Theoretical study of N–H· · ·O hydrogen bonding properties and cooperativity effects in linear acetamide clusters

Abstract

We investigated geometry, energy, \({\nu_{{\text{N--H}}}}\) harmonic frequencies, 14N nuclear quadrupole coupling tensors, and \({n_{\rm O}\to \sigma _{{\text{N--H}}}^\ast}\) charge transfer properties of (acetamide) n clusters, with n = 1 − 7, by means of second-order Moller-Plesset perturbation theory (MP2) and DFT method. Dependency of dimer stabilization energies and equilibrium geometries on various levels of theory was examined. B3LYP/6-311++G** calculations revealed that for acetamide clusters, the average hydrogen-bonding energy per monomer increases from −26.85 kJ mol−1 in dimer to −35.12 kJ mol−1 in heptamer; i.e., 31% cooperativity enhancement. The n-dependent trend of \({\nu_{{\text{N--H}}}\,{and}\,^{14}}\) N nuclear quadrupole coupling values were reasonably correlated with cooperative effects in \({r_{{\text{N--H}}}}\) bond distance. It was also found that intermolecular \({n_{\rm O}\to \sigma_{{\text{N--H}}}^\ast}\) charge transfer plays a key role in cooperative changes of geometry, binding energy, \({\nu_{{\text{N--H}}}}\) harmonic frequencies, and 14N electric field gradient tensors of acetamide clusters. There is a good linear correlation between 14N quadrupole coupling constants, C Q (14N), and the strength of Fock matrix elements (F ij ). Regarding the \({n_{\rm O}\to \sigma_{{\text{N--H}}}^\ast}\) interaction, the capability of the acetamide clusters for electron localization, at the N–H· · ·O bond critical point, depends on the cluster size and thereby leads to cooperative changes in the N–H· · ·O length and strength, N–H stretching frequencies, and 14N quadrupole coupling tensors.