THE INTRIGUING BEHAVIOR OF (IONIZED) OXALACETIC ACID INVESTIGATED BY TANDEM MASS-SPECTROMETRY

Tandem mass spectrometry based experiments on commercial oxalacetic ac id (OAA) samples confirm the indirect evidence from the elegant study of Flint et al. (J. Org. Chem. 57 (1992) 7270) that solid OAA exists s olely in the (Z)-enol form of HOOCC(H)=C(OH)COOH. It is further shown that: (1) the samples contain a minor impurity in the low percent rang e, assigned as a dehydration product of 4-hydroxy-4-methyl-2-ketogluta ric acid; (2) careful evaporation of solid OAA samples yields mass spe ctra representative of the (Z)-enol form. The (E)-enol is not present in the solid, nor is its ion generated in the gas phase via isomerizat ion of the ionized (Z)-enol form. However, even under gentle sample in troduction conditions, a partial ketonization of the neutral (Z)-enol takes place. The resulting keto OAA molecules are either ionized intac t or decarboxylate to yield a mixture of alpha-hydroxyacrylic acid, CH 2=C(OH)COOH and its keto isomer pyruvic acid, CH3C(=O)COOH. From compu tational quantum chemistry (CBS-4) and experimental data, ionic enthal pies of formation of 95 and 109 kcal/mol were derived for CH2=C(OH)COO H.+ and CH3C(=O)COOH.+, respectively; and (3) under less controlled sa mple introduction conditions, thermal dehydration may take place to yi eld C4H2O4 molecules whose mass spectral characteristics are indisting uishable from those of ionized hydroxymaleic anhydride. (C) 1997 Elsev ier Science B.V.