A systematic study on hydrogen bonding interaction between formamide and cytosine

A systematic computational study was carried out to characterize the hydrogen bonding of complexes formed between formamide and cytosine by DFT calculations. The computations were performed mainly with the B3LYP/6-311++G(d,p) level. Seven stable cyclic structures are found on the potential energy surface, in which four structures have two normal hydrogen bonds and the others have only one normal hydrogen bond with a very weak hydrogen bond that can be neglected. In the four structures with two normal hydrogen bonds, two have seven-membered rings, and the others have an eight-membered ring. The eight-membered ring is preferred to the seven-membered one by analyzing the hydrogen bond lengths and the interaction energies. The infrared spectrum frequencies, vibrational frequency shifts and charge number are also reported.