Proximity effects in the electron ionisation mass spectra of substituted cinnamamides

Abstract

The electron ionisation mass spectra of an extensive set of 53 ionised monosubstituted and disubstituted cinnamamides [XC₆H₄CH=CHCONH₂, X = H, F, Cl, Br, I, CH₃, CH₃O, CF₃, NO₂, CH₃CH₂, (CH₃)₂CH and (CH₃)₃C; and XYC₆H₃CH=CHCONH₂, 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 XYC₆H₃CH=CHCONH₂: 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 ^·NH₂ results in the formation of a substituted cinnamoyl cation, [XC₆H₄CH=CHCO]⁺ or [XYC₆H₃CH=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 CH₃^· by cleavage of a 4-alkyl group to give a benzylic cation, [R¹R²CC₆H₄CH=CHCONH₂]⁺ (R¹, R² = H, CH₃).

Keywords: Electron ionisation; cinnamamides; cyclisation; isomerisation; proximity effects; simple cleavage; substituent effects.