Effect of hydrogen bonding and cooperativity on stretching force constants of formamide

Abstract

Stretching force constants for formamide and its seven associated species involving two to four molecules hydrogen-bonded through linear and cyclic configurations and 10 structures containing formamide hydrogen-bonded with one to five water molecules are reported. Since ab initio calculations are rather inconvenient to perform on such big clusters and are time-consuming, CNINDO MO calculations were carried out using the gradient method. The results demonstrate, on the one hand, the feasibility of semiempirical calculations for the evaluation of trends in force constants for big clusters where generally ab initio calculations become much involved and, on the other hand, explain the effect of hydrogen bonding and cooperativity on force constants and vibrational spectra of biologically important systems composed of formamide in the condensed phase and its aqueous solutions. The C=O and N-H stretching force constants are found to reduce significantly on hydrogen bonding. The reduction in force constant is further enhanced when two cyclic dimers become associated through a linear hydrogen bond. The results indicate justification for the stabilization of the formamide structure with two cyclic dimers hydrogen-bonded together. The reduction in the force constants on hydrogen bonding also reflect the cooperativity contribution. The C=O and C-N stretching force constants for the structures corresponding to formamide in liquid and aqueous solution phases are in agreement with the experimental vibrational frequencies reported.