Pressure and Temperature Dependence of the Recombination ofp-Fluorobenzyl Radicals

Abstract

The rate constants of the recombination reaction of p-fluorobenzyl radicals, p-F-C6H4CH2 + p-F-C6H4CH2 (+M) --> C14H12F2 (+M), have been measured over the pressure range 0.2-800 bar and the temperature range 255-420 K. Helium, argon, and CO2 were employed as bath gases (M). At pressures below 0.9 bar in Ar and CO2, and 40 bar in He, the rate constant k1 showed no dependence on the pressure and the nature of the bath gas, clearly indicating that it had reached the limiting high-pressure value of the energy-transfer (ET) mechanism (k(1,infinity)ET). A value of k(1,infinity)ET(T) = (4.3 +/- 0.5) x 10(-11) (T/300 K)(-0.2) cm3 molecule(-1) s(-1) was determined. At pressures above about 5 bar, the k1 values in Ar and CO2 were found to gradually increase in a pressure range where according to energy-transfer mechanism, they should remain at the constant value k(1,infinity)ET. The enhancement of the recombination rate constant beyond the value k(1,infinity)ET increased in the order He < Ar < CO2, and it became more pronounced with decreasing temperature. The dependences of k1 on pressure, temperature, and the bath gas were similar to previous observations in the recombination of benzyl radicals. The effect of fluorine-substitution of the benzyl ring on k1 values is discussed. The present results confirm the significant role of radical complexes in the recombination kinetics of benzyl-type radicals in the gas-liquid transition range. The observations on a rate enhancement beyond the experimental value of k(1,infinity)ET at elevated densities up to the onset of diffusion-control are consistently explained by the kinetic contribution of a "radical-complex" mechanism which is solely based on standard van der Waals interaction between radicals and bath gases.

Pressure Effects on the Relaxation of an Excited Ethane Molecule in High-Pressure Bath Gases

“…In contrast, E nrmvib is characterized by one initial decay rate (approximately comparable to the slow E nrmrot rate) with a nonlinear pressure dependence for Ar at 300 K but a linear pressure dependence for the three bath gases at 800 K. However, if Ar at 300 K had been plotted over the reduced density range of the 800 K bath gases, the nonlinear behavior of initial decay would not have been evident. The results for ethane in a 300 K Ar bath are quite similar to previous results for nitromethane in a 300 K Ar bath. , The work presented here implies that a change in the density of the bath gas, by changing its pressure and/or temperature, can be used to control molecular vibrational energy relaxation and thus reaction rates and mechanisms, beyond the IBC approximation …”

“…53,54 The work presented here implies that a change in the density of the bath gas, by changing its pressure and/or temperature, can be used to control molecular vibrational energy relaxation and thus reaction rates and mechanisms, beyond the IBC approximation. 62 The result of a linear pressure dependence for the initial rotational relaxation rate at 300 K but an irregular, nonlinear pressure dependence of the same rate at 800 K was unexpected. To understand this result, we elaborated on a statistical model previously developed in our study of CH 3 NO 2 relaxation.…”

Experimental and Modeling Study of the Temperature and Pressure Dependence of the Reaction C2H5 + O2 (+ M) → C2H5O2 (+ M)

“…In doing this we were aware of the possibility that the reaction mechanism may change, becoming a superposition of the radical-complex (RC) and the “normal” energy transfer (ET) mechanism. High pressure experiments on other reactions have provided evidence for the onset of a RC mechanism, for the reaction CH 3 + O 2 → CH 3 O 2 in Ar near 300 bar, for the reactions 2C 7 H 7 → C 14 H 14 and 2C 7 H 6 F → C 14 H 12 F 2 in Ar near 10 bar. − Reaction might fall in between, such that studies of the high pressure end of the falloff curve of reaction may provide information on the RC contribution as well.…”

Radical probe system for in situ measurements of radical densities of hydrogen, oxygen, and nitrogen

“…The underlying reason is due to the fact that recombination of radicals on the surface depends on numerous factors such as temperature, pressure, impurities present, chamber size, surface morphology and coating thickness. Such factors play key roles in energy transfer mechanisms of association, dissociation and collisional energy transfer in low pressure regimes [24]. Impurities poison catalytic surface and probe response to the temperature charges decreases over time.…”