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.

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Structure,Thermochemistry and Reactivity of Protonated Glycolaldehyde