Spherical harmonics representation of the potential energy surface for the H2⋯H2 van der Waals complex.

We perform a study of the molecular anisotropy for the H2⋯H2 van der Waals system using a spherical harmonics expansion. We use six leading stable configurations to construct our analytical potential energy surface (PES) from ab initio calculations guided qualitatively by the symmetry-adapted perturbation theory (SAPT) analyses. We extrapolate the energies of the PES performed at the CCSD(T)/aug-cc-pVnZ (n = 2 and 3) levels to the complete basis set (CBS) limit. To best fit the shallow potential energy surface of each leading configuration with the intermolecular distance, it was employed an extended version of the Rydberg potential. To assess the quality of our extrapolated analytical PES, we calculate the second virial coefficients, which are in relatively good agreement with the experimental data. As a result, the spherical harmonics coefficients obtained might be of considerable relevance in spectroscopy and dynamics applications.

Theoretical study of the H + HCN → H + HCN process.

We present a theoretical study on the detailed mechanism and kinetics of the H + HCN → H + HNC process. The potential energy surface was calculated at the complete basis set quantum chemical method, CBS-QB3. The vibrational frequencies and geometries for four isomers (H 2CN, cis-HCNH, trans-HCNH, CNH 2), and seven saddle points (TSn where n = 1 - 7) are very important and must be considered during the process of formation of the HNC in the reaction were calculated at the B3LYP/6-311G(2d,d,p) level, within CBS-QB3 method. Three different pathways (PW1, PW2, and PW3) were analyzed and the results from the potential energy surface calculations were used to solve the master equation. The results were employed to calculate the thermal rate constant and pathways branching ratio of the title reaction over the temperature range of 300 up to 3000 K. The rate constants for reaction H + HCN → H + HNC were fitted by the modified Arrhenius expressions. Our calculations indicate that the formation of the HNC preferentially occurs via formation of cis-HCNH, the fitted expression is k P W2(T) = 9.98 × 10-22 T 2.41 exp(-7.62 kcal.mol-1/R T) while the predicted overall rate constant k O v e r a l l (T) = 9.45 × 10-21 T 2.15 exp(-8.56 kcal.mol-1/R T) in cm 3 molecule -1 s -1. Graphical Abstract (a) Potential energy surface, (b) thermal rate constants as a function of temperature and

Rate constant calculations of the C2 + HCN → CCCN+H addition via the Master Equation.

The addition of C2 to HCN is of relevant interest in astrochemistry. We studied the pathways of this addition to produce CCCN and estimated its reaction rate using the Master Equation in the circumstellar environment. From the results of this study, it was possible to show that a different pathway in the Surface Potential Energy-PES can also be investigated. In a circumstellar envelop environment, with temperatures varying between 1000 K and 2000 K, the abundances of these species are favorable to this kind of addition, and our branching ratio for the rate constant showed that the new pathway is more favorable in comparison with other possibilities for this range of temperatures in this environment, and must be taken into account in any computation of the rate constant. Graphical Abstract Branching ratios of pathways involved in the C2 + HCN → CCCN+H addition, at a temperature range of 1000-2000 K.