Self-consistent perturbation theory of diamagnetism

Simple and Effective Identification of Local 6π- and Global [4n + 2] Aromaticity of Macrocyclic Conjugated Hydrocarbons by 1H/13C Chemical Shifts and the Corresponding Ring Current Effect

Structures, 1H/13C chemical shifts, and the ring current effects (spatial magnetic properties: through-space NMR shieldings [TSNMRSs]) of various π-conjugated macrocyclic hydrocarbons and the corresponding charged analogues have been calculated at the B3LYP/6-311G(d,p) theory level using the GIAO perturbation method and employing the nucleus-independent chemical shift (NICS) characterization. The spatial magnetic properties (TSNMRS) are visualized as iso-chemical shielding surfaces (ICSSs) of various size and direction and together with especially the δ(1H)/ppm chemical shifts employed to unequivocally qualify and quantify local 6π-aromaticity of individual benzenoid building blocks and the global ([4n + 2], n > 1) aromaticity of the macrocyclic ring.

Osmaborane Clusters with B4 and B5 Rings Stabilized in the Coordination Sphere of {OsLn} (Ln = η5-C5Me5 or H2(PPh3)2)

New synthetic routes have been developed to synthesize osmaborane clusters featuring B4 and B5 rings in the coordination spheres of osmium. Thermolysis of [Os(PPh3)3Cl2], 1 in the presence of excess of [BH3·THF] led to the formation of [Os(PPh3)2H2(η4-B4H8)], 2 along with [HOs(PPh3)2B5H10], 3. Cluster 2 features a planar tetraborane ring coordinated to an osmium center in an η4 fashion. Cluster 3 can be considered an osmium analogue of hexaborane(10), in which the osmium center is situated at the base of the pentagonal pyramid geometry. In a different synthetic protocol, we have carried out the metathesis reaction of [Cp*OsBr2]2 (Cp* = η5-pentamethylcyclopentadienyl), 5 with [LiBH4·THF] followed by thermolysis in the presence of [BH3·THF] that generated [Cp*Os(η5-B5H10)], 6. Cluster 6 has a planar pentaborane ring that is stabilized in the coordination sphere of osmium, making it a boron analogue of osmocene. Clusters 2 and 6 are the first examples of structurally characterized planar B4 and B5 rings, respectively, that are stabilized in the coordination sphere of osmium. All the synthesized molecules were characterized using multinuclear NMR and IR spectroscopy, mass spectrometry, and single crystal X-ray diffraction analyses. Theoretical calculations were carried out to visualize the electronic structures and bonding scenarios in 2 and 6.

Transient radical species and oxygen colorimetric indicators grounded on phenothiazinium dyes

This study investigates four phenothiazinium dyes including methylene blue (MB) and three analogues containing auxochrome variation 2-4 as oxygen colorimetric indicators prepared by their incorporation into two types of oxygen permeable materials containing: a) polyvinyl alcohol substrate with additional TiO2 photocatalysts (PhOxIn), and b) carboxymethylcellulose polymer matrix containing glucose and KOH (ChOxIn). In vacuum packages where volumetric concentrations of oxygen were below 0.01 %, the leuco forms of the tested phenothiazinum dyes were readily oxidised, initiating the colour turn from white to blue. The redox processes involved were explored by experimental electron paramagnetic resonance EPR and UV-vis spectroscopic methods and further supported by theoretical quantum chemistry. The EPR experiments showed that the chemical oxidation of the leuco-form of the phenothiazinium dyes 1-4 produced transient aminyl radical species with the single electron mainly located at the nitrogen atom of the heterocyclic core. The formation of these transient aminyl species was also perceived by UV-vis spectroscopy, their absorption maxima situated in the 552-592 nm range being also supported by TD-DFT theoretical calculations.

Predicting 35-Cl electric field gradient tensors in crystalline solids using cluster and fragment-corrected planewave density functional theory

Planewave-corrected methods have proven effective for accurately modeling nuclear magnetic resonance (NMR) parameters in crystalline systems. Recent work extended the application of planewave-corrected calculations beyond the second row, predicting EFG tensor parameters for 35Cl using a simple molecular correction to projector augmented-wave (PAW) density functional theory (DFT). Here we extend this work using fragment and cluster-based calculations coupled with polarizable continuum (PCM) methods to improve further the accuracy of planewave-corrected 35Cl EFG tensor calculations. Benchmark data from a test set comprised of 105 individual 35Cl EFG tensor principal components for chlorine-containing molecular crystals and crystalline chloride salts shows fragment-corrected planewave calculations using the PBE0 hybrid density functional improve the accuracy of predicted EFG tensor components by 30 % relative to traditional planewave calculations. We compare the influence of different geometry optimization methods and density functionals on the accuracy of predicted 35Cl EFG tensor parameters. Four cases of spectral assignment are presented to demonstrate the utility of improving the accuracy of predicted 35Cl EFG tensor parameters.

The Azide-Wittig Reaction

The introduction of heteroatoms into conjugated organic molecules is an important strategy to tune their reactivity and physical properties. In this realm triazabutadienes (TBDs) of the general from R2C=N-N=NR' are an interesting class of compounds, however, general synthetic protocols for their generation are limited. Based on the serendipitous finding that the sterically encumbered azide Mes*N3 (Mes*=2,4,6-tBu3C6H2) reacted with PMe3 in the presence of an aromatic aldehyde to form a TBD, we now report on the "Azide-Wittig" reaction. This azide-Wittig reaction is shown to be a versatile tool for the synthesis of a variety of TBDs, tolerating a wide range of aldehydes and organic azides as coupling partners. The preference for azide-Wittig, rather than aza-Wittig reactivity was rationalized using computational methods. This study shows how kinetic control can significantly alter the reaction pathway, thereby switching from an aza-Wittig to an azide-Wittig regime.

The Quest for Stable Borozene Core in Main-Group Capped Inverse Sandwich Complexes, [(HE)2B6H6]2- (E=B, Al, Ga, In, and Tl)

The ubiquitous chemistry of benzene led us to explore ways to stabilise analogous borozene, by capping them with appropriate groups. The mismatch in overlap of ring-cap fragment molecular orbitals in [(HB)2B6H6]2- is overcome by replacing the two BH caps with higher congeners of boron. We calculated the relative energies of all the polyhedral structural candidates for [(HE)2B6H6]2- (E=Al-Tl) and found hexagonal bipyramid (HBP) to be more stable with Al-H caps. A global minimum search also gives HBP as the most stable structure for [Al2B6H8]2-. The capped B6H6 ring in [(HAl)2B6H6]2- has aromaticity comparable to that of benzene.

(±)-Salvicatone A: A Pair of C27-Meroterpenoid Enantiomers with Skeletons from the Roots and Rhizomes of Salvia castanea Diels f. tomentosa Stib

(±)-Salvicatone A (1), a C27-meroterpenoid featuring a unique 6/6/6/6/6-pentacyclic carbon skeleton with a 7,8,8a,9,10,10a-hexahydropyren-1 (6H)-one motif, was isolated from the roots and rhizomes of Salvia castanea Diels f. tomentosa Stib. Its structure was characterized by comprehensive spectroscopic analyses along with computer-assisted structure elucidation, including ACD/structure elucidator and quantum chemical calculations with 1H/13C NMR and electronic circular dichroism. Biogenetically, compound 1 was constructed from decarboxylation following [4 + 2] Diels-Alder cycloaddition reaction between caffeic acid and miltirone analogue. Bioassays showed that (-)-1 and (+)-1 inhibited nitric oxide production in lipopolysaccharide-induced RAW264.7 macrophage cells with an IC50 value of 6.48 ± 1.25 and 15.76 ± 5.55 μM, respectively. The structure-based virtual screening based on the pharmacophores in ePharmaLib, as well as the molecular docking and molecular dynamics simulations study, implied that (-)-1 and (+)-1 may potentially bind to retinoic acid receptor-related orphan receptor C to exert anti-inflammatory activities.

Low-Scaling, Efficient and Memory Optimized Computation of Nuclear Magnetic Resonance Shieldings within the Random Phase Approximation Using Cholesky-Decomposed Densities and an Attenuated Coulomb Metric

An efficient method for the computation of nuclear magnetic resonance (NMR) shielding tensors within the random phase approximation (RPA) is presented based on our recently introduced resolution-of-the-identity (RI) atomic orbital RPA NMR method [Drontschenko, V. J. Chem. Theory Comput. 2023, 19, 7542-7554] utilizing Cholesky decomposed density type matrices and employing an attenuated Coulomb RI metric. The introduced sparsity is efficiently exploited using sparse matrix algebra. This allows for an efficient and low-scaling computation of RPA NMR shielding tensors. Furthermore, we introduce a batching method for the computation of memory demanding intermediates that accounts for their sparsity. This extends the applicability of our method to even larger systems that would have been out of reach before, such as, e.g., a DNA strand with 260 atoms and 3408 atomic orbital basis functions.

Caffeine-Legal Natural Stimulant with Open Research Perspective: Spectroscopic and Theoretical Characterization

Caffeine is an alkaloid with a purine structure and has been well known for centuries due to its presence in popular drinks-tea and coffee. However, the structural and spectroscopic parameters of this compound, as well as its chemical and biological activities, are still not fully known. In this study, for the first time, we report on the measured oxygen-17 NMR spectra of this stimulant. To support the assignment of our experimental NMR data, extensive quantum chemical calculations of NMR parameters, including nuclear magnetic shielding constants and indirect spin-spin coupling constants, were performed. In a theoretical study, using nine efficient density functionals (B3LYP, BLYP, BP86, CAM-B3LYP, LC-BLYP, M06, PBE0, TPSSh, wB97x), and in combination with a large and flexible correlation-consistent aug-cc-pVTZ basis set, the structure and NMR parameters were predicted for a free molecule of caffeine and in chloroform, DMSO and water. A polarized continuum model (PCM) was used to include a solvent effect. As a result, an optimal methodology was developed for predicting reliable NMR data, suitable for studies of known, as well as newly discovered, purines and similar alkaloids. The results of the current work could be used in future basic and applied studies, including NMR identification and intermolecular interactions of caffeine in various raw materials, like plants and food, as well as in the structural and spectroscopic characterization of new compounds with similar structures.

Influence of Selective Deoxyfluorination on the Molecular Structure of Type-2 N-Acetyllactosamine

N-Acetyllactosamine is a common saccharide motif found in various biologically active glycans. This motif usually works as a backbone for additional modifications and thus significantly influences glycan conformational behavior and biological activity. In this work, we have investigated the type-2 N-acetyllactosamine scaffold using the complete series of its monodeoxyfluorinated analogs. These glycomimetics have been studied by molecular mechanics, quantum mechanics, X-ray crystallography, and various NMR techniques, which have provided a comprehensive and complete insight into the role of individual hydroxyl groups in the conformational behavior and lipophilicity of N-acetyllactosamine.

Direct evidence for a deprotonated lysine serving as a H-bond "acceptor" in a photoreceptor protein

Deprotonation or suppression of the pKa of the amino group of a lysine sidechain is a widely recognized phenomenon whereby the sidechain amino group transiently can act as a nucleophile at the active site of enzymatic reactions. However, a deprotonated lysine and its molecular interactions have not been directly experimentally detected. Here, we demonstrate a deprotonated lysine stably serving as an "acceptor" in a H-bond between the photosensor protein RcaE and its chromophore. Signal splitting and trans-H-bond J coupling observed by NMR spectroscopy provide direct evidence that Lys261 is deprotonated and serves as a H-bond acceptor for the chromophore NH group. Quantum mechanical/molecular mechanical calculations also indicate that this H-bond exists stably. Interestingly, the sidechain amino group of the lysine can act as both donor and acceptor. The remarkable shift in the H-bond characteristics arises from a decrease in solvation, triggered by photoisomerization. Our results provide insights into the dual role of this lysine. This mechanism has broad implications for other biological reactions in which lysine plays a role.

1H and 13C NMR spectra of infinitene and the ring current effect of the aromatic molecule

The spatial magnetic properties (through-space NMR shieldings-TSNMRSs-actually the ring current effect in 1H NMR spectroscopy) of the recently synthesized infinitene (the helically twisted [12]circulene) have been calculated using the GIAO perturbation method employing the nucleus-independent chemical shift (NICS) concept and visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. Both 1H and 13C chemical shifts of infinitene and the aromaticity of this esthetically very appealing molecule have been studied subject to the ring current effect thus obtained. This spatial magnetic response property of TSNMRSs dominates the different magnitude of 1H and 13C chemical shifts, especially in the cross-over section of infinitene, which is unequivocally classified as an aromatic molecule based on the deshielding belt of its ring current effect. Differences in aromaticity of infinitene compared with isolated benzene can also be qualified and quantified on the magnetic criterion.

Stilbene-pyridazinone hybrids: design, synthesis and in vitro antiplatelet activity screening

A series of stilbene analogues, in which a phenyl ring was replaced by the pyridazin-3(2H)-one nucleus, was designed and synthesized to be explored as platelet aggregation inhibitors. The proposed stilbene-pyridazinone hybrids were successfully obtained from simple starting materials and by Wittig's reaction. Most of the target compounds displayed improved in vitro activity in comparison with the standard drug, resveratrol, highlighting as the most potent the analogues 10d and 10e, with inhibition percentages of 94.15 % at 100 µM and 100 % at 50 µM, respectively. The pharmacokinetic and toxicity (ADME/T) properties of the novel hybrids were also estimated with the SwissADME and ProTox-II web servers.

Pt(II) Bis(pyrrole-imine) complexes: Luminescent probes and cytotoxicity in MCF-7 cells†

Four Pt(II) bis(pyrrole-imine) Schiff base chelates (1-4) were synthesised by previously reported methods, through a condensation reaction, and the novel crystal structure of 2,2'-{propane-1,3-diylbis[nitrilo(E)methylylidene]}bis(pyrrol-1-ido)platinum(II) (1) was obtained. Pt(II) complexes 1-4 exhibited phosphorescence, with increased luminescence in anaerobic solvents or when bound to human serum albumin (HSA). One of the complexes shows a 15.6-fold increase in quantum yield when bound to HSA and could be used to detect HSA concentrations as low as 5 nM. Pt(II) complexes 1-3 was investigated as potential theranostic agents in MCF-7 breast cancer cells, but only complex 3 exhibited cytotoxicity when irradiated with UV light (λ355nmExcitation). Interestingly, the cytotoxicity of complex 1 was unresponsive to UV light irradiation. This indicates that only complex 3 can be considered a potential photosensitising agent.

Structural characterization and keto-enol tautomerization of 4-substituted pyrazolone derivatives with DFT approach

The synthesis of two pyrazolone derivative compounds, PYR-I(4-Acetyl-1-(4-chlorophenyl)-3-isopropyl-1H-pyrazol-5(4H)-one) and PYR-II1-(4-Chlorophenyl))-3-isopropyl-5-oxo-4,5-5-dihydro-1H-pyrazole-4-carbaldehyde, their characterization by FT-IR, NMR, UV-Vis and GC-MS techniques, and the evaluation of the keto-enol tautomerization process of the structures along with the DFT approach and spectral data were reported in this paper. Spectral findings indicated that PYR-I was stable at the keto state. The IR spectrum recorded in solid form showed that the PYR-II structure was stable in the enol state, while the NMR spectrum in the solution medium showed that it was stable in the keto state. DFT-based analyses were realized with the B3LYP hybrid functional and the 6-311++G(d,p) basis set. The modelled keto, transition and enol state molecular geometries of structures were optimized in the gas phase and different solvent media and the total energy and dipole moment values were investigated at the specified theoretical level. The possible keto-enol tautomerism mechanism of the structures was evaluated through some thermodynamic parameters such as the difference in free Gibbs energy (ΔG), enthalpy (ΔH), entropy (ΔS), and predictive tautomeric equilibrium constants (Keq), acidity constants (pKa) and percentages of tautomers at 298.15 K and 1 atm pressure. The results of these analyses based on the DFT approach indicated that the keto-enol tautomer equilibrium heavily favours the keto form for PYR-I and the enol form for PYR-II in all cases. Moreover, natural bond orbital (NBO) analysis was performed for the tautomers, and the chemical reactivity profiles of the most stable tautomers were examined with the values of frontier molecular orbital energy and some reactivity descriptors.

Hydrogen bonding patterns and cooperativity in polyproline II helical bundles

Hydrogen bond cooperativity (HBC) plays an important role in stabilizing protein assemblies built by α-helices and β-sheets, the most common secondary structures. However, whether HBC exists in other types of protein secondary structures such as polyproline II (PPII) helices remains unexplored. This is intriguing, since PPII systems as assembling blocks are continuously emerging across multiple fields. Here, using a combination of computational chemistry tools and molecular modeling corroborated by experimental observables, we characterize the distinct H-bonding patterns present in PPII helical bundles and establish that HBC stabilizes intermolecular PPII helices as seen in other protein assemblies such as amyloid fibrils. In addition to cooperative interactions in canonical CO···HN H-bonds, we show that analogous interactions in non-canonical CO···HαCα H-bonds are relevant in Gly-rich PPII bundles, thus compensating for the inability of glycine residues to create hydrophobic cores. Our results provide a mechanistic explanation for the assembly of these bundles.

Experimental Observation of a Terminal Borylene-Dinitrogen Adduct via Cleavage of a 1,2,3,4,5-Diboratriazoline

While azides do not react with simple alkenes except under harsh conditions, a diboron alkene analogue, the doubly cyclic alkyl(amino)carbene (CAAC)-stabilized dicyanodiborene 1, reacts spontaneously with organic azides (7-10 equiv.) at room temperature to yield two equivalents of stable CAAC-imino(cyano)boranes (2-R). NMR-spectroscopic monitoring of the reaction mixtures shows the initial formation of a 1 : 1 mixture of 2-R and a relatively long-lived intermediate (Int), which in the presence of excess azide is converted into a second equivalent of 2-R. In the absence of excess azide, however, Int decomposes to 3, the product of an intramolecular C-H activation by a putative dicoordinate borylene intermediate "(CAAC)B(CN)". Mechanistic insights from trapping experiments, NMR-spectroscopic and high-resolution mass spectrometry data, as well as DFT computations reveal that Int is the terminal borylene end-on-dinitrogen adduct [(CAAC)B(CN)(η1-N2)]. The formation of the iminoboranes 2-R from diborene 1 and RN3 proceeds via an azide-diborene Huisgen-type [3+2] cycloaddition reaction, followed by a retro-[3+2] cycloaddition, yielding 2-R and [(CAAC)B(CN)(η1-N2)]. The latter then undergoes either N2 extrusion and intramolecular C-H activation to generate 3, or a Staudinger-type reaction with a second equivalent of azide to generate a second equivalent of the iminoborane 2-R.

Tricyanomethane or Dicyanoketenimine-Silylation makes the Difference

Pseudohalides such as tricyanomethanide, [C(CN)3]-, are well known in chemistry, biochemistry and industrial chemistry. The protonated species HC(CN)3, a classic hydrogen pseudohalide Brønsted acid, is a very strong acid with a pKa value of -5. However, HC(CN)3 is difficult to handle as it tends to decompose rapidly or, more precisely, to oligo- and polymerize. Therefore, silylated pseudohalide compounds with the [Me3Si]+ as the "big organometallic proton" have become interesting, exhibiting similar chemical properties but better kinetic protection. Here, the stepwise silylation of the pseudohalide anion [C(CN)3]- is reported, forming the heavier homologue of HC(CN)3, namely [Me3Si][C(CN)3], and in presence of two additional [Me3Si]+ cations even the dicationic species [(Me3Si-NC)3C]2+ as stable [B(C6F5)4]- salt. Surprisingly, in contrast to the protonated species HC(CN)3, in which the proton is bound to the central carbon atom of [C(CN)3]-, silylation of the [C(CN)3]- anion occurs at one of the three terminal nitrogen atoms, thus forming the long-sought dicyanoketenimine [Me3Si-NC-C(CN)2]. All further silylation steps take place exclusively on the terminal N atoms of the three CN groups and not on the central carbon atom, until the intriguing, highly symmetrical dication, [(Me3Si-NC)3C]2+, is finally generated. The experimental data are supported by quantum chemical calculations in terms of thermodynamics and chemical bonding.

Analysis of bonding motifs in unusual molecules II: infinitene

The bonding structures of infinitene, the Chemical and Engineering News 2021 Molecule of the Year, is studied by means of oriented quasi-atomic orbitals (QUAOs) to assess the degree of aromaticity within the molecule. It is found that the angularity introduced into infinitene when it takes on the helical shape of the infinity symbol has a profound effect on bond order, delocalization of bonding interactions, and the aromatic character of the system. In kekulene, a planar isomer of infinitene, the bonding analysis shows fluctuations of pocketed delocalization of bonding interactions in π-sextets associated with Clar's rule. Conversely, much smaller fluctuations are observed between the adjacent rings of infinitene. The observations drawn from the quasi-atomic bonding analysis support the idea that there is aromatic character across the entire infinitene molecule, not just localized around individual rings as in kekulene.

Core-electron contributions to the magnetic response of molecules with heavy elements and their significance in aromaticity assessments

This study delves into the magnetic response of core electrons and their influence on the global magnetic response of planar and three-dimensional systems containing heavy elements, employing the removing valence electron (RVE) approximation. We also explore electronic aromaticity indices to understand the potential role of core electrons on electron delocalization in the absence of an external perturbation. The study reveals that core electrons significantly contribute to the overall magnetic response, especially to the magnetic shielding, affecting the interpretation of aromaticity. In contrast, the calculation of the electronic aromaticity indices suggests a negligible participation of the core electrons on electron delocalization. Despite their widespread use, the study emphasizes caution in labeling systems as strongly aromatic based solely on shielding function computations. It is noteworthy to emphasize the limitations associated with each aromaticity criterion; particularly in the context of magnetic shielding function calculations, the core-electron effect contamination is undeniable. Hence, the integration of various criteria becomes imperative for attaining a comprehensive understanding of magnetic responses within complex systems.

Quantum chemical package Jaguar: A survey of recent developments and unique features

This paper is dedicated to the quantum chemical package Jaguar, which is commercial software developed and distributed by Schrödinger, Inc. We discuss Jaguar's scientific features that are relevant to chemical research as well as describe those aspects of the program that are pertinent to the user interface, the organization of the computer code, and its maintenance and testing. Among the scientific topics that feature prominently in this paper are the quantum chemical methods grounded in the pseudospectral approach. A number of multistep workflows dependent on Jaguar are covered: prediction of protonation equilibria in aqueous solutions (particularly calculations of tautomeric stability and pKa), reactivity predictions based on automated transition state search, assembly of Boltzmann-averaged spectra such as vibrational and electronic circular dichroism, as well as nuclear magnetic resonance. Discussed also are quantum chemical calculations that are oriented toward materials science applications, in particular, prediction of properties of optoelectronic materials and organic semiconductors, and molecular catalyst design. The topic of treatment of conformations inevitably comes up in real world research projects and is considered as part of all the workflows mentioned above. In addition, we examine the role of machine learning methods in quantum chemical calculations performed by Jaguar, from auxiliary functions that return the approximate calculation runtime in a user interface, to prediction of actual molecular properties. The current work is second in a series of reviews of Jaguar, the first having been published more than ten years ago. Thus, this paper serves as a rare milestone on the path that is being traversed by Jaguar's development in more than thirty years of its existence.

Extended shortwave infrared absorbing antiaromatic fluorenium-indolizine chromophores

Shortwave infrared (SWIR, 1000-1700 nm) and extended SWIR (ESWIR, 1700-2700 nm) absorbing materials are valuable for applications including fluorescence based biological imaging, photodetectors, and light emitting diodes. Currently, ESWIR absorbing materials are largely dominated by inorganic semiconductors which are often costly both in raw materials and manufacturing processes used to produce them. The development of ESWIR absorbing organic molecules is thus of interest due to the tunability, solution processability, and low cost of organic materials compared to their inorganic counterparts. Herein, through the combination of heterocyclic indolizine donors and an antiaromatic fluorene core, a series of organic chromophores with absorption maxima ranging from 1470-2088 nm (0.84-0.59 eV) and absorption onsets ranging from 1693-2596 nm (0.73-0.48 eV) are designed and synthesized. The photophysical and electrochemical properties of these chromophores, referred to as FluIndz herein, are described via absorption spectroscopy in 17 solvents, cyclic voltammetry, solution photostability, and transient absorption spectroscopy. Molecular orbital energies, predicted electronic transitions, and antiaromaticity are compared to higher energy absorbing chromophores using density functional theory. The presence of thermally accessible diradical states is demonstrated using density functional theory and EPR spectroscopy, while XRD crystallography confirms structural connectivity and existence as a single molecule. Overall, the FluIndz chromophore scaffold exhibits a rational means to access organic chromophores with extremely narrow optical gaps.

Accurate NMR Shieldings with σ-Functionals

In recent years, density-functional methods relying on a new type of fifth-rung correlation functionals called σ-functionals have been introduced. σ-Functionals are technically closely related to the random phase approximation and require the same computational effort but yield distinctively higher accuracies for reaction and transition state energies of main group chemistry and even outperform double-hybrid functionals for these energies. In this work, we systematically investigate how accurate σ-functionals can describe nuclear magnetic resonance (NMR) shieldings. It turns out that σ-functionals yield very accurate NMR shieldings, even though in their optimization, exclusively, energies are employed as reference data and response properties such as NMR shieldings are not involved at all. This shows that σ-functionals combine universal applicability with accuracy. Indeed, the NMR shieldings from a σ-functional using input orbitals and eigenvalues from Kohn-Sham calculations with the exchange-correlation functional of Perdew, Burke and Ernzerhof (PBE) turned out to be the most accurate ones among the NMR shieldings calculated with various density-functional methods including methods using double-hybrid functionals. That σ-functionals can be used for calculating both reliable energies and response properties like NMR shieldings characterizes them as all-purpose functionals, which is appealing from an application point of view.

Combined Theoretical and Experimental Investigations: Design, Synthesis, Characterization, and In Vitro Cytotoxic Activity Assessment of a Complex of a Novel Ureacellobiose Drug Carrier with the Anticancer Drug Carmustine

Drug delivery systems (DDSs) are used to transport drugs which are characterized by some pharmaceutical problems to the specific target site, enhancing therapeutic efficacy and reducing off-target accumulation in the body. In this work, one of the recently synthesized molecules, 1,10-N,N'-bis-(β-ᴅ-ureidocellobiosyl)-4,7,13,16-tetraoxa-1,10-diazacyclooctadecane (TN), was tested as a potential drug carrier towards the anticancer drug carmustine. For this purpose, different techniques were used, from synthesis and calculations to cytotoxicity assessment. Our results showed that TN is characterized by a very compact geometry, which significantly impacts its complexation properties. Although it forms a very stable complex with carmustine, it adopts a non-inclusion geometry, as verified by both experimental and theoretical NMR analyses. The cytotoxicity study performed for all analyzed molecules (TN; carmustine; TN:carmustine complex) towards normal and cancer (breast and colon) cells revealed that TN is not toxic and that the formation of complexes with carmustine reduces the toxicity of carmustine to normal cells.

Helically Chiral π-Expanded Azocines Through Regioselective Beckmann Rearrangement and Their Charged States

We report a synthetic approach to π-expanded [6]helicenes incorporating tropone and azocine units in combination with a 5-membered ring, which exhibit intriguing structural, electronic, and chiroptical properties. The regioselective Beckmann rearrangement allows the isolation of helical scaffolds containing 8-membered lactam, azocine, and amine units. As shown by X-ray crystallographic analysis, the incorporation of tropone or azocine units leads to highly distorted [6]helicene moieties, with distinct packing motifs in the solid state. The compounds exhibit promising optoelectronic properties with considerable photoluminescence quantum yields and tunable emission wavelengths depending on the relative position of the nitrogen center within the polycyclic framework. Separation of the enantiomers by chiral high-performance liquid chromatography (HPLC) allowed characterization of their chiroptical properties by circular dichroism (CD) and circularly polarized luminescence (CPL) spectroscopy. The azocine compounds feature manifold redox chemistry, allowing for the characterization of the corresponding radical anions and cations as well as the dications and dianions, with near-infrared (NIR) absorption bands extending beyond 3000 nm. Detailed theoretical studies provided insights into the aromaticity evolution upon reduction and oxidation, suggesting that the steric strain prevents the azocine unit from undergoing aromatization, while the indene moiety dominates the observed redox chemistry.

Modeling solvent effects and convergence of 31P-NMR shielding calculations with COBRAMM

Solvent effects on 31P-NMR parameters for triphenylphosphine oxide and triphenylphosphine in chloroform have been extensively investigated by testing different solvation models. The solvent is described implicitly, mixed implicitly/explicitly, and using full explicit models. Polarizable continuum model (PCM), molecular dynamic (MD) simulations, and hybrid quantum mechanics/molecular mechanics (QM/MM) calculations are used to disclose the effects of solute/solvent interactions and, more generally, the role of the embedding in NMR simulations. The results show the beneficial effect of carrying out QM/MM optimizations on top of geometries directly extracted from classical MD simulations, used to ensure representative conformational sampling. The nuclear shielding convergence has been tested against a different number of snapshots and with the inclusion of solvent shells into the QM region. An automated MD//QM/MM//GIAO protocol, implemented in the COBRAMM package, is here proposed and tested on trimethyl phosphite showing that our approach boosts the convergence of nuclear shielding satisfactorily. The present work aims to be a stepping-stone to assess proper QM/MM computational strategies in simulating chemical shifts in non-homogeneous systems like supramolecular and biological systems.

Computational studies on a selection of phosphite esters as antioxidants for polymeric materials

CONTEXT

Phosphite esters, a class of organo-phosphorus compounds, are widely used as non-discolouring antioxidants in many polymeric products. Apart from normal radical scavenging, they prevent the splitting of hydroperoxides (ROOH), one of the initial products of autoxidation, from forming extremely reactive free radicals such as alkoxy (RO.) and hydroxy (.OH) radicals. The inherent molecular properties of antioxidants and the chemistry of their action are essential for researchers working in this field of science. Four organo-phosphorous compounds well-known for their antioxidant activity are selected here for theoretical analysis: Tri(m-methylphenyl) phosphite (m-TMPP), Tri(4-methyl-2,6-di-tert-butylphenyl) phosphite (TMdtBPP), Tri(allylphenyl) phosphite (TAPP) and Tri(mercaptobenzothiazoyl) thiophosphate (TMBTTP). The antioxidant activity exhibited by these compounds is theoretically verified, and the results are consistent with the available experimental data. Such theoretical predictions offer advantages in scientific research, particularly when researchers need to select certain molecules as antioxidants for experiments from a pool of molecular systems.

METHODS

The chemical computations presented in this report are done in Gaussian 16 program package. The procedure of density functional theory (DFT) with the model chemistry B3LYP/6-31G(d,p) is used to generate computational data. Global reactivity indices, thermochemical data, Fukui functions, molecular electrostatic potential and NMR spectra are computed for the chosen molecular systems from their optimized geometries.

Phosphaarsenes - Combining Phospha- and Arsa-Wittig-Reagents

Dipnictenes of the type RPn=PnR (Pn=P, As, Sb, Bi) can be viewed as dimers of the corresponding pnictinidenes R-Pn. Phosphanylidene- and arsanylidenephosphoranes (R-Pn(PMe3); Pn=P, As) have been shown to be versatile synthetic surrogates for the delivery of pnictinidene fragments. We now report that thermal treatment of 1 : 1 mixtures of R-P(PMe3) and R'-As(PMe3) gives access to arsaphosphenes of the type RP=AsR'. Three examples are presented and the properties and reactivity of Mes*P=AsDipTer (1) (Mes*=2,4,6-tBu3-C6H2; DipTer=2,6-(2,6-iPr2C6H3)2-C6H3) were studied in detail. Solid state 31P NMR spectroscopy revealed a large 31P NMR chemical shift anisotropy with a span of ca. 920 ppm for 1 while computational methods were employed to investigate this pronounced magnetic deshielding of the P atom in 1. In the presence of the carbene IMe4 (IMe4=:C(MeNCMe)2) 1 is shown to be split into the corresponding NHC adducts Mes*P(IMe4) and DipTerAs(IMe4), which is additionally shown for diarsenes.

Phospha-RosIndolizine Dye with Shortwave Infrared (SWIR) Absorption and Emission

Shortwave infrared (SWIR, 1000-1700 nm) absorbing and emitting dyes are needed for infrared diodes and sensors used in a wide variety of industrial and medical applications. Herein, an electron-withdrawing phosphine oxide (P═O) substituted xanthene is coupled with strong indolizine donors to produce a SWIR absorbing (λabs = 1294 nm in DCM) and emitting (λemis = 1450 nm in DCM) dye called PRos1450. The unique properties of this dye are characterized via photophysical, electrochemical, and computational analyses.

Unveiling the Unusual Mn(CO)3 Migration in a Manganese Cyclohexenyl Complex by DFT Computations

Homogeneous catalysis involving a transition metal agostic interaction (TM…H…C) is an attractive strategy for C-H bond activation, in which the transition metal agostic intermediates serve as the critical component. To investigate the roles of manganese agostic intermediates in the unusual migration of the Mn(CO)3 fragment in the (exo-phenyl)(η3-cyclohexenyl)manganese tricarbonyl [(Ph)(η3-C6H8)Mn(CO)3] (complex 1) under the protonation of tetrafluoroboric acid-diethyl ether (HBF4.Et2O), a comprehensive density functional theory (DFT) theoretical study was performed. The computational results showed that formation of the [(cyclohex-3-enyl)-η6-benzene]manganese tricarbonyl complex [(C6H9)(η6-Ph)Mn(CO)3+][BF4] (complex 2) was achieved via a series of mono-agostic and di-agostic intermediates. The overall rate-limiting step for this unusual migration of the Mn(CO)3 fragment is the formation of the di-agostic (η2-phenyl)manganese complex 8 (4 → 5 → 8) with a Gibbs barrier of 15.4 kcal mol-1. The agostic intermediates with TM…H…C agostic interactions were well-characterized by geometry parameters, Atoms-In-Molecules (AIM) analyses, and the Natural Adaptive Orbitals (NAdOs). The located pathways in the current study successfully explained the experimental observations, and the findings on the TM…H…C agostic interaction provided a new aspect of the catalytic reaction with the manganese complex.

Indenoannulated Tridecacyclene: An All-Carbon Seven-Stage Redox-Amphoter

We disclose an indenoannulated tridecacyclene comprising a central cyclooctatetraene moiety with multiple adjacent pentagonal rings which is accessible in a concise synthetic sequence. The saddle-shaped geometry of the non-benzenoid polycyclic scaffold and its unique packing behavior in the solid state were characterized by X-ray crystallography. In electrochemical studies, the compound undergoes seven reversible redox events comprising five reductions and two oxidations. The dicationic and dianionic species obtained by chemical oxidation and reduction, respectively, were characterized spectroscopically in solution. Density functional theory calculations were applied to provide insights into aromaticity evolution in the respective charged species, highlighting the beneficial effect of the non-benzenoid moieties on charge stabilization.

Trapping an Elusive Phosphanyl-Phosphaalumene

We describe our efforts to access a compound with an Al=P double bond by reaction of Al(Nacnac) towards [H2CN(Dipp)]2P(PCO) (Nacnac=HC[C(Me)N(Dipp)]2; Dipp=2,6-iPr2C6H3). Our observations are consistent with the formation of a transient phosphanyl-phosphaalumene at low temperatures (-70 °C), however this species was found to readily undergo intramolecular C-H activation of the β-diketiminato ligand upon warming to room temperature. The reactivity of the transient complex toward small molecules including dihydrogen, carbon dioxide, phosphaketenes, amines and silanes could be explored at low temperatures, showcasing that the target compound can react as both a frustrated Lewis pair (via the pendant phosphanyl moiety) or in hydroelementation reactions of the Al=P bond. The elusive target molecule could be trapped by addition of a Lewis base (tetrahydrofuran) affording an isolable molecular species that reacts in an analogous fashion to the base-free compound.

Unexpected discovery: "A new 3,3'-bipyrazolo[3,4-b]pyridine scaffold and its comprehensive analysis"

In this study, a novel 3,3'-bipyrazolo [3,4-b]pyridine-type structure was synthesized from 5-acetylamino-3-methyl-1-phenylpyrazole using the Vilsmeier-Haack reaction as a key step. The spectroscopic properties and structural elucidation of the compound were determined with the use of FT-IR, HRMS, 1H NMR, and 13C NMR. Likewise, the theoretical analysis of the IR and NMR spectra allowed peaks to be assigned and a solid correlation was demonstrated between the experimental and theoretical results. Finally, ab initio calculations based on the density functional theory method at the B3LYP/6-311G (d,p) level of theory were used to determine the conformational energy barrier, facilitating the identification of the most probable conformers of the synthesized compound. Overall, our findings contribute to the understanding of bipyrazolo [3,4-b]pyridine derivatives.

Unexpected Intermolecular C-H···O Hydrogen Bonds and 1H NMR Chemical Shifts in a Key Linker for Fluorine-18 Labeling of Dimeric Drugs

The compound 2-{[(trifluoromethyl)sulfonyl]oxy}propane-1,3-diyl bis(4-methylbenzenesulfonate) (TPB) is a crucial intermediate in the synthesis of 18F-radiolabeled cromolyn derivatives. In this work, we combine 1H NMR spectroscopy, X-ray crystallography, ab initio molecular dynamics, and NMR calculations to examine the structure, interactions, and solvation dynamics of the TPB molecule. In CDCl3, the CH2 groups within its glyceryl-derived linker exhibit a single set of proton signals in the 1H NMR measurements. However, when TPB is dissolved in DMSO-d6, distinct splitting patterns emerge despite its seemingly symmetric chemical structure. Crystallographic analysis further unveils the absence of overall symmetry in its three-dimensional arrangement. To elucidate these unique NMR features, we carry out ab initio molecular dynamics simulations and characterize the solvation structures and dynamics of TPB in CHCl3 and DMSO solutions. In contrast to the predominantly nonpolar nature of the CHCl3 solvents, DMSO directly participates in C-H···O hydrogen-bonding interactions with the solute molecule, leading to the splitting of its -CH2 chemical shifts into two distinct distributions. The comprehensive understanding of the structure and solvation interactions of TPB provides essential insights into its application in the radiofluorination reactions of cromolyn derivatives and holds promise for the future development of radiolabeled dimeric drugs.

Predicting 51V nuclear magnetic resonance observables in molecular crystals

Solid-state nuclear magnetic resonance (NMR) spectroscopy and quantum chemical density functional theory (DFT) calculations are widely used to characterize vanadium centers in biological and pharmaceutically relevant compounds. Several techniques have been recently developed to improve the accuracy of predicted NMR parameters obtained from DFT. Fragment-based and planewave-corrected methods employing hybrid density functionals are particularly effective tools for solid-state applications. A recent benchmark study involving molecular crystal compounds found that fragment-based NMR calculations using hybrid density functionals improve the accuracy of predicted 51V chemical shieldings by 20% relative to traditional planewave methods. This work extends the previous study, including a careful analysis of 51V chemical shift anisotropy, electric field gradient calculations, and a more extensive test set. The accuracy of planewave-corrected techniques and recently developed fragment-based methods using electrostatic embedding based on the polarized continuum model (PCM) are found to be highly competitive with previous methods. Planewave-corrected methods achieve a 34% improvement in the errors of predicted 51V chemical shieldings relative to planewave. Additionally, planewave-corrected and fragment-based calculations were performed using PCM embedding, improving the accuracy of predicted 51V chemical shielding (CS) tensor principal values by 30% andC q values by 15% relative to traditional planewave methods. The performance of these methods is further examined using a redox-active oxovandium complex and a common 51V NMR reference compound.

Aromaticity of six-membered nitro energetic compounds through molecular electrostatic potential, magnetic, electronic delocalization and reactivity-based indices

The electron density depletion associated with π-hole at the ring center typical of energetic compounds was clearly revealed by the molecular electrostatic potential (ESP). In addition, the spatial arrangement of NO2 groups appears to affect the ESP value in the ring center, and therefore sensitivity to detonation. Indeed, for monocyclic nitrobenzene compounds with the same number of NO2 groups, the ESP value in the ring center decreases as the NO2 groups are more closely spaced. As expected, the central rings become less aromatic as NO2 groups are added. The MCI, PDI, PLR, NICSzz(1), FLU indices are all strongly correlated with the ESP values observed in the ring center of the set of nitrobenzenes. Aromaticity indices based on electron delocalization criteria appear to be very sensitive to small variations in aromaticity. Among magnetic-based indices, only NICSzz(1) is capable to predict small changes in aromaticity. The PLR index derived from conceptual DFT is quite relevant for predicting small variations in aromaticity. According to our results, the most suitable aromaticity index is not based on a single criterion, and that selecting it is more subtle. Therefore, it is important to combine information from several criteria to obtain a more complete description of the aromaticity of the studied compounds.

Evaluation of DFT methods for predicting geometries and NMR spectra of Bi(III) dithiocarbamate complexes with antitumor properties

CONTEXT

Bismuth complexes with dithiocarbamate ligands have attracted attention because of their biological applications, such as antimicrobial, antileishmanial, and anticancer properties. These complexes have high cytotoxic activity against cancer cells, being more active than the standard drugs cisplatin, doxorubicin, and tamoxifen. In the present study, we investigated the ability of some DFT methods to reproduce the geometries and NMR spectra of the Bi(III) dithiocarbamate complexes, selected based on their proven antitumor activity. Our investigation revealed that the M06-L/def2-TZVP/ECP/CPCM method presented good accuracy in predicting geometries, while the TPSSh/def2-SVP/ECP/CPCM method proved effective in analyzing the 13C NMR spectra of these molecules. In general, all examined methods exhibited comparable performance in predicting 1H NMR signals.

METHODS

Calculations were performed with the Gaussian 09 program using the def2-SVP and def2-TZVP basis sets, employing relativistic effective core potential (ECP) for Bi and using the CPCM solvent model. The exchange-correlation functionals BP86, PBE, OLYP, M06-L, B3LYP, B3LYP-D3, M06-2X, TPSSh, CAM-B3LYP, and ωB97XD were used in the study. Geometry optimizations were started from crystallographic structures available at the Cambridge Structural Database. The theoretical results were compared with experimental data using the mean root-mean-square deviation (RMSD), mean absolute deviations (MAD), and linear correlation coefficient (R2).

Changing aromatic properties through stacking: the face-to-face dimer of Ni(II) bis(pentafluorophenyl)norcorrole

Nuclear magnetic resonance (NMR) shielding constants have been calculated for Ni(II) bis(pentafluorophenyl)norcorrole and its face-to-face stacked dimer at the Hartree-Fock (HF), second-order Møller-Plesset perturbation theory (MP2), complete-active-space self-consistent-field (CASSCF) levels as well as at density functional theory (DFT) levels using several functionals. The calculated 1H NMR shielding constants agree rather well with the experimental ones. The shielding constants of N and Ni calculated at DFT, HF, and MP2 levels differ from those obtained in the CASSCF calculations due to near-degeneracy effects at the Ni atom. The calculated magnetically induced current densities show that the monomer is antiaromatic, sustaining a strong global paratropic ring current, and the dimer is aromatic, sustaining a strong diatropic ring current. Qualitatively the same current density is obtained at the employed levels of theory. The most accurate ring-current strengths are probably obtained at the MP2 level. The aromatic dimer has a short intermolecular distance of less than 3 Å. The intermolecular interaction changes the nature of the frontier orbitals leading to a formal double bond between the norcorrole macrocycles.

Antiproliferative and Pro-Apoptotic Activity and Tubulin Dynamics Modulation of 1H-Benzimidazol-2-yl Hydrazones in Human Breast Cancer Cell Line MDA-MB-231

(1) Background: The aim of the work is the evaluation of in vitro antiproliferative and pro-apoptotic activity of four benzimidazole derivatives containing colchicine-like and catechol-like moieties with methyl group substitution in the benzimidazole ring against highly invasive breast cancer cell line MDA-MB-231 and their related impairment of tubulin dynamics. (2) Methods: The antiproliferative activity was assessed with the MTT assay. Alterations in tubulin polymerization were evaluated with an in vitro tubulin polymerization assay and a docking analysis. (3) Results: All derivatives showed time-dependent cytotoxicity with IC50 varying from 40 to 60 μM after 48 h and between 13 and 20 μM after 72 h. Immunofluorescent and DAPI staining revealed the pro-apoptotic potential of benzimidazole derivatives and their effect on tubulin dynamics in living cells. Compound 5d prevented tubulin aggregation and blocked mitosis, highlighting the importance of the methyl group and the colchicine-like fragment. (4) Conclusions: The benzimidazole derivatives demonstrated moderate cytotoxicity towards MDA-MB-231 by retarding the initial phase of tubulin polymerization. The derivative 5d containing a colchicine-like moiety and methyl group substitution in the benzimidazole ring showed potential as an antiproliferative agent and microtubule destabilizer by facilitating faster microtubule aggregation and disrupting cellular and nuclear integrity.

Origin of 17O NMR chemical shifts based on molecular orbital theory: paramagnetic terms of the pre-α, α and β effects from orbital-to-orbital transitions, along with the effects from vinyl, carbonyl and carboxyl groups

17O NMR chemical shifts (δ(O)) were analysed based on the molecular orbital (MO) theory, using the diamagnetic, paramagnetic and total absolute magnetic shielding tensors (σd(O), σp(O) and σt(O), respectively). O2- was selected as the standard for the analysis. An excellent relationship was observed between σd(O) and the charges on O for O6+, O4+, O2+, O0 and O2-. The data from H2O, HO+, HO- and H3O+ were on the correlation line. However, such relationship was not observed for the oxygen species, other than above. The pre-α, α and β effects were evaluated bases on σt(O), where the pre-α effect arises from the protonation to a lone pair orbital on O2-, for an example. The 30-40 ppm and 20-40 ppm (downfield shifts) were predicted for the pre-α and β effects, respectively, whereas the values for the α effect was very small in magnitude, where the effect from the hydrogen bond formation should be considered. Similarly, the carbonyl effect in H2C[double bond, length as m-dash]O and the carboxyl effects in H(HO)C[double bond, length as m-dash]O were evaluated from MeOH, together with H2C[double bond, length as m-dash]CHOH from CH3CH2OH. Very large downfield shifts of 752, 425 and 207 ppm were predicted for H2C[double bond, length as m-dash]O*, H(HO)C[double bond, length as m-dash]O* and H(HO*)C[double bond, length as m-dash]O, respectively, together with the 81 ppm downfield shift for H2C[double bond, length as m-dash]CHO*H. The origin of the effect were visualized based on the occupied-to-unoccupied orbital transitions. As a result, the origin of the 17O NMR chemical shifts (δ(17O)) can be more easily imaged and understand through the image of the effects. The results would help to understand the role of O in the specific position of a compound in question and the mechanisms to arise the shift values also for the experimental scientists. The aim of this study is to establish the plain rules founded in theory for δ(17O), containing the origin, which has been achieved through the treatments.

Identification and quantification of local antiaromaticity in polycyclic aromatic hydrocarbons (PAHs) based on the magnetic criterion

The spatial magnetic properties, through-space NMR shieldings (TSNMRSs, actually the ring current effect in 1H NMR spectroscopy), of a selection of entirely antiaromatic and aromatic polycyclic conjugated hydrocarbons (PCHs), and aromatic PCHs with antiaromatic components, have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept and visualized as iso-chemical-shielding surfaces (ICSSs) of various sizes and directions. Using both in-plane and above/below-plane ICSS data, polycyclic aromatic hydrocarbons can be readily distinguished from polycyclic antiaromatic ones, even when antiaromatic components are present in the polycyclic aromatic hydrocarbons (PAHs). These antiaromatic zones can also be attributed to internal components of the in-plane deshielding belt present in aromatic compounds and possible partial antiaromatic ring current effects in the same place. This makes it possible to unequivocally confirm correctly assigned or adjust incorrectly assigned antiaromaticity of individual rings in the same molecule.

Aromaticity in the Electronic Ground and Lowest Triplet States of Molecules with Fused Thiophene Rings

Analysis of the variations of the off-nucleus isotropic magnetic shielding, σiso(r), around thiophene, thienothiophenes, dithienothiophenes and sulflowers in their electronic ground (S0) and lowest triplet (T1) states reveals that some of the features of aromaticity and bonding in these molecules do not fit in with predictions based on the popular Hückel's and Baird's rules. Despite having 4n π electrons, the S0 states of the sulflowers are shown to be aromatic, due to the local aromaticities of the individual thiophene rings. To reduce its T1 antiaromaticity, the geometry of thiophene changes considerably between S0 and T1: In addition to losing planarity, the carbon-carbon two 'double' and one 'single' bonds in S0 turn into two 'single' and one 'double' bonds in T1. Well-defined Baird-style aromaticity reversals are observed between the S0 and T1 states of only three of the twelve thiophene-based compounds investigated in this work, in contrast, the sulflower with six thiophene rings which is weakly aromatic in S0 becomes more aromatic in T1. The results suggest that the change in aromaticity between the S0 and T1 states in longer chains of fused rings is likely to affect mostly the central ring (or the pair of central rings); rings sufficiently far away from the central ring(s) can retain aromatic character.

Chiral Jahn-Teller Distortion in Quasi-Planar Boron Clusters

In this work, we have observed that some chiral boron clusters (B16-, B20-, B24-, and B28-) can simultaneously have helical molecular orbitals and helical spin densities; these seem to be the first compounds discovered to have this intriguing property. We show that chiral Jahn-Teller distortion of quasi-planar boron clusters drives the formation of the helical molecular spin densities in these clusters and show that elongation/enhancement in helical molecular orbitals can be achieved by simply adding more building blocks via a linker. Aromaticity of these boron clusters is discussed. Chiral boron clusters may find potential applications in spintronics, such as molecular magnets.

Close Stacking of Antiaromatic Ni(II) Norcorrole Originating from a Four-Electron Multicentered Bonding Interaction

A π-conjugated molecule with one electronic spin often forms a π-stacked dimer through molecular orbital interactions between two unpaired electrons. The bonding is recognized as a multicentered two-electron interaction between the two π-conjugated molecules. Here, we disclose a multicentered bonding interaction between two antiaromatic molecules involving four electrons. We have synthesized an antiaromatic porphyrin analogue, Ni(II) bis(pentafluorophenyl)norcorrole. Its dimer adopts a face-to-face stacked structure with an extremely short stacking distance of 2.97 Å. The close stacking originates from a multicenter four-electron bonding interaction between the two molecules. The bonding electrons were experimentally observed via synchrotron X-ray diffraction analysis and corroborated by theoretical calculations. The intermolecular interaction of the molecular orbitals imparts the stacked dimer with aromatic character that is distinctly different from that of its monomer.

The σ+π dual aromaticity of typical bi-tetrazole ring molecule TKX-50

Two complexes of dihydroxylammonium 5,5'-bistetrazole-1,1'-diolate (TKX-50) were employed to evaluate the aromaticity of their tetrazole rings via deep analysis such as the electronic structure, the ZZ component of the natural chemical shielding tensor (NICSZZ) and component orbitals, localized orbital locator purely contributed by σ-orbitals (LOL-σ) and localized orbital locator purely contributed by π-orbitals (LOL-π), the anisotropy of the induced current density (AICD) and the ZZ component of iso-chemical shielding surface (ICSSZZ) of these tetrazole rings thereof. The conclusion shows: that all tetrazole rings and bi-tetrazole rings in complexes have strong σ and a comparable strength π double aromaticity; all these magnetic shields almost symmetrically increase from the central axis to the tetrazole ring atoms; tetrazole rings in complex II show a little stronger dual aromaticity than that in complex I mainly due to the different orientation of the fragment 2 encompassing two hydroxylamine groups resulting in different effects on the contributions of σ orbitals and π orbitals to total aromaticity of tetrazole rings thereof; the difference in aromaticity is fundamentally caused by the atoms O with stronger electron-withdrawing than atom N in fragment 2 interact with bi-tetrazole ring through O in complex I but through N in complex II.

Towards 2D Borane Chemistry in Hexagonal Cyclic Compounds

A comprehensive comparison between known benzene mono-substituted compounds R-Ph and the corresponding isoelectronic unknown R-cyclohexaborane(12) molecules is carried out from a geometric and electronic structure point of view, with R={H, BH2, CH3, NH2, OH, F ; AlH2, SiH3, PH2, SH, Cl ; NO2, OCH3}. We suggest new chemical names for the 2D borane compounds and analyze the geometric and electronic structure carbon vs. boron comparatives by means of HOMO-LUMO gaps, bonding schemes, electron density topological properties and predicted NMR chemical shifts. The predictions on the properties in planar hexagonal cyclic boranes may help in the design of synthesis procedures for these yet-unkown compounds.

Exploring Geometrical, Electronic and Spectroscopic Properties of 2-Nitroimidazole-Based Radiopharmaceuticals via Computational Chemistry Methods

Tumor hypoxia plays an important role in the clinical management and treatment planning of various cancers. The use of 2-nitroimidazole-based radiopharmaceuticals has been the most successful for positron emission tomography (PET) and single photon emission computed tomography (SPECT) imaging probes, offering noninvasive means to assess tumor hypoxia. In this study we performed detailed computational investigations of the most used compounds for PET imaging, focusing on those derived from 2-nitroimidazole: fluoromisonidazole (FMISO), fluoroazomycin arabinoside (FAZA), fluoroetanidazole (FETA), fluoroerythronitroimidazole (FETNIM) and 2-(2-nitroimidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)acetamide (EF5). Conformational analysis, structural parameters, vibrational IR and Raman properties (within both harmonic and anharmonic approximations), as well as the NMR shielding tensors and spin-spin coupling constants were obtained by density functional theory (DFT) calculations and then correlated with experimental findings, where available. Furthermore, time-dependent DFT computations reveal insight into the excited states of the compounds. Our results predict a significant change in the conformational landscape of most of the investigated compounds when transitioning from the gas phase to aqueous solution. According to computational data, the 2-nitroimidazole moiety determines to a large extent the spectroscopic properties of its derivatives. Due to the limited structural information available in the current literature for the investigated compounds, the findings presented herein deepen the current understanding of the electronic structures of these five radiopharmaceuticals.

Fragment-Based Deep Learning for Simultaneous Prediction of Polarizabilities and NMR Shieldings of Macromolecules and Their Aggregates

Simultaneous prediction of the molecular response properties, such as polarizability and the NMR shielding constant, at a low computational cost is an unresolved issue. We propose to combine a linear-scaling generalized energy-based fragmentation (GEBF) method and deep learning (DL) with both molecular and atomic information-theoretic approach (ITA) quantities as effective descriptors. In GEBF, the total molecular polarizability can be assembled as a linear combination of the corresponding quantities calculated from a set of small embedded subsystems in GEBF. In the new GEBF-DL(ITA) protocol, one can predict subsystem polarizabilities based on the corresponding molecular wave function (thus electron density and ITA quantities) and DL model rather than calculate them from the computationally intensive coupled-perturbed Hartree-Fock or Kohn-Sham equations and finally obtain the total molecular polarizability via a linear combination equation. As a proof-of-concept application, we predict the molecular polarizabilities of large proteins and protein aggregates. GEBF-DL(ITA) is shown to be as accurate enough as GEBF, with mean absolute percentage error <1%. For the largest protein aggregate (>4000 atoms), GEBF-DL(ITA) gains a speedup ratio of 3 compared with GEBF. It is anticipated that when more advanced electronic structure methods are used, this advantage will be more appealing. Moreover, one can also predict the NMR chemical shieldings of proteins with reasonably good accuracy. Overall, the cost-efficient GEBF-DL(ITA) protocol should be a robust theoretical tool for simultaneously predicting polarizabilities and NMR shieldings of large systems.

Implementation and First Evaluation of Strong-Correlation-Corrected Local Hybrid Functionals for the Calculation of NMR Shieldings and Shifts

Local hybrid functionals containing strong-correlation factors (scLHs) and range-separated local hybrids (RSLHs) have been integrated into an efficient coupled-perturbed Kohn-Sham implementation for the calculation of nuclear shielding constants. Several scLHs and the ωLH22t RSLH have then been evaluated for the first time for the extended NS372 benchmark set of main-group shieldings and shifts and the TM70 benchmark of 3d transition-metal shifts. The effects of the strong-correlation corrections have been analyzed with respect to the spatial distribution of the sc-factors, which locally diminish exact-exchange admixture at certain regions in a molecule. The scLH22t, scLH23t-mBR, and scLH23t-mBR-P functionals, which contain a "damped" strong-correlation factor to retain the excellent performance of the underlying LH20t functional for weakly correlated situations, tend to make smaller corrections to shieldings and shifts than the "undamped" scLH22ta functional. While the latter functional can also deteriorate agreement with the reference data in certain weakly correlated cases, it provides overall better performance, in particular for systems where static correlation is appreciable. This pertains only to a minority of systems in the NS372 main-group test set but to many more systems in the TM70 transition-metal test set, in particular for high-oxidation-state complexes, e.g., Cr(+VI) complexes and other systems with stretched bonds. Another undamped scLH, the simpler LDA-based scLH21ct-SVWN-m, also tends to provide significant improvements in many cases. The differences between the functionals and species can be rationalized on the basis of one-dimensional plots of the strong-correlation factors, augmented by isosurface plots of the fractional orbital density (FOD). Position-dependent exact-exchange admixture is thus shown to provide substantial flexibility in treating response properties like NMR shifts for both weakly and strongly correlated systems.

Crystal, Solution, and Computational Study of the Structure of Ortho-Lithium N,N-Diisopropyl-P,P-Diphenylphosphinothioic Amide

The first crystal structure of an ortho-lithium phosphinothioic amide complexed with tetramethylethylenediamine 12 is reported. The complex consists of a spirane in which the spiro-lithium is N,N- and C,S-chelated by the diamine and organophosphorus ligands, respectively. The analogous ortho anion 14 obtained by Sn(IV)/Li transmetallation in THF has also been synthesized. Nuclear magnetic resonance study of both anions showed that they exist as monomers in solution and are involved in dynamic processes including the restricted rotation around the P-N bond. 14 is converted at room temperature by nucleophilic cyclization to the dearomatized anion 15, which evolves after a few hours to the benzophosphindole sulfide 16. Density functional theory calculations supported the aggregation state in solution and were used to explore the conformational space of anion 12, the mechanism of ortho-lithiation directed by P(X)-N (X=O, S) groups, and the mechanism of formation of 15.