Autocatalytic Isomerizations of the Two Most Stable Conformers of Carbonic Acid in Vapor Phase: Double Hydrogen Transfer in Carbonic Acid Homodimers

The <i>cis–cis</i> [(<i>cc</i>)] and <i>cis–trans</i> [(<i>ct</i>)] conformers of carbonic acid (H₂CO₃) are known as the two most stable conformers based on the different orientations of two OH functional groups present in the molecule. To explain the interconversion of the (<i>cc</i>)-conformer to its (<i>ct</i>)-conformer, the rotation of one of the two indistinguishable OH functional groups present in the (<i>cc</i>)-conformer has been shown until now as the effective isomerization mechanism. Moreover, the (<i>ct</i>)-conformer, which is slightly energetically disfavored over the (<i>cc</i>)-conformer, has been considered as the starting point for the decomposition of H₂CO₃ into CO₂ and H₂O molecules. Experimentally, on the other hand, the infrared (IR) and Raman spectroscopy of the crystalline H₂CO₃ polymorphs suggest that the most possible basic building blocks of H₂CO₃ polymorphs consist of only and exclusively the (<i>cc</i>)-conformers. However, the sublimations of these crystalline H₂CO₃ polymorphs result both the (<i>cc</i>)- and (<i>ct</i>)-conformers in the vapor phase with the (<i>cc</i>)-conformer being the major species. In this article, we first report the high level ab initio calculations investigating the energetics of the autocatlytic isomerization mechanism between the two most stable conformers of carbonic acid in the vapor phase. The calculations have been performed at the MP2 level of theory in conjunction with aug-cc-pVDZ, aug-cc-pVTZ, and 6-311++G­(3df,3pd) basis sets. The results of the present study specifically and strongly suggest that double hydrogen transfer within the eight-membered cyclic doubly hydrogen-bonded (H-bonded) ring interface of the H₂CO₃ homodimer formed between two (<i>cc</i>)-conformers is ultimately the starting mechanism for the isomerization of the (<i>cc</i>)-conformer to its (<i>ct</i>)-conformer, especially, during the sublimation of the H₂CO₃ polymorphs, which result in the vapor phase concentration of the (<i>cc</i>)-conformer at the highest levels