Collision of molecular anions of benzenedicarboxylic esters with oxygen ina triple quadrupole mass spectrometer

A series of symmetrical phthalate, isophthalate, and terephthalate ester mo lecular anions were reacted with oxygen in the collision cell of a triple q uadrupole mass spectrometer to produce characteristic [M - R](-) fragments that can be used to identify these compounds. The [M - R](-)/M-. intensity ratios decreased for homologous esters in the following order: phthalates > isophthalates much greater than terephthalates. Based on the [M - R](-) io n intensities for different alkyl substituents, on O-18(2) labeling experim ents, and on the reactivity of bis(t-butylcyclohexyl)phthalates, it was con cluded that M-R anions are generated through an S(N)2 nucleophilic displace ment at the alpha carbon of the saturated alkyl substituent. For the phenyl ester, the reaction proceeds through attack at the carbonyl carbon. (C) 19 99 American Society for Mass Spectrometry.