| Abstract
| - 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, νsp(∥). These transitions lie much lower in energy than anticipated by a recently reported empirical trend which found the νsp(∥) 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.
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