Experimental and theoretical studies of the unimolecular decomposition of nitrosobenzene: High-pressure rate constants and the C-N bond strength

The unimolecular decomposition of nitrosobenzene has been studied at 553-648 K with and without added NO under atmospheric pressure. Kinetic modeling of the measured C₆H₅NO decay rates by including the rapid reverse reaction and minor secondary processes yielded the high-pressure first-order rate constant for the decomposition C₆H₅NO → C₆H₅ + NO (1), k^∞ ₁ = (1.42 ± 0.13) × 10¹⁷ exp [-(55 060 ± 1080)/RT] s^-1, where the activation energy is given in units of cal/mol. With the thermodynamics third-law method, employing the values of k^∞ ₁ and those of the reverse rate constant measured in our earlier study by the cavity ring-down technique between 298 and 500 K, we obtained the C-N bond dissociation energy, D^○ ₀ (C₆H₅-NO) = 54.2 kcal/mol at 0 K, with an estimated error of ±0.5 kcal/mol. This new, larger bond dissociation energy is fully consistent with the quantum mechanically predicted value of 53.8-55.4 kcal/mol using a modified Gaussian-2 method. Our high-pressure rate constant was shown to be consistent with those reported recently by Horn et al. (ref 13) for both forward and reverse reactions after proper correction for the pressure falloff effect.