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

Abstract

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 transitionstate properties (G3B3, HL1, E o = 84.6 kcal/mol) are used to construct a Rice-RamspergerKassel-Marcus (RRKM) model whose fit to the rate measurements suggests a E 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 E 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, P τ 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. C

C–H Bond Strengths and Acidities in Aromatic Systems: Effects of Nitrogen Incorporation in Mono-, Di-, and Triazines

“…There has been increasing interest in understanding the pyrolysis of nitrogen-containing heterocycles, especially five-membered (azole) and six-membered (azine) compounds. − These nitrogen-rich species are abundant in coal , and represent the fundamental unit of many high-energy-density materials. , Combustion of these compounds leads to the production of nitrogen oxide (NO x ) species, which play an important role in the formation and degradation of tropospheric ozone . A number of studies have focused on the pyrolysis of azine molecules containing one, two, and three nitrogen atoms (referred to as pyridine, diazines, and triazines, respectively). − One of the main goals of these studies was to determine the strength and reactivity of specific C–H bonds that were shown to be critical in the decomposition process. For example, the generally accepted mechanism for decomposition of pyridine is initiated by the cleavage of the weakest C–H bondthe bond adjacent to the nitrogen atom .…”

“…Experimental bond strength values provide data points with which theoreticians can compare their models. However, few experimental studies of gaseous azines exist, and they have been largely limited to shock tube experiments that rely on complex modeling of the data, resulting in fairly large error bars. ,,,− Moreover, to date there are no experimental determinations of the BDEs of 1,2-diazine or any triazines. The experimental shock tube results were later complemented by a comprehensive theoretical investigation by Barckholtz et al focused on computing the C–H and N–H BDEs of aromatic hydrocarbons .…”

Three-Body photodissociation of thionyl chloride

“…Numerous molecular systems became objects for the study of three-body photodissociation dynamics [10][11][12][13][14]. Among them, acetone [15,16] is probably the best-studies molecule.…”