Solution to the pi-Distortivity Problem

摘要

Traditionally,,the delocalized pi system of benzene is believed to be responsible for its perfectly symmetric D-6h geometry. However, it has also been suggested :that the pi system prefers a distorted D-3h geometry. Arguments for this have been based on clever use of VB methods as well as through shifts in the frequency of the distortive b(2u) mode. Evidence has been provided through different ways of partitioning the total electronic energy between the sigma and the pi systems. These Methods are plagued by the fact that there is no, unique way to partition the energy; leading to questions regarding the validity of the conclusions. Here we note that even though energy cannot be partitioned exactly, force acting on a nucleus depends only on the Single particle density and can hence be partitioned exactly. Using good-quality wave functions that are numerically found to obey the Hellmann-Feynman theorem to good accuracy, we calculate the sigma and pi components of the force and provide conclusive evidence of pi-distortivity at the HE level. Our approach provides an, unambiguous way,to approach the problem with wave functions that account for electron correlation. Our calculations suggest that the conclusion is, valid at the MP2 level, too.