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

摘要

Theoretical calculations plus the solvent dependence of the n1nH, n13nC NMR and IR spectra were used to determine thenconformational equilibrium in N-methyl-2-fluoroacetamide (NMFA) and N-methyl-2-fluoropropionamide (NMFP).nAb initio calculations were used to identify the stable rotamers and obtain their geometries and the application ofnsolvation theory on the n1nJCF coupling constant gave the conformer populations in the solvents studied. In NMFA abninitio calculations at the CBS-Q level yielded only two stable rotamers, the cis and trans, with ∆E(cis–trans) = 19.7 kJnmolnu00011n. The presence of two conformers was confirmed by the FTIR spectra. Assuming these forms, the observedncouplings when analysed by solvation theory gave ∆E = 21.3 kJ molnu00011n in the vapour phase, decreasing to 8.9 kJ molnu00011nin CDCl3 and to 0.8 kJ molnu00011n in DMSO. For NMFP the B3LYP calculations at the 6-311u0002u0002g(2df,2p) level gave onlynthe trans rotamer as stable, while the gauche form was a plateau in the potential energy surface. However the FTIRnspectra clearly showed the presence of two conformers. A minimum for the gauche rotamer was only found when thenSCRF (self consistent reaction field) routine was included in the theoretical calculations. The equilibrium in NMFPnwas therefore analysed by solvation theory in terms of the trans and gauche rotamers to give ∆E(gauche–trans) = 15.9nkJ molnu00011n in the vapour phase, decreasing to 10.8 kJ molnu00011n in CCl4 and to 0.5 kJ molnu00011n in DMSO.