Structure, Thermochemistry and Reactivity of Protonated Glycolaldehyde

Structures and relative energies of various conformers of the simplest sugar, glycolaldehyde, 1, and its protonated form, [1H]+, were investigated by ab initio molecular orbital calculations. The 298 K heats of formation of the most stable conformers, deduced from the atomization energies at the G2 level, are equal to Δ fH°(1) = −324.8 kJ mol−1 and Δ fH°[1H]+ = 426.0 kJ mol−1. The corresponding proton affinity value is PA(1) = 779.8 kJ mol−1, in perfect agreement with the experimental determination of 783.3 ± 3.8 kJ mol−1 obtained by the kinetic method. A gas-phase basicity value, GB(1), of 745–748 kJ mol−1 is also deduced from theory and experiment. The exclusive dissociation channel of protonated glycolaldehyde, [1H]+, is water loss which leads essentially to the acylium ion [CH3CO]+. The corresponding potential energy profile, investigated at the MP2/6–31G* level, reveals a route via a [CH3CO]+ / water complex after an energy determining step involving a simultaneous 1,2-hydrogen migration and C–O bond elongation. The critical energy of the reaction, evaluated at the G2(MP2,SVP)level, is 170 kJ mol −1 above the most stable conformation of the [1H]+ ion. The 298 K heats of formation of the three most stable [C2H3O]+ ions have been calculated at the G2 level: Δ fH°[CH3CO]+ = 655.0 kJ mol−1, Δ fH°[CH2COH]+ = 833.0 kJ mol−1, Δ fH°[c-CH2CHO]+ = 886.2 kJ mol−1.