POTENTIAL-ENERGY SURFACE AND VIBRATIONAL FREQUENCIES OF CARBONIC-ACID

Ab initio quantum chemical calculations have been performed for the ca rbonic acid molecule. Results obtained at the QCISD(T)/6-311++G* leve l show that formation of gas phase H2CO3 from water and carbon dioxide is endoergic by 43.4 kJ/mol, including zero-point energy corrections. The barrier to this reaction is 217 kJ/mol. The calculations reveal g lobal and local minima associated with rotational isomers of the molec ule. Transition states have been characterized for interconversion of the isomers by internal rotation and by intramolecular proton transfer reactions. Calculated vibrational frequencies and infrared band inten sities are compared with recent experimental studies of condensed phas e carbonic acid prepared by two different synthetic routes.