“…To elucidate the subtle balance between intramolecular and two different kinds of intermolecular hydrogen bonding, it makes sense to study the simplest prototype,11 which has even been detected in interstellar space:12, 13 glycolaldehyde (CH(O)CH 2 OH). Aldol reactions of glycolaldehyde catalyzed by dipeptides can lead to pentose sugars in an enantioselective way,14 adding to the importance of glycolaldehyde in prebiotic chemistry. In solid form, glycolaldehyde adopts a cyclic dimeric hemiacetal arrangement,15, 16 but in the gas phase, it provides the two essential functional groups for this study, a hydroxy group directly neighboring a carbonyl group, with which it can form a relatively strong internal hydrogen bond 17–20.…”