Mt. Nguyen et al., ANOTHER LOOK AT THE DECOMPOSITION OF METHYL AZIDE AND METHANIMINE - HOW IS HCN FORMED, Journal of physical chemistry, 100(16), 1996, pp. 6499-6503
Ab initio molecular orbital calculations have been used to study the d
ecomposition of methyl azide (CH3N3), methanimine, and its isomers (CH
3N) in both lowest lying singlet and triplet states. Geometries were o
ptimized using UMP2/6-31G(d,p) level of theory while energies of the s
tationary points on potential energy surfaces were obtained from QCISD
(T) calculations with larger 6-311++G(d,p) and 6-311++G(3df,2p) basis
sets and corrected for zero-point energies. The temperature dependence
of the rate constants of various dissociative processes has also been
calculated using the conventional transition-state theory. While the
decomposition of methyl azide occurs, in the singlet state, through a
concerted motion of N-2 elimination with hydrogen shift, giving methan
imine, the triplet methyl azide does not exist as a discrete species b
ut falls apart, giving triplet methylnitrene plus N-2. Starting from s
inglet methanimine, 1,1-H-2 elimination giving HNC is found to be favo
red over 1,2-H-2 elimination giving HCN, a 1,2-H shift yielding aminoc
arbene, and N-H bond cleavage producing the H2CN radical. The hot HNC
molecule is expected to rearrange rapidly to HCN. From singlet aminoca
rbene (HCNH2), 1,2-H-2 loss giving HNC is also a less energy-demanding
step than the 1,2-H-2 loss, generating HCN. Overall, it appears that,
in the lowest singlet state, HCN is not directly formed upon fragment
ation of methanimine but rather from rearrangement of HNC which is the
primary product. In the triplet state, the HCN formation from either
methylnitrene or methanimine passes through successive losses of H ato
ms.