Inversion of stationary point levels in a flat transition structure region on account of vibrational energy.

초록

Characterization of the stationary points in a transition structure region of the potential energy surface for the HCOCl + Cl- reaction has been carried out at the MP2/6-311 + G** level. Two stationary points are found: an intermediate with Cs symmetry and all positive eigenvalues in the Hessian and a transition state with C1 symmetry and only one negative eigenvalue in the Hessian. The MP2/6-311 + G** electronic energies of these two only differ by δΔE(=ΔETS - ΔEInt) = 0.01 kcal mol-1. The zero-point and thermal energy corrections to the electronic energy lead to an inversion of levels: δΔEZPE = -0.06 and δΔH = -0.59 kcal mol-1, which becomes normal when the -TΔS term is added to obtain δΔG = + 0.90 kcal mol-1 (+0.80 kcal mol-1 at the G2(+)MP2 level). This inversion arises solely from one less vibrational mode in the TS on the flat transition structure region.