Proton transfer and tautomerization in 7-hydroxyquinoline⋅(NH3)n clusters: Structure and energetics at the self-consistent field level

The structures and energetics of 23 different 7-hydroxyquinoline⋅(NH3)n clusters with n=1–6 are investigated using ab initio calculations at the SCF/6-31G** level. Clusters based on both the enol and keto tautomers of 7-hydroxyquinoline (7-HQ) are investigated, as well as ion-pair (zwitterionic) clusters resulting from proton transfer from 7-HQ to the (NH3)n cluster. In all cases, the solvent cluster forms hydrogen bonds at both the -O-H and N positions of 7-HQ. For n=2–4, ammonia-chain clusters are found, for n⩾4, additional hydrogen-bond topologies appear such as bifurcated chains, cycles and mixed chain/cycles. Although the bare keto 7-HQ is calculated to be 17 kcal/mol less stable than the enol form, the energy difference decreases continuously to 11 kcal/mol for 7-HQ⋅(NH3)6 with increasing n. For one of the enol clusters involving six NH3 molecules, proton transfer can occur from either the -O-H or N-H group to the cluster, yielding a locally stable zwitterion cluster 7-HQ−⋅(NH3)6H+. For the keto tautomer, locally stable ion-pair clusters were found for n=2, 5, and 6, but these are calculated to be considerably less stable than the neutral enol forms.