Both studies 1,2 agree qualitatively in the estimates of the various critical energies separating each isomer and the dissociation products. However discrepancies of no less than 50 kJ/mol were apparent for some elementary steps, particularly hydrogen atom migrations. Furthermore, conflicting conclusions were drawn concerning the nature of the energy determining step for dissociation 3 of 1 Á . To clarify these questions, in view of the moderate size of this system, it was of interest to reinvestigate the energetic of reactions of Scheme 1 using a more sophisticated theoretical method such as the G2 theory. 4 COMPUTATIONAL SECTIONIt seems well established that accurate heats of formation of open-shell systems can be attained at the G2 level of theory 4a for species with low spin contamination. 5 Standard G2 theory employs a geometry optimized at the MP2(full)/ 6-31G(d) level and a scaled HF/6-31G(d) ZPE. A base energy calculated at the MP4/6-311G(d,p) level is corrected by several additivity approximations to QCISD(T) and to 6-311G(3df,2p) basis set. In an attempt to account for residual basis set deficiencies, G2 theory introduces higherlevel corrections (HLC) that depends on the number of paired and unpaired electrons. In the more economical G2(MP2) approach, the basis set-extension energy corrections are obtained at the 2nd order Molle-Plesset level.

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.

The gas-phase basicities of a representative set of hydroxy- and methoxycarbonyl compounds (hydroxyacetone, 1, 3-hydroxybutanone, 2, 3-hydroxy-3-methylbutanone, 3, 1-hydroxy-2-butanone, 4, 4-hydroxy-2-butanone, 5, 5-hydroxy-2-pentanone, 6, methoxyacetone, 7, 3-methoxy-2-butanone, 8, 4-methoxy-2-butanone, 9, and 5-methoxy-2-pentanone, 10) were experimentally determined by the equilibrium method using Fourier transform ion cyclotron resonance and high-pressure mass spectrometry techniques. The latter method allows the measurement of proton transfer equilibrium constants at various temperatures and thus the estimate of both the proton affinities and the protonation entropies of the relevant species. Quantum chemical calculations at the G3 and the B3LYP/6-311+G(3df,2p)//6-31G(d) levels of theory were undertaken in order to find the most stable structures of the neutrals 1-10 and their protonated forms. Conformational and vibrational analyses have been done with the aim of obtaining a theoretical estimate of the protonation entropies.