Dynamics of nonadiabatic reactions (theory). I. Branching ratios for early and late seams

Abstract

The 3D classical trajectory surface hopping (TSH) method has been applied in a ‘‘model’’ study of factors governing nonadiabatic reaction, A+BC→AB+C* and →AB+C. In the diabatic approximation the potential-energy surfaces (pes) were a LEPS surface for F+H2 (→AB+C*) and a repulsive pes Vrep (→AB+C). These intersected in the exit valley to give an early or a late seam (E or L, perpendicular to the exit valley). The splitting at the avoided crossing 2ε was adjusted to ε=1.26 or 5.02 kcal/mol. The ratio of reactive cross sections onto the upper and lower adiabatic pes ρ* was investigated for mass combinations H+HL, L+HL, L+HH, and H+LL with E and L seams, and for small and large ε. The effect on ρ* of reapportioning a constant total energy (ETOT=13.84 kcal/mol) between reagent translation T and vibration V was examined for these 16 cases. Since the velocity in the coordinate of separation increased with increased T (yielding increased product translation; ΔT→ΔT′) ρ* also tended to increase with T. The extreme mass combinations H+HL and L+HH exhibited modified ρ* due to markedly differing widths in the entry and exit valley. The strongly skewed pes for H+LL led to multiple crossing of the seam which reduced ρ*. For other mass combinations ρ* was reduced by the inability of the low T′ component of the product to hop across the 2ε gap. In all cases ρ* was an index of the local dynamics at the seam, and hence shed light on the intermediate motions en route to the asymptotic outcome V′, R′, T′.