Protonated Carbonic Acid and the Trihydroxymethyl Radical in the Gas Phase. A Neutralization−Reionization Mass Spectrometric and ab Initio/RRKM Study

Abstract

Gas-phase protonation of carbonic acid is predicted to occur preferentially at the carbonyl oxygen with the 298 K proton affinity that was calculated as PA = 741 and 775 kJ mol-1 for the syn- and anti-conformer, respectively. The hydroxyl groups are less basic with a topical PA = 660 kJ mol-1. The standard enthalpy of formation of gas-phase carbonic acid was calculated as = −614 kJ mol-1. Collisional neutralization of protonated carbonic acid, anti-1+, yields transient trihydroxymethyl radicals that dissociate rapidly by loss of a hydrogen atom, so that no survivor species are observed on a 360 ns time scale. High-level ab initio calculations with G2(MP2), G2, B3-MP2, and QCISD(T)/6-311+G(3df,2p) find two stable conformers of trihydroxymethyl radicals, anti-1 and syn-1, that interconvert rapidly by O−H bond rotations. The lowest-energy dissociation of anti-1 is loss of a hydrogen atom which requires 93 kJ mol-1 in the transition state and forms the anti-conformer of carbonic acid with an overall reaction ...