Understanding the rate of spin-forbidden thermolysis of HN3 and CH3N3

Abstract

The pyrolysis of the simplest azides HN(3) and CH(3)N(3) has been studied computationally. Nitrogen extrusion leads to the production of NH or CH(3)N. The azides have singlet ground states but the nitrenes CH(3)N and NH have triplet ground states. The competition between spin-allowed decomposition to the excited state singlet nitrenes and the spin-forbidden N(2) loss is explored using accurate electronic structure methods (CASSCF/cc-pVTZ and MR-AQCC/cc-pVTZ) as well as statistical rate theories. Nonadiabatic rate theories are used for the dissociation leading to the triplet nitrenes. For HN(3), (3)NH formation is predicted to dominate at low energy, and the calculated rate constant agrees very well with energy-resolved experimental measurements. Under thermal conditions, however, the singlet and triplet pathways are predicted to occur competitively, with the spin-allowed product increasingly favored at higher temperatures. For CH(3)N(3) thermolysis, spin-allowed dissociation to form (1)CH(3)N should largely dominate at all temperatures, with spin-forbidden formation of (3)CH(3)N almost negligible. Singlet methyl nitrene is very unstable and should rearrange to CH(2)NH immediately upon formation, and the latter species may lose H(2) competitively with vibrational cooling, depending on temperature and pressure.

Spin‐Forbidden Branching in the Mechanism of the Intrinsic Haber–Weiss Reaction

“…[17] According to the transition probability based on the Landau-Zener model appliedt os pinforbidden reactions, if, for instance, relativelyl ow velocities (in comparison with the spin-orbit coupling between the two spin states) are achieved,t he probability of spin conversion can be increased, as can be seen from Equation (1) of Ref. [17].Apreference for as pin-forbidden pathwayh as been observed previously for ar eaction between species containing light elements, [18] and many cases have been illustrated in gas-phase ion chemistry. [19] Ap otential energy calculation along the O···O bond distance of hydrogen peroxidea nd one of the O···OH bond angles showsaflat region involving 2 Int-1, 2 TS-2, and 2/4 MECP (see Figure 3).…”

“…. A preference for a spin‐forbidden pathway has been observed previously for a reaction between species containing light elements, and many cases have been illustrated in gas‐phase ion chemistry…”

Spin‐forbidden reactions: computational insight into mechanisms and kinetics

“…Instead the reactions proceed through two different MECPs, one en route to H 2 loss and one en route to hydrogen migration. Another exception is the concerted bond‐breaking and spin‐state change that appears to occur during decomposition of HN 3 , as mentioned above.…”

“…This can be computed using simple expressions such as Landau-Zener theory, 1 based upon calculations of the electronic coupling terms between the potential energy surfaces of different spin-typically, the spin-orbit coupling matrix element. This spin-orbit coupling matrix element has been computed for a number of systems, 23,29,41 including reactions of the FeO + ion mentioned earlier. 39,40 Finally, ρ*(E − E*)…”

“…29 Tests of this theory for a number of simple reactions suggest that it performs well in comparison with more complicated theoretical methods, such as trajectory surface hopping methods or nonadiabatic nuclear quantum dynamics. 29,41 Of course, as a statistical rate theory that is strongly related to transition state theory, it suffers from many of the same assumptions and approximations as the latter, and cannot be expected to yield exact results. Sensitivity tests, however, suggest that the magnitude of the typical errors arising from the different approximations is of the order of a factor of two for the rate constant.…”