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

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 enthalpy, Δ H rxn,0 = 18 kJ mol -1 . Four other [C,H 3,O 3 ] isomers are found by calculations to be local energy minima, e.g., hydrogen-bonded complexes [O C O···H···OH 2 ] • ( 2 ) and [HO H···O C OH] • ( 3 ), and dihydroxymethoxy radicals anti- 4 and syn- 4 . Of these, 2 was generated transiently in the gas phase and found to dissociate rapidly to water and CO 2 . Kinetic isotope effects on dissociations of anti- 1 and syn- 1 are analyzed by RRKM calculations and used to estimate the radical internal energy, which can be expressed as a simple sum of the cationic precursor internal energy and the energy gained by Franck−Condon effects on collisional electron transfer.