Conformational analysis. Part 35. NMR, solvation and theoretical investigation of rotational isomerism in methyl fluoroacetate and methyl difluoroacetate

The solvent and temperature dependence of the C-13 NMR spectra of methyl fluoroacetate (MFA) and methyl difluoroacetate (MDFA) are reported and the (1)J(CF) coupling analysed in terms of the conformer couplings and energies. Density Functional Theory (DFT) calculations were used to obtain the conformer geometries and solvation theory gave the solvent dependence of the conformer energies. In MFA the DFT method at the B3LYP/6-311+G(d,p) level gave only two energy minima for the cis (F-C-C=O 0 degrees) and trans (F-C-C=O 180 degrees) conformers of ca. equal energy. The gauche conformer was not a minimum in the energy surface. The FTIR spectra of MFA support this result as two resolved carbonyl bands are observed whose relative intensity changes markedly with solvent polarity. Assuming only these forms, the observed coupling when analysed by solvation theory leads to the energy difference (E-cis-epsilonE(trans)) between the cis and trans conformers of 0.90 kcal mol(-1) in the vapour phase, decreasing to 0.41 kcal mol(-1) in CCl4 and -0.71 kcal mol(-1) in DMSO. In MDFA the DFT calculations gave two minima for the cis (H-C-C=O 0 degrees) and gauche (H-C-C=O 141.9 degrees) conformers with an energy difference (E-cis-E-gauche) of 0.2 kcal mol(-1). The FTIR spectra of MDFA support this result as in the non-polar solvent (CCl4) two resolved bands are observed but in solvents of medium and high polarity the carbonyl absorption appears as a single band. Assuming only the two forms, the observed coupling when analysed by solvation theory leads to the energy difference (E-cis-E-gauche) between the cis and gauche conformers of 0.0 kcal mol(-1) in the vapour phase, increasing to 0.46 kcal mol(-1) in CCl4 and 1.12 kcal mol(-1) in DMSO.