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 greater than or equal to4, 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)(6)H+. 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. (C) 2001 American Institute of Physics.