Thermal dissociation of alkyl nitrites and recombination of alkyl radicals

Abstract

The thermal dissociations of three alkyl nitrites (propyl, n-butyl and i-butyl) have been studied in a diaphragmless shock tube by laser schlieren densitometry over 700< T<1000K and 60<P<240 Torr. The nitrites act as clean, thermal sources of alkyl radicals that create the radicals at much lower temperatures than are accessible with other precursors. The decomposition of the three nitrites proceeds via a common mechanism, which is discussed. Rate coefficients for dissociation of the alkyl nitrites to an alkoxy radical and NO, k 1 , were obtained with an uncertainty of 30%. Rice-Ramsperger-Kassel-Marcus/Master Equation (RRKM/ME) calculations for dissociation of the nitrites were performed and provide excellent simulations of the experimental results. The high-pressure limit rate coefficients from the RRKM/ME calculations are: k 1,C3H7ONO = (5.69±1.71)×10 21 T -1.60 exp(-21458/T) s -1 ; k 1,n-C4H9ONO = (9.91±2.97)×10 21 T -1.59 exp(-21588/T) s -1 ; k 1,i-C4H9ONO = (3.92±1.18)×10 21 T -1.58 exp(-21162/T) s -1 . At the low temperatures of this work, the alkyl radicals are thermally stable and thus their recombination reactions were also studied. The recombination rate coefficients for ethyl, n-propyl and i-propyl radicals (uncertainties of 50%) are in excellent agreement with literature values, particularly recent theoretical results, and extend the range of experiments to higher temperatures.

A modular, multi-diagnostic, automated shock tube for gas-phase chemistry

“…23 The thermodynamic data for isobutyl nitrite was generated from group additivity estimates using the RMG website, rmg.mit.edu., 24 consistent with the sources of thermodynamic data reported in Ref. 3. The yellow lines represent simulations using the mechanism of Randazzo et al 3 The downward spike in Figs.…”

“…Dissociation of isobutyl nitrite was previously reported by Randazzo et al 3 The reaction occurs at comparably low temperatures, due to the weak O-NO bond (∼41 kcal/mol). The dissociation of isobutyl nitrite proceeds via steps R1 and R2 to form NO, formaldehyde, and an isopropyl radical iC 4 H 9 ONO ⇌ iC 4 H 9 O + NO, The rate of reaction R2 is much faster than reaction R1; 3 the early part of the LS experiments are essentially only sensitive to R1 allowing an accurate measure of its rate coefficient.…”

“…21 For comparison, Kiefer and Shah reported a value of RL = 27.01 for cyclohexene 4 and Randazzo et al reported RL = 27.0 for isobutyl nitrite. 3 The differences in the values of RL for the reactant species between the original values and those used here vary by no more than 5%. The contribution of the reactant RL to the overall value of KL is proportional to the mole fraction (2%) and so the minor updates to the values are RL should have a negligible impact on the experimental density gradient.…”

“…Twenty-two experiments were conducted in which isobutyl nitrite at 2% molar fraction in argon was shock heated to a nominal incident shock pressure, P 2 , of 120 Torr in the temperature range 700 K < T 2 < 1000 K. Sample LS plots are shown in Figs. 7(a) 3 was used without alteration. The simulations were implemented in Cantera 22 with thermodynamic data for all species except isobutyl nitrite taken from the Burcat database.…”

“…The capabilities of this new apparatus are demonstrated by replication of previously measured decomposition rate constants for isobutyl nitrite 3 and cyclohexene 4 by laser-schlieren densitometry (LS). The LS technique is most suitable for low pressures (<1 atm) but tests the salient features of the BST.…”