Structure, reactivity and thermochemical properties of protonated lactic acid

• Published in: Journal of mass spectrometry. 2004-06, Vol.39 (6), p.613-620
• Main Authors: Berruyer-Penaud, Florence, Bouchoux, Guy, Payen, Olivier, Sablier, Michel
• Format: Article
• Language: English
• Subjects: Atomic and molecular physics; Bond strengths, dissociation energies; Chemical Sciences; Exact sciences and technology; gas-phase basicity; ion-neutral complexes; lactic acid; molecular orbital calculations; Molecular properties and interactions with photons; or physical chemistry; Physics; Properties of molecules and molecular ions; proton affinity; Theoretical and
• ISSN: 1076-5174; 1096-9888
• DOI: 10.1002/jms.627

The protonation energetics of lactic acid (LA) were experimentally determined by the kinetic method including the entropy effect. The values (proton affinity, PA(LA) = 817.4 ± 4.3 kJ mol−1; protonation entropy, ΔS°p(LA) = −2 ± 5 J K−1 mol−1; gas‐phase basicity, GB(LA) = 784.5 ± 4.5 kJ mol−1) agree satisfactorily with computed G2(MP2) expectations (PA(LA) = 811.8 kJ mol−1; ΔS°p(LA) = −7.1 J K−1 mol−1; GB(LA) = 777.4 kJ mol−1). The fragmentation behaviour of protonated lactic acid (LAH+) is dominated by carbon monoxide loss followed by elimination of a water molecule. Direct dehydration of LAH+ is only a high‐energy process hardly competitive with the CO loss. A complete mechanistic scheme, based on MP2/6–31G* calculations, is proposed; it involves isomerization of the various protonated forms of LA and the passage through the ion–neutral complex between the 2‐hydroxypropyl acylium cation and a water molecule. Copyright © 2004 John Wiley & Sons, Ltd.