DFT study of ground state proton transfer in 2-pyridone/2-hydroxypyridine-ammonia clusters

DFT calculations in the ground state have been performed for 2-hydroxypyridine-ammonia clusters 2HP · (NH3)n (n = 1 to 5) and their tautomers. Structures, relative energies and vibrational frequencies of these species were obtained at the B3LYP/6-31++G** level of theory. It was found that some structural parameters such as intra- and intermolecular distances and vibrational frequencies of O-H and N-H stretching, serve as probes and indicators of proton transfer in these clusters. The study of neutral forms shows that intermolecular proton transfer to the (NH3)n occurs for n = 4 and 5 in enol and keto clusters, respectively. For enol form, ion-pair cluster calculated for n = 4 is less stable than the neutral form by 7.1 kJ/mol. While, for keto form, it is calculated to be 25.4 kJ/mol less stable than the neutral form.