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

摘要

We report Ar-predissociation vibrational spectra of the binary proton-bound hydrates of acetonitrile (AN), AN·H~+·OH2 and AN·D~+·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·H~+·OH2 vibrations involving the parallel displacement of the shared proton along the heavy atom axis, v_(sp)(||). These transitions lie much lower in energy than anticipated by a recently reported empirical trend which found the v_(sp)(||) fundamentals to be strongly correlated with the difference in proton affinities (ΔPA) between the two tethered molecules (Roscioli et al., Science, 2007, 316, 249). The different behavior of the AN·H~+·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 (ΔPA) trend.