Proximity effects in the electron ionisation mass spectra of substituted cinnamamides.

The electron ionisation mass spectra of an extensive set of 53 ionised monosubstituted and disubstituted cinnamamides [XCHCH=CHCONH, X = H, F, Cl, Br, I, CH, CHO, CF, NO, CHCH, (CH)CH and (CH)C; and XYCHCH=CHCONH, X = Y = Cl; and X, Y = F, Cl or Br] are reported and discussed. Particular attention is paid to the significance of loss of the substituent, X, from the 2-position, via a rearrangement that is sometimes known as a proximity effect, which has been reported for a range of radical-cations, but is shown in this work to be especially important for ionised cinnamamides. When X is in the 2-position of the aromatic ring, [M - X] is formed to a far greater extent than [M - H]; in contrast, when X is in the 3-position or 4-position, [M - H] is generally much more important than [M - X]. Parallel trends are found in the spectra of XYCHCH=CHCONH: the signal for [M - X] dominates that for [M - Y] when X is in the 2-position and Y in the 4-position or 5-position, irrespective of the nature of X and Y. Further insight is obtained by studying the competition between expulsion of X and alternative fragmentations that may be described as simple cleavages. Loss of NH results in the formation of a substituted cinnamoyl cation, [XCHCH=CHCO] or [XYCHCH=CHCO]; this process competes far less effectively with the proximity effect when X is in the 2-position than when it is in the 3-position or 4-position. Additional information has been obtained by investigating the competition between formation of [M - H] by the proximity effect and loss of CH by cleavage of a 4-alkyl group to give a benzylic cation, [RRCCHCH=CHCONH] (R, R = H, CH).