Conformational analysis in N-methylfluoroamides. A theoretical, NMR and IR investigation

Abstract

Theoretical calculations plus the solvent dependence of the ¹H, ¹³C NMR and IR spectra were used to determine the conformational equilibrium in N-methyl-2-fluoroacetamide (NMFA) and N-methyl-2-fluoropropionamide (NMFP). Ab initio calculations were used to identify the stable rotamers and obtain their geometries and the application of solvation theory on the ¹J_CF coupling constant gave the conformer populations in the solvents studied. In NMFA ab initio calculations at the CBS-Q level yielded only two stable rotamers, the cis and trans, with ΔE(cis–trans) = 19.7 kJ mol⁻¹. The presence of two conformers was confirmed by the FTIR spectra. Assuming these forms, the observed couplings when analysed by solvation theory gave ΔE = 21.3 kJ mol⁻¹ in the vapour phase, decreasing to 8.9 kJ mol⁻¹ in CDCl₃ and to 0.8 kJ mol⁻¹ in DMSO. For NMFP the B3LYP calculations at the 6-311++g(2df,2p) level gave only the trans rotamer as stable, while the gauche form was a plateau in the potential energy surface. However the FTIR spectra clearly showed the presence of two conformers. A minimum for the gauche rotamer was only found when the SCRF (self consistent reaction field) routine was included in the theoretical calculations. The equilibrium in NMFP was therefore analysed by solvation theory in terms of the trans and gauche rotamers to give ΔE(gauche–trans) = 15.9 kJ mol⁻¹ in the vapour phase, decreasing to 10.8 kJ mol⁻¹ in CCl₄ and to 0.5 kJ mol⁻¹ in DMSO.