The Role of Catalysis in Alkanediol Decomposition: Implications for General Detection of Alkanediols and Their Formation in the Atmosphere.

Quantum chemical calculations have been carried out to investigate the gas-phase structure, stability, and decomposition of the two simplest alkanediols, methanediol and 1,1-ethanediol, in the presence of various catalysts. Three different conformers for monomeric alkanediols namely cis, trans, and trans' were considered. The calculations reveal that alkanediols may exist not only as monomers but also as dimers that have high binding energies of 7-11 kcal/mol due to hydrogen bonding among the oxygenate functionalities. Some of these dimers have high dipole moments and, thus, may be more easily detected experimentally than the monomers of alkanediols. For the decomposition of alkanediols, the calculations dominantly favor dehydration over dehydrogenation. The relatively low barrier for the decomposition of 1,1-ethanediol suggests that the structure of an alkanediol plays a role in its decomposition. Though the dehydration of alkanediols with or without water catalyst involves large barriers, organic and inorganic acids, the hydroperoxyl radical catalytically influences the reaction to such an extent that the dehydration reaction either involves significantly reduced barriers or essentially becomes barrierless. Considering that alkanediols contain hydroxyl groups and their dimers have high binding energies, the gas-phase dehydration may be self-driven. Because acids are present in significant amounts in the troposphere, results suggest that diol dehydration may be facile under atmospheric conditions.