Infrared spectroscopy of the cluster ions H+3⋅(H2)n

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Abstract
The vibrational spectra of the clusters H+3(H2)n were observed near 4000 cm−1 by vibrational predissociation spectroscopy. Spectra of mass‐selected clusters were obtained by trapping the ions in a radio frequency ion trap, exciting vibrational transitions of the cluster ions to predissociating levels, and detecting the fragment ions with a mass spectrometer. Low resolution bands of the solvent H2 stretches were observed for the clusters of one to six H2 coordinated to an H+3 ion. The red shift of these vibrations relative to the monomer H2 frequency supported the model of H+9 as an H+3 with a complete inner solvation shell of three H2, one bound to each corner of the ion. Two additional bands of H+5 were observed, one assigned as the H+3 symmetric stretch, and the other as a combination or overtone band. High‐resolution scans (0.5 and 0.08 cm−1) of H+n, n=5, 7, and 9 yielded no observable rotational structure, a result of either spectral congestion or rapid cluster dissociation. The band contour of the H+5 band changed upon cooling the internal degrees of freedom, but the peaks remained featureless. The observed frequencies of H+7 and H+9 agreed well with ab initio predictions, but those of H+5 did not. This deviation is discussed in terms of the large expected anharmonicity of the proton bound dimer H+5.