Intramolecular hydrogen binding interactions in 8-hydroxyquinoline, both in its zwitterionic tautomer and in the rotamer without the intramolecular hydrogen bond (IHB), have been computed using the B3LYP and MPW1K density functionals. The rotation of the OOH bond and intramolecular proton transfer reactions were studied theoretically. The following theory levels have been applied: B3LYP/6-31G(d,p), B3LYP/6-311ϩϩG(d,p), MPW1K/6-311ϩϩG(d,p), and MPW1K/6-311ϩϩG(2d,3p)//MPW1K/6-311ϩϩG(d,p). Natural bond orbital (NBO) analysis has also been carried out. The effect of medium (benzene, chloroform, tetrahydrofuran, 1,2-dichloroethane, acetone, water) was simulated using the self-consistent reaction field (SCRF) method within the framework of the polarizable continuum model (PCM), at the MPW1K/6-311ϩϩG(d,p) level. The evolution of geometry, relative energies, heights of rotation (around the OOH bond) and tautomerization barriers, IHB energies, and ⌬G(solv) have been systematically investigated. The results obtained have shown the failure to neglect some changes of the above characteristics in polar media with respect to the gaseous phase. The series of stability of the forms under study in the gaseous phase remains the same in solution. Thus, in spite of the important role of the solvent electrostatic effects, the intrinsic stability of those species overcomes the solvent effects.

A theoretical quantum chemical study of the intramolecular hydrogen bonding interactions in 8-mercaptoquinoline has been carried out. Special attention has been paid to the rotation of S-H bond and intramolecular proton-transfer reactions. Therewith, the B3LYP/6-311++G(d,p), B3LYP/6-31+G(2d,2p), MPW1K/6-311++G(d,p), MPW1K/6-31+G(2d,2p), BH&HLYP/6-311++G(d,p), and G96LYP/6-311++G(d,p) methods have been used. By means of the Onsager and PCM reaction field methods, the effects of solvent on hydrogen-bond energies, conformational equilibria, rotational barriers, and tautomerism in aqueous solution have been studied. These simulations were done at the MPW1K/6-311++G(d,p) and B3LYP/6-311++G(d,p) levels. Natural-bond orbital analysis has been performed to study the intramolecular hydrogen bond (IHB) in the gaseous phase and in aqueous medium. The stability of forms under consideration in solution does not coincide with that in the gaseous phase, underlining a great importance of the electrostatic influence of solvent. Double-proton transfer in the prototropic tautomerization of 8-mercaptoquinoline, one water molecule complex in the gaseous phase and in solution, has been systematically studied. The double-proton transfer occurs concertedly and synchronously. The water-assisted tautomerization is kinetically less, but thermodynamically more favorable, compared to that of the single-proton transfer. As in the case with single-proton transfer, for water-assisted reaction, the tautomerization energies and barrier heights decrease with the increase in dielectric constant, which implies faster and more complete tautomerization of 8-mercaptoquinoline in a polar solvent.

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By means of the MNDO/H, I-MNDO, MNDO/HB and MNDO/M methods, the values of the enthalpy of intramolecular hydrogen bond (IHB) formation for series of substances have been computed. The applicability of these methods for a semi-quantitative estimation of the IHB enthalpy in compounds with a planar quasicycle has been shown. .