Accurate heats of formation and acidities for H₃PO₄, H₂so₄, and H₂CO₃ from Ab initio electronic structure calculations

Atomization energies and heats of formation at 0 and 298 K for carbonic acid, sulfuric acid and phosphoric acid were calculated using coupled cluster theory including noniterative, quasi-pertubative triple excitations with large basis sets. Optimized geometries and harmonic vibrational frequencies were calculated at the MP2/aug-cc-p VTZ level. Atomization energies were obtained at the CCSD(T) level by using the aug-cc-pV(n+d)Z basis sets where the +d corresponds to the inclusion of tight d function for second row atoms. The results show that high-level calculations of gas phase acidities in conjuction with continuum solvent models can be used to constrain the pK_a's of very strong acids in aqueous solution.