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We present a new method taking explicitly into account the coupling between rotation and bending of a nonlinear triatomic molecule colliding with an atom. This approach based on a rigid-bender treatment of the triatomic molecule was originally developed for the case of triatomic molecule linear at equilibrium. It is here extended to the case of a colliding bent triatomic molecule at equilibrium and applied to the case of the para-H2+ H2O inelastic collision using a new H2O-para-H2adiabatically reduced 4D potential. The results of the method for purely rotational transitions are compared to those of rigid-rotor calculations while vibrational quenching rates of the first exited bending level are calculated for the first time at the close-coupling level.

The C + C2H2 reaction is a key process in interstellar cloud chemistry. In a crossed-beam scattering experiment approaching the low-collision energies that characterize these environments, we determined relative differential cross sections by detecting the H-atom product. High-level ab initio calculations of the reaction energies of two competing pathways, leading to cyclic and linear C3H, were also performed. Both channels are clearly distinguishable: the integral cross section of the c-C3H + H channel monotonically decreases with increasing relative translational energy whilst the l-C3H + H channel exhibits a translational energy threshold. Moreover, a comparison of the H-atom yields from the C + C2H2 and C + C2H4 reactions shows that the C3 + H2 nonadiabatic channel dominates. These results are consistent with the calculated enthalpies and corroborate earlier low-temperature kinetic experiments. Branching ratios of the three reaction pathways are given in the T = 15-300 K temperature domain for inclusion in astrochemical databases.

In this paper we report the first theoretical study of the title reaction. A global, single-valued model of the ground-state potential energy surface has been obtained by fitting to an extensive set of high-level ab initiocalculations. The surface is found to be attractive apart from linear geometries where energy barriers appear due to conical intersections. This model was then used to calculate the reactive reactant state selected cross sections for collision energies ranging from threshold up to 4000 cm−1. These calculations were performed using our version of the Baer’s approach of the RIOSA-NIP method which is based on the use of a negative imaginary potential. We find that the reaction probability is extremely oscillatory as a function of kinetic energy as it is a case for insertion reactions with a low exoergicity. The resulting reaction rate coefficient is found to first increase slowly as a function of temperature up to a broad maximum around 20 K and then to decrease slowly when temperature keeps increasing.

The influence of electronically nonadiabatic transitions in the C(3P_g) + CH(X 2Π) → C₂(X 1Σ+_g, a 3Π_u) + H(2S_g) reaction is investigated by using Tully's fewest-switches version of the trajectory surface hopping method. A diabatic model of the first two 2A' potential energy surfaces coupled by a conical intersection is used. The diatomic CH has the internal state (ν = 0, j = 0) and batches of 20 000 trajectories are computed for four collision energies, E = 0.1, 0.3, 0.5 and 0.7 eV. We find that the reaction dynamics does not exhibit a statistical character, despite the existence of deep wells along the reaction path. Only the distribution of scattering angle shows a good agreement between trajectories and phase space theory results. A strong excess of vibrational energy is disposed on both electronic products C₂(X 1Σ+_g) and C₂(a 3Π_u), correlated with a lack of recoil energy. With all trajectories starting on a single potential surface, we obtain an electronic branching ratio X : a close to 2 : 3, only slightly dependent on the collision energy.

We report two new global analytical potential energy surfaces (PESs) corresponding to the lowest adiabatic 1²A' and 2²A' states of NO₂. High quality ab initio calculations were performed at a MRCI (multi-reference configuration interaction) level using a polarised triple-zeta basis set. Energies were computed on a grid of 650 and 595 energies for the 1²A' and 2²A' states respectively. These were used to construct global analytic PESs with the DMBE (double many-body expansion) method, paying special attention to the O(³P)+NO(²Π) channel. Additionally, a SEC correction (scaled external correction) was applied in order to obtain the correct dissociation energy in this channel for the ground 1²A' state. Particular attention was paid to the inclusion of an anisotropic long-range description of the potential, which was absent in the previously available global PESs. Geometries and energies of the main stationary points of these ²A' surfaces compare very well with other theoretical and experimental data.

Realistic analytical representations of the twelve lowest singlet and triplet electronic adiabatic potential energy curves of C2molecule are given in this article. The corresponding electronic states are correlated with C atoms both in their 3P state. A new set of high level MRCI calculations coupled with a double many-body expansion analytical fitting based on the extended Hartree–Fock approximate correlation energy model have been used in this work. Using RKR data available in the literature, comparison is made between our results and RKR turning points concerning the four lowest singlet states X1Σ+g, A1Πu, B1Δgand B′1Σ+gof C2. The agreement is very satisfying.

Potential energy surfaces for the first three electronic states of the reaction C(³P)+CH(X²Π)→C₂+H have been constructed from a new set of high level ab initio data which are of the multireference configuration interaction variety and were carried out using a polarised triple-zeta basis set. These are the X²Σ⁺ and the A²Π states, and lead to the formation of C₂(X¹Σ_g⁺) and C₂(a³Π_u) considering an adiabatic dissociation process. Each adiabatic potential is expressed within the double many-body expansion (DMBE) scheme which is based, in this case, on the extended Hartree–Fock approximate correlation energy model (EHFACE). Moreover, a quasiclassical trajectory study of the title reaction has been performed for each of the three potential energy surfaces, yielding the corresponding rate constants.

Tunneling probabilities for the reactions O + HD → OH + D and O + DH → OD + H have been calculated by semiclassical dynamical methods and compared to accurate quantal calculations for the same potential energy surface. The results are used to test the reliability of variational transition state theory with the least-action semiclassical method for tunneling probabilities for the prediction of intramolecular kinetic isotope effects.

The 18 lowest potential energy surfaces of C2H have been investigated with the complete active space multiconfigurational self-consistent-field method. We restricted our study to the doublet and quartet spin multiplicities. Twelve surfaces are issued from the ground-state reactants, while the six others are issued from the first excited state of the reactants. The approach of C toward CH shows no barrier for 6 of the 12 surfaces, obviously making the reaction possible at very low temperatures. The study of the potential energy curves along the reactant and product channels shows that the X, A, a, b, and c states of C2 are expected to be populated by the title reaction, even at very low temperatures. Moreover, six new equilibrium structures corresponding to the excited states of C2H are predicted.