Very‐low‐pressure pyrolysis of nitroso‐ and pentafluoronitrosobenzene CNO bond dissociation energies

Abstract

The high-pressure absolute rate constants for the decomposition of nitrosobenzene and pentafluoronitrosobenzene were determined using the very-low-pressure pyrolysis (VLPP) technique. Bond dissociation energies of DHo(C6Hs-NO) = 51.5 f 1 kcal/mole and DH" (C&s-NO) = 50.5 f 1 kcal/mole could be deduced if the radical combination rate constant is set at log k,(M-'.sec-') = 10.0 f 0.5 for both systems and the activation energy for combination is taken as 0 kcal/mole at 298'K. AH,O(CF,H~NO), AHjo(C+jF~NO), and Aff/'(C6F5') could be estimated from our kinetic data and group additivity. The values are 48.1 =t 1, -160 f 2, and -130.9 Z!Z 2 kcal/mole, respectively. C-X bond dissociation energies of several perfluorinated phenyl compounds, DHo(CsF5-X), were obtained from the reported values of AHjo(C6F&) and our estimated AHjo(C&5.) [X = H, CHB, NO, Cl, F. CFa, I, and OH].

Experimental and Theoretical Studies of the Unimolecular Decomposition of Nitrosobenzene: High-Pressure Rate Constants and the C−N Bond Strength

“…Recently, the unimolecular decomposition of C 6 H 5 NO was studied by Horn et al in shock waves in the 800−1000 K temperature range at pressures between 0.9 and 4.4 atm. Their first-order rate constant obtained near 2 atm can be represented by k 1 = 10 15.29±0.53 e -(24720±1110) / T s -1 , which appears to be in accord with the extrapolated result of Choo and co-workers have also measured the rate constant for the association of C 6 H 5 with NO near 10 Torr in the 420−820 K range using a fast-flow reactor under pseudo-first-order conditions.…”

“…The reaction mechanism used for kinetic modeling is presented in Table . The Arrhenius plot of the modeled unimolecular decomposition rate constants is presented in Figure and compared with previously reported data. , A weighted least-squares analysis of our data covering the 553−648 K temperature range gave rise to the expression = (1.42 ± 0.13) × 10 17 exp[−(55 060 ± 1080)/ RT ] s -1 , where R = 1.987 cal/(mol K).

2 Arrhenius plot of nitrosobenzene unimolecular decomposition reaction: (▿) ref ; (▵) this work with NO added; (○) this work without NO added; (dotted line) ref ; (solid line) this work according to eq II.

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“…For providing a more accurate prediction of C 6 H 5 concentration, we have carried out a series of measurements for the rate constant of the C 6 H 5 NO decomposition reaction by Fourier transform infrared (FTIR) spectrometry. The study was performed under atmospheric conditions, and its result reveals that the activation energy for the decomposition reaction is higher by as much as 6 kcal/mol than those reported earlier. , In order to confirm this surprising finding, we performed a high-level ab initio molecular orbital (MO) calculation, employing the Gaussian-2 method (G2M) recently developed by Mebel and co-workers . The experimental and theoretical results are presented below.…”

“…Arrhenius plot of nitrosobenzene unimolecular decomposition reaction: (▿) ref ; (▵) this work with NO added; (○) this work without NO added; (dotted line) ref ; (solid line) this work according to eq II.…”

“…The rate constant for the unimolecular decomposition of nitrosobenzene, C 6 H 5 NO → C 6 H 5 + NO (1), was first measured by Choo et al. employing the very-low-pressure pyrolysis (VLPP) method . Their extrapolated high-pressure rate constant by means of the Rice−Ramsperger−Kassel−Marcus (RRKM) theory, = 2.5 × 10 15 exp(−24 660/ T ) s -1 , has provided the widely accepted C−N bond dissociation energy D ° 298 (C 6 H 5 −NO) = 51 ± 1 kcal/mol …”