THE DECARBOXYLATION AND DEHYDRATION REACTIONS OF MONOMERIC FORMIC-ACID

Two unimolecular dissociation reactions of formic acid have been investigated theoretically. The decarboxylation reaction yields molecular hydrogen and carbon dioxide, while the dehydration reaction produces water and carbon monoxide. The 1,2-hydrogen shift rearrangement from formic acid to dihydroxymethylene has also been considered. Methods ranged from double zeta plus polarization self-consistent field to triple zeta plus double polarization coupled cluster singles and doubles. For certain key structures, the coupled cluster method including single, double, and linearized triple excitations (CCSDT-1) was applied as well with the double zeta plus polarization basis set. A barrier height of approximately 71 kcal mol-1 with zero point vibrational energy correction is predicted for the dissociation to molecular hydrogen and carbon dioxide. A rather comparable value of approximately 68 kcal mol-1 is predicted for the barrier to the dehydration reaction. The 1,2-hydrogen shift transition state is somewhat higher in energy at approximately 79 kcal mol-1. These predicted energy barriers are discussed with reference to the existing experimental results on the thermal decomposition of formic acid.