Acid-base thermochemistry of gaseous aliphatic α-aminoacidsElectronic supplementary information (ESI) available: B3LYP/6-31G(d) H°0, G3MP2B3 H°0, H°298 and G°298 values of the complete set of investigated conformers are given in Tables S1(Glycine), S2(Alanine), S3(Valine), S4(Leucine), S5(Isoleucine) and S6(Proline). Geometries of the most stable conformers of neutral, protonated and deprotonated aliphatic aminoacids are illustrated in Fig. S1 to S6. See DOI: 10.1039/c0cp00775g

Acid-base thermochemistry of isolated aliphatic amino acids (denoted AAA ): glycine, alanine, valine, leucine, isoleucine and proline has been examined theoretically by quantum chemical computations at the G3MP2B3 level. Conformational analysis on neutral, protonated and deprotonated species has been used to identify the lowest energy conformers and to estimate the population of conformers expected to be present at thermal equilibrium at 298 K. Comparison of the G3MP2B3 theoretical proton affinities, PA, and Δ H acid with experimental results is shown to be correct if experimental thermochemistry is re-evaluated and adapted to the most recent acidity-basicity scales. From this point of view, a set of evaluated proton affinities of 887, 902, 915, 916, 919 and 941 kJ mol −1 , and a set of evaluated Δ H acid of 1433, 1430, 1423, 1423, 1422 and 1426 kJ mol −1 , is proposed for glycine, alanine, valine, leucine, isoleucine and proline, respectively. Correlations with structural parameters (Taft's σ α polarizability parameter and molecular size) suggest that polarizability of the side chain is the major origin of the increase in PA and decrease in Δ H acid along the homologous series glycine, alanine, valine and leucine/isoleucine. Heats of formation of gaseous species AAA , AAAH + and [AAA−H] − were computed at the G3MP2B3 level. The present study provides previously unavailable Δ f H ° 298 for the ionized species AAAH + and [AAA−H] − . Comparison with Benson's estimate, and correlation with molecular size, show that several experimental Δ f H ° 298 values of neutral or gaseous AAA might be erroneous. G3MP2B3 calculations on glycine, alanine, valine, leucine, isoleucine and proline provide new and accurate gas phase acidity and basicity values.