The role of charge‐site location in fragmenting ions. 1—[CH3 CHXCOOCH3]+˙ and [CH2XCH2 COOCH3]+˙ ions and the structures of daughter ions derived from the loss of X (X = I, Br, Cl and CH3)

The structures of the m/z 87, [C4H7O2]+, ions generated by dissociative ionization of CH3CGXCOOCH3 and XCH2CH2COOCH3 (X = CH3, Cl, Br, and I) have been investigated via their unimolecular and collisionally activated fragmentations and by apperance energy measurements. For both precursors loss of X = CH3 produced, via H atom transfer, ions of structure {article}{empty}{document}CH₂ = CH C⁺ ( OH ) OCH₃{document} (a), ΔHf = 386 kj mol−1. In marked contrast, loss of I˙ from ionized CH3CHICOOCH3 and ICH2CH2COOCH3 proceeded without rearrangement to yield respectively ions of structure {article}{empty}{document}CH₃ C⁺ HCOOCH₃{document} (b), ΔHf = 480 kJ mol−1 and (c), ΔHf = 450 kJ mol−1. These different fragmentation behaviours are explained via photoelectron spectra which show that the formal charge site in the precursor ion is at the carbonyl oxygen when X = CH3 but at the halogen atom when X = I. The precursor molecules X = Cl and Br display both of the above characteristics, CH3CHXCOOCH3 yielding mixtures of a and b and XCH2CH2COOCH3 producing a and c ions.