An NMR, IR and theoretical investigation of the methyl effect on conformational isomerism in 3‐fluoro‐3‐methyl‐2‐butanone and 1‐fluoro‐3,3‐dimethyl‐2‐butanone

Abstract

AbstractThe solvent dependence of the 1H and 13C NMR spectra of 3‐fluoro‐3‐methyl‐2‐butanone (FMB) and 1‐fluoro‐3,3‐dimethyl‐2‐butanone (FDMB) was examined and the 4JHF, 1JCF and 2JCF couplings are reported. Density functional theory (DFT) at the B3LYP/6–311 ++ G(2df,2p) level with ZPE (zero point energy) corrections was used to obtain the conformer geometries. In both FMB and FDMB, the DFT method gave only two minima for cis (FCCO, 0°) and trans (FCCO, 180°) rotamers. Assuming the cis and trans forms, the observed couplings in FMB when analysed by solvation theory gave the energy difference E cis − E trans of 3.80 kcal mol−1 (1 kcal = 4.184 kJ) in the vapour phase (cf. the DFT value of 3.21 kcal mol−1), decreasing to 2.6 kcal mol−1 in CCl4 and to 0.27 kcal mol−1 in DMSO. In FDMB the observed couplings when analysed similarly by solvation theory gave E cis − E trans = 1.80 kcal mol−1 in the vapour phase, decreasing to 0.47 kcal mol−1 in CCl4 and to −1.25 kcal mol−1 in DMSO. The introduction of a methyl group geminal to the fluorine atom shifts the conformational equilibrium towards the trans rotamer, in contrast to no significant effect when the methyl group is introduced at the α‐carbon further from the fluorine atom. Copyright © 2002 John Wiley & Sons, Ltd.