No abstract

The gas phase infrared spectra of the hydrated hydronium cluster ions H3O+⋅(H2O)n(n=1, 2, 3) have been observed from 3550 to 3800 cm−1. The new spectroscopic method developed for this study is a two color laser scheme consisting of a tunable cw infrared laser with 0.5 cm−1 resolution used to excite the O–H stretching vibrations and a cw CO2 laser that dissociates the vibrationally excited cluster ion through a multiphoton process. The apparatus is a tandem mass spectrometer with a radio frequency ion trap that utilizes the following scheme: the cluster ion to be studied is first mass selected; spectroscopic interrogation then occurs in the radio frequency ion trap; finally, a fragment ion is selected and detected using ion counting techniques. The vibrational spectra obtained in this manner are compared with that taken previously using a weakly bound H2 ‘‘messenger.’’ A spectrum of H7 O+3 taken using a neon messenger is also presented. Ab initio structure and frequency predictions by Remington and Schaefer are compared with the experimental results.

Infrared spectra of hydrated hydronium ions weakly bound to an H2 molecule, specifically H7O+3 ⋅H2 and H9O+4 ⋅H2, have been observed. Mass-selected parent ions, trapped in a radio frequency ion trap, are excited by a tunable infrared laser; following absorption, the complex predissociates with loss of the H2, and the resulting fragment ions are detected. Spectra have been taken from 3000 to 4000 cm−1, with a resolution of 1.2 cm−1. They are compared to recent theoretical and experimental spectra of the hydronium ion hydrates alone. Binding an H2 molecule to these clusters should only weakly perturb their vibrations; if so, our spectra should be similar to spectra of the hydrated hydronium ions H7O+3 and H9O+4.

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.