation, highlighting the importance of accurately describing the initial valence excitation to simulate the excited-state X-ray absorption spectra.

Photoswitching Molecules Functionalized with Optical Cycling Centers Provide a Novel Platform for Studying Chemical Transformations in Ultracold Molecules

A novel molecular structure that bridges the fields of molecular optical cycling and molecular photoswitching is presented. It is based on a photoswitching molecule azobenzene functionalized with one and two CaO- groups, which can act as optical cycling centers (OCCs). This paper characterizes the electronic structure of the resulting model systems, focusing on three questions: (1) how the electronic states of the photoswitch are impacted by a functionalization with an OCC; (2) how the states of the OCC are impacted by the scaffold of the photoswitch; and (3) whether the OCC can serve as a spectroscopic probe of isomerization. The experimental feasibility of the proposed design and the advantages that organic synthesis can offer in the further functionalization of this molecular scaffold are also discussed. This work brings into the field of molecular optical cycling a new dimension of chemical complexity intrinsic to only polyatomic molecules.

Thermally Initiated Formation of Criegee Intermediate CH2OO in the Oxidation of Ethane

Criegee intermediates (CIs) play an important role in atmospheric chemistry as a transient source of the OH radical through their formation by the ozonolysis of unsaturated organic compounds. Here, we report thermally initiated formation of the smallest CI (CH2OO) in the oxidation of ethane (CH3CH3) that may be relevant to combustion and flames. The SiO2/SiC oxidation microreactor is heated to 1800 K and has a short residence time of ∼100 μs. The CH2OO we observe is likely formed in a lower-temperature region near the microreactor's exit. Plausible mechanisms for CH2OO formation and retention under these conditions mediated by methylperoxy (CH3OO) radicals are discussed. Pure rotational spectra of CH2OO and other intermediates (HO2, CH3CHO, CH2CHOH, c-CH2OCH2, CH3CH2CHO, CH3OOH, and HCOOH) are detected with a chirped-pulse Fourier transform millimeter-wave spectrometer operating in the frequency range of 60-90 GHz. Detection occurs in a molecular beam, where the species are supersonically cooled to 5 K.

Unraveling the effect of reagent vibrational excitation on the scattering mechanism of the benchmark H + H2 → H2 + H hydrogen exchange reaction in the coupled 12E' ground electronic manifold

The hydrogen exchange reaction, H + H2 → H2 + H, along with its isotopic variants, has been the cornerstone for the development of new and novel dynamical mechanisms of gas-phase bimolecular reactions since the 1930s. The dynamics of this reaction are theoretically investigated in this work to elucidate the effect of reagent vibrational excitation on differential cross sections (DCSs) in a nonadiabatic situation. The dynamical calculations are carried out using a time-dependent quantum mechanical method, both on the lower adiabatic potential energy surface and employing a two-state coupled diabatic theoretical model to explicitly include all the nonadiabatic couplings present in the 12E' ground electronic manifold of the H3 system. Towards this effort, the Boothroyd-Keogh-Martin-Peterson (BKMP2) surface of the lower adiabatic component is coupled with the double many-body expansion (DMBE) surface of the upper one. The smooth variation of energy along the D3h seam of the conical intersections is explicitly confirmed. The coupled two-state calculations are performed only for H2 (v = 3-4, j = 0), as the minimum of the conical intersections becomes energetically accessible for these vibrational levels in the considered energy range. Initial state-selected total and state-to-state DCSs are calculated to elucidate various mechanistic aspects of reagent vibrational excitation. The latter enhances the forward scattering and makes the backward scattering less prominent. Important roles of collision energy in the vibrational energy disposal of both forward- and backward-scattered products are examined. Analysis of the state-to-state DCSs of the vibrationally excited reagents reveals an important correlation among scattering angle, and the product rotational angular momentum and its helicity state. Such an analysis establishes a novel mechanism for the forward scattering of the reaction.

Significant Chiral Asymmetry Observed in Neutral Amino Acid Ultraviolet Photolysis

The origin of homochirality in biological organisms remains an open question. Some suggest that its origin might be extraterrestrial, specifically due to the exposure of chiral molecules to circularly polarized photons in interstellar space, which could cause an initial population imbalance leading to the homochirality observed today. However, this extraterrestrial hypothesis has not been widely accepted, largely due to the belief that molecular optical rotatory dispersion is too insignificant to create the substantial imbalance required for homochirality. Here we report experimental evidence that specific conformers of neutral amino acids exhibit significant asymmetry in the chiral destroying dissociation rate induced by circularly polarized photons. The observed anisotropy factor for the lowest energy conformer of leucine was remarkably large, reaching 0.1─a factor of 13 times larger than observed for zwitterionic leucine in solid films, and nearly 40 times greater than the anisotropy reported in the electronic absorption spectrum of gas-phase leucine ensembles at room temperature. This significant finding indicates that even if reported anisotropy values in the electronic absorption spectrum are low, the dissociation asymmetry of certain conformers can still be substantial. An anisotropy factor of 0.1 could result in an initial enantiomeric excess exceeding 10%, even with a 90% extent of reaction. This discovery suggests that asymmetric photodissociation of amino acids may have been a crucial factor in the emergence of biological homochirality.

Semiclassical description of nuclear quantum effects in solvated and condensed phase molecular systems

In this perspective we deal with the challenge of investigating nuclear quantum effects in solvated and condensed phase molecular systems in a computationally affordable way. To this end, semiclassical methods are promising theoretical approaches, as we demonstrate through vibrational spectroscopy and reaction kinetics. We show that quantum vibrational features can be found in hydrates of carbonyl compounds and microsolvated amino acids, and we report quantum estimates of the low-temperature reaction rate constant of a unimolecular reaction taking place in a noble-gas matrix. The hallmark of semiclassical methods is their ability to include nuclear quantum effects into classical molecular dynamics simulations. For this reason, unlike other popular methods, semiclassical approaches are able to account also for real-time quantum contributions and are expected to point out the importance of nuclear quantum effects in complex systems for a wider range of chemical properties.

Cationic Coordination Modification Drives Birefringence and Nonlinear Effect Double Lifting in Sulfate

As nonlinear optical (NLO) crystals, sulfates have the superiority of transparency for ultraviolet (UV) light, but they are often troubled by small nonlinear coefficients and birefringence owing to the high symmetry of the [SO4]2- group. By introducing two neutral diethylenetriamine (DETA) molecules to replace the six coordinated water molecules of the [Zn(H2O)6]2+ complex cation in [Zn(H2O)6](SO4)(H2O), a new sulfate with an acentric structure, namely, [Zn(DETA)2](SO4)(H2O)3, has been designed and synthesized. Structural investigation reveals that the coordination modification of Zn2+ ion tremendously enhances its intraoctahedral distortion. The formed distorted [Zn(DETA)2]2+ cations and the [SO4]2- groups feature an optimized arrangement, endowing [Zn(DETA)2](SO4)(H2O)3 with enhancements in both second harmonic generation (SHG) intensity, from undetectable to 0.25 × KH2PO4 (KDP), and birefringence, from 0.014 to 0.042 at 1064 nm. Despite the slight compromise made in the light transmission range, [Zn(DETA)2](SO4)(H2O)3 still possesses a short absorption edge of 220 nm, retaining the majority of the light transmission range in the solar-blind region. Our work provides a novel and applicable approach of cationic coordination modification to improve the nonlinear optical coefficient and birefringence of sulfates.

Exploring New Algorithms for Molecular Vibrational Spectroscopy Using Physics-Informed Program Synthesis

Inductive program synthesis (PS) has recently begun to emerge as a useful new approach to automatically generate algorithms in quantum chemistry, as demonstrated in recent applications to the vibrational Schrödinger equation for simple model systems with one or two degrees-of-freedom. Here, we report a new physics-informed approach to inductive PS that is more conducive to the generation of discrete variable representation algorithms for real molecular systems. The new framework ensures separability of the kinetic and potential operators and does not require an exact solution to compare synthesized algorithmic predictions with. Algorithms with a tridiagonal matrix structure are generated via a variational-based stochastic optimization procedure. Crucially, through an extensive testing procedure, we demonstrate that variationally synthesized algorithms perform just as well as those generated using a target function. Assuming a direct product representation of normal coordinates, these algorithms are applied to three triatomic molecules. In total, we identify a set of seven PS algorithms that accurately reproduce the vibrational spectra of H2O, NO2, and SO2, as predicted by Colbert-Miller and sine-DVR algorithms.

Nonadiabatic ab initio chemical reaction dynamics for the photoisomerization reaction of 3,5-dimethylisoxazole via the S1 electronic state

Nonadiabatic ab initio molecular dynamics simulations were performed to explore the photoisomerization pathway from isoxazole (iso-OXA) to oxazole (OXA), considering four electronic states. The XMS-CASPT2 and SA4-CASSCF theories were employed to describe these electronic structures, which were caused by 12 electrons in 11 orbitals with the cc-pVDZ + sp diffuse basis set; the Gaussian s- and p-type diffuse functions were extracted from Dunning's aug-cc-pVDZ function. The potential energy and its gradient at each time step were computed on-the-fly at these levels in the time evolution of the classical trajectory. When the two electronic states were close to each other, the trajectory surface hopping (TSH) judgment between the two adjacent states was carried out by the anteater procedure based on the Zhu-Nakamura formula (ZN-TSH). The two different excited state lifetimes were found to exist in the first electronic state (S1), estimated at 10.77 and 119.81 fs. Upon photoexcitation, the N-O bond breaks and energetically relaxes to the ground state (S0). In the pathway leading to the main product, azirine formation, the 5-membered ring retains a planar structure while undergoing a non-adiabatic transition with an increasing N-O bond distance. Furthermore, it was verified that a 1,2-shift takes place in the pathway that results in the production of ketenimine, causing a nonadiabatic transition.

Competition between C-H⋯S and S-H⋯Cl H-bonds in a CHCl3-H2S complex: a combined matrix isolation IR spectroscopic and quantum chemical investigation

H-bonded complexes between CHCl3 and H2S have been studied in a cold and inert argon matrix using IR spectroscopy. Both molecules were found to act as both a H-bond donor and acceptor, resulting in two different conformers. The more stable one (binding energy 3.25 kcal mol-1) was bound by a C-H⋯S H-bond, while the less stable one (1.90 kcal mol-1) by a S-H⋯Cl H-bond. The H-bonded complex formation has been confirmed by monitoring the spectral changes in νC-H and νS-H fundamental vibrations. The νC-H mode exhibited red shifts by 22.7 cm-1 upon C-H⋯S H-bond formation, while the formation of a S-H⋯Cl H-bond resulted in 24.2 and 25.4 cm-1 red shifts in the νS-H modes. The barrier for conversion of the less stable conformer to the more stable one was found to be 0.6 kcal mol-1. The rigid matrix environment prevented any detectable population transfer that required significant relative movement of the monomeric moieties. Dispersion interaction was found to significantly contribute to the overall stabilization of both conformers, more so for the S-H⋯Cl H-bonded one.

The Changing Phases of Bromopentafluorobenzene

As part of a much larger study on non-covalent interactions in binary adducts, we have explored the solid-state structures of bromopentafluorobenzene (C6F5Br) using differential scanning calorimetry (DSC), variable-temperature powder X-ray diffraction (VT-PXRD), and single-crystal X-ray diffraction (SXD). DSC data initially indicated a single solid-state phase below the freezing point, but revealed additional weak transitions upon heating. The crystal structures of three solid-state phases have been solved. The SXD data showed that phases I and IV are centrosymmetric, whilst phase II is polar. However, the structure of phase III remains elusive due to the changing phase behaviour of C6F5Br that is determined as much as by kinetics as thermodynamics. The results underline the need for multiple analytical techniques to study non-covalent interactions and offer valuable data for refining computational models in crystal structure prediction and machine learning. A comparison with the iodinated counterpart is also made.

Systematic Investigation of Electronic States and Bond Properties of LnO, LnO+, LnS, and LnS+ (Ln = La-Lu) by Spin-Orbit Multiconfiguration Perturbation Theory

The electronic structures of lanthanide monoxides (LnO/LnO+) and monosulfides (LnS/LnS+) for all lanthanide series elements (Ln = La-Lu) have been systematically analyzed with sophisticated quantum chemical calculations. The ground electronic configuration has been determined to be Ln 4fn6s1 or 4fn+1 for the neutral molecules and Ln 4fn for the cations. The low-lying energy states resulting from spin-orbit coupling and ligand field effects have been resolved using spin-orbit multiconfiguration quasi-degenerate second-order perturbation theory calculations. The ionization energies of LnO (5.20-7.06 eV) are about 0.3-2.2 eV lower than those of LnS (5.54-9.22 eV) due to the difference in the Ln 6s and 4f orbital energies from which an electron is removed during the ionization process. The bond dissociation energies (BDEs) have been computed by the state-averaged general multiconfigurational perturbation theory and the completely renormalized coupled-cluster [CR-CC(2,3)] methods. The BDEs are highly dependent on the lanthanide elements as several factors of the lanthanides affect the bond dissociation. The calculated bond lengths and energies agree well with available experimental values and are systematically predicted for the series of lanthanide monoxides and monosulfides where experimental values are not available. Furthermore, the LS terms of low-lying energy states and their corresponding bond properties have been clarified in detail to systematize the similarities and differences of the lanthanide compounds.

Spectroscopy and Excited-State Dynamics of Methyl Ferulate in Molecular Beams

The spectroscopic and dynamic properties of methyl ferulate─a naturally occurring ultraviolet-protecting filter─and microsolvated methyl ferulate have been studied under molecular beam conditions using resonance-enhanced multiphoton ionization spectroscopy in combination with quantum chemical calculations. We demonstrate and rationalize how the phenyl substitution pattern affects the state ordering of the lower excited singlet state manifold and what the underlying reason is for the conformation-dependent Franck-Condon (FC) activity in the UV-excitation spectra. Studies on microsolvated methyl ferulate reveal potential coordination sites and the influence of such coordination on the spectroscopic properties. Our quantum chemical studies also enable us to obtain a quantitative understanding of the dominant excited-state decay routes of the photoexcited ππ* state involving a ∼3 ns intersystem crossing pathway to the triplet manifold─which is much slower than found for coumarates─and a relatively fast intersystem crossing back to the ground state (∼30 ns). We show that a common T1/S0 crossing can very well explain the observation that T1 lifetimes are quasi-independent of the phenyl substitution pattern.

Activation and Functionalization of the Uranyl Ion by Electrochemical Reduction

Interconversion of the oxidation states of uranium enables separations and reactivity schemes involving this element and contributes to technologies for recycling of spent nuclear fuels. The redox behaviors of uranium species impact these processes, but use of electrochemical methods to drive reactions of molecular uranium complexes and to obtain molecular insights into the outcomes of electrode-driven reactions has received far less attention than it deserves. Here, we show that electro-reduction of the uranyl ion (UO22+) can be used to promote stepwise functionalization of the typically unreactive oxo groups with exogenous triphenylborane (BPh3) serving as a moderate electrophile, avoiding the conventional requirement for a chemical reductant. Parallel electroanalytical, spectrochemical, and chemical reactivity studies, supported by spectroscopic findings and structural data from X-ray diffraction analysis on key reduced and borylated products, demonstrate that our electrochemical approach largely avoids undesired cross reactions and disproportionation pathways; these usually impact the multicomponent systems needed for uranyl functionalization chemistry. Joint computational studies have been used to map the changes associated with U-O activation and to quantify the free energy differences related to key reactions. Taken together, the results suggest that electrochemical methods can be used for selective interconversion of molecular actinide species, reminiscent of methods commonly employed in transition metal redox catalysis.

Chiral recognition of amino acids through homochiral metallacycle [ZnCl2L]2

Chiral recognition holds tremendous significance in both life science and chemistry. The ability to differentiate between enantiomers is crucial because one enantiomer typically holds greater biological relevance while its counterpart is often not only unnecessary but also potentially harmful. In this regard, homochiral metallacycle [ZnCl2L]2 is used in this study to understand and differentiate between the R and S enantiomers of amino acids (alanine, proline, serine, and valine). The electronic, geometric, and thermodynamic stabilities of the amino acid enantiomers inside the metallacycle are determined through various analyses. The greater interaction energy (Eint) is obtained for the ser@metallacycle complexes i.e., -33.03 and -30.75 kcal mol-1, respectively for the S and R enantiomers. The highest chiral discrimination energy of 3.11 kcal mol-1 is achieved for ala@metallacycle complexes. Regarding the electronic properties, the frontier molecular orbital (FMO) analysis indicates that the energy gap decreases after complexation, which is confirmed through density of states (DOS) analysis. Moreover, natural bond orbital (NBO) analysis determines the amount and direction of charge transfer i.e., from metallacycle towards amino acids. The maximum NBO charge transfer is observed for S-pro@metallacycle complex i.e., -0.291 |e|. Electron density difference (EDD) analysis further proves the direction of charge transfer. Noncovalent interaction index (NCI) and quantum theory of atoms in molecules (QTAIM) analyses demonstrate that the noncovalent interactions present between the host and guest are the weak van der Waals forces and hydrogen bonding. The results of NCI and QTAIM analyses for all the complexes are in alignment with those of the interaction energy (Eint) and chiral discrimination energy (Echir) analyses, i.e., significantly greater non-bonding interactions are observed for the complexes with greater Echir, i.e., for ala@metallacycle. Overall, our analyses demonstrate the excellent chiral discrimination ability of metallacycle towards chiral molecules, i.e., for enantiomers of amino acids through host-guest supramolecular chemistry.

Revealing Local Structure of Angiotensin Receptor-Neprilysin Inhibitor (S086) Drug Cocrystal by Linear and Nonlinear Infrared Spectroscopies

Structurally knowing the active sites of a drug is important for understanding its therapeutic functions. S086 is a novel angiotensin receptor-neprilysin inhibitor that consists of the molecular moieties of EXP3174 (the active metabolite of the angiotensin receptor blocker losartan) and sacubitril (a neprilysin inhibitor prodrug) in a 1:1 molar ratio. There are two forms of cocrystals of S086, namely, ξ-crystal and α-crystal, which were formed both via intermolecular coordination bonding to calcium ions, with the aid of internal water. The binding state of multiple carboxyl anions (COO-) to Ca2+ of EXP3174 and sacubitril was examined in this study using infrared (IR) absorption spectroscopy, in which the asymmetric stretching (as) and symmetric stretching (ss) modes of the COO- groups were used as IR probes. Ultrafast two-dimensional (2D) IR spectroscopy was utilized for spectrally assigning the origin of multiple COO- groups by the presence or absence of interchromophore vibrational coupling. Key structural variation between the two crystal forms was found: in the unit cell of ξ-crystal, the ratio of "bridging" and "bidentate" types of COO- binding to Ca2+ for four EXP3174 molecules is 2:2, while the ratio is predicted to be 3:1 in the case of α-crystal. However, in both crystals, four sacubitril molecules are believed to similarly form a "trident" type of COO- binding to Ca2+. Our study demonstrates that linear and nonlinear IR spectroscopies can be used to characterize local crystal structures of drugs and reveal subtle difference between similar crystal structures.

N-Aromatic Complexation in Tetraphenyl Porphyrin Iron (III)-Pyridine: Evidence of Spin-Flip via Gas-Phase Electronic Spectroscopy

There is growing interest in the electronic properties of metalloporphyrins especially in relation to their interactions with other molecular species in their local environment. Here, UV-VIS laser photodissociation spectroscopy in vacuo has been applied to an iron-centred metalloporphyrin (FeTPP+) and its N-aromatic adduct with pyridine (py) to determine the electronic effect of complexation. Both the metalloporphyrin (FeTPP+) and pyridine adduct (FeTPP+⋅py) absorb strongly across the spectral region studied (652-302 nm: 1.91-4.10 eV). Notably, a large blue shift was observed for the dominant Soret band (41 nm) upon complexation (0.47±0.02 eV), indicative of strong pyridine binding. Significant differences in the profiles (i. e. number and position of bands) of the electronic spectra are evident comparing FeTPP+ and FeTPP+⋅py. Time-dependent density functional theory calculations were used to assign the spectra, revealing that the FeIII spin-state flips from S=3/2 to S=5/2 upon complexation with pyridine. For FeTPP+, all bright spectral transitions are found to be π-π* in character, with electron density variously distributed across the porphyrin and/or its phenyl substituents. Similar electronic excitations are observed for FeTPP+⋅py, with an additional bright transition which involves charge transfer from the porphyrin to the pyridine moiety.

Assessing Nonadiabatic Dynamics Methods in Long Timescales

Nonadiabatic dynamics simulations complement time-resolved experiments by revealing ultrafast excited-state mechanistic information in photochemical reactions. Understanding the relaxation mechanisms of photoexcited molecules finds application in energy, material, and medicinal research. However, with substantial computational costs, the nonadiabatic dynamics simulations have been restricted to ultrafast timescales, typically less than a few picoseconds, thus neglecting a wide range of photoactivated processes occurring in much longer timescales. Before developing new methodologies, we must ask: How well do the popular nonadiabatic dynamics methods perform in a long timescale simulation? In this study, we employ the multiconfiguration time-dependent Hartree (MCTDH) and its multilayer variants (ML-MCTDH), ab initio multiple spawning (AIMS), and fewest-switches surface hopping (FSSH) methodologies to simulate the excited-states dynamics of a weakly coupled multidimensional Spin-Boson model Hamiltonian designed for a long timescale decay behavior. Our study assures that despite having very different theoretical backgrounds, all the above methods deliver qualitatively similar results. While quantum dynamics would be very costly for long timescale simulations, the trajectory-based approaches are paving the way for future advancements.

Confinement Effects of Two-Dimensional Surfaces on Water Adsorption and Dissociation over Pt(111)

It has been established that the confined space created by stacking a two dimensional (2D) surface atop a metal catalyst serves as a nano-reactor. According to recent research, when a graphene (Gr) overlayer encloses a catalyst from above, the activation barrier for the water dissociation reaction, a process with major industrial significance, decreases. In order to investigate how the effect of confinement varies among different two-dimensional (2D) materials, we study the adsorption and dissociation barriers of water molecule on (111) under graphene, hexagonal boron nitride (h-BN), and heptazine-based graphitic carbon nitride (g-C3N4) layers using density functional theory calculations. Our findings reveal that the strength of adsorption does not decrease consistently with a reduction in the height of the 2D overlayer. Furthermore, a smaller barrier is not always the consequence of poorer adsorption of the reactant. We also examine the effect of confinement on the shape of the reaction path, on the frequencies of vibrational modes, and on the rate constants derived using the harmonic transition state theory. Overall, all three 2D surfaces cause a decrease in barrier height and a weakening of adsorption, though to differing degrees due to a mix of mechanical, geometric and electronic variables.

Temperature effects on the branching dynamics in the model ambimodal (6 + 4)/(4 + 2) intramolecular cycloaddition reaction

C14H20 (tetradecapentaene) is a simple model system exhibiting post transition-state behavior, wherein both the (6 + 4) and (4 + 2) cycloaddition products are formed from one ambimocal transition state structure. We studied the bifurcation dynamics starting from the two ambimodal transition state structures, the chair-form and boat-form, using the quasi-classical trajectory, classical molecular dynamics, and ring-polymer molecular dynamics methods on the parameter-optimized semiempirical GFN2-xTB potential energy surface. It was found that the calculated branching fractions differ between the chair-form and boat-form due to the different nature in the IRC pathways. We also investigated the effects of temperature on bifurcation dynamics and found that, at higher temperatures, trajectories stay longer in the intermediate region of the potential energy surface.

Full-dimensional coupled-channel statistical approach to atom-triatom systems and applications to H/D + O3 reaction

The statistical quantum model (SQM), which assumes that the reactivity is controlled by entrance/exit channel quantum capture probabilities, is well suited for chemical reactions with a long-lived intermediate complex. In this work, a time-independent coupled-channel implementation of the SQM approach is developed for atom-triatom systems in full dimensionality. As SQM treats the capture dynamics quantum mechanically, it is capable of handling quantum effects such as tunneling. A detailed study of the H/D + O3 capture dynamics was performed by applying the newly developed SQM method on an accurate global potential energy surface. Agreement with previous ring polymer molecular dynamics (RPMD) results on the same potential energy surface is excellent except for very low temperatures. The SQM results are also in reasonably good agreement with available experimental rate coefficients. The strong H/D kinetic isotope effect underscores the dominant role of quantum tunneling under an entrance channel barrier at low temperatures.

Time-resolved Coulomb explosion imaging of vibrational wave packets in alkali dimers on helium nanodroplets

Vibrational wave packets are created in the lowest triplet state 13Σu+ of K2 and Rb2 residing on the surface of helium nanodroplets, through non-resonant stimulated impulsive Raman scattering induced by a moderately intense near-infrared laser pulse. A delayed, intense 50-fs laser pulse doubly ionizes the alkali dimers via multiphoton absorption and thereby causes them to Coulomb explode into a pair of alkali ions Ak+. From the kinetic energy distribution P(Ekin) of the Ak+ fragment ions, measured at a large number of delays, we determine the time-dependent internuclear distribution P(R, t), which represents the modulus square of the wave packet within the accuracy of the experiment. For both K2 and Rb2, P(R, t) exhibits a periodic oscillatory structure throughout the respective 300 and 100 ps observation times. The oscillatory structure is reflected in the time-dependent mean value of R, ⟨R⟩(t). The Fourier transformation of ⟨R⟩(t) shows that the wave packets are composed mainly of the vibrational ground state and the first excited vibrational state, in agreement with numerical simulations. In the case of K2, the oscillations are observed for 300 ps, corresponding to more than 180 vibrational periods with an amplitude that decreases gradually from 0.035 to 0.020 Å. Using time-resolved spectral analysis, we find that the decay time of the amplitude is ∼260 ps. The decrease is ascribed to the weak coupling between the vibrating dimers and the droplet.