An unusual pathway for the elimination of hydrogen cyanide from benzonitrile upon electron impact as revealed by 13C and 15N labelling

It is shown by ¹⁵N and specific ¹³C labelling that ～50% of the molecules of hydrogen cyanide, eliminated within ～10^?6 s upon electron impact of benzonitrile, contains the original cyano carbon atom, whereas the remaining percentage contains one of the phenyl ring carbon atoms at random. This is even more dramatic for the molecular ions of benzonitrile which decompose in the first and second field-free regions of the VG Micromass ZAB-2F high-field mass spectrometer used. Then only 5–7% of the eliminated molecules of hydrogen cyanide contains the original cyano carbon atom. A cycloaddition-cycloreversion process in the molecular ions, leading to ionized 1-cyano-1,3-hexadien-5-yne as an intermediate in the hydrogen cyanide loss, is proposed to explain this.