Conformational analysis, Part 32.† NMR, solvation and theoretical investigation of conformational isomerism in 3-fluorobutan-2-one and 3,3-difluorobutan-2-one

Abstract

The solvent and temperature dependence of the 1H and 13C NMR spectra of 3-fluorobutan-2-one (FB) and 3,3-difluorobutan-2-one (DFB) are reported and the 4JHF, 1JCF and 2JCF couplings analysed using ab initio calculations and solvation theory. The solvent dependence of the IR spectra (carbonyl band) was also measured. In FB, ab initio theory at the 6-31G**/MP2 level gives only two energy minima for the cis (F–C–CO 22°) and trans (F–C–CO 178°) rotamers. The gauche rotamer was not a minimum in the energy surface. Assuming only the cis and trans forms, the observed couplings when analysed by solvation theory lead to the energy difference (Ecis – Etrans) between the cis and trans rotamers of 3.7 kcal mol–1 in the vapour phase, decreasing to 2.5 kcal mol–1 in CCl4 and to 0.1 kcal mol–1 in DMSO. In all solvents used the trans rotamer is more stable than the cis. The vapour state energy difference compares very well with that calculated [3.67 kcal mol–1 including a zero-point energy correction (ZPE)]. In DFB ab initio calculations at this level and also at (6-311G**/MP2 and ZPE) gave only one minimum in the potential energy surface corresponding to the cis rotamer (C–C–CO 0°). The 1H and 13C NMR data, 4JHF, 1JCF and 2JCF couplings do not change with solvent confirming that there is only one rotamer in solution for DFB, in agreement with the ab initio calculations.