FALLOFF BEHAVIOR IN THE THERMAL-DISSOCIATION RATE OF N2O

The striking difference in falloff behavior (pressure dependence) observed between N2O and Ar as collision partners in the thermal dissociation rate of N2O is noted. A mechanism proposed by Lindars and Hinshelwood [Proc. R. Sec. London, Ser. A 1955, A231, 178] involves transitions, both unimolecular and collision-induced, from the ground singlet electronic state to a dissociating triplet state. The mechanism was fit to a wide range of experimental data for neat N2O and N2O-Ar mixtures (273 individual data points), accounting quantitatively for the observed behavior [0.066 average rms deviation in In(k)]. Results derived for the limiting high- and low-pressure (M = Ar) rate constants exceed current recommended values by factors of similar to 1.4 and similar to 2, respectively, over the temperature range 925-2000 K. An alternative mechanistic interpretation involving a finite rate for intramolecular vibrational relaxation is discussed.