Estimation of individual NH•••X (X = N, O) hydrogen bonding energies in some complexes involving multiple hydrogen bonds using NBO calculations

The aim of this work is to estimate the NII center dot center dot center dot X (X = N, O) individual hydrogen bond energies (E-HB) in several systems involving multiple hydrogen bonds using donor-acceptor interaction energies (E-(2)) by natural bond orbital analysis at MP2/6-311++G(d,p) level of theory. The substituted dimers of formylformamide and 2-iminoacetamide are selected to search for simple relations between E-(2) and individual EHB. The coefficient of determination between EHB and E-(2) in the base of linear equation is acceptable. The achieved equations are validated by using them on the substituted formylformamide and 2-iminoacetamide dimers and then applied to some multiple hydrogen-bonded systems. Frequency calculations indicate that there are the elongation of H-N bond due to formation of complex and redshift of stretching mode. With our method, the strength of each hydrogen bond is in good agreement with geometrical parameters and the results obtained by atoms in molecules and nuclear magnetic resonance analyses, indicating accuracy of the presented method.