H-bonded complexes of uracil with parent nitrosamine: A quantum chemical study

The structure, stability and proton transfer in H-bonded complexes formed from interaction between uracil (U) and NH2NO (NA) have been investigated using B3LYP, B3PW91 and MP2 methods with a wide range of basis sets. With four preferential interaction sites in the vicinity of the uracil, eight cyclic complexes (UN1-8) with two intermolecular hydrogen bonds N(C)HU⋯O(N)NA and HNA⋯OU were found on the potential energy surface. RAHB mechanism is exhibited in all the complexes. Four 8- and four 7-membered ring complexes have electronic interaction energies (IEs) in the range of −44.0 to −63.2 kJ/mol and −42.5 to −60.9 kJ/mol at MP2/6-311++G(2d,2p) levels, respectively. The most stable complex is formed via NH bond of U with highest acidity and CO group of U with lowest proton affinity. Our results show that intermolecular interactions are affected more by acidity of proton donor group in U than by proton affinity of proton acceptor group. We found a relationship between H-bond distances and the corresponding frequency shifts. Thermodynamic properties correspond to the process of proton transfer between monomers have been calculated at various levels of theory. NBO and AIM analyses confirm that the charge transfer takes place from NA to U in UN1-6 and vice versa in UN7-8 which CH is a proton donor group in U. There is a correlation between intermolecular charge transfer energies and interaction energies and electron densities at H-bond critical points. AIM analysis shows that H-bond contacts are electrostatic in nature and covalent nature of proton donor groups decreases upon complex formation.