BENZENE FORMS HYDROGEN BONDS WITH WATER

Abstract

Fully rotationally resolved spectra of three isotopic species of 1:1 clusters of benzene with water ($H_{2}O, D_{2}O$, and HDO) are fit to yield moments of inertia which demonstrate unambiguously that water is positioned above the benzene plane in nearly free internal rotation with both hydrogens pointing toward the $\pi$-cloud. Ab initio calculations $(MP2/6.31G^{\ast \ast}$ with basis set superposition error corrections) predict a binding energy of $D_{e}\gtrsim 1.78$ kilocalorie per mole. In both the experimental and theoretical structures, water is situated nearly 1 angstrom inside the van der Waals contacts of the monomers, a clear manifestation of hydrogen bond formation in this simple model of aqueous.$\pi$ electron interactions.