Ab initio calculations of coupled potential energy surfaces for the Cl(2P3/2,2P1/2)+H2 reaction

Three-dimensional potential energy surfaces for the entrance channel of the title reaction have been computed using accurate multi-reference configuration wavefunctions and a very large basis set. The three adiabatic potential surfaces correlating asymptotically with Cl(P-2)+H-2 ((2)Sigma(+) and H-2 for collinear geometries) have been transformed to a diabatic representation, which leads to a fourth coupling potential for non-linear geometries. In addition, the spin-orbit coupling surfaces have been computed using the Breit-Pauli Hamiltonian. The six resulting potentials (four electrostatic and two spin-orbit) have been fitted to analytical functions. The diabatic (2)Pi potential has a minimum in the entrance channel at collinear geometries, while the diabatic (2)Sigma potential has a well for a T-shaped (C-2v) structure. This different anisotropy leads to a conical intersection of the two (2)A' ((2)Sigma(+), (2)Pi) adiabatic potentials at intermediate H-2-Cl distances. The adiabatic spin-orbit potentials, which correlate asymptotically with Cl(P-2(1/2,3/2))+H-2 are obtained by diagonalizing the full electrostatic+spin-orbit Hamiltonian. Spin-orbit coupling increases the adiabatic barrier height by 0.84 kcal mol(-1). Implications for the non-adiabatic reaction dynamics are discussed.