Potential nonrigidity of the NO3 radical

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.

Wavefunction-based quantum-chemicalab initiocalculations for core electron binding energies of small open shell molecules

Core electron binding energies (CEBEs), i.e. ionization energies of 1s core orbitals, are calculated by means of wavefunction-based quantum-chemicalab initiomethods for a series of small open-shell molecules containing first-row atoms. The calculations are performed in three steps: (a) Koopmans' theorem, where the orbitals of the electronic ground state are used unchanged also for the ions, (b) Hartree-Fock or self consistent field (SCF) approximation in which the orbitals are allowed to relax after 1s ionization (ΔSCF), (c) dynamic correlation effects on top of SCF. For open-shell molecules 1s ionization leads to ions in several spin states, mostly to a pair of a triplet and a singlet state. In several cases one or both of these ionic states are only poorly described by a single-reference SCF wavefunction, therefore a multi-reference complete active space self consistent field (CAS-SCF) wavefunction is used instead. The correlation effects are evaluated by means of our multi-reference coupled electron pair approximation program. The accuracy of the calculated CEBEs is in the order of 0.1-0.4 eV. This is in agreement with experimental results for NO and O₂. But there exist only very few gas phase data for CEBEs of open-shell molecules.

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.

High-Resolution Photoelectron Spectroscopy of Cryogenically Cooled NO3̅

High-resolution anion photoelectron spectra of cryogenically cooled NO₃̅ anions obtained using slow photoelectron velocity-map imaging are presented and provide new insight into the vibronic structure of the corresponding neutral radical. A combination of improved spectral resolution, measurement of energy-dependent intensity effects, temperature control, and comparison to theory allows for full assignment of the vibronic features observed in this spectrum. We obtain a refined electron affinity of 3.9289(14) eV for NO₃. Further, the appearance of Franck-Condon forbidden transitions from vibrationally cold anions to neutral states with excitation along the NO₃ ν₄ mode confirms that these features arise from vibronic coupling with the B̃²E' excited state of NO₃ and are not hot bands, as has been suggested. Together, the suite of experimental and simulated results provides clear evidence that the ν₃ fundamental of NO₃ resides near 1050 cm^-1, addressing a long-standing controversy surrounding this vibrational assignment.

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.

Reactions of the methylsulfinyl radical [CH3(O)S˙] with oxygen (3O2) in solid argon

The atmospherically highly relevant methylsulfinyl radical (CH₃(O)S˙) reacts with molecular oxygen in cryogenic argon matrices and forms the methylsulfinylperoxyl radical (CH₃(O)SOO˙). The later was characterized by IR and UV/Vis spectroscopy, including isotopic labelling studies.

On the vibronic level structure in the NO3 radical. I. The ground electronic state

The model Hamiltonian approach of Koppel et al. [Adv. Chem. Phys. 57, 59 (1984)] is used to analyze the electronic spectroscopy of the nitrate radical (NO3). Simulations of negative ion photodetachment of NO3-, the X 2A2'<--B 2E' dispersed fluorescence spectrum of NO3, and the B 2E'<--X 2A2' absorption spectrum are all in qualitative agreement with experiment, indicating that the model Hamiltonian contains most or all of the essential physics that govern the strongly coupled X 2A2' and B 2E' electronic states of the radical. All 14 bands seen in the dispersed fluorescence spectrum below 2600 cm-1 are assigned based on the simulations, filling in a few gaps left by previous work, and 7 additional bands below 4000 cm-1 are tentatively assigned. The assignment is predicated on the assumption that the nu3 level of NO3 is near 1000 cm-1 rather than 1492 cm-1 as is presently believed. Support for this reassignment (which associates the 1492 cm-1 band with the nu1+nu4 level) comes from both the model Hamiltonian spectrum and a Fourier-transform infrared feature at 2585 cm-1 that is consistent with the large and positive cross anharmonicity between nu1 and nu4 needed for the alternative 1492 cm-1 assignment. An apparent systematic deficiency exists in the treatment of the model Hamiltonian for levels involving nu4. A discussion of the correlation between energy levels in the rigid D3h and C2v limits is illustrative, and provides insight into just how hard it is to treat the degenerate bending coordinate (q4) of NO3 accurately.

Ab initio study of the mechanism of the atmospheric reaction: NO2 + O3 --> NO3 + O2

The atmospheric reaction NO2 + O3 --> NO3 + O2 (1) has been investigated theoretically by using the MP2, G2, G2Q, QCISD, QCISD(T), CCSD(T), CASSCF, and CASPT2 methods with various basis sets. The results show that the reaction pathway can be divided in two different parts at the MP2 level of theory. At this level, the mechanism proceeds along two transition states (TS1 and TS2) separated by an intermediate, designated as A. However, when the single-reference higher correlated QCISD methodology has been employed, the minimum A and the transition state TS2 are not found on the hypersurface of potential energy, which confirms a direct reaction mechanism. Single-reference high correlated and multiconfigurational methods consistently predict the barrier height of reaction (1) to be within the range 2.5-6.1 kcal mol(-1), in reasonable agreement with experimental data. The calculated reaction enthalpy is -24.6 kcal mol(-1) and the reaction rate calculated at the highest CASPT2 level, of k = 6.9 x 10(-18) cm(3) molecule(-1) s(-1). Both results can be regarded also as accurate predictions of the methodology employed in this article.

Peroxynitrate and peroxynitrite: a complete basis set investigation of similarities and differences between these NOx species

Peroxynitric acid/peroxynitrate (PNA) rivals peroxynitrous acid/peroxynitrite (PNI) in importance as a reactive oxygen species. These species possess similar two-electron oxidative behavior. On the other hand, stark differences exist in the stability of these molecules as a function of pH and in the presence of CO(2), and also in the types of bond homolysis reactions that PNA and PNI may undergo. Using CBS-QB3 theory, we examine these similarities and differences. The activation barriers for two-electron oxidation of NH(3), H(2)S, and H(2)C=CH(2) by PNA and PNI are found to be generally similar. The O-O BDE of O(2)NOOCO(2-) is predicted to be 26 kcal/mol greater than that of ONOOCO(2-). This accounts for the insensitivity of PNA to the presence of CO(2). Likewise, the O-O BDE of O(2)NOOH is predicted to be 19 kcal/mol greater than that of ONOOH, in excellent agreement with experiment. The fundamental principle underlying the large differences in O-O BDEs between PNA and PNI species is that the NO(2) that is formed from PNI can relax from the (2)B(2) excited state to the (2)A(1) ground state, whereas no such comparable state change occurs with NO(3) from PNA. Decomposition of the anions O(x)NOO(-) is more complex, with the energetics influenced by solvation. ONOO(-) can homolyze to yield NO/O(2-); however, this pathway represents a thermodynamic "dead end" since the radical pairs generated by homolysis should mostly revert to starting material. However, decomposition of O(2)NOO(-) yields the stable products NO(2-)/(3)O(2), a couple that is more stable than starting material. This may occur either by initial formation of NO(2)/O(2-) or NO(2-)/(1)O(2), with the latter intermediates thermodynamically favored both in the gas phase and in solution. Given the extremely fast back-reaction of the homolysis products, heterolysis probably dominates the observed O(2)NOO(-) decomposition kinetics. This is in agreement with the first of two "kinetically indistinguishable" mechanistic possibilities proposed for O(2)NOO(-) decomposition (Goldstein, S.; Czapski, G.; Lind, J.; Merényi, G. Inorg. Chem. 1998, 37, 3943-3947).