Energetics and Mechanisms for the Unimolecular Dissociation of Protonated Trioses and Relationship to Proton-Mediated Formaldehyde Polymerization to Carbohydrates in Interstellar Environments

摘要

We report the unimolecular decomposition of protonated glyceralde-hyde, [HOCH_2CH(OH)CHO]H~+, and protonated dihydroxyacetone, [HOCH_2-C(O)CH_2OH]H~+. On the basis of mass spectrometric experiments and computational quantum chemistry, we have found that these isomeric ions interconvert freely at energies below that required for their unimolecular decompositions. The losses of formaldehyde and water (the latter also followed by CO loss) are the dominating processes, with formaldehyde loss having the lower energetic threshold. The reverse of the formaldehyde loss, namely, the addition of formaldehyde to protonated glycolaldehyde, appears to be an inefficient reaction at low temperature and pressure in the gas phase, leading to dissociation products. The relevance of these findings to interstellar chemistry and prebiotic chemistry is discussed, and it is concluded that the suggestion made in the literature that successive addition of formaldehyde by proton-assisted reactions should account for interstellar carbohydrates most likely is incorrect.