An ab initio study of crystal field effects, part 3: 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 electrostatic field of the proper symmetry. Attention is paid to the way the electrostatic 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 after ak-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 better 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 effects of hydrogen bonding. It was shown that the C=O bond lengthens by about 0.011 å per H-bond it accepts, while the N-C bond diminishes by about 0.015 å per H-bond it donates.

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