Cooperativity and ground-state proton transfer in 7-hydroxyimidazo[1,2-a]pyridine·ammonia clusters: DFT study

DFT/B3LYP (6-311+G**) has been used to study twelve different geometries of 7-hydroxyimidazo[1,2-a]pyridine·(NH3)n clusters with n=1-6. Ion-pair (Zwitterionic) clusters resulting from proton transfer from 7-hydroxyimidazo[1,2-a]pyridine (7HIP) to the (NH3)n cluster have also been investigated. The structure, energetics and relative stabilities of the geometries were compared and analyzed in gaseous phase. The principle of maximum hardness has been tested by calculating chemical hardness and chemical potential to confirm the more stable of different conformers and predict their order of stability. In all cases, the solvent cluster form hydrogen bonds at -O-H or N(2) positions, or at both -O-H and N(2) positions of 7HIP. For n=2-4, ammonia-chain clusters are found, for n=4-6 other topologies appear such as cycle and mixed chain/cycle. The study of neutral forms shows that intermolecular proton transfer from the -O-H group of 7HIP to the (NH3)n occurs for the critical size nc=5 yielding a locally stable zwitterions 7HIP−·(NH3)4NH4+. Some structural parameters such as intra- and intermolecular distances and vibrational frequency of -O-H stretching serving as indicators of proton transfer have been investigated. The natural bond orbital analysis (NBO) of the more stable conformers revealed specifically, that for a given number n of ammonia molecules, the stabilization energy E(2) is largest for LP(1)NA→BD*O15-H16 donor-acceptor interaction (n=2-5), and this value increases with increasing n (22.74 kJ/mol for n=2, 35.38 kJ/mol for n=3, 35.57 kJ/mol for n=4 and 53.54 kJ/mol for n=5), showing that electron density is directly transferred to the -O-H antibonding orbital, which result in weakening and elongating of the -O-H bond and red-shift.