Linear bending in propynyl cation, allene, and propyne systems: do they have flexible structures? an ab initio 4 – 31 + G molecular orbital study

Abstract

The linear bending of propyne, allene, and propynyl cation structures has been studied with SCF MO procedures. Ab initio gradient calculations are used to determine structural information. Force constants are evaluated. The results show fairly flexible structures under linear bending. The propynyl cation has a marked anisotropy; the out-plane linear bending (the plane is defined by the CH₂ group) is the softer mode. The electronic structure for the cation presents a fairly strong nucleophilic centre at the C_β atom. Interestingly, 10° bending of propyne or allene does not require more than 1.10 kcal mol^–1. For the alkynyl cation 0.6 kcal mol^–1 is required. The theoretical results show that linear bending makes an integral part of the reactivity properties of all these species.