A detailed study on the symmetry breaking and its effect on the potential surface of NO3

Mechanistic Inside into the Gas-Phase NO3·+HO2· Reaction

The reaction between NO 3 · and HO 2 · is critical in nighttime atmospheric chemistry. This reaction is believed to occur via two channels: one leading to the formation of HNO3 and the other forming OH · . A long-standing question regarding this reaction is whether this reaction occurs through HNO3 path or OH · or both at a time. Some indirect experiments proposed the HNO3 path as the exclusive path, while some suggested the possibility of both the paths. In the present work, using high-level quantum chemical and kinetics calculations, we have tried to shed light on the mechanism of this reaction. We have incorporated full triple excitation and partial quadratic excitation corrections at the coupled-cluster level for our energetics part, whereas a master equation approach has been employed to obtain the rate constants within 213-400 K. Our investigation suggests that it is only the OH · path through which the reaction takes place.

Reply to the 'Comment on "Theoretical study of the NO3 radical reaction with CH2ClBr, CH2ICl, CH2BrI, CHCl2Br, and CHClBr2"' by C. J. Nielsen and Y. Tang, Phys. Chem. Chem. Phys., 2022, , DOI: 10.1039/D2CP03013F

In this Reply, we answer the main argument raised in the Comment about the energy of the NO₃ radical and its influence in the reaction profiles of the reaction of the NO₃ radical with CH₂ClBr, CH₂ICl, CH₂BrI, CHCl₂Br, and CHClBr₂ by C. J. Nielsen and Y. Tang. The optimized geometry of the NO₃ radical has been obtained using 49 DFT functionals: 26 functionals predict a minimum with D_3h symmetry and 23 with C_2v symmetry. The former functionals have been used to calculate the thermodynamic values of three reactions (X + HNO₃ → XH + NO₃, X= OH, CH₃ and CCl₃) and compared with experimental data. Those functionals with smaller errors have been used to recalculate the barriers of the reaction of NO₃ with CH₂ClBr, CH₂ICl, CH₂BrI, CHCl₂Br, and CHClBr₂. The results show differences of 10.5 kJ mol^-1 when compared to those obtained with the M08HX functional.

Comment on "Theoretical study of the NO3 radical reaction with CH2ClBr, CH2ICl, CH2BrI, CHCl2Br, and CHClBr2" by I. Alkorta, J. M. C. Plane, J. Elguero, J. Z. Dávalos, A. U. Acuña and A. Saiz-Lopez, Phys. Chem. Chem. Phys. 2022, , 14365

This comment addresses a systematic error in the potential energy surfaces of the title reactions presented in the original article by Alkorta et al. The NO₃ radical has D_3h symmetry in the electronic ground state while the M08HX functional employed in the original article predicts an incorrect C_2v geometry and energy. By combining thermodynamic data for the OH + HNO₃ → H₂O + NO₃ reaction with spectroscopic data and results from M08HX calculations on HNO₃, H₂O and the OH radical, the ground state NO₃ radical energy is estimated to be 37 kJ mol^-1 lower than reported for the C_2v geometry.

Simulation of the photodetachment spectra of the nitrate anion (NO3-) in the B 2E' energy range and non-adiabatic electronic population dynamics of NO3

The photodetachment spectrum of the nitrate anion (NO3-) in the energy range of the NO3 second excited state is simulated from first principles using quantum wave packet dynamics. The prediction...

Probing Interfacial Effects on Ionization Energies: The Surprising Banality of Anion-Water Hydrogen Bonding at the Air/Water Interface

Liquid microjet photoelectron spectroscopy is an increasingly common technique to measure vertical ionization energies (VIEs) of aqueous solutes, but the interpretation of these experiments is subject to questions regarding sensitivity to bulk versus interfacial solvation environments. We have computed aqueous-phase VIEs for a set of inorganic anions, using a combination of molecular dynamics simulations and electronic structure calculations, with results that are in excellent agreement with experiment regardless of whether the simulation data are restricted to ions at the air/water interface or to those in bulk aqueous solution. Although the computed VIEs are sensitive to ion-water hydrogen bonding, we find that the short-range solvation structure is sufficiently similar in both environments that it proves impossible to discriminate between the two on the basis of the VIE, a conclusion that has important implications for the interpretation of liquid-phase photoelectron spectroscopy. More generally, analysis of the simulation data suggests that the surface activity of soft anions is largely a second or third solvation shell effect, arising from disruption of water-water hydrogen bonds and not from significant changes in first-shell anion-water hydrogen bonding.

Exploring spin symmetry-breaking effects for static field ionization of atoms: Is there an analog to the Coulson-Fischer point in bond dissociation?

Löwdin's symmetry dilemma is an ubiquitous issue in approximate quantum chemistry. In the context of Hartree-Fock (HF) theory, the use of Slater determinants with some imposed constraints to preserve symmetries of the exact problem may lead to physically unreasonable potential energy surfaces. On the other hand, lifting these constraints leads to the so-called broken symmetry solutions that usually provide better energetics, at the cost of losing information about good quantum numbers that describe the state of the system. This behavior has previously been extensively studied in the context of bond dissociation. This paper studies the behavior of different classes of HF spin polarized solutions (restricted, unrestricted, and generalized) in the context of ionization by strong static electric fields. We find that, for simple two electron systems, unrestricted Hartree-Fock (UHF) is able to provide a qualitatively good description of states involved during the ionization process (neutral, singly ionized, and doubly ionized states), whereas RHF fails to describe the singly ionized state. For more complex systems, even though UHF is able to capture some of the expected characteristics of the ionized states, it is constrained to a single M_s (diabatic) manifold in the energy surface as a function of field intensity. In this case, a better qualitative picture can be painted by using generalized Hartree-Fock as it is able to explore different spin manifolds and follow the lowest solution due to lack of collinearity constraints on the spin quantization axis.

Insight into Hydrogen Abstractions by Nitrate Radical: Structural, Solvent Effects, and Evidence for a Polar Transition State

The role of a polarized transition state and solvent effects on nitrate radical reactions was examined with a previously under-reported class of substrates, ethers, for their atmospheric implications. Absolute rate constants for hydrogen abstraction from a series of alcohols, ethers, and alkanes by nitrate radical have been measured in acetonitrile, water, and mixtures of these two solvents. Across all of these classes of compounds, using a modified form of the Evans-Polanyi relationship, it is demonstrated that the observed structure/reactivity trends can be reconciled by considering the number of abstractable hydrogens, strength of the C-H bond, and ionization potential (IP) of the substrate. Hydrogen abstractions by nitrate radical occur with low selectivity and are characterized by an early transition state (α ≈ 0.3). The dependence of the rate constant on IP suggests a polar transition state with some degree (<10%) of charge transfer. These conclusions stand for reactions conducted in solution (CH₃CN and H₂O) as well as gas-phase values. Because of this polar transition state, the rate constants increase going from the gas phase to a polar solvent, and the magnitude of the increase is consistent with Kirkwood theory.

Accurate quantum dynamics simulation of the photodetachment spectrum of the nitrate anion (NO3 -) based on an artificial neural network diabatic potential model

The photodetachment spectrum of the nitrate anion (NO₃ ^-) is simulated from first principles using wavepacket quantum dynamics propagation and a newly developed accurate full-dimensional fully coupled five state diabatic potential model. This model utilizes the recently proposed complete nuclear permutation inversion invariant artificial neural network diabatization technique [D. M. G. Williams and W. Eisfeld, J. Phys. Chem. A 124, 7608 (2020)]. The quantum dynamics simulations are designed such that temperature effects and the impact of near threshold detachment are taken into account. Thus, the two available experiments at high temperature and at cryogenic temperature using the slow electron velocity-map imaging technique can be reproduced in very good agreement. These results clearly show the relevance of hot bands and vibronic coupling between the X̃ ²A₂ ^' ground state and the B̃ ²E' excited state of the neutral radical. This together with the recent experiment at low temperature gives further support for the proper assignment of the ν₃ fundamental, which has been debated for many years. An assignment of a not yet discussed hot band line is also proposed.

The role of electron-nuclear coupling on multi-state photoelectron spectra, scattering processes and phase transitions

We present first principle based beyond Born-Oppenheimer (BBO) theory and its applications on various models as well as realistic spectroscopic and scattering processes, where the Jahn-Teller (JT) theory is brought in conjunction with the BBO approach on the phase transition of lanthanide complexes. Over one and half decades, our development of BBO theory is demonstrated with ab initio calculations on representative molecules of spectroscopic interest (NO2 radical, Na3 and K3 clusters, NO3 radical, C6H6+ and 1,3,5-C6H3F3+ radical cations) as well as triatomic reactive scattering processes (H+ + H2 and F + H2). Such an approach exhibits the effect of JT, Renner-Teller (RT) and pseudo Jahn-Teller (PJT) type of interactions. While implementing the BBO theory, we generate highly accurate diabatic potential energy surfaces (PESs) to carry out quantum dynamics calculation and find excellent agreement with experimental photoelectron spectra of spectroscopic systems and cross-sections/rate constants of scattering processes. On the other hand, such electron-nuclear couplings incorporated through JT theory play a crucial role in dictating higher energy satellite transitions in the dielectric function spectra of the LaMnO3 complex. Overall, this article thoroughly sketches the current perspective of the BBO approach and its connection with JT theory with various applications on physical and chemical processes.

Atmospheric Chemistry of Methyl Isocyanide-An Experimental and Theoretical Study

The reaction of CH3NC with OH radicals was studied in smog chamber experiments employing PTR-ToF-MS and long-path FTIR detection. The rate coefficient was determined to be kCH3NC+OH = (7.9 ± 0.6) × 10-11 cm3 molecule-1 s-1 at 298 ± 3 K and 1013 ± 10 hPa; methyl isocyanate was the sole observed product of the reaction. The experimental results are supported by CCSD(T*)-F12a/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ quantum chemistry calculations showing the reaction to proceed primarily via electrophilic addition to the isocyanide carbon atom. On the basis of the quantum chemical data, the kinetics of the OH reaction was simulated using a master equation model revealing the rate coefficient to be nearly independent of pressure at tropospheric conditions and having a negative temperature dependence with kOH = 4.2 × 10-11 cm3 molecule-1 s-1 at 298 K. Additional quantum chemistry calculations on the CH3NC reactions with O3 and NO3 show that these reactions are of little importance under atmospheric conditions. The atmospheric fate of methyl isocyanide is discussed.

Topological Effects in Vibronically Coupled Degenerate Electronic States: A Case Study on Nitrate and Benzene Radical Cation

We carry out detailed investigation for topological effects of two molecular systems, NO₃ radical and C₆H₆ ⁺ (Bz⁺) radical cation, where the dressed adiabatic, dressed diabatic, and adiabatic-via-dressed diabatic potential energy curves (PECs) are generated employing ab initio calculated adiabatic and diabatic potential energy surfaces (PESs). We have implemented beyond Born-Oppenheimer (BBO) theory for constructing smooth, single-valued, and continuous diabatic PESs for five coupled electronic states [J. Phys. Chem. A 2017, 121, 6314-6326]. In the case of NO₃ radical, the nonadiabatic coupling terms (NACTs) among the low-lying five electronic states, namely, X̃ ²A₂ ^' (1²B₂), A~ ²E″ (1²A₂ and 1²B₁), and B~ ²E' (1²A₁ and 2²B₂), bear the signature of Jahn-Teller (JT) interactions, pseudo JT (PJT) interactions, and accidental conical intersections (CIs). Similarly, Bz⁺ radical cation also exhibits JT, PJT, and accidental CIs in the interested domain of nuclear configuration space. In order to generate dressed PECs, two components of degenerate in-plane asymmetric stretching modes are selectively chosen for both the molecular species (Q _3x -Q _3y pair for NO₃ radical and Q _16x -Q _16y pair for Bz⁺ radical cation). The JT coupling between the electronic states is essentially originated through the asymmetric stretching normal mode pair, where the coupling elements exhibit symmetric and nonlinear functional behavior along Q _3x and Q _16x normal modes.

The gas-phase structure of the asymmetric, trans-dinitrogen tetroxide (N2O4), formed by dimerization of nitrogen dioxide (NO2), from rotational spectroscopy and ab initio quantum chemistry

We report the first experimental gas-phase observation of an asymmetric, trans-N₂O₄ formed by the dimerization of NO₂. In additional to the dominant ¹⁴N₂¹⁶O₄ species, rotational transitions have been observed for all species with single ¹⁵N and ¹⁸O substitutions as well as several multiply substituted isotopologues. These transitions were used to determine a complete substitution structure as well as an r₀ structure from the fitted zero-point averaged rotational constants. The determined structure is found to be that of an ON-O-NO₂ linkage with the shared oxygen atom closer to the NO₂ than the NO (1.42 vs 1.61 Å). The structure is found to be nearly planar with a trans O-N-O-N linkage. From the spectra of the ¹⁴N¹⁵NO₄ species, we were able to determine the nuclear quadrupole coupling constants for each specific nitrogen atom. The equilibrium structure determined by ab initio quantum chemistry calculations is in excellent agreement with the experimentally determined structure. No spectral evidence of the predicted asymmetric, cis-N₂O₄ was found in the spectra.

Isomer Identification in Flames with Double-Imaging Photoelectron/Photoion Coincidence Spectroscopy (i2PEPICO) using Measured and Calculated Reference Photoelectron Spectra

Double-imaging photoelectron/photoion coincidence (i2PEPICO) spectroscopy using a multiplexing, time-efficient, fixed-photon-energy approach offers important opportunities of gas-phase analysis. Building on successful applications in combustion systems that have demonstrated the discriminative power of this technique, we attempt here to push the limits of its application further to more chemically complex combustion examples. The present investigation is devoted to identifying and potentially quantifying compounds featuring five heavy atoms in laminar, premixed low-pressure flames of hydrocarbon and oxygenated fuels and their mixtures. In these combustion examples from flames of cyclopentene, iso-pentane, iso-pentane blended with dimethyl ether (DME), and diethyl ether (DEE), we focus on the unambiguous assignment and quantitative detection of species with the sum formulae C5H6, C5H7, C5H8, C5H10, and C4H8O in the respective isomer mixtures, attempting to provide answers to specific chemical questions for each of these examples. To analyze the obtained i2PEPICO results from these combustion situations, photoelectron spectra (PES) from pure reference compounds, including several examples previously unavailable in the literature, were recorded with the same experimental setup as used in the flame measurements. In addition, PES of two species where reference spectra have not been obtained, namely 2-methyl-1-butene (C5H10) and the 2-cyclopentenyl radical (C5H7), were calculated on the basis of high-level ab initio calculations and Franck-Condon (FC) simulations. These reference measurements and quantum chemical calculations support the early fuel decomposition scheme in the cyclopentene flame towards 2-cyclopentenyl as the dominant fuel radical as well as the prevalence of branched intermediates in the early fuel destruction reactions in the iso-pentane flame, with only minor influences from DME addition. Furthermore, the presence of ethyl vinyl ether (EVE) in DEE flames that was predicted by a recent DEE combustion mechanism could be confirmed unambiguously. While combustion measurements using i2PEPICO can be readily obtained in isomer-rich situations, we wish to highlight the crucial need for high-quality reference information to assign and evaluate the obtained spectra.

Five Electronic State Beyond Born-Oppenheimer Equations and Their Applications to Nitrate and Benzene Radical Cation

We present explicit form of Adiabatic to Diabatic Transformation (ADT) equations and expressions of non-adiabatic coupling terms (NACTs) for a coupled five-state electronic manifold in terms of ADT angles between electronic wave functions. ADT matrices eliminate the numerical instability arising from singularity of NACTs and transform the adiabatic Schrödinger equation to its diabatic form. Two real molecular systems NO₃ and C₆H₆⁺ (Bz⁺) are selectively chosen for the demonstration of workability of those equations. We examine the NACTs among the lowest five electronic states of the NO₃ radical [X̃²A₂^' (1²B₂), òE″ (1²A₂ and 1²B₁) and B̃²E' (1²A₁ and 2²B₂)], in which all types of non-adiabatic interactions, that is, Jahn-Teller (JT) interactions, Pseudo Jahn-Teller (PJT) interactions, and accidental conical intersections (CIs) are present. On the other hand, lowest five electronic states of Bz⁺ [X̃²E_1g (1²B_3g and 1²B_2g), B̃²E_2g (1²A_g and 1²B_1g), and C̃²A_2u (1²B_1u)] depict similar kind of complex feature of non-adiabatic effects. For NO₃ radical, the two components of degenerate in-plane asymmetric stretching mode are taken as a plane of nuclear configuration space (CS), whereas in case of Bz⁺, two pairs are chosen: One is the pair of components of degenerate in-plane asymmetric stretching mode, and the other one is constituted with one of the components each from out-of-plane degenerate bend and in-plane degenerate asymmetric stretching modes. We calculate ab initio adiabatic potential energy surfaces (PESs) and NACTs among the lowest five electronic states at the CASSCF level using MOLPRO quantum chemistry package. Subsequently, the ADT is performed using those newly developed equations to validate the positions of the CIs, evaluate the ADT angles and construct smooth, symmetric, and continuous diabatic PESs for both the molecular systems.

Photoelectron Spectroscopy and Ab Initio Calculations of CS3(-) Isomers: Carbon Trisulfide and Carbon Disulfide S-Sulfide Anions

Carbon sulfides are known as a class of binary compounds that can exist in various isomeric and/or polymeric forms. As for a sulfur-rich compound with the composition formula CS3, two possible constitutional isomers have been proposed experimentally or theoretically for the neutral species and its corresponding radical cation and anion. Although the previous studies claim that one isomer has a carbon trisulfide (CS3) C-centered configuration and the other has a carbon disulfide S-sulfide (SCSS) chain configuration, they have not yet been fully identified by a spectroscopic method. In this study, we have prepared the anions of those isomers in the gas phase by employing two types of reactions: dissociative electron attachment to 1,3-dithiole-2-thione for CS3(-) formation and the S(-) + CS2 ion-molecule reaction for SCSS(-). Photoelectron spectroscopic measurements reveal that the reactions result in the production of two anionic species that can be well distinguished by their vertical detachment energy. With the aid of ab initio calculations, they are identified distinctively as the anions of carbon trisulfide and carbon disulfide S-sulfide.

CuNO2 and Cu(+)NO2 Revisited: A Comparative ab Initio and DFT Study

We have reinvestigated CuNO2 and Cu(+)NO2 at ab initio as well as at pure and hybrid DFT levels of approximation employing large ANO basis sets. The systems were fully optimized using the CCSD(T), QCISD(T), BPW91, PBE, PBE0, and B3LYP methods. Several stationary points (minima and transition structures) were found on the related potential energy surfaces (PES). The C2v bidentate η(2)-O,O isomer is calculated to be the most stable species on the CuNO2 PES, followed by two monodentate isomers [Formula: see text] the Cs η(1)-O and C2v η(1)-N species which are higher in energy by 12 and 14 kcal/mol, respectively, at CCSD(T)/Basis-II (where Basis-II is 21s15p10d6f4g/8s7p5d3f2g for Cu; 14s9p4d3f/5s4p3d2f for O and N). On the Cu(+)NO2 PES, the Cs monodentate η(1)-O trans (0 kcal/mol) and cis (+3 kcal/mol at CCSD(T)/Basis-II) isomers are found, followed by the C2v monodentate η(1)-N isomer (+14 kcal/mol at CCSD/Basis-II). In contrast to the pure DFT, the hybrid DFT methods perform reasonably well for predicting the relative stabilities (except for η(1)-N of CuNO2) and structures; however, their predictions of the bond dissociation energies are less reliable (for CuNO2 the difference is as much as 10 kcal/mol compared to the CCSD(T) values). The performance of the QCISD(T) method was analyzed, and, furthermore, the issue of symmetry breaking was investigated.

Negative ion photoelectron spectroscopy confirms the prediction that D3h carbon trioxide (CO3) has a singlet ground state

The CO₃ radical anion (CO₃˙^–) has been formed by electrospraying carbonate dianion (CO₃^2–) into the gas phase.

The CO₃ radical anion (CO₃˙^–) has been formed by electrospraying carbonate dianion (CO₃^2–) into the gas phase. The negative ion photoelectron (NIPE) spectrum of CO₃˙^– shows that, unlike the isoelectronic trimethylenemethane [C(CH₂)₃], D_3h carbon trioxide (CO₃) has a singlet ground state. From the NIPE spectrum, the electron affinity of D_3h singlet CO₃ was, for the first time, directly determined to be EA = 4.06 ± 0.03 eV, and the energy difference between the D_3h singlet and the lowest triplet was measured as ΔE_ST = – 17.8 ± 0.9 kcal mol^–1. B3LYP, CCSD(T), and CASPT2 calculations all find that the two lowest triplet states of CO₃ are very close in energy, a prediction that is confirmed by the relative intensities of the bands in the NIPE spectrum of CO₃˙^–. The 560 cm^–1 vibrational progression, seen in the low energy region of the triplet band, enables the identification of the lowest, Jahn–Teller-distorted, triplet state as ³A₁, in which both unpaired electrons reside in σ MOs, rather than ³A₂, in which one unpaired electron occupies the b₂ σ MO, and the other occupies the b₁ π MO.

Even-odd product variation of the C(n)(+) + D(2) (n = 4-9) reaction: complexity of the linear carbon cation electronic states

We have studied reactions between linear Cn(+) (n = 4-9) and D2, using ion mobility mass spectrometry techniques and quantum chemical calculations in order to understand the complex reactivity of the linear cluster cations. Only linear CnD(+) products were observed for the odd (n = 5, 7, 9) linear clusters, while CnD2(+) was the main product for the even clusters. For the reaction rate constants determined for these two channels, we obtained the following two features: (1) the rate constant decreases with the size n, and (2) even-sized clusters have lower rate constants than neighboring odd-sized clusters. In the theoretical calculations using the CCSD(T) and B3LYP methods with the cc-pVTZ basis, we found that a low lying (2)Σ state in odd clusters may play an important role in these reactions. This opposes the previous interpretation that the (2)Πg/u state is the dominant electronic state for linear Cn(+) (n = 4-9) clusters. We showed that a barrierless radical abstraction forming CnD(+) occurs through a direct head on approach for the (2)Σ state Cn(+). In contrast, a carbene-like insertion forming CnD2(+) occurs through a sideways approach for the (2)Πg/u state Cn(+). We have concluded that the higher rate constants for the odd clusters come from the existence of symmetry broken (2)Σ states which are absent in even linear clusters.

A theoretical study of the mechanism of the atmospherically relevant reaction of chlorine atoms with methyl nitrate, and calculation of the reaction rate coefficients at temperatures relevant to the troposphere

Computed rate coefficients of the atmospherically important Cl + CH₃ONO₂ → HCl + CH₂ONO₂ reaction reported for the first time.

Structure, stability, and properties of the trans peroxo nitrate radical: the importance of nondynamic correlation

We report a comparative single-reference and multireference coupled-cluster investigation on the structure, potential energy surface, and IR spectroscopic properties of the trans peroxo nitrate radical, one of the key intermediates in stratospheric NOX chemistry. The previous single-reference ab initio studies predicted an unbound structure for the trans peroxo nitrate radical. However, our Fock space multireference coupled-cluster calculation confirms a bound structure for the trans peroxo nitrate radical, in accordance with the experimental results reported earlier. Further, the analysis of the potential energy surface in FSMRCC method indicates a well-behaved minima, contrary to the shallow minima predicted by the single-reference coupled-cluster method. The harmonic force field analysis, of various possible isomers of peroxo nitrate also reveals that only the trans structure leads to the experimentally observed IR peak at 1840 cm(-1). The present study highlights the critical importance of nondynamic correlation in predicting the structure and properties of high-energy stratospheric NOx radicals.

Atmospheric formation of the NO3 radical from gas-phase reaction of HNO3 acid with the NH2 radical: proton-coupled electron-transfer versus hydrogen atom transfer mechanisms

The amidogen radical abstracts the hydrogen from nitric acid through a proton coupled electron transfer mechanism rather than by an hydrogen atom transfer process.