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 [XC 6 H 4 CH=CHCONH 2 , X = H, F, Cl, Br, I, CH 3 , CH 3 O, CF 3 , NO 2 , CH 3 CH 2 , (CH 3 ) 2 CH and (CH 3 ) 3 C; and XYC 6 H 3 CH=CHCONH 2 , 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 XYC 6 H 3 CH=CHCONH 2 : 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 2 results in the formation of a substituted cinnamoyl cation, [XC 6 H 4 CH=CHCO] + or [XYC 6 H 3 CH=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 3 · by cleavage of a 4-alkyl group to give a benzylic cation, [R 1 R 2 CC 6 H 4 CH=CHCONH 2 ] + (R 1 , R 2 = H, CH 3 ).