An ab initio study of the ground potential energy surface (PES) of the $\mathrm{O}{(}^3\mathrm{P)+}{\mathrm{CH}}_{\mathrm{4}}{\mathrm{\to OH+CH}}_{\mathrm{3}}$ reaction has been performed using the second- and fourth-order Mo/ller–Plesset methods with a large basis set. A triatomic analytical ground PES with the methyl group treated as an atom of 15.0 a.m.u. has been derived. This PES has been employed to study the kinetics [variational transition state theory (VTST) and quasiclassical trajectory (QCT) rate constants] and dynamics (QCT method) of the reaction. The ab initio points have also been used directly to calculate the VTST rate constant considering all atoms of the system. The best VTST methods used lead to a good agreement with the experimental rate constant for 1000–2500 K, but QCT rate constant values are about one-third the experimental ones for 1500–2500 K. The cold QCT $\mathrm{OH}\text{(v=0)}$ rotational distribution arising from the simulation of the reaction with $\mathrm{O}{(}^3\mathrm{P)}$ atoms produced in the photodissociation of ${\mathrm{NO}}_{\mathrm{2}}$ at 248 nm is in good agreement with experiment, while the very small QCT $\mathrm{OH}\text{(v=1)}$ population obtained is consistent with measurements. The triatomic PES model derived in this work may be used in studies of the kinetics and dynamics under conditions where the methyl group motions are not strongly coupled to the motions leading to reaction.

In this work we have carried out ab initio electronic structure calculations, CASSCF/CASPT2 and $\mathrm{CASSCF/MRCI-SD+Q}$ with several Pople’s and correlation-consistent Dunning’s basis sets, of the planar cis- and trans-NO dimers for the lowest eight electronic (singlet and triplet) states. The geometry, frequencies, dipole moment, binding energy, and vertical excitation energies are predicted with an accuracy close to or even better than the best reported ab initio previous results for some of these properties, and in very good agreement with the available experimental data. CASPT2 optimized geometries show the existence of at least four shallow NO-dimers (i.e., two $\mathit{cis}-{\mathrm{(NO)}}_{\mathrm{2}}$ ${(}^1{A}_1$ and ${}^3{B}_2)$ and two $\mathit{trans}-{\mathrm{(NO)}}_{\mathrm{2}}$ ${(}^1{A}_g$ and ${}^3{A}_u\mathrm{)),}$ although CASSCF optimization with CASPT2 pointwise calculations indicate the existence of other less stable dimers, on the excited states. Vertical excitation energies were calculated for these four dimers. For the cis-NO dimer, the ordering and the energy spacings between the excited states (i.e., ${}^1{A}_1,$ ${}^3{B}_2,$ ${}^1{B}_2,$ 2nd ${}^1{A}_1,$ ${}^1{A}_2,$ ${}^3{A}_2,$ ${}^3{B}_1,$ 2nd ${}^3{B}_1)$ are very similar to those found in a recent MRCI-SD study. The singlet cis-NO dimer ${(}^1{A}_1)$ is the most stable one in almost quantitative accord with the experimental data, and in disagreement with previous density functional theory studies. A nonplanar transition state for the singlet trans ↔ cis isomerization has also been fully characterized. This leads to an almost negligible energy barrier which would originate a rapid isomerization to the most stable cis-NO dimer at low temperatures, in accord with the experimental difficulties to measure the properties of the trans-NO dimer. Not only are basis set superposition error corrections necessary to evaluate accurately the binding energies, but also to determine the NN distance of these symmetrical dimers. Some problems regarding the symmetry of the wave function were found for the symmetrical NO dimers and for the $\mathrm{NO+NO}$ asymptote, and several approximate solutions were proposed.

We report two new analytical fits of the ground potential energy surface (PES) ${(}^2{A}^{\prime })$ and the first excited PES ${(}^4{A}^{\prime })$ involved into the title reaction and its reverse, using ab initio electronic structure calculations from Papers I and II along with new grids of ab initio points by means of the second-order perturbation theory on CASSCF wave function [CASPT2 (17,12) G2/aug-cc-pVTZ] reported here (1250 points for the ${}^2{A}^{\prime }$ PES and 910 points for the ${}^4{A}^{\prime }$ PES). Some experimental data were also introduced to better account for the exoergicity and the experimental rate constant at 300 K. The final root-mean-square deviations of the fits were 1.06 and 1.67 kcal/mol for ${}^2{A}^{\prime }$ and the ${}^4{A}^{\prime }$ PESs, respectively, for the NOO $C_s$ abstraction and insertion regions of the PESs. Thermal rate constants were calculated (300–5000 K) for both the direct and reverse reactions by means of the variational transition state theory with the inclusion of a microcanonical optimized multidimensional tunneling correction, obtaining a very good agreement with the experimental data within all the temperature range. The new analytical ${}^2{A}^{\prime }$ PES presents several stationary points not introduced in previous analytical surfaces, and describes accurately the ${\mathrm{NO}}_2$ ${\mathrm{(X }}^2{A}_1)$ minimum, which seems to be very accessible according to the trajectories run in a preliminary quasiclassical trajectory study. The new analytical ${}^4{A}^{\prime }$ PES has a lower energy barrier than the previous one, which increases significantly the contribution of this PES to the total rate constant at high temperatures. Moreover, the new analytical PESs not only describe accurately the $C_s$ regions of the NOO system but also the ONO $C_{\text{2v}}$ or near $C_{\text{2v}}$ regions.

The N(⁴S_u)+NO(X ²Π)→N₂(X ¹Σ_g⁺)+O(³P_g) reaction plays an important role in the upper atmosphere chemistry and as a calibration system for discharge flow systems. Surprisingly, very little theoretical and experimental work has been devoted to the characterization of the dynamical features of this system. In this work a Sorbie–Murrell expression for the lowest ³A‘ potential energy surface (PES) connecting reactants in their ground electronic states based upon the fitting of an accurate ab initio CI grid of points has been derived. The PES fitted shows no barrier to reaction with respect to the reactants asymptote in accordance with experimental findings and becomes highly repulsive as the NNO angle is varied away from the saddle point geometry. The results of preliminary quasiclassical trajectory calculations on this surface reproduce very well the experimental energy disposal in products, even though the vibrational distribution derived from trajectories seems to be a bit cooler than the experimental data. Moreover, thermal rate constants derived from trajectories are in excellent accordance with experimental values.

This work presents two new analytical fits of the ground potential energy surface (PES) ${(}^3{A}^{″})$ and the first excited PES ${(}^3{A}^{\prime })$ involved in the title reaction, considering the N-abstraction (1) and the O-abstraction (2) reaction channels, and the reverse reaction (−1). The PESs are derived from ab initio electronic structure calculations by means of second-order perturbation theory on a complete active-space self-consistent-field wave function (CASPT2 method). Stationary points and extensive grids of ab initio points (about 5600 points for the ${}^3{A}^{″}$ PES and 4900 points for the ${}^3{A}^{\prime }$ PES) were fitted along with some diatomic spectroscopic data to better account for the experimental exoergicity. Thermal rate constants were calculated (200–5000 K) for all mentioned reaction processes by means of the variational transition-state theory with the inclusion of a semiclassical tunneling correction. Excellent agreement with the experimental data was observed for reaction (1) and its reverse, within all the temperature range, substantially improving the results derived from previous analytical PESs. The contribution of the ${}^3{A}^{\prime }$ PES to the reaction rate constant ${\mathrm{(k}}_1)$ was small even at high temperatures (e.g., only 10.8% at 2500 K). Moreover, the main contribution to reaction rate constant ${\mathrm{(k}}_2)$ was due to the ${}^3{A}^{\prime }$ PES, differing from what happens for reaction (1). The O-abstraction reaction channel accounts for a 3.0% of the total reaction ${\mathrm{(k=k}}_1{\mathrm{+k}}_2)$ at 5000 K, consistent with the very limited experimental information available.