Why is Benzene Soluble in Water? Role of OH/pi Interaction in Solvation

摘要

The XH/pi interaction (X = C, N, or O) plays an essential role in a variety of fundamental processes in condensed phase, and it attracts broad interests in the fields of chemistry and biochemistry in recent years. This issue has a direct relevance to an intriguing phenomenon that a benzene molecule exhibits a negative solvation free energy of -0.87 kcal/mol in ambient water though it is a typical nonpolar organic solute. In this work, we developed a novel method to analyze the free energy delta mu due to the electron density fluctuation of a solute in solution to clarify the mechanism responsible for the affinity of benzene to bulk water. Explicitly, the free energy delta mu is decomposed into contributions from sigma and pi electrons in pi-conjugated systems on the basis of the QM/MM method combined with a theory of solutions. With our analyses, the free energy delta mu(pi) arising from the fluctuation of pi electrons in benzene was obtained as -0.94 kcal/mol and found to be the major source of the affinity of benzene to water. Thus, the role of pi electrons in hydration is quantified for the first time with our analyses. Our method was applied to phenyl methyl ether (PME) in water solution to examine the substituent effects of the electron donating group (EDG) on the hydration of a pi-conjugated system. The delocalization effect of the pi electrons on hydration was also investigated performing the decomposition analyses for ethene and 1,3-butadiene molecules in water solutions. It was revealed that the stabilization due to delta mu(pi) for butadiene (-0.76 kcal/mol) is about three times as large as that for ethene (-0.26 kcal/mol), which suggests the importance of the delocalization effect of the pi electrons in mediating the affinity to polar solvent.