Conformational properties, torsional potential, and vibrational force field for methacryloyl fluoride: An a b i n i t i o investigation

Abstract

We have examined the structure, torsional potentials, vibrational spectra, and harmonic force fields for the s-cis and s-trans isomers of methacryloyl fluoride with the aim of understanding some of the conformational properties of these molecules and their relationship to macroscopic polymer properties. The calculations have been performed at the SCF level with a split valence basis set. We find the structure to be in reasonably good agreement with experiment and as expected find the structural variations between the two isomers to be slight. However, we do suggest a reexamination of the carbonyl geometry in light of our studies. Our calculations show the energy difference between the s-cis and the s-trans isomers to be less than 1 kcal/mol at both the split valence and the split valence polarized levels, favoring the s-trans form. An examination of the torsional potential reveals that a rigid rotor approximation provides an adequate description of the motion of either the methyl group, or of the COF moiety. For the methyl group we find a barrier of 1.5 kcal/mol, with the preferred conformation having the hydrogen atom eclipsed with respect to the carbon–carbon double bond. Our best calculations show that the torsional barrier to interconvert the s-trans to the s-cis form is 7.0 kcal/mol, somewhat higher than the recent experimental values of 6.5 and 5.1 kcal/mol. This study also points out the need to include the difference in zero point energies between reactants and the transition state when evaluating torsional potentials. Our calculated vibrational spectra show the similarities and differences between these two isomers, and suggest areas where the assignments should be further examined. In particular, we suggest that some of the low frequency torsional modes deserve further scrutiny. A fit of our data to a three term Fourier series shows that we are able to reproduce the experimentally derived barrier, even though a direct determination shows that the predicted barrier lies higher.