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

The three‐body dissociation of 1,3,5‐triazine (s‐triazine, s‐C 3 H 3 N 3 → 3HCN) has been observed in incident shock waves with the laser‐schlieren technique. The experiments use 5% triazine/Kr and cover 1630–2350 K for 100–600 Torr. These experiments show dissociation rates with strong falloff and a slight but fully expected pressure dependence. The dissociation is without secondary reaction save for a possible, but rather unlikely, contribution from the isomerization HCN → HNC. Electronic structure calculations of the transition‐state properties (G3B3, HL1, E o = 84.6 kcal/mol) are used to construct a Rice–Ramsperger–Kassel–Marcus (RRKM) model whose fit to the rate measurements suggests a 〈ΔE〉 down of 1200 cm −1 . However, a seemingly better fit is achieved using the barrier of 81 kcal/mol proposed by Dyakov et al. (J. Phys. Chem. A 2007, 111, 9591–9599). With this barrier k ∞ (s −1 ) = 5.3 × 10 16 exp(−86.6(kcal/mol)/RT), and the fit now accepts the more routine 〈ΔE〉 down = 126(T/298) 0.9 . It seems the dissociation most likely occurs by a direct, one‐step, “triple” dissociation to 3HCN, although the present experiments cannot rule out a multistep process. Vibrational relaxation of the triazine was also examined in 5% and 20% mixtures with Kr over 770–1500 K for pressures between 6 and 14 Torr. Relaxation is very fast, with a slight inverse temperature dependence, Pτ rising from 100 to 200 ns‐atm over the full temperature range. Integrated gradients are in good accord with calculated total changes in density, indicating a single exponential relaxation. A separate investigation of relaxation in the related molecule pyrazine (500–1300 K, in 1% and 5% in Kr, between 13 and 66 Torr) is included. Again relaxation is rapid, but here the temperature dependence seems more normal, the relaxation times decreasing slightly with temperature. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 211–220, 2010