Radical routes to interstellar glycolaldehyde. The possibility of stereoselectivity in gas-phase polymerization reactions involving CH2O and ˙CH2OH

Abstract

A previous report that the interstellar molecule glycolaldehyde (HOCH₂CHO) can be made from hydroxymethylene (HOCH:) and formaldehyde has been revisited at the CCSD(T)/6-311++G(3df,2p)//MP2/6-311++G(3df,2p) level of theory. This reaction competes with the formation of acetic acid and methylformate, molecules which have also been detected in interstellar clouds. Other possible modes of formation of glycolaldehyde by radical/radical reactions have been shown to be viable theoretically as follows:HO˙ + ˙CH₂CHO → HOCH₂CHO [ΔG_r(298 K) = −303 kJ mol⁻¹]HOCH₂˙ + ˙CHO → HOCH₂CHO (−259 kJ mol⁻¹)

The species in these two processes are known interstellar molecules. Key radicals ˙CH₂CHO and ˙CH₂OH in these sequences have been shown to be stable for the microsecond duration of neutralization/reionization experiments in the dual collision cells of a VG ZAB 2HF mass spectrometer. The polymerization reaction HOCH₂CH˙OH + nCH₂O → HOCH₂[CH(OH)]_n˙CHOH (n = 1 to 3) has been studied theoretically and shown to be energetically feasible, as is the cyclization reaction of HOCH₂[(CH₂OH)₄]˙CHOH (in the presence of one molecule of water at the reacting centre) to form glucose. The probability of such a reaction sequence is small even if polymerization were to occur in interstellar ice containing a significant concentration of CH₂O. The large number of stereoisomers produced by such a reaction sequence makes the formation of a particular sugar, again for example glucose, an inefficient synthesis. The possibility of stereoselectivity occurring during the polymerization was investigated for two diastereoisomers of HOCH₂[(CHOH)]₂˙CHOH. No significant difference was found in the transition state energies for addition of CH₂O to these two diastereoisomers, but a barrier difference of 12 kJ mol⁻¹ was found for the H transfer reactions ˙OCH₂[(CHOH)]₂CH₂OH → HOCH₂[(CHOH)₂˙CHOH of the two diastereoisomers.