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 DeltaHdegrees(f.298)= -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, DeltaH(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-(OHOH2)-H-...-O-...](.) (2) and [HO-(HO)-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 CO2. 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.