Intermolecular Proton Binding in the Presence of a Large Electric Dipole: Ar-Tagged Vibrational Predissociation Spectroscopy of the CH3CN•H+•OH2 and CH3CN•D+•OD2 Complexes

We report Ar-predissociation vibrational spectra of the binary proton-bound hydrates of acetonitrile (AN), AN center dot H+center dot OH2 and AN center dot D+center dot OD2, in the 600-3800 cm(-1) energy range. This complex was specifically chosen to explore the nature of the intermolecular proton bond when there is a large difference between the electric dipole moments of the two tethered molecules. Sharp, isotope-dependent bands in the vicinity of 1000 cm(-1) are traced to AN center dot H+center dot OH2 vibrations involving the parallel displacement of the shared proton along the heavy atom axis, nu(sp)(parallel to). These transitions lie much lower in energy than anticipated by a recently reported empirical trend which found the nu(sp)(parallel to) fundamentals to be strongly correlated with the difference in proton affinities (Delta PA) between the two tethered molecules (Roscioli et al., Science, 2007, 316, 249). The different behavior of the AN center dot H+center dot OH2 complex is discussed in the context of the recent theoretical prediction (Fridgen, J. Phys. Chem A., 2006, 110, 6122) that a large disparity in dipole moments would lead to such a deviation from the reported (Delta PA) trend.