Gas-phase reactivity of [arene—Cr]+adduct ions: Differentiation of disubstituted benzene isomers

Abstract

The gas-phase reactivity of isomeric [arene-Cr+] adduct ions was studied by tandem mass spectrometry. Ortho, meta and para-disubstituted benzene-chromium ions were generated by electron impact-induced loss of three CO groups from corresponding (ρ6-arene)tricarbonylchromium complexes. Six series of substituted toluene-chromium ions [RC6H4CH3-Cr]+ (R = Cl, F, OCH3, CO2CH3, COCH3 and CONH2) and a series of ethylanisoles [C2H5C6H4OCH3-Cr]+ were considered. Comparison of metastable decomposition and/or collision-induced dissociation spectra allows the characterization of the ortho isomer through specific fragmentations, often owing to the ability of Cr+ to form a bridge between the two substituents, and differentiation of the meta from the para isomer. The method described here relies on the abundance of a diagnostic fragment ion which is present in the spectrum of one isomer but is absent or shows different relative intensities in the spectra of the other isomers. Deuterium labelling was used to demonstrate some specific fragmentation mechanisms.