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

Characterization of the stationary points in a flat 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.