Kinetics of association and dissociation in a weakly bound system: NO + NO2⇌ N2O3

Abstract

Extensive kinetic measurements have been made on the system, NO + NO₂ N₂O₃(1, –1), Equilibrium mixtures of nitrogen oxides containing an excess of NO at 208 ± 5 K were perturbed by flash photolysis and the rate of relaxation back to equilibrium was monitored by observing the transmittance of a line from a CO c.w. laser selected to coincide with the ν₁ band of N₂O₃. Measurements were made through the following pressure ranges (in Torr) of third-body gases: He (190–6970), Ne (248–6970), Ar (190–500), N₂(166–401) and CF₄(132–441). The results have been fitted to fall-off curves constructed according to methods developed by Troe yielding the following high- and low-pressure limiting rate constants: (k₁^∞/10^–12 cm³ molecule^–1 s^–1)= 3.4 ± 1; (k_1,M°/10^–33 cm³ molecule^–1 s^–1)= 4.2_–0.2^+0.3(M = He), 5.0_–0.3^+0.45(Ne), 6.7_–0.5^+0.6(Ar), 9.1_–0.6^+0.8(N₂) and 13.1_–0.9^+1.1(CF₄). Despite the weakness of the N—N bond, it appears that the kinetics are well represented by the rate theories formulated on the basis of rapid and complete randomisation of internal energy. This finding is discussed in the light of information (a) provided by high-resolution spectra of the ν₁, 2ν₁, 3ν₁ and 4ν₁ bands of N₂O₃ and (b) from measurements on the vibrational relaxation of NO(v= 1) by NO₂.