The conformational behaviour of 2-fluoro and of 2,6-difluoroacetophenone implied by proximate spin–spin coupling constants and MO calculations

Abstract

The temperature dependence of [Formula: see text], the nuclear spin–spin coupling constant over five formal bonds between the methyl protons and the 19F nucleus in 2-fluoroacetophenone and 2,6-difluoroacetophenone, is modelled on the assumption that 5J is a proximate coupling and that the STO 3G MO potential functions describe the population distributions of the rotamers defined by rotation about the exocyclic sp2–sp2 carbon–carbon bond. It is assumed that 5J has a cos4 θ dependence between 0 and 90°, where θ is the angle by which the acetyl group twists out of the plane of the benzene plane. The potential function is obtained from extensive geometry optimization procedures for a range of θ values. At 305 K, nonplanar conformations are substantially populated in 2-fluoroacetophenone, according to this model, which is also consistent with the idea that the 2,6-difluoro derivative has a markedly nonplanar ground state. The model reproduces the large 5J in the monofluoro relative to the difluoro compound, as well as the much larger temperature dependence in the former.