Structure, thermochemistry and reactivity of protonated glycolaldehyde

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Eur. J. Mass Spectrom. 7, 351 - 357 (2001)

Structure, thermochemistry and reactivity of protonated glycolaldehyde

Guy Bouchoux
Département de Chimie, Laboratoire des Mécanismes Réactionnels, UMR CNRS 7651, Ecole Polytechnique, 91128 Palaiseau cedex, France
Florence Penaud-Berruyer
Laboratoire de Chimie Physique, Groupe de Chimie Théorique, UMR CNRS 8000, Bâtiment 490, Université Paris Sud, 91405 Orsay cedex, France
William Bertrand
Laboratoire dEtudes de Traces Organiques, Institut Pasteur de Lille, CNRSSP, 930 boulevard Lahure, 59505 Douai cedex, France

ABSTRACT:

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 D_fH°(1) = 324.8 kJ mol¹ and D_fH°[1H]₊ = 426.0 kJ mol¹. The corresponding proton affinity value is PA(1) = 779.8 kJ mol¹, in perfect agreement with the experimental determination of 783.3 ± 3.8 kJ mol¹ obtained by the kinetic method. A gas-phase basicity value, GB(1), of 745748 kJ mol¹ 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 MP2/631G* level, reveals a route via a [CH₃CO]⁺ / water complex after an energy determining step involving a simultaneous 1,2-hydrogen migration and CO bond elongation. The critical energy of the reaction, evaluated at the G2(MP2,SVP) level, is 170 kJ mol¹ 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: D_fH°[CH₃CO]⁺ = 655.0 kJ mol¹, D_fH°[CH₂COH]⁺ = 833.0 kJ mol¹, D_fH°[c-CH₂CHO]⁺ = 886.2 kJ mol¹.

Keywords: glycolaldehyde, proton affinity, basicity, G2 calculations, dehydration of protonated glycolaldehyde

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