Infrared spectra of the solvated hydronium ion: Vibrational predissociation spectroscopy of mass-selected H₃O⁺·(H₂O)_n·(H ₂)_m

Abstract

Infrared spectra of the mass-selected clusters H3O+·(H2O)n·(H 2) and H3O+·(H2)n (n = 1, 2, and 3) were observed by vibrational predissociation spectroscopy. The clusters were mass-selected and then trapped in a radio frequency ion trap. Cluster dissociation by loss of H2 followed excitation of OH or H2 stretches. Spectra were recorded by detecting fragment ions as a function of laser frequency. From spectra of H3O+·(H2O)n·(H 2), we were able to determine the spectrum of the hydrated hydronium ion H3O+·(H2O)n, because the H2 formed weak complexes with the hydrates. Spectra in the OH stretching region (3000-4000 cm-1) were observed at a resolution of 1.3 cm-1 for clusters n = 1, 2, and 3. The structure of the clusters and the perturbing effect of the H2 were inferred from a comparison with recent unpublished ab initio vibrational frequencies calculated by Remington and Schaefer. Observation of the hydrogen-bonded OH stretch in H3O+·(H2O)3 at 2670 cm-1 confirmed the assignment of the low-resolution spectrum obtained by Schwarz. Infrared spectra of the hydronium ion solvated by H2 were also reported. In the clusters H3O+·(H2)n (n = 1, 2, and 3), all OH and H2 stretching vibrations were observed. In several bands of H3O+·(H2), rotational structure was partially resolved, with the resolution limited by the laser line width of 0.8 cm-1. © 1990 American Chemical Society.