Isomerization of the molecular ion of cyclohexyl cyanide to an ionized bicycloheptane imine species competing with direct ring-opening

Abstract

The mechanisms of the dissociation reactions of the molecular ion of cyclohexyl cyanide have been studied with nitrogen-15, carbon-13 and deuterium labelling in combination with tandem mass spectrometric experiments. The most important unimolecular reactions of ionized cyclohexyl cyanide involve the losses of CH • 3, NH 3, HCN, C 3H 3N and C 4H • 7. These reactions are all significant for the molecular ions dissociating in the ion source, whereas the metastable molecular ions react only by the competing losses of CH • 3, NH 3 and HCN. The results of the labelling experiments reveal that the carbon atoms at the 2- and 6-positions of the ring become structurally indistinguishable from the carbon atoms at the 3- and 5-positions prior to the losses of CH • 3 and HCN. This finding is suggested to be a result of a rearrangement of the molecular ion of cyclohexyl cyanide with formation of a symmetrical and ionized [2,2,1]-bicycloheptane-7-imine as the key intermediate. The elimination of C 3H 3N is indicated to involve a direct ring-opening of the molecular ion of cyclohexyl cyanide followed by dissociation to yield a C 4H +· 8 ion. The loss of a C 4H •; radical may involve the same initial step followed by an H-shift in the ring-opened species prior to dissociation.