A total of 816 ab initio points at the PUMP4/6-311G(2d) level were used to derive an analytical expression for the lowest 3 AЈ adiabatic potential energy surface ͑PES͒ of the reaction O͑ 3 P͒ϩCS͑X 1 ⌺ ϩ ͒→CO͑X 1 ⌺ ϩ ͒ϩS͑ 3 P͒. Thermal rate constants calculated using the variational transition state theory and semiclassical tunneling correction were used as a tool to determine the optimum analytical surface. This was done by comparing the calculated rate constant at room and lower temperatures with the experimental values. The best analytical surface ͑PES 3͒ reproduces the rate constant at low temperatures well. However, it has not been possible to obtain an analytical PES capable of reproducing both the rate constant at 300 K and the activation energy ͑150-300 K range͒. At higher temperatures, the contribution of the lowest 3 AЉ adiabatic potential energy surface to the rate constant seems to be important to reproduce the experimental data. At present, the PES 3 is the most suitable analytical surface to be used for kinetic and dynamical single surface studies.

In this work we have carried out ab initio electronic structure calculations on the ground (2A@) potential energy surface (PES) involved in the H(2S) ] ClF and the F(2P) ] HCl reactions. Transition states and van der Waals minima have been characterized and have been used along with a grid of approximately 3400 ab initio [PUMP2/6-311G(3d2f,3p2d)] points to derive an analytical PES. The global root-mean-square deviation of the Ðt (2.66 kcal mol~1) is within the range of the estimated ab initio accuracy. The saddle-point energies of this Ðtted PES were locally scaled to reproduce the thermal rate constants at 300 K of these reactions considering the H isotope. Calculated variational transition state theory rate constants with the inclusion of a microcanonical optimized multidimensional tunneling correction are in good accord with experiments at di †erent temperatures, both for reactions with H and D isotopes. A small H/D kinetic isotope e †ect is predicted to have a similar extension for the three reactions depending on the temperature and (k H /k D B 1È2) according to the available experimental results.

In this work we have studied at an ab initio level the lowest 2A′ potential energy surface (PES) of the HCl2 system. This PES is involved in the H(2S)+Cl2(X 1Σg+)→HCl(X 1Σ+)+Cl(2P) and Cl(2P)+HCl(X 1Σ+)→HCl(X 1Σ+)+Cl(2P) gas phase elementary chemical reactions. The former reaction is an important chemical laser while the second one is the most frequently used prototype of heavy–light–heavy reaction. A large number of points on the 2A′ PES have been calculated at the PUMP2/6-311G(3d2 f,3p2d) ab initio level. The ab initio calculations show the existence of two angular transition states with negligible or very small barriers to collinearity. This and other properties of the PES are in agreement with previous studies. An analytical expression based on a many-body expansion has been used to obtain a satisfactory fit of the 740 ab initio points calculated, with a root-mean-square deviation within the range of the estimated ab initio method error margin. This analytical representation of the 2A′ PES has been used to evaluate the variational transition state theory thermal rate constants of the above-mentioned reactions, including also the Cl+DCl reaction, and quite good agreement has been obtained when comparing with experimental results. The analytical PES obtained in this work is suitable for use in studies on the kinetics and dynamics of the HCl2 system.

No abstract

No abstract