Proton sharing in hydronium-nitrogen clusters probed with infrared spectroscopy

Cluster ions containing hydronium and multiple nitrogen molecules, e.g., H3O+(N-2)(n)(n = 1-4) are produced in a supersonic molecular beam using a pulsed discharge source. Ions are mass analyzed and size-selected using a reflectron time-of-flight mass spectrometer. Selected ions are investigated with infrared laser photodissociation spectroscopy in the 2000-4000 cm(-1) region. Photodissociation occurs by the loss of a single nitrogen molecule from each cluster. The infrared spectra contain free-OH vibrations, hydrogen bonding O-H vibrations, combination bands between the latter vibrations and the low-frequency intermolecular stretches, and an N-N stretch in the n = 4 clusters. The n = 1 cluster has partially resolved rotational structure, confirming that its structure is that of end-on addition of N2 to one of the hydrogens of hydronium. The hydrogen bonding bands have broad linewidths and are significantly red-shifted from the free-OH vibrations. The red-shift decreases when more nitrogens are added, as the shared-proton interaction is distributed over the three hydrogen binding sites. Proton sharing in this system is highly biased toward the water moiety, but the nitrogen interaction is significant enough to induce significant vibrational shifts compared to other weakly bound complexes with hydronium (e.g., argon). (C) 2010 Elsevier B.V. All rights reserved.