Gas chromatographic-mass spectrometric determination of a phenylcinnamic acid isomers: Practical and theoretical aspects

Abstract

The Perkin condensation of benzaldehyde and phenylacetic acid was performed in the presence of acetic anhydride and triethylamine. After silylation, the resulting mixture was analysed with a Hewlett-Packard gas chromatograph equipped with a mass-selective detector. Excellent baseline separation was achieved for the α-phenylcinnamic acid trimethyisilyl ester (2,3-diphenylpropenoic acid trimethylsilyl ester) isomers. The fragments were identical in the mass spectra of the isomers; however, the relative intensities differed appreciably. The identification of most peaks was straightforward; nevertheless, non-trivial fragments were also formed via bond scissions and subsequent rearrangements of the McLafferty type. Semi-empirical quantum chemical calculations were also performed and bond orders were determined for the fully optimized geometries of various silyl ester derivatives of the neutral molecules and of the parent ions. 'Rese were aimed at mapping the strength of bonds expected to break in the mass-selective detector. It was found that the bond orders in the isomers do not differ significantly, hence the fragmentation patterns would be essentially the same, irrespective of the silylating agent. Major cleavage routes could be predicted, however. Predictions based on calculations were verified experimentally.