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 ⁴J_HF, ¹J_CF and ²J_CF 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 (E_cis – E_trans) 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, ⁴J_HF, ¹J_CF and ²J_CF couplings do not change with solvent confirming that there is only one rotamer in solution for DFB, in agreement with the ab initio calculations.