Gas Phase Kinetics and Equilibrium of Allyl Radical Reactions with NO and NO₂

Abstract

Allyl radical reactions with NO and NO 2 were studied in direct, time-resolved experiments in a temperature controlled tubular flow reactor connected to a laser photolysis/photoionization mass spectrometer (LP-PIMS). In the C 3 H 5 + NO reaction 1, a dependence on the bath gas density was observed in the determined rate coefficients and pressure falloff parametrizations were performed. The obtained rate coefficients vary between 0.30− 14.2 × 10 −12 cm 3 s −1 (T = 188−363 K, p = 0.39−23.78 Torr He) and possess a negative temperature dependence. The rate coefficients of the C 3 H 5 + NO 2 reaction 2 did not show a dependence on the bath gas density in the range used (p = 0.47−3.38 Torr, T = 201−363 K), and they can be expressed as a function of temperature with k(C 3 H 5 + NO 2 ) = (3.97 ± 0.84) × 10 −11 × (T/300 K) −1.55±0.05 cm 3 s −1 . In the C 3 H 5 + NO reaction, above 410 K the observed C 3 H 5 radical signal did not decay to the signal background, indicating equilibrium between C 3 H 5 + NO and C 3 H 5 NO. This allowed the C 3 H 5 + NO ⇄ C 3 H 5 NO equilibrium to be studied and the equilibrium constants of the reaction between 414 and 500 K to be determined. With the standard second-and third-law analysis, the enthalpy and entropy of the C 3 H 5 + NO ⇄ C 3 H 5 NO reaction were obtained. Combined with the calculated standard entropy of reaction (ΔS°2 98 = 137.2 J mol −1 K −1 ), the third-law analysis resulted in ΔH°2 98 = 102.4 ± 3.2 kJ mol −1 for the C 3 H 5 −NO bond dissociation enthalpy. ■ INTRODUCTIONReactive hydrocarbon free radicals and nitrogen oxides (NO x = NO + NO 2 ) are produced in several common environments. Energy released by burning of hydrocarbons creates a variety of reactive intermediates, including unsaturated alkenyl radicals, and combustion under atmospheric conditions leads to the formation of nitrogen oxides. 1,2 Oxides of nitrogen are primary anthropogenic pollutants but also have natural sources in the atmosphere. 3,4 The principles governing mutual reactions of alkenyl radicals and NO x are important for the understanding of hydrocarbon oxidation mechanisms and optimization of combustion processes.Allyl radical (C 3 H 5 ) is the simplest conjugated, resonancestabilized alkenyl radical. Alkenyl radicals are formed in hydrogen abstraction reactions by reactive species (e.g., OH or other radicals) from carbon atoms at the β-position to the double bond in alkenes and by pyrolysis of larger hydrocarbons at elevated temperatures. 5−7 Small unsaturated hydrocarbon radicals with resonance-stabilized structures are thermodynamically more stable than similar saturated radicals lacking resonance stabilization. Consequently, they reach higher concentrations under combustion conditions and play a role in the molecular weight growth chemistry. They are identified as precursors for polycyclic aromatic hydrocarbons (PAHs) and, subsequently, for soot formation. 8−10 In the present work, two reactions of allyl radical with nitrogen oxides were investigated:(1)Both reactions have been stu...

Kinetics of Several Oxygen-Containing Carbon-Centered Free Radical Reactions with Nitric Oxide

“…This type of result has been obtained for a few other R + NO reactions measured in our setup, and the radical decay has been returned to "a normal type of behavior" (i.e., R decay to the prephotolysis background; Figure 2) by using lower ionization energies. The nitroso compounds are generally expected to have relatively low R−NO bond energies, which may result in an equilibrium in the reaction (i.e., R + NO ↔ RNO) at correspondingly low temperatures, 3,23 though for the saturated 1). The density change of about a factor of 3 increases the reaction rate coefficient roughly about 50%.…”

Kinetics of the Reaction of the Cyclopentadienyl Radical with Nitrogen Dioxide

“…In particular, the strong decrease in k 1 between 400 and 800 K, by a factor of ca. 7, corresponds to an effective large negative activation energy of −13 kJ mol –1 , much larger than −3.4 and −2.4 kJ mol –1 for the reactions of the C 3 H 5 and C 3 H 3 delocalized radicals with NO 2 , respectively. , The model explains this dramatic decrease in k 1 via the switch from the fast addition mostly to the N site of NO 2 at low temperatures () to the slower addition to the O sites at higher temperatures (resulting in channels –), where channel is unimportant because of the falloff.…”

“…7, corresponds to an effective large negative activation energy of −13 kJ mol −1 , much larger than −3.4 and −2.4 kJ mol −1 for the reactions of the C 3 H 5 and C 3 H 3 delocalized radicals with NO 2 , respectively. 40,41 The model explains this dramatic decrease in k 1 via the switch from the fast addition mostly the N site of NO 2 at low temperatures (1a) to the slower addition to the O sites at higher temperatures (resulting in channels 1b−1d), where channel 1a is unimportant because of the falloff. In a reversible reaction of addition, under certain conditions, one can generally expect to see a nonexponential kinetics of reactants decay corresponding to relaxation to equilibrium.…”

“…Both reactions exhibit negative temperature dependences of the rate constants and the absence of a pressure dependence at the room temperature. Experimental data of these studies do not allow us to distinguish the channels of addition of the respective radicals to the N or one of the O sites of NO 2 ; therefore, the only analogy one can derive from refs − is that the overall radical + NO 2 reaction rate constant can be expected to have negative temperature dependence. In the absence of any channel-specific data, the following expressions were selected for the high-pressure-limit rate constants of the addition of cyclopentadienyl radical to the two sites of NO 2 : Here, reaction numbers and are assigned to addition to the N and the O sites of NO 2 to form initial adducts, to avoid any confusion with the notations used for reactions –, which refer to the more complex processes of addition, stabilization, and chemically activated isomerization and decomposition: …”

“…No experimental or theoretical information is available on these high-pressure-limit rate constants. The only two cognate reactions studied experimentally are those of NO 2 with the allyl (C 3 H 5 ) , and the propargyl (C 3 H 3 ) delocalized radicals. In both cases, reactions were studied at low temperatures (201–363 and 220–336 K, respectively) and low pressures (0.5–6 Torr).…”