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

Abstract

Resonant ion-dip infrared spectra of C 6 H 6-H 2 O and C 6 H 6-HOD have been recorded in the OH stretch fundamental region. The spectra provide further evidence for the unique, large-amplitude motions present in these hydrogen-bonded complexes. In C 6 H 6-H 2 O, transitions out of the lowest ortho ͑⌸͒ and para ͑⌺͒ ground state levels are observed. A transition at 3634 cm Ϫ1 is assigned as an unresolved pair of parallel transitions ͑⌺→⌺ and ⌸→⌸͒ involving the symmetric stretch fundamental ͑at 3657 cm Ϫ1 in free H 2 O͒. In the antisymmetric stretch region, transitions at 3713, 3748, and 3774 cm Ϫ1 are assigned as ⌸→⌺, ⌺→⌸, and ⌸→⌬ transitions, respectively. The spacing of the transitions is consistent with nearly free internal rotation of H 2 O 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 H 2 O on benzene. Surprisingly, in C 6 H 6-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(vϭ0) and OH(vϭ1) 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°.

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“…[76] An alternative approach is provided by IR-UV double resonance experiments. [77][78][79][80][81][82][83] This pump-probe method rests on the use of two lasers. The UV laser (or probe) is tuned to an electronic transition of a given species, while the IR laser (pump) is scanned in the region of interest.…”

The World of Non-Covalent Interactions: 2006

“…Most frequently, an H-bond is of the X-H...Y type, where X is the electronegative element and Y is the contact with excess of electrons. H-bonds with X, Y = F, O and N are the most frequent and most extensively studied [166][167][168] , though X-H...π hydrogen bonds (for X = O and C) have also been observed [169][170][171][172][173][174][175][176] .…”

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