Mass spectrometry of cycloheptatriene derivatives-II: Ring contraction of the molecular ion of 7-cyanocycloheptatriene. Participation of ring carbon in the loss of HCN (or HNC)

At an electron energy of 70 eV (nominal) both the [M]+·‐ion and the [MH]+‐ion of the title compound expel hydrogen cyanide. 13C labelling in the cyano group shows that these ions lose—within experimental error—only H13CN when decomposing in the ion source, i.e. when possessing a relatively high internal energy. In the first and second field free region, however, as well as in the ion source at low ionizing energies (9 eV, nominal), H12CN is also eliminated. This phenomenon may be explained by ring contraction of the molecular ion to a six membered ring, possibly initiated by the formation of a norcaradiene structure, which may then rearrange further to a species of higher symmetry. This ring contraction is supported by the occurrence of peaks of low intensity at m/e 78 and m/e 77, due to fragments which are generated in one step from the molecular ion.