Kinetics of the CH₃O₂ + NO Reaction:  Temperature Dependence of the Overall Rate Constant and an Improved Upper Limit for the CH₃ONO₂ Branching Channel

Abstract

The overall rate constant and an upper limit for the CH 3 ONO 2 product channel for the CH 3 O 2 + NO reaction have been measured using the turbulent flow technique with high-pressure chemical ionization mass spectrometry for the detection of reactants and products. At room temperature and 100 Torr pressure, the rate constant (and the two standard deviation error limit) was determined to be (7.8 ( 2.2) × 10 -12 cm 3 molecule -1 s -1 . The temperature dependence of the rate constant was investigated between 295 and 203 K at pressures of either 100 or 200 Torr, and the data was fit to the following Arrhenius expression: (9.2 -3.9+6.0 × 10 -13 ) exp[(600 ( 140)/T] cm 3 molecule -1 s -1 . Although the room-temperature rate constant value agrees well with the current recommendation for atmospheric modeling, our values for the rate constant at the lowest temperatures accessed in this study (203 K) are about 50% higher than the same recommendation. No CH 3 -ONO 2 product was detected from the CH 3 O 2 + NO reaction (using direct CH 3 ONO 2 detection methods for the first time), but an improved upper limit of 0.03 (at 295 K and 100 torr) for this branching channel was determined.

Sensitizing effects of NOx on CH4 oxidation at high pressure

“…In case of NO + C 2 H 5 OO, the stabilization channel to C 2 H 5 ONO 2 contributes only ∼1% at ambient conditions [110,111]. Room-temperature measurements of k 6 [106][107][108][109][112][113][114][115][116] and k 7 [99,111,115,[117][118][119] are generally in good agreement. Atkinson et al [98] proposed temperature-dependent rate expressions based on data from 200-400 K [108,109,111,118,119] with corrections to match the average values at 298 K.…”

“…Lesar et al noted that the second adduct, CH 3 ONO * 2 , could be stabilized under suitable temperature and pressure conditions. However, the available experimental data indicate that this stabilization channel should be neglected unless very low temperatures are applied [106][107][108][109]. In case of NO + C 2 H 5 OO, the stabilization channel to C 2 H 5 ONO 2 contributes only ∼1% at ambient conditions [110,111].…”

Proton transfer mass spectrometry studies of peroxy radicals

“…It has been shown that small RO 2 species can be ionized according to Eq. (4) for n as high as four although the ionization efficiency is not known [17][18][19][20]. Furthermore, the detection efficiency for products such as the alkyl nitrates as well as other oxidation products (e.g., the multifunctional species) are not known.…”

“…Chemical ionization with water-proton clusters has been studied extensively in the laboratory [1,2]. With respect to atmospheric chemistry, the technique has been used for quantification of organic compounds in ambient air (e.g., [3][4][5][6][7]), for the elucidation of hydrocarbon oxidation mechanisms in chamber systems (e.g., [8][9][10][11][12][13][14][15][16]); or for kinetics studies of organic peroxy radicals ("RO 2 " where R is an organic group) such as CH 3 O 2 [17,18], CH 3 CH 2 O 2 [19], and C 3 H 7 O 2 [20]. These initial forays into the use of chemical ionization for the study of peroxy radicals employed ionization conditions (high pressures and low electric fields) such that the water proton clusters were large, i.e., H + (H 2 O) n with n > 3.…”

Computational study of the perhalogenated methyl nitrates CX3ONO2, CXxY3−xONO2(X, Y = F, Cl)

“…The alkyl radicals produced by photodissociation or oxidation of hydrocarbons and halocarbons [4,5] or addition of an atmospheric oxidant X (X=OH,Cl) to an alkene double bond [6,7] CH 4 although being a minor pathway, has been difficult to quantify so far. The alkyl nitrates are also the association minima in the potential energy surface of the reaction between alkoxy radicals and nitrogen dioxide RO + NO 2 → RONO 2 → products (2) Due to the importance of reaction (1) the subject has known numerous experimental investigations [8][9][10][11][12][13], many of which have investigated the reactions of the halogenated peroxy radicals [14][15][16] and particularly, the fluorinated ones [17][18][19][20][21][22][23][24][25]. The branching ratio for RONO 2 formation has been reported to vary systematically with carbon number and increase from <0.014 for ethyl to >0.32 for octyl radicals [5,24].…”