Resonant ion-dip infrared spectroscopy of benzene–H2O and benzene–HOD

Abstract

Resonant ion-dip infrared spectra of C6H6–H2O and C6H6–HOD have been recorded in the OH stretch fundamental region. The spectra provide further evidence for the unique, large-amplitude motions present in these π hydrogen-bonded complexes. In C6H6–H2O, transitions out of the lowest ortho (Π) and para (Σ) ground state levels are observed. A transition at 3634 cm−1 is assigned as an unresolved pair of parallel transitions (Σ→Σ and Π→Π) involving the symmetric stretch fundamental (at 3657 cm−1 in free H2O). In the antisymmetric stretch region, transitions at 3713, 3748, and 3774 cm−1 are assigned as Π→Σ, Σ→Π, and Π→Δ transitions, respectively. The spacing of the transitions is consistent with nearly free internal rotation of H2O about benzene’s sixfold axis in both ground and vibrationally excited states. The intensities of combination bands depends critically on the mixing of some local mode character into the symmetric and antisymmetric stretches at asymmetric positions of H2O on benzene. Surprisingly, in C6H6–HOD, five transitions are observed in the OH stretch region, all arising from the ground state zero point level. Even more unusual, the higher-energy combination bands are many times stronger than the OH stretch fundamental. The local mode OH stretch has components both parallel and perpendicular to benzene’s sixfold axis, leading to strong parallel and perpendicular transitions in the spectrum. A two-dimensional model involving free internal rotation and torsion of HOD in its plane is used to account for the qualitative appearance of the spectrum. The form of the OH(v=0) and OH(v=1) torsional potentials which reproduce the qualitative features of the spectrum are slightly asymmetric, double-minimum potentials with large-amplitude excursions for HOD over nearly 180°.