Identification of HOC•HC(O)H, HOCH2C•O, and HOCH2CH2O• Intermediates in the Reaction of H plus Glycolaldehyde in Solid Para-Hydrogen and Its Implication to the Interstellar Formation of Complex Sugars

Glycolaldehyde [HOCH2C(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 HOCH2CH2O center dot (1), (HOCH2CHOH)-H-center dot (2), (HOCH2CO)-O-center dot (3), (HOCHC)-H-center dot(O)H (4), and (OCH2C)-C-center dot(O)H (5) 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 Cis-cis conformer of GA (Cc-GA) in solid p-H-2 at 3.2 K and identified IR spectra of radicals Cc-(HOCH2CO)-O-center dot (3) and Cc-(HOCHC)-H-center dot(O)H (4) produced from H abstraction as well as closed-shell HOCHCO (6) produced via consecutive H abstraction of GA. In addition, Cc-HOCH2CH2O center dot (1) and (CH2OH)-H-center dot + H2CO (7) were produced through the H addition and the H-induced fragmentation channels, respectively. In darkness, when only H-tunneling reactions occurred, the formation of (3) was major and that of (1) was minor. In contrast, during IR irradiation to produce H atoms with higher energy, the formation of (4) and (CH2OH)-H-center dot + H2CO (7) became important. We also successfully converted most Cc-GA to the second-lowest-energy conformer Trans-trans-GA (Tt-GA) by prolonged IR irradiation at 2827 nm to investigate H + Tt-GA; Tt-(HOCH2CO)-O-center dot (3 '), Tt-(HOCHC)-H-center dot(O)H (4 '), HOCHCO (6), Tt-HOCH2CH2O center dot (1 '), and (CH2OH)-H-center dot + H2CO (7) were observed. We discuss possible routes for the formation of higher-order sugars or related compounds involving (7), (1), (3), and (4), but neither (2), which was proposed previously, nor (5) 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.