Collisional excitation of C2H- by H2: New interaction potential and scattering calculations.

Interstellar anions play an important role in astrochemistry as being tracers of the physical and chemical conditions in cold molecular clouds and circumstellar gas. The local thermodynamic equilibrium is generally not fulfilled in media where anions are detected and radiative and collisional data are required to model the observed lines. The C2H- anion has not yet been detected in the interstellar medium; however, collisional data could be used for non-LTE models that would help in identifying the most intense lines. For this purpose, we have computed the first 4D potential energy surface (PES) of the C2H--H2 complex using an explicitly correlated coupled-cluster approach. The PES is characterized by a single deep minimum with a well-depth of 924.96 cm-1. From this interaction potential, we derived excitation cross sections and rate coefficients of C2H- induced by collisions with para- and ortho-H2. The results obtained for collisions with para-H2 are compared to previous calculations performed using a 2D-PES obtained from an average over H2 rotations.

Understanding propyl cyanide and its isomers formation: ab initio study of the spectroscopy and reaction mechanisms.

Recent detection of propyl cyanide (C3H7CN) with both linear and branched structures has stimulated many experimental and theoretical studies. In this theoretical work, we present the spectroscopic properties of the far infrared spectra of these species and we investigate their different paths of formation in the gas phase. Our spectroscopic study concerns the far infrared spectra of iso, anti and gauche propyl cyanide isomers. The equilibrium structures and the potential energy surfaces are calculated using explicitly correlated cluster ab initio methods (CCSD(T)-F12) and a variational procedure designed for non-rigid species and large amplitude motions. Accurate rotational constants, centrifugal distortion constants, potential energy barriers and surfaces are provided. The rovibrational parameters in the ground vibrational states compare very well with experimental data. The low energy vibrational levels correspond to torsional modes. Far infrared energies are calculated up to 500 cm-1 using the variational approach and the vibrational second order theory (VPT2), and a good agreement with previous experimental values is found. We have also investigated the gas phase formation of the different C3H7CN isomers. After several trials of reacting gaseous species, we considered that a possible formation route of the C3H7CN isomers can be from the bimolecular reaction of HCN with propene. At the UMP2(full)/aug-cc-pVTZ level of theory, this reaction involves two steps for each isomer; the first one corresponds to the association of the two radicals while the second one corresponds to H transfer. From highly correlated ab initio calculations by means of CCSD(T)/aug-cc-pVTZ//UMP2(full)/aug-cc-pVTZ, the geometries, energetics and minimum energy paths of the reactions are obtained. Also, the first step's transition state disappears, as the diradical minimum is much less stabilized with UCCSD(T) than by UMP2(full). We employ the zero curvature tunneling and canonical variational (CVT/ZCT) semiclassical method to predict rate constants for propyl cyanide isomers formation in the gas phase. However, due to the presence of a significant barrier, this reaction leads to very small rate constants. Very recently, we probed reaction mechanisms involving radical addition and we found that these reactions are barrierless and highly exothermic leading to expected fast reactive processes for the formation of propyl cyanide products. The kinetics of such diradical reactions is under study.

A new ab initio potential energy surface for the collisional excitation of N2H(+) by H2.

We compute a new potential energy surface (PES) for the study of the inelastic collisions between N2H(+) and H2 molecules. A preliminary study of the reactivity of N2H(+) with H2 shows that neglecting reactive channels in collisional excitation studies is certainly valid at low temperatures. The four dimensional (4D) N2H(+)-H2 PES is obtained from electronic structure calculations using the coupled cluster with single, double, and perturbative triple excitation level of theory. The atoms are described by the augmented correlation consistent triple zeta basis set. Both molecules were treated as rigid rotors. The potential energy surface exhibits a well depth of ≃2530 cm(-1). Considering this very deep well, it appears that converged scattering calculations that take into account the rotational structure of both N2H(+) and H2 should be very difficult to carry out. To overcome this difficulty, the "adiabatic-hindered-rotor" treatment, which allows para-H2(j = 0) to be treated as if it were spherical, was used in order to reduce the scattering calculations to a 2D problem. The validity of this approach is checked and we find that cross sections and rate coefficients computed from the adiabatic reduced surface are in very good agreement with the full 4D calculations.

Rotational excitation of CN(X (2)Sigma(+)) by He: Theory and comparison with experiments.

Rotational excitation of the CN(X (2)Sigma(+)) molecule with He is investigated. We present a new two-dimensional potential energy surface (PES) for the He-CN system, calculated at an internuclear CN distance frozen at its experimental equilibrium distance. This PES was obtained using an open-shell, coupled-cluster method including all single and double excitations, as well as the perturbative contributions of connected triple excitations [RCCSD(T)]. Bond functions were placed at mid-distance between the center of mass of the CN molecule and He atom for a better description of the van der Waals interaction. State-to-state collisional excitation cross sections of the fine-structure levels of CN by He are calculated for energies up to 2500 cm(-1), which yield after thermal averaging, rate coefficients up to 350 K. The exact spin splitting of the energy levels is taken into account. The propensity rules between fine-structure levels are studied, and it is shown that the rate constants for Deltaj=DeltaN transitions are much larger than those for Deltaj not equalDeltaN transitions, as expected from theoretical considerations. Our calculated rate coefficients are compared to experimental results at 295 K of Fei et al. [J. Chem. Phys. 100, 1190 (1994)]. The excellent agreement confirms the accuracy of the PESs and of the scattering calculations.