J. Phys. Chem. 1996, 100, 993-997
993
Reaction of Isopropyl Peroxy Radicals with NO over the Temperature Range 201-401 K Ju1 rg Eberhard,† Peter W. Villalta,‡ and Carleton J. Howard* Aeronomy Laboratory, EnVironmental Research Laboratories, National Oceanic and Atmospheric Administration, Boulder, Colorado, 80303 ReceiVed: July 5, 1995X
The rate constant for the gas-phase reaction of isopropyl peroxy radicals with NO has been measured over the temperature range of 201-401 K using chemical ionization mass spectrometric detection of the peroxy radical. The temperature dependent expression for the rate constant was found to be k(T) ) (2.7 ( 0.5) × 10-12 exp{(360 ( 60)/T} cm3 molecule-1 s-1 which gives a rate constant of k ) (9.0 ( 1.5) × 10-12 cm3 molecule-1 s-1 at 298 K. This value is a factor 1.8-2.6 higher than previous measurements.
Introduction Propyl peroxy radicals are formed in the atmosphere through the OH radical initiated oxidation of propane.1 At room temperature, in 95% of the cases the OH radical will abstract a hydrogen atom from the secondary carbon atom of propane to form isopropyl radicals.1
(CH3)2CH2 + OH f (CH3)2CH + H2O
(1)
The i-C3H7 radical reacts with O2 to form isopropyl peroxy radicals.
(CH3)2CH + O2 + M f (CH3)2CHOO + M
(2)
In polluted air i-C3H7O2 radicals will react predominantly with NO.
(CH3)2CHOO + NO f (CH3)2CHO + NO2
(3a)
(CH3)2CHOO + NO + M f (CH3)2CHONO2 + M (3b) Reaction 3a is an important step in the formation of photochemical smog since the subsequent photolysis of NO2 leads to formation of ozone. Reaction 3b acts as a sink for both the alkyl peroxy radicals and NOx. The branching ratio k3b/(k3a + k3b) is reported to be about 0.04 under tropospheric conditions.2 The overall room temperature rate constant for reaction 3 has been determined previously,3,4 although the evaluated recommendation is somewhat higher.5 From experimental studies of the reactions of NO with i-C3H7O2, t-C4H9O2,4 (CH3)3CCH2O2, and (CH3)3CC(CH3)2CH2O2 6 radicals, it has been concluded that the rates of RO2 + NO reactions decrease with increasing alkyl chain length and branching, whereas the evaluated recommendations5 for the C1 to C3 radicals do not suggest such a trend. In the present study our primary objectives are to resolve the disagreements among the available data for reaction 3 and to investigate the temperature dependence of the reaction. A secondary objective is to investigate the product formation from that reaction. This work is part of an ongoing effort in our laboratory to develop detection methods for peroxy radicals using chemical ionization mass spectrometry (CIMS). Previ† Fellow of the Schweizerischer Nationalfonds zur Fo ¨ rderung der wissenschaftlichen Forschung. ‡ Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO. X Abstract published in AdVance ACS Abstracts, December 15, 1995.
0022-3654/96/20100-0993$12.00/0
ously, we reported a study of the CH3O2 + NO reaction using CIMS with O2+ reagent ions.7 In the present study we detect i-C3H7O2 radicals using CIMS with O2- reagent ions. Experimental Section Apparatus. The experimental system has been used previously to study the CH3O2 + NO reaction.7 The apparatus consists of a neutral flow tube reactor coupled to an ion flow tube/quadrupole mass spectrometer commonly referred to as a flowing afterglow.8 Both the flowing afterglow technique8 and flow tube kinetic measurements using chemical ionization mass spectrometry9,10 have been described previously. Recent papers describe most details of the ion flow tube11 and the neutral flow tube7 used in this study. Radical Generation. The radical source consists of a 20 cm long × 1.27 cm o.d. quartz furnace with a platinum wire wound around a 5 cm long section to electrically heat the tube. A mixture of isobutyl nitrite (Aldrich, g97%) and He was passed through the furnace heated to 873 K, to yield isopropyl radicals, formaldehyde, and NO, which has been observed to be the major reaction channel.12 ∆
(CH3)2CHCH2ONO 98 (CH3)2CH + H2CO + NO (4) Oxygen was added right after the heated zone to form isopropyl peroxy radicals.
(CH3)2CH + O2 + M f (CH3)2CHOO + M
(5)
To minimize loss of radicals on the glass surface of the radical source, a Teflon sleeve and halocarbon wax were used to cover the wall to within 3 cm of the heated region. Experimental Conditions. He (g99.9999%) flow rates of 12-25 cm3(STP) s-1 (STP ≡ 273 K, 1 atm) and pressures of 1.5-4.5 Torr were used in the neutral flow tube. These conditions resulted in flow velocities of 800-2400 cm s-1. NO flows ranging from 2.1 × 10-4 to 2.4 × 10-3 cm3(STP) s-1 were introduced to the flow tube through the movable injector yielding [NO] ) 6.1 × 1011 to 1.3 × 1013 molecules cm-3 in the neutral tube. The NO was delivered to the flow tube in two ways, either as pure NO or out of Pyrex bulbs containing NO/He mixtures. The NO bulbs were prepared manometrically by diluting pure NO with He. The NO bulbs were then calibrated absolutely by comparing the change in the O2+ signal, owing to the charge transfer reaction of O2+ with NO, when © 1996 American Chemical Society
994 J. Phys. Chem., Vol. 100, No. 3, 1996
Eberhard et al.
using the NO/He mixtures and known flows of pure NO. The manometric determinations of the NO concentrations were usually about 5-8% lower than the CIMS calibration. Possible reasons for this discrepancy are (i) an error in the calibration of the pressure gauge used to prepare the bulbs, (ii) loss of NO in the glass bulb,13 and (iii) an error in the calibration using CIMS. As it was not possible to check the calibration of the pressure gauge at that time, we assumed the calibration using CIMS to be more reliable. The NO (g99.0%) used was passed through a dry ice cooled silica gel trap at about 300 Torr to reduce the nitrogen oxide impurities. The isobutyl nitrite was eluted from a reservoir at 196 K with a flow of He at 0.25-0.70 cm3(STP) s-1. Under these conditions the nitrite is completely decomposed in the furnace, as indicated by the disappearance of the signals from the reaction of the nitrite with O2- ions. For optimization of the production of i-C3H7O2 radicals, an additional He flow of 0.6-3.0 cm3(STP) s-1 was added after the reservoir and before the radical source. O2 (g99.99%) was added to the source reactor at a flow rate of 1 cm3(STP) s-1, resulting in O2 concentrations in the radical source of 1 × 1016 to 1 × 1017 molecules cm-3. Under these conditions .99.99% of the i-C3H7 radicals are converted to i-C3H7O2 radicals before they enter the reaction zone of the neutral flow tube.5 The initial concentrations of i-C3H7O2 radicals were in the range 5 × 1010 to 7 × 1011 molecules cm-3. These values were estimated using CIMS from the increase in the NO2 concentration upon reaction of i-C3H7O2 radicals with excess NO. For this estimate no correction was applied for contributions to the NO2- signal from isopropyl nitrite which is formed through reaction of isopropoxy radicals with NO. The NO2 concentration was estimated by comparing the observed signal for NO2-, obtained by charge transfer from O2- to NO2, with the signal when introducing a calibrated NO2/ He mixture directly into the ion tube. The O2, He, and NO2/ He flows were measured with calibrated mass flowmeters. For the investigation of the product formation additional experiments were carried out at 298 K with an excess of O2 (1-2 Torr) in the neutral flow tube. The ion flow tube was operated with a He flow rate of 100 cm3(STP) s-1 and at a pressure of 0.6 Torr. A flow of about 15 cm3(STP) s-1 of O2 (g99.99%) was added just downstream of the filament to produce the O2- reagent ions. To suppress the presence of O- ions, a flow of about 5 cm3(STP) s-1 of N2O was added 24 cm downstream of the O2 inlet. The ion chemistry is described in the following section. Detection Scheme. By addition of O2 to the ion tube the oxygen species O-, O2-, O3-, and O4- are formed. The O2ions are generated by electron attachment to O2.
e + O2 + He f O2- + He k6 ) (9 ( 3) × 10-31 cm6 molecule-2 s-1
(6) 298 K 14
Similarly to reaction 6 the O- ion is made by dissociative attachment of an energetic electron to O2.
eq + O2 f O- + O
k8 ) (5.5 ( 1.6) × 10-31 cm6 molecule-2 s-1
k9 ) 2.6 × 10-31 cm6 molecule-2 s-1
(9) 301 K 16
Since O- ions are more reactive than O2- ions and since they lead to the formation of O3- through reaction 8, N2O was added to the ion tube leading to the following reaction sequence.
O- + N2O f NO- + NO k10 ) (2.2 ( 0.4) × 10-10 cm3 molecule-1 s-1
(10) 298 K 17
NO- + O2 f O2- + NO k11 ) (5 ( 5) × 10-10 cm3 molecule-1 s-1
(11) 285 K 18
The addition of N2O has the effect of suppressing both O- and O3- ions in favor of O2-, the reagent ion in these experiments. Upon generation of i-C3H7O2 radicals, a signal at m/e 75 was observed. This signal was attributed to the parent negative ion of the i-C3H7O2 radical based on its dependence on heating the radical source to generate i-C3H7 radicals, on the addition of O2 and isobutyl nitrite to the radical source, and on its depletion upon addition of NO in the neutral flow tube. While the m/e 75 signal increased upon addition of O2 to the neutral tube, a m/e 75 signal could be produced without O2 in the neutral flow tube. This could be attributed to either the reaction of propyl radicals with O4- in the ion tube,
(CH3)2CH + O4- f [(CH3)2CHOO]- + O2
(12)
or the reaction of i-C3H7 radicals with excess O2 present in the ion tube and subsequent charge transfer reaction.
(CH3)2CHOO + O2- f [(CH3)2CHOO]- + O2 (13) Under our experimental conditions reaction 13 seems to be more likely. It was shown that the signal arises from the i-C3H7 radicals as it disappeared when the i-C3H7 radicals were scavenged by reaction with NO or NO2 or by radical selfreaction when the residence time was increased in the neutral flow reactor. Under typical experimental conditions an unidentified small background in the m/e 75 signal was observed. Some of the measurements had to be corrected for this background. Other than this, interference in the m/e 75 signal from other ion-molecule reactions is unlikely because an interference would require a change in the signal with NO reaction time in the neutral tube. Under our various experimental conditions we did not find any significant trends in the decay plots that would indicate any such interference. For the investigation of the product formation an additional experiment was carried out at 298 K where methanol was added to the ion tube to generate CH3OH2+, which will transfer a proton to compounds having higher proton affinities than methanol. Results
(7)
The ions generated in reactions 6 and 7 can react with O2 to yield O3- and O4- ions, respectively.
O- + O2 + He f O3- + He
O2- + O2 + He f O4- + He
(8) 298 K 15
Rate Constant. The reaction of isopropyl peroxy radicals with NO was studied at temperatures ranging from 201 to 401 K. The i-C3H7O2 radicals were generated in the radical source at a fixed position on the flow tube. The NO was added through the movable injector in excess [NO] g 5[i-C3H7O2] so that the reaction kinetics were pseudo-first order in the peroxy radical concentration. The variation of the radical concentration with reaction time is described by Eqs I and II,
Reaction of Isopropyl Peroxy Radicals with NO
J. Phys. Chem., Vol. 100, No. 3, 1996 995
d[(CH3)2CHOO]/dt ) -k[(CH3)2CHOO]
(I)
ln[(CH3)2CHOO] ) -kt + c
(II)
where k ) k3[NO] - kw, c is a constant, k3 is the bimolecular rate coefficient for reaction 3, and kw is the first-order rate coefficient for loss of peroxy radicals on the wall of the movable injector. The wall loss on the injector enters because the amount of injector surface exposed to the radical stream changes as the injector is moved. The first-order rate coefficient kw is determined by measuring the peroxy radical decay when [NO] ) 0 and is found to be small with values
