THE DECARBONYLATION OF IONIZED BETA-HYDROXYPYRUVIC ACID - THE HYDROGEN-BRIDGED RADICAL-CATION [CH2=O-H-O=C-OH](.+) STUDIED BY EXPERIMENT AND THEORY

The intriguing gas-phase ion chemistry of beta-hydroxypyruvic acid (HP A), HOCH2C(=O)COOH, has been investigated using tandem mass spectromet ry (metastable ion (MI) and (multiple) collision-induced dissociation (CID) experiments, neutralization-reionization mass spectrometry (NRMS ), O-18 and D isotopic labelling on both the acid and its methyl ester ) in conjunction with computational chemistry (ab initio MO and densit y functional theories). HPA does not enolize upon evaporation, but it retains its keto structure. When ionized, decarbonylation occurs and, depending on the internal-energy content, this dissociation reaction p roceeds via two distinct routes. The source-generated, high-energy ion s lose the keto C=O, not via a least-motion extrusion into ionized gly colic acid, HOCH2COOH.+, but via a rearrangement that yields the title H-bridged radical cation CH2=O ... H ... O=C-OH.+ for which Delta H-f (0) = 99 +/- 3 kcal/mol. The long-lived low-energy ions enolize prior to decarbonylation and lose the carboxyl C=O. Again, this is not a lea st-motion extrusion (which would produce the most stable isomer, HOC(H )=C(OH)(2)(.+), Delta H-f(0) = 73 kcal/mol) but a rearrangement yieldi ng the ion-dipole complex HOC(H)C=C=O.+/H2O. The methyl ester of HPA b ehaves analogously, yielding CH2=O ... H ... O=C-OCH3.+ and HOC(H)C=C= O.+/CH3OH upon decarbonylation of the high-and low-energy ions, respec tively. Decarboxylation into the ylidion CH2OH2.+ characterizes the di ssociation chemistry of both the title H-bridged ion and its glycolic acid isomer HOCH2COOH.+. A computational analysis of this reaction (wh ich satisfies the experimental observations) leads to the proposal tha t the decarboxylation of the acid occurs via CH2-O(H)... H ... O=C=O. as the key intermediate, whereas the title H-bridged ion follows a hi gher energy route that involves ion-dipole rotations leading to the io nized carbene HO(H-2)CO-C-OH.+ and the distonic ion H2O-C(H-2)-O-C=O. as key intermediates.