Benzene forms hydrogen bonds with water

Fully rotationally resolved spectra of three isotopic species of 1:1 clusters of benzene with water (H₂O, D₂O, and HDO) were fit to yield moments of inertia that demonstrate unambiguously that water is positioned above the benzene plane in nearly free internal rotation with both hydrogen atoms pointing toward the π cloud. Ab initio calculations (MP2 level of electron correlation and 6-31G** basis set with basis set superposition error corrections) predict a binding energy D_e ≳ 1.78 kilocalories per mole. In both the experimental and theoretical structures, water is situated nearly 1 angstrom within the van der Waals contacts of the monomers, a clear manifestation of hydrogen bond formation in this simple model of aqueous-π electron interactions.