The role of charge‐site location in fragmenting ions. 1—[CH 3 CHXCOOCH 3 ] +˙ and [CH 2 XCH 2 COOCH 3 ] +˙ ions and the structures of daughter ions derived from the loss of X (X = I, Br, Cl and CH 3 )

The structures of the m /z 87, [C 4 H 7 O 2 ] + , ions generated by dissociative ionization of CH 3 CGXCOOCH 3 and XCH 2 CH 2 COOCH 3 (X = CH 3 , 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 = CH 3 produced, via H atom transfer, ions of structure \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH_2 = CH}\mathop {\rm C}\limits^{\rm + } \left({{\rm OH}} \right){\rm OCH}_{\rm 3} $\end{document} ( a ), Δ H f = 386 kj mol −1 . In marked contrast, loss of I ˙ from ionized CH 3 CHICOOCH 3 and ICH 2 CH 2 COOCH 3 proceeded without rearrangement to yield respectively ions of structure \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH_3}\mathop {\rm C}\limits^{\rm + } {{\rm HCOOCH_3}} $\end{document} ( b ), Δ H f = 480 kJ mol −1 and ( c ), Δ H f = 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 = CH 3 but at the halogen atom when X = I. The precursor molecules X = Cl and Br display both of the above characteristics, CH 3 CHXCOOCH 3 yielding mixtures of a and b and XCH 2 CH 2 COOCH 3 producing a and c ions.