화학공학소재연구정보센터
Journal of Physical Chemistry, Vol.98, No.44, 11353-11361, 1994
Tautomeric Equilibria of 3-Hydroxypyrazole in the Gas-Phase and in Solution - A Theoretical-Study Combining Ab-Initio Quantum-Mechanics and Monte-Carlo Simulation Methods
Ab initio geometry optimizations were carried out with the inclusion of electron correlation at the second-order Moller-Plesset (MP2) level for eight tautomers of 3-hydroxypyrazole, and some important structural features due to electron correlation effects are discussed. To obtain a more definitive estimate of the relative stabilities for the five major tautomeric forms A, B1, B2, F, and G in the gas phase, energy calculations were performed at various levels of electron correlation up to coupled-cluster single and double excitation (CCSD) and with a hierarchy of basis sets up to 6-311+G(3df,2p). The effects of basis set and electron correlation are profound due to the slow convergence of the relative energies. Combining our best estimate of the relative energies with zero-point vibrational energy corrections, thermal corrections, and entropic contributions, we conclude that B1 is the most stable form in the gas phase and G the least stable one with a free energy of 7.6 kcal/mol higher than that of B1. The tautomeric equilibria in a polar solvent with epsilon = 40 and in water with epsilon = 78.3 were studied using the self-consistent reaction field (SCRF) theory. The inability of the ab initio continuum model to treat first-hydration-shell effects, which are dominated by hydrogen bonding, resulted in a significant underestimate of the hydration free energy for G. In an effort to include both the solute polarization and the first-hydration-shel1 effects, we combined Monte Carlo simulation/statistical perturbation theory (MC/SPT) with the SCRF method to investigate the hydration effects on the system. Partial charges that correspond to the relaxed charge distribution in a polar medium with epsilon = 78.3 were used in the MC simulations, together with standard OPLS Lennard-Jones parameters.