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

We investigated geometry, energy, nu(N)-H harmonic frequencies, N-14 nuclear quadrupole coupling tensors, and n(O) -> sigma(N-H)* 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 HN- and N-14 nuclear quadrupole coupling values were reasonably correlated with cooperative effects in r(N-H) bond distance. It was also found that intermolecular n(O) -> sigma(N-H)* charge transfer plays a key role in cooperative changes of geometry, binding energy, nu(N-H) harmonic frequencies, and N-14 electric field gradient tensors of acetamide clusters. There is a good linear correlation between N-14 quadrupole coupling constants, C-Q(N-14), and the strength of Fock matrix elements (F-ij). Regarding the n(O) -> sigma(N-H)* interaction, the capability of the acetamide clusters for electron localization, at the N-H center dot center dot center dot O bond critical point, depends on the cluster size and thereby leads to cooperative changes in the N-H center dot center dot center dot O length and strength, N-H stretching frequencies, and N-14 quadrupole coupling tensors.