ISOMERIZATION OF THE MOLECULAR ION OF CYCLOHEXYL CYANIDE TO AN IONIZED BICYCLOHEPTANE IMINE SPECIES COMPETING WITH DIRECT RING-OPENING

The mechanisms of the dissociation reactions of the molecular ion of c yclohexyl cyanide have been studied with nitrogen-15, carbon-13 and de uterium labelling in combination with tandem mass spectrometric experi ments. The most important unimolecular reactions of ionized cyclohexyl cyanide involve the losses of CH3., NH3, HCN, C3H3N and C4H7.. These reactions are all significant for the molecular ions dissociating in t he ion source, whereas the metastable molecular ions react only by the competing losses of CH3., NH3 and HCN. The results of the labelling e xperiments reveal that the carbon atoms at the 2- and 6-positions of t he ring become structurally indistinguishable from the carbon atoms at the 3- and 5-positions prior to the losses of CH3. and HCN. This find ing is suggested to be a result of a rearrangement of the molecular io n of cyclohexyl cyanide with formation of a symmetrical and ionized [2 ,2,1]-bicycloheptane-7-imine as the key intermediate. The elimination of C3H3N is indicated to involve a direct ring-opening of the molecula r ion of cyclohexyl cyanide followed by dissociation to yield a C4H8+. ion. The loss of a C4H7. radical may involve the same initial step fo llowed by an H-shift in the ring-opened species prior to dissociation. (C) 1997 Elsevier Science B.V.