An Ab Initio Study of Crystal Field Effects, Part 3t: Solid- and Gas-Phase Geometry of Formamide, Modeling the Changes in a Peptide Group Due to Hydrogen Bonds

A model of the solid state of formamide is constructed by optimizing a central molecule in an elect rostat ic field of the proper symmetry. Attent ion is paid to the way the electrostat ic charges are obtained. Point charges obtained from a Mulliken population analysis yield a final set of atomic charges in the central molecule that agree reasonably well with those obtained experimentally af ter a K-refinement of formamide. Point charges from a so-called stockholder partitioning agree slightly less. Furthermore, the simple crystal field adaptation of standard ab initio methods reproduces within experimental limits the differences in C-O and C -N lengths, observed between the gas-phase and the solid state geometry. Again, a Mulliken field agrees slightly bet ter than a stockholder field, but the difference in performance is statistically insignificant. In a survey of 221 high-quality single-crystal x-ray determinations of compounds containing the peptide group N C = O , we found evidence supporting quantitatively the conclusion that the increase of C = O and the decrease of C -N bond length in the gas-to-solid transition is dominated by the ef fects of hydrogen bonding. It was shown that the C = O bond lengthens by about 0.011 • per H-bond it accepts, while the N-C bond diminishes by about 0.015 A per H-bond it donates.