Gas-phase deprotonation of p-hydroxybenzoic acid investigated by IR spectroscopy: solution-phase structure is retained upon ESI.

The gas-phase structure of the conjugate base of p-hydroxybenzoic acid (and related compounds) and the influence of the solvent used in its generation by electrospray ionization have recently been under debate. While the phenoxide structure is known to be lower in energy in the gas phase, the carboxylate structure is favored in aqueous solution, fuelling the controversy. Here we probe the structure of this gas-phase anion by IR spectroscopy and show that its structure is determined by the protic or aprotic nature of the solvent, which suggests that it is the solution-phase structure that is transferred to the gas phase.