Accurate ab initio potential energy surface, dynamics, and thermochemistry of the F+CH4→HF+CH3 reaction

An accurate full-dimensional global potential energy surface (PES) for the F+CH4 -> HF+CH3 reaction has been developed based on 19 384 UCCSD(T)/aug-cc-pVTZ quality ab initio energy points obtained by an efficient composite method employing explicit UCCSD(T)/aug-cc-pVDZ and UMP2/aug-cc-pVXZ [X=D,T] computations. The PES contains a first-order saddle point, (CH4- -F)(SP), separating reactants from products, and also minima describing the van der Waals complexes, (CH4- - -F)(vdW) and (CH3- - -HF)(vdW), in the entrance and exit channels, respectively. The structures of these stationary points, as well as those of the reactants and products have been computed and the corresponding energies have been determined using basis set extrapolation techniques considering (a) electron correlation beyond the CCSD(T) level, (b) effects of the scalar relativity and the spin-orbit couplings, (c) diagonal Born-Oppenheimer corrections (DBOC), and (d) zero-point vibrational energies and thermal correction to the enthalpy at 298 K. The resulting saddle point barrier and ground state vibrationally adiabatic barrier heights (V-SP and V-VAGS), dissociation energy of (CH3- - -HF)(vdW) (D-e and D-0), and the reaction enthalpy (Delta H-e(degrees), Delta H-0(degrees), and Delta H-298(degrees)) are (240 +/- 40 and 245 +/- 200 cm(-1)), (1070 +/- 10 and 460 +/- 50 cm(-1)), and (-10000 +/- 50, -11200 +/- 80, and -11000 +/- 80 cm(-1)), respectively. Variational vibrational calculations have been carried out for (CH3- - -HF)(vdW) in full (12) dimensions. Quasiclassical trajectory calculations of the reaction using the new PES are reported. The computed HF vibrational and rotational distributions are in excellent agreement with experiment.