Protonated sulfuric acid: Vibrational signatures of the naked ion in the near- and Mid-IR

The species formally obtained from the protonation of sulfuric acid is examined as a naked ion in the gas phase, and its IR spectral features are reported. The major fundamental vibrational bands have been acquired in both the 700-1700 and the 3400-3800 cm^-1 spectral ranges using state of the art equipment at the CLIO facility, namely, FT-ICR mass spectrometry coupled to the tunable IR beam of either a free electron laser or a tabletop optical parametric oscillator/amplifier (OPO/OPA) laser source. In this way, the structural features of the sampled ion could be ascertained and found to conform to the most stable isomer on the [H₃,S,O₄]⁺ potential energy surface. The experimental IR multiple photon dissociation (IRMPD) spectrum is in fact consistent with the IR spectrum calculated for the trihydroxyoxosulfonium cation OS(OH)₃⁺, providing a conclusive structural probe.