Computational Study of the Isomerizations and Fragmentations of the Singlet and Triplet CH2CH2NH Diradicals

Pathways for the thermal unimolecular reactions of the CH2CH2NH diradicals, both singlet and triplet, have been investigated by ab initio MO calculations using 4-31G basis functions. The minimum-energy paths for possible isomerizations and fragmentations are traced by the SCF gradient procedure, and multireference double-excitation (MRD) CI calculations carried out for all the stationary structures located. The results of calculation show that the energetically most favorable unimolecular reaction possible to the triplet diradical is an α-hydrogen detachment giving rise to CH2=CH–NH (2A″), whereas that to the singlet diradical is a 1,2-hydrogen migration to give CH3CH=NH (1A′). The activation barrier heights calculated for the two processes are 103 and 74 kJ mol−1, respectively. It is concluded that, in the gas phase, CH3CN and HCN should be formed concurrently by the α-bond cleavages of the doublet radical CH3CH=N (2A′), which will arise most readily from the vibrationally hot CH3CH=NH (1A′).