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

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.

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