CBS-4 ab initio results on the formation and stability of N-aminoazetidine radical cation and derivatives

Experimental evidence has recently indicated the role of substituted azetidine radical cations in the electron ionization mass spectrometric fragmentation of N,N-dimethylhydrazone derivatives of 5- and 6-unsaturated straight-chain aldehydes. Although the 2+2 cycloaddition of ethylene and methyleneamine to form neutral azetidine has a very high energy barrier and is only mildly exothermic, CBS-4 ab initio calculations show that formation of the proposed radical cation intermediate is strongly exothermic with a low barrier. Further calculations show that the general energetics remain essentially unchanged even when substituents of the type present in the mass spectrometric studies are included. These calculations demonstrate that the proposed new mass spectral fragmentation mechanism is both thermodynamically and kinetically feasible.