Geometric and electronic similarities between transition structures for electrocyclizations and sigmatropic hydrogen shifts

Abstract

This paper reports new theoretical evidence that supports previous proposals concerning the similarity between transition structures for electrocyclizations and sigmatropic hydrogen shifts. This evidence was obtained using two recently proposed methodologies, namely the so-called generalized population analysis and the formalism of molecular quantum similarity indices. Analysis of multicenter bond indices shows that the transition structures for cationic [1,n] hydrogen shifts do indeed have three-center indices that are similar to those of other three-center carbocations. In addition, the close resemblance of the electronic structures of electrocyclic and sigmatropic transition structures that differ by only a proton is supported by the values of their quantum molecular similarity indices.