Analytical potential energy surface for the reaction with intermediate complexes NH₃ + Cl → NH₂ + HCl: Application to the kinetics study

Abstract

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 highlevel ab initio electronic structure calculations obtained at the CCSD(T) ¼ FULL/aug-ccpVTZ//CCSD(T) ¼ 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

Dynamics study of the OH + NH3 hydrogen abstraction reaction using QCT calculations based on an analytical potential energy surface

“…This endothermic reaction evolves through two reactant complexes in the entrance channel, a saddle point and a product complex in the exit channel. Based exclusively on high-level ab initio calculations, we developed an analytical PES, PES-2010, 4 which was tested against a great variety of kinetics and dynamics properties. Kinetically, 4 this PES-2010 reproduced experimental thermal forward rate constants in the common temperature range; and dynamically 5,6 we found different mechanisms of reaction depending on the collision energy due to the presence of the reactant well: indirect at low (<3 kcal mol −1 ) and direct at high (>5 kcal mol −1 ) collision energies.…”

Constructing Potential Energy Surfaces for Polyatomic Systems: Recent Progress and New Problems

“…This represents 0.38, 0.83, and 1.17 kcal mol −1 , respectively, for F, Cl, and Br. Our group considered these approaches in the construction of the surface for the F( 2 P) + CH 4 [42], Cl( 2 P) + CH 4 [49], and Cl( 2 P) + NH 3 [94,95] reactions. Note, however, that this approach is to some extent incoherent in the sense that as we move away from the saddle point towards reactants the energy asymptotically tends to zero, the energy of the lower state of the halogen atom.…”

“…In addition, we have also studied reactions with asymmetrically substituted methane, H + CH 3 Cl and Cl + CHClF 2 [98,99], which represent another challenge in the PES construction for polyatomic systems, because in addition to the aforementioned problems, the possibility of several reaction channels needs to be taken into account. Although the older surfaces were semiempirical, that is, they combined theoretical and experimental information in the fitting procedure due to computational limitations at that time, the newer surfaces, those developed from 2007 onward, are based exclusively on quantum mechanical information: H + CH 4 [41], H + CCl 4 [97], H + NH 3 [39], and Cl( 2 P) + NH 3 [95].…”

“…Thus, for the five-body systems we removed the dependency on one of the hydrogen atoms of CH 4 , and for the seven-body system, OH + CH 4 , we added additional Morse and harmonic terms to describe the OH bond and <HOH angle, while for the asymmetric reactions we allowed the four atoms bonded to the carbon atom to vary independently. In addition, we improved the analytical form when building the Cl + NH 3 surface by allowing the equilibrium N-H distance to vary along the reaction path [95].…”

“…In addition, the presence of several maxima and minima requires a fine evaluation of the energies, gradients, and Hessians needed for a correct description of the PES. This, obviously, represents a complication in the construction of the PES, and only very few global surfaces have as yet been developed [26,95].…”