Ab Initio Treatment of the Chemical Reaction Precursor Complex Br(²P)−HCN. 1. Adiabatic and Diabatic Potential Surfaces^†

The three adiabatic potential surfaces of the Br(²P)−HCN complex that correlate to the ²P ground state of the Br atom were calculated ab initio. With the aid of a geometry-dependent diabatic mixing angle, also calculated ab initio, these adiabatic potential surfaces were transformed into a set of four diabatic potential surfaces required to define the full 3 × 3 matrix of diabatic potentials. Each of these diabatic potential surfaces was expanded in terms of the appropriate spherical harmonics in the atom-linear molecule Jacobi angle θ. The dependence of the expansion coefficients on the distance R between Br and the HCN center of mass and on the CH bond length was fit to an analytic form. For HCN in its equilibrium geometry, the global minimum with D_e = 800.4 cm^-1 and R_e = 6.908a₀ corresponds to a linear Br−NCH geometry, with an electronic ground state of Σ symmetry. A local minimum with D_e = 415.1 cm^-1, R_e = 8.730a₀, and a twofold degenerate Π ground state is found for the linear Br−HCN geometry. The binding energy, D_e, depends strongly on the CH bond length for the Br−HCN complex and much less strongly for the Br−NCH complex, with a longer CH bond giving stronger binding for both complexes. Spin−orbit coupling was included and diabatic states were constructed that correlate to the ground ²P_3/2 and excited ²P_1/2 spin−orbit states of the Br atom. For the ground spin−orbit state with electronic angular momentum j = (³/₂) the minimum in the potential for projection quantum number ω = ±(³/₂) coincides with the local minimum for linear Br−HCN of the spin-free case. The minimum in the potential for projection quantum number ω = ±(¹/₂) occurs for linear Br−NCH but is considerably less deep than the global minimum of the spin-free case. According to the lowest spin−orbit coupling included adiabatic potential the two linear isomers, Br−NCH and Br−HCN, are about equally stable. In the subsequent paper, we use these potentials in calculations of the rovibronic states of the Br−HCN complex.