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.