Ab Initio Study of Reactions of sym-Triazine

Abstract

Ab initio calculations were performed to investigate reaction mechanisms for formation and decomposition of the six-membered ring C3N3H3, known as sym-triazine. MP2 geometry optimizations with QCISD(T) energy refinements for critical points on the potential energy surface were calculated with the 6-31G**, 6-311++G** and cc-pVTZ basis sets. Good agreement is found for MP2 geometries and frequencies of sym-triazine and HCN when compared with the corresponding experimental values. Two decomposition mechanisms of sym-triazine, the concerted triple dissociation (sym-triazine → 3 HCN) and the stepwise decomposition (sym-triazine → H2C2N2 + HCN → 3 HCN) were investigated. All calculations show that the lowest energy decomposition mechanism is the concerted triple dissociation. Our best calculations predict the zero-point-energy-corrected barrier to decomposition to be 81.2 kcal/mol. The calculated reaction enthalpy is 35.5 kcal/mol, 7.7 kcal/mol lower than experiment. Intrinsic reaction coordinate calculations leading from the transition state of the concerted triple dissociation reaction to three HCN molecules led to a minimum on the potential energy surface. The corresponding structure is a cyclic (HCN)3 cluster. The temperature-corrected formation enthalpy of the cluster is −8.7 kcal/mol relative to three isolated HCN molecules. The zero-point-corrected barrier to formation of sym-triazine from the cluster is 58.1 kcal/mol. QCISD(T) energy refinements did not differ significantly from the MP2 results.

Shock tube study of 1,3,5‐triazine dissociation and relaxation and relaxation of pyrazine

“…A small contribution to the density gradient from HCN isomerization may occur at the highest temperatures, but the absence of such effect in direct HCN experiments 48 and the need for an unreasonably large rate here would seem to rule this out. The best agreement between RRKM calculations and experiment is with a barrier of 81 kcal/mol as was suggested in the calculations of Pai et al 13 and Dyakov et al 15, with a rather routine 〈Δ E 〉 down = 126( T /298) 0.9 .…”

“…This last mechanism is supported by the computational studies of Politzer and coworkers 8 as well as the classical dynamics calculations of Sewell and Thompson 9. There is experimental and theoretical evidence that the aromatics 1,3,5‐triazine (s‐triazine) 10–16 and s‐tetrazine 17–20 also undergo three‐body unimolecular decomposition when they are highly vibrationally excited on the ground electronic surface, as they are in a thermal reaction. However, the dissociation of the infamous substituted triazine, melamine, 2,4,6‐triamino‐1,3,5‐triazine, evidently follows a radical path 21.…”

“…10 indicates a high 〈Δ E 〉 down of 1200 cm −1 . However, both Dyakov et al 30 and Pai et al 13 have proposed a lower barrier of 81 kcal/mol: Using this, a new RRKM calculation was attempted altering only the barrier and 〈Δ E 〉 down , and this is compared to experiment in Fig. 11, where the fit is seen to be slightly improved.…”

“…Such experiments should provide a good barrier and may well suggest a choice between various calculations. For this, we have chosen s‐triazine, where it seems the triple‐product channel is well established for the thermal reaction 10–16. In any case, we note that any possible contribution from the chain reaction, as might be initiated by CH fission, can be clearly detected as acceleration with the laser‐schlieren technique, as was seen in the studied one‐ and two‐nitrogen heterocycles 23–26.…”

“…The three‐body dissociation of s‐triazine (1) is believed to be the dominant channel 10–16, But a two‐step path has also been considered 13,16: In any case, HCN is the sole product of this dissociation and laser‐schlieren (LS) experiments cannot distinguish between these routes as long as the dimer dissociation is fast. Note that the result of s‐triazine dissociation would presumably be HCN not HNC.…”