Identification of HOCHC(O)H, HOCHCO, and HOCHCHO Intermediates in the Reaction of H + Glycolaldehyde in Solid -Hydrogen and Its Implication to the Interstellar Formation of Complex Sugars.

Glycolaldehyde [HOCHC(O)H, GA], the primitive sugar-like molecule detected in the interstellar medium (ISM), is a potential precursor for the synthesis of complex sugars. Despite its importance, the mechanism governing the formation of these higher-order sugars from GA under interstellar circumstances remains elusive. Radical intermediates HOCHCHO (), HOCHCHOH (), HOCHCO (), HOCHC(O)H (), and OCHC(O)H () derived from GA could be potential precursors for the formation of glyceraldehyde (aldose sugar), dihydroxyacetone (ketose sugar), and ethylene glycol (sugar alcohol) in dark regions of ISM. However, the spectral identification of these intermediates and their roles were little investigated. We conducted reactions involving H atoms and the - conformer of GA (-GA) in solid -H at 3.2 K and identified IR spectra of radicals -HOCHCO () and -HOCHC(O)H () produced from H abstraction as well as closed-shell HOCHCO () produced via consecutive H abstraction of GA. In addition, -HOCHCHO () and CHOH + HCO () were produced through the H addition and the H-induced fragmentation channels, respectively. In darkness, when only H-tunneling reactions occurred, the formation of () was major and that of () was minor. In contrast, during IR irradiation to produce H atoms with higher energy, the formation of () and CHOH + HCO () became important. We also successfully converted most GA to the second-lowest-energy conformer --GA (-GA) by prolonged IR irradiation at 2827 nm to investigate H + -GA; -HOCHCO (), -HOCHC(O)H ( HOCHCO (), -HOCHCHO (), and CHOH + HCO () were observed. We discuss possible routes for the formation of higher-order sugars or related compounds involving (), (), (), and (), but neither (), which was proposed previously, nor () plays a significant role in H + GA. Such previously unreported rich chemistry in the reaction of H + GA, with four channels of three distinct types, indicates the multiple roles that GA might play in astronomical chemistry.