Analytical potential energy surface for the reaction with intermediate complexes NH _3 + Cl → NH _2 + HCl: Application to the kinetics study

摘要

We present for the first time an analytical potential energy surface (PES) for the reaction of hydrogen abstraction from ammonia by a chlorine atom. It has a very complicated shape with various maxima and minima. The functional form used in the development of the PES considered the stretching and bending nuclear motions, and the parameters in the calibration process were fitted to reproduce exclusively high-level ab initio electronic structure calculations obtained at the CCSD(T) = FULL/aug-cc-pVTZ//CCSD(T) = FC/cc-pVTZ single point level. Thus, the surface is completely symmetric with respect to the permutation of the three ammonia hydrogen atoms, and no experimental information is used in the process. The ab initio information used in the fit includes a wide spectrum of properties (equilibrium geometries, relative energies, and vibrational frequencies) of the reactants, products, saddle point, intermediate complexes in the entry and exit channels, points on the reaction path, and points on the reaction swath. By comparison with the reference results, we show that the resulting PES reproduces not only the ab initio data used in the fitting procedure but also other thermochemical and kinetics results computed at the same ab initio level, which were not used in the fit-equilibrium constants, rate constants, and kinetic isotope effects. This represents a severe test for the new surface. As a first application, we perform an extensive kinetics study using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range, 200-2000 K, on this analytical PES. The forward rate constants reproduce the sparse experimental measurements, while the reverse ones reproduce the change of activation energy with temperature reported in another theoretical study, although unfortunately there are no experimental data for comparison. Finally, we analyze the influence of the intermediate complexes and the spin-orbit correction on the kinetics results. In summary, these results indicate that the PES adequately describes this reaction, and the reasonable agreement with experiment lends further confidence to this new surface.