17846
J. Phys. Chem. 1996, 100, 17846-17854
Kinetic Investigations of the Reactions of CD3O2 with NO and NO3 at 298 K Frank Helleis, Geert K. Moortgat, and John N. Crowley* Max-Planck-Institut fu¨ r Chemie, DiVision of Atmospheric Chemistry, Postfach 3060, 55020 Mainz, Germany ReceiVed: July 1, 1996; In Final Form: September 4, 1996X
The reactions of methylperoxy radicals with NO3 and NO were investigated at room temperature using the method of discharge flow/mass spectrometry. For reaction with NO, data were obtained for normal and deuterated methylperoxy. k(CH3O2 + NO) ) (7.5 ( 1.0) × 10-12 cm3 molecule-1 s-1 and k(CD3O2 + NO) ) (8.6 ( 1.0) × 10-12 cm3 molecule-1 s-1. The rate constant for reaction of CD3O2 with NO3 was found to be k(CD3O2 + NO3 f CD3O + NO2 + O2) ) (1.3 ( 0.2) × 10-12 cm3 molecule-1 s-1. In addition, rate constants were obtained for the reaction of CD3O with NO3: k(CD3O + NO3 f CD3O2 + NO2) ) (3.7 ( 0.8) × 10-12 cm3 molecule-1 s-1, k(CD3O + NO3 f DCDO + DNO3) ) (5.6 ( 1.0) × 10-13 cm3 molecule-1 s-1.
Introduction Recent reviews have drawn attention to the important roles of NO3 1 and peroxy radicals2,3 both in tropospheric and in stratospheric processes. In daylight, peroxy radicals are generated by the reaction of OH radicals with organic compounds such as CH4 and CO, leading to formation of the methylperoxy radical (CH3O2) and HO2, respectively. As photolysis of O3 in the presence of H2O is the major source of OH radicals, a strong diurnal profile for RO2 may be expected. In the polluted troposphere, peroxy radicals (RO2) will react with NO to perturb the NO2/NO/O3 photostationary state and lead to an increase in O 3.
The CH3O radical is rapidly converted to HO2, which may react further with NO3 to regenerate OH:
CH3O + O2 f HO2 + HCHO
(6)
HO2 + NO3 f OH + NO2 + O2
(7a)
f HNO3 + O2
(7b)
In contrast, the NO3 radical, which absorbs light strongly between 600 and 700 nm, is photolyzed rapidly during the day and is present in elevated concentrations (10 ppt) only at night. The major source of NO3 is the reaction between O3 and NO2. At first glance, it would therefore seem unlikely that the reaction between peroxy radicals and NO3 will be significant in the atmosphere. However, known reactions of the NO3 radical include those with organic species which may lead to peroxy radical formation in the presence of O2. The fate of the peroxy radicals thus produced at night is different from that at day due to the absence of NO. Possible reaction partners include O3, NO2 in a reversible process, and NO3. A modeling study4 has demonstrated that nighttime reactions of NO3 with peroxy radicals such as CH3O2, generated in this particular study Via a reaction between NO3 and CH3SCH3, can convert RO2 to RO and thus to HO2 and OH. The model used a rate constant of 2.3 × 10-12 cm3 molecule-1 s-1 for a bimolecular reaction between NO3 and CH3O2 that leads to CH3O production:
The OH radical thus formed can regenerate a peroxy radical to complete the cycle, and the NO3 radical can be regarded as playing the same role at night as NO does during the day. Recent field measurements5 made at night in a forested area reveal high concentrations of RO2 (up to 40 ppt), and an anticorrelation with the simultaneously measured NO3 radical. This may be taken as a strong indication of nighttime RO2/ NO3 interaction, similar to that proposed by Platt et al.4 Kinetic data for reaction 7 have been obtained in four separate studies, and the results are in reasonable agreement6-9 with a k298 of ca. 3.5 × 10-12 cm3 molecule-1 s-1 10 with the radical channel (7a) dominant. At the onset of this research, our understanding of the kinetics of the reaction between CH3O2 and NO3 was less satisfactory. At that time, only one study had been made of this reaction Via a rather indirect method involving computer modeling of a complex reaction sequence initiated by the modulated photolysis of HNO3 in the presence of CH4/O2.11 An additional, nonphotolytic source of NO3 radicals has since been identified by the same authors12 and may cast some doubt on the interpretation of their kinetic data. This work was undertaken in order to delineate these uncertainties and we have chosen a more direct method utilizing flow tube/mass spectrometric methods which enable both the methylperoxy radical and NO3 to be simultaneously detected with high selectivity and sensitivity. More recent studies of the reaction between methylperoxy and NO3 have also utilized flow-tube methods, but combined with detection of the CH3O product.13,14 We compare the new and old literature results along with the results from the present study in more detail later. We also present room temperature rate constants for the reactions between CH3O2 and CD3O2 with NO.
CH3O2 + NO3 f CH3O + NO2 + O2
CH3O2 + NO f CH3O + NO2
(8)
CD3O2 + NO f CD3O + NO2
(9)
RO2 + NO f RO + NO2
(1)
RO + O2 f HO2 + R′CHO
(2)
HO2 + NO f OH + NO2
(3)
NO2 + hν + O2 f NO + O3
(4)
(5)
* Address correspondence to this author. X Abstract published in AdVance ACS Abstracts, October 15, 1996.
S0022-3654(96)01059-3 CCC: $12.00
© 1996 American Chemical Society
Reactions of CD3O2 with NO and NO3 at 298 K
J. Phys. Chem., Vol. 100, No. 45, 1996 17847
Figure 2. Titration of NO3 with NO. The end point of this particular titration is at [NO] ) [NO3] ) 4.8 × 1013 molecules cm-3. Figure 1. Schematic diagram of the discharge-flow experiment showing source regions for the reactants and inlet ports for NO titrant. MW ) microwave.
These rate constants were measured to establish whether the reactivity of CH3O2 and CD3O2 toward NO are identical. This enables the kinetic data for the reaction between CD3O2 and NO3,
CD3O2 + NO3 f products
(10)
which was used as surrogate for CH3O2 + NO3 in the present study, to be applied to the reaction of CH3O2 with NO3. The data for CH3O2 also serves to strengthen the database for this important reaction. Experimental Section A schematic diagram of the discharge flow/mass spectrometer system is shown in Figure 1. The main reactor is a 80 cm long by 3 cm i.d. Pyrex flow tube, the temperature of which could be controlled by passing a thermostated fluid through an outer jacket. The inner wall of the reactor was coated with halocarbon wax. Linear flow velocities of either ca. 5 or 10 m s-1 were established within the reactor by variation of the bulk flow rate, consisting predominantly of He, and by throttling a rotary pump. Calibrated flow rates were maintained either by Tylan mass flow controllers (type F60) or by self-built constant pressure flow controllers which could be calibrated on-line using the dp/dt technique. The sample region and ionization regions were pumped by a 7 in. oil diffusion pump, and by a turbomolecular pump, respectively. All experiments were carried out at a total pressure of 1.7 Torr and at 298 K. Radical and stable species, continuously sampled at the downstream end of the flow tube Via a two-stage beam inlet system, were detected by modulated molecular beam mass spectrometry. Beam modulation at 213 Hz was provided by a tuning fork type chopper. Ions, generated by electron bombardment (15 eV, 0.1-1 mA), were selected by a quadrupole mass filter (Riber/Balzers) and detected by a channeltron electron multiplier. The spectrometer was run in selective ion monitoring mode (m/e ) 50 and m/e ) 62 were monitored during kinetic runs) with mass-dependent variation of the emission current and signal integration times. Variable reaction times were obtained by the software-controlled translation of the sliding injector over a maximum distance of ca. 65 cm. Both CD3O2 and NO3 were detected as their parent ions at 50 and 62 amu, respectively. Experiments with CH3O2 were
impossible due to the coincidence of a comparatively strong signal from the N17O2 fragment ion of HNO3 with the parent ion of CH3O2 (47 amu). The detection limits (S/N ) 1) for CD3O2 (m/e ) 50) was 5 × 109 molecules cm-3 for an integration time of 30 s. 1. Generation of NO3 and CD3O2. Both radicals were made in titration reactions of molecular precursors with F atoms generated by passing 0.005% (injector) or 0.1% (side arm) mixtures of F2 in He through 2450 Hz microwave discharges run at 18 W microwave power. Ceramic (Al2O3) inserts were used to reduce F atom loss at the glass surfaces where the microwave cavity was attached to glass tubing. NO3 radicals, generated by the rapid titration of F atoms with a large excess of HNO3, k11 ) 2.4 × 10-11 cm3 molecule-1 s-1 15-17 (lifetime of F ≈ 70 µs), were added to the reactor Via a fixed side arm coated with Halocarbon wax positioned at the upstream end of the flow tube.
F + HNO3 f HF + NO3
(11)
All experiments were carried out under pseudo-first-order conditions with NO3 in excess and extraction of kinetic parameters required the measurement of absolute NO3 concentrations. This was achieved by adding various known amounts of NO to the flow tube and determining the titration point ([NO3] ) [NO]) of the reaction:
NO3 + NO f 2 NO2
(12)
The validity of this method has been discussed in detail by Mellouki et al.15 and Canosa-Mas et al.18 Care was taken to ensure that the reaction between F and NO315,16,19,20 was avoided during titration experiments by keeping the ratio HNO3/NO3 greater than 10. Once the response of the mass spectrometer to NO3 at 62 amu was established, this mass could be recorded along with CD3O2 decay profiles in all experiments. A correction for a slight drift in response over the course of several hours was made. Typical results from a titration experiment ([NO3]i ) (4.8 ( 0.2) × 1013 molecules cm-3) are displayed in Figure 2. CD3O2 radicals, generated in reactions 13 and 14 were added to the reactor through a 120 cm long sliding double-injector concentric to the main reactor flow tube.
F + CD4 f CD3 + DF
(13)
CD3 + O2 + M f CD3O2 + M
(14)
17848 J. Phys. Chem., Vol. 100, No. 45, 1996
Helleis et al.
The methylperoxy source is constricted and therefore runs at elevated pressures (ca. 4 Torr) compared to the flow tube. This increases the efficiency with which O2 titrates the CD3 radical and reduces the generation of CD3O Via
CD3 + CD3O2 f 2CD3O
(15)
Calculations, which neglected wall loss of CD3O, showed that the [CD3O]0 exiting the injector was less than 5 × 1010 molecules cm-3. The methylperoxy radical generation scheme has been described in detail in a recent publication.21 The concentration of CD3O2 in the main reactor was determined by adding an excess of NO, measuring the product NO2, assuming a stoichiometry of 1 for the reaction, and calibrating the NO2 signal with a known flow of NO2/He.
CD3O2 + NO f CD3O + NO2
(16)
Typical reactor concentrations of radical and precursor species were [NO3] ) (2-8) × 1013, [CD3O2] ) 5 × 1011, [CD4] ) (1.6-3.5) × 1013, [O2] ) 5.5 × 1015 molecules cm-3. 2. Materials. The gases and purification methods employed in these experiments were as follows: He (Linde, 99.999%), was passed through a liquid nitrogen trap and an oxysorb column; O2 (Linde, 99.999%), CH4, (Linde, 99.9%), and a 1% mixture of F2 in He (Linde) were used without further purification. Anhydrous HNO3 was prepared by the room temperature distillation of a H2SO4/KNO3 mixture and stored at -30 °C in the dark to prevent thermal or photolytic decomposition. NO (Matheson, 99.5%) was purified by fractional distillation to remove all traces of NO2 and higher oxides. After purification the mass spectrum of NO revealed the presence of less than 1% NO2. NO2 was prepared by adding a large excess of O2 to pure NO, and the unreacted O2 was removed by distillation at ca. -50 °C. NO2 was stored as a nominal 1% mixture in He. The true concentration of NO2 was found by correcting for the equilibrium with N2O4. Results 1. Reaction of CD3O2 with NO3. All experiments were carried out under pseudo-first-order conditions with the NO3 radical in large excess. Under these conditions the decay of CD3O2 should be described by
[CD3O2]t ) [CD3O2]0 exp(-k10[NO3]0t)
(i)
where k10 is the total (product channel independent) rate constant for the reaction between CD3O2 and NO3. As described previously,21 a small residual was present under the methylperoxy signal at m/e ) 50. The size of this residual was determined at regular intervals by addition of large excess concentrations of NO ([NO]0 > 1 × 1014 molecules cm-3) at the top of the flow tube and amounted to 5-10% of the total signal at zero reaction time ([CH3O2]0). Once corrected, the m/e ) 50 concentration profiles were observed to follow an exponential decay. Assuming pseudo-first-order kinetics, decay rates, kobs, are obtained from the slopes of plots of ln(CD3O2+ signal) vs time as shown in Figure 3 for a series of measurements at various [NO3]0. No systematic variation of the NO3 signal was observed over the entire reaction zone. The experimental conditions and results of this analysis are given in Table 1. Measured CD3O2 first-order decay constants, kobs, were corrected for diffusion effects by22
k1st ) kobs(1 + kobsDeff/V2)
(ii)
Figure 3. Decays of CD3O2 in the presence of varying amounts of NO3. The initial concentration of NO3 was (A) 0; (B) 5.52 × 1012; (C) 1.12 × 1013; (D) 2.25 × 1013; (E) 3.26 × 1013; (F) 4.91 × 1013; (G) 6.63 × 1013; (H) 8.24 × 1013 molecules cm-3.
TABLE 1: Experimental Results and Conditions for (a) the CD3O2 + NO3 Experiments and (b) the CH3O2/CD3O2 + NO Experimentsa (a) CD3O2 + NO3 [NO3] -d ln[CD3O2]/dt [NO3] -d ln[CD3O2]/dt (molecules cm-3) (s-1) (molecules cm-3) (s-1) 3.26 × 1013 0 4.40 × 1012 9.10 × 1012 1.86 × 1013 3.53 × 1013 6.18 × 1013 0 7.40 × 1012 1.48 × 1013 2.88 × 1013 5.25 × 1013 8.18 × 1013 0 0
13.6 1.80 6.41 8.09 9.86 13.2 17.7 1.97 6.61 9.70 11.8 16.4 23.9 1.48 1.21
0 4.91 × 1013 6.63 × 1013 8.24 × 1013 9.74 × 1013 0 5.62 × 1012 1.12 × 1013 2.25 × 1013 2.51 × 1013 4.46 × 1013 5.66 × 1013 0
1.58 16.9 20.0 22.6 24.3 2.30 7.14 9.53 12.5 11.51* 16.4* 16.1* 1.21*
(b) CH3O2/CD3O2 + NO -d ln[CH3O2]/dt -d ln[CD3O2]/dt [NO] [NO] (s-1) (s-1) molecules cm-3 molecules cm-3 0 3.78 × 1012 4.25 × 1012 5.13 × 1012 5.90 × 1012 7.13 × 1012 9.54 × 1012 1.29 × 1013 1.66 × 1013
1.80 31.3 38.2 43.1 51.1 59.6 80.1 101 128
0 5.20 × 1012 7.97 × 1012 9.09 × 1012 1.10 × 1013 1.21 × 1013 1.41 × 1013
2.50 45.7 65.8 79.40 93.6 106 121
a All NO3 data obtained at 298 K and 1.73 Torr of He and linear velocity of 990 cm s-1 except those marked with an *: pressure, 1.70 Torr, linear velocity ) 529 cm s-1. All NO data obtained at 298 K, 1.70 Torr and a linear velocity of 995 cm s-1. All decay rates are corrected for diffusion effects as described in the text. The correction was always less than 1% for the NO3 experiments and less than 2% for the NO experiments.
where k1st is the corrected first-order decay rate (s-1), V is the linear velocity (cm s-1), and Deff is an effective diffusion coefficient (cm2 s-1),
Deff ) D12 + r2V2/48D12
(iii)
where r is the flow tube radius (cm) and D12 the pressure- and temperature-dependent binary diffusion coefficients for CH3O2/ He. D12 was calculated as described by Hirschfelder et al.23
Reactions of CD3O2 with NO and NO3 at 298 K
J. Phys. Chem., Vol. 100, No. 45, 1996 17849 negligible ([CD3O2]0 ) 5 × 1011 molecules cm-3, k ) 3.6 × 10-13 cm3 molecule-1 s-1). These observations show that our pseudo-first-order analysis is not appropriate for this chemical system and may be taken as an indication of secondary chemistry that distorts the CD3O2 profile. However, exponential decay characteristics were observed over almost the whole range of reaction times in which our measurements were made. In order to understand this we must consider the possible products of reactions 10. By drawing analogy to the reaction between NO3 and HO2 (7a) we assume that a major product channel is reaction 10a (∆Hr ) -40 kJ mol-1). We consider other product channels later.
CD3O2 + NO3 f CD3O + NO2 + O2
Figure 4. Plot of pseudo-first-order decay rate of CD3O2 against [NO3]0.
(10a)
The OH radical generated in reaction 7a is known to react rapidly with NO3 to regenerate HO2.10 The same cycle may also apply to reaction 10, i.e., the CD3O product of reaction 10a may react with NO3 to regenerate CD3O2. Strong evidence for coupling of reactions 10a and 17a (using non-deuterated analogues) has recently been provided by independent kinetic investigations of the title reaction using flow tubes and laserinduced fluorescence (LIF) to detect CH3O radicals.13,14 The LIF work indicates room temperature rate constants of ca. 1 × 10-12 and 2 × 10-12 cm3 molecule-1 s-1 for the non-deuterated analogues of reactions 10a and 17a, respectively. In addition, CD3O2 has been observed following mixing of CD3O and NO3 in recent discharge-flow mass spectrometer experiments.24
CD3O + NO3 f CD3O2 + NO2
(17a)
If we consider only reactions 10a and 17a, the net result is
2NO3 f 2NO2 + O2 Figure 5. Decay profile of CD3O2 with various amounts of NO. [NO]i was (A) 0; (B) 5.20 × 1012; (C) 6.72 × 1012; (D) 7.97 × 1012, (E) 9.09 × 1012; (F) 1.10 × 1013; (G) 1.21 × 1013; (H) 1.41 × 1013 molecules cm-3.
using a reduced collision integral and the combined collision parameter for CO2/He. The bimolecular rate constant (k10) is related to k1st by
k10 ) k1st/[NO3]0
(iv)
and was initially derived from linear plots of k1st vs [NO3]0 as shown in Figure 4. From this analysis a value for the rate constant of reaction 10 of (2.0 ( 0.1) × 10-13 cm3 molecule-1 s-1 at 298 K was obtained. However, a close inspection of Figure 3 reveals that the firstorder decays for CD3O2 in various excess amounts of NO3 do not have a common intercept at zero reaction time (injector at 0 cm). Indeed, if extrapolated, the curves would meet at ca. -15 cm. In addition, a large positive intercept (k1st ) 6 s-1 at [NO3]0 ) 0) is apparent in Figure 4. The first of these effects may result from a poorly defined contact point (due to mixing phenomena) of the injector and main reactor gas flows, although a value of -15 cm seems rather too high to be explained by this. In order to test this supposition, experiments were carried out under identical flow conditions but with NO as reactant in excess. The results in Figure 5 clearly indicate that the contact time of the gases is indeed well defined with all decays meeting at ca. 0 ( 1 cm. The 6 s-1 intercept in Figure 4 is possibly due to losses of CD3O2 or CD3O at the wall, or reaction of CD3O with O2 (see later). In these experiments, low initial concentrations render the loss of CD3O2 due to self-reaction
(18)
and no change in CD3O2 once CD3O has reached a steady state concentration which is determined by the rates of reactions 10a and 17a. Additional reactions of CD3O2 that do not generate CD3O will deplete its steady state concentration and result in a slow decay. A further loss process of CD3O to form stable products that is first order in NO3 will also result in an exponential decay of CD3O2 if equilibrium between CD3O2 and CD3O (reactions 10a and 17a) is rapidly established compared to the rate of loss of CD3O. To recap, our observed exponential decay of CD3O2 is not governed solely by [NO3]0 and k10 (pseudo-first-order analysis) but reflects changing fluxes through reactions 10a and 17a due to first-order loss processes of either CD3O or CD3O2. The initially rapid drop in CD3O2 concentration at short reaction times in our experiments is due to removal by reaction with NO3 before the CD3O radical concentration, and thus the rate of reaction 17a is high enough to regenerate significant amounts of CD3O2; i.e., it describes the approach to equilibrium. We first make the assumption that reaction 10b (which is exothermic by ca. 280 kJ mol-1) is responsible for the removal of CD3O2 from equilibrium:
CD3O2 + NO3 f DCDO + DONO + O2
(10b)
With this assumption, the slope of the fit to the data in Figure 4 (the [NO3]-dependent loss of CD3O2) is equated to the rate constant of this reaction, i.e., k(10b) ) (2.0 ( 0.1) × 10-13 cm3 molecule-1 s-1. The intercept represents [NO3]-independent loss of CD3O2 either directly or Via loss of CD3O which may react with O2 or decompose at the wall. We consider not only a reaction of CD3O2 with NO3 that does not generate CD3O but also the reaction of CD3O with
17850 J. Phys. Chem., Vol. 100, No. 45, 1996
Helleis et al.
NO3 to generate products that are not CD3O2, e.g.
CD3O + NO3 f DCDO + DNO3
(17b)
The rate constant necessary for this reaction to reproduce the observed CD3O2 loss rate will differ from the value of 2.0 × 10-13 cm3 molecule-1 s-1 above unless the equilibrium concentrations of CD3O2 and CD3O are equal, which would imply equal rate constants for k10a and k17a in the absence of other loss reactions of CD3O, such as wall loss. As CD3O could not be detected in these experiments, the wall loss rate of CD3O was not measured. Our approach is to extract the kinetic data from these experiments using numerical simulation. First, the data are put on a concentration scale. Absolute CD3O2 concentrations in the absence of NO3 were measured in two experiments, in both cases the concentration was found to be close to 5 × 1011 molecules cm-3. However, for all decays an associated firstorder loss rate of CD3O2 was measured in the absence of NO3. The comparison of back extrapolated wall loss decays to t ) 0 enables us to normalize all data to be consistent with this initial concentration. The incorporation of the extrapolated initial concentration (see Figure 6) then gives us the curvature in our plots (which we have treated thus far as purely first order) which makes our analysis sensitive to the rate constants that describe the approach to equilibrium. The departure from equilibrium is controlled by the reactive loss processes of CD3O2 and CD3O that do not generate CD3O and CD3O2, respectively. For CD3O, this may include a constant for the wall loss and reaction with O2. There exists only one determination of the rate constant for CD3O with O2 and this was derived Via product analysis in a complex reaction scheme25 and appears to be unrealistically low when compared with accepted values for CH3O with O2.10 We assume initially that k(CD3O + O2) ) k(CH3O + O2) ) 1.9 × 10-15 cm3 molecule-1 s-1 and examine later the effect of reducing this rate coefficient to take into account the expected kinetic isotope effect.
CD3O + O2 f DCDO + DO2
(19)
CD3O + wall f products
(20)
In addition, though less important, CD3O may undergo selfreaction (21) or may react with CD3O2 (22):
CD3O + CD3O f CD2O + CD3OD
(21)
CD3O + CD3O2 f products
(22)
The complete reaction scheme is given in Table 2. The results of a fit to a single data set are shown in Figure 6 which displays the CD3O2 decay in the absence and presence of NO3 ([NO3]0 ) 2.88 × 1013 molecules cm-3) and the simulated CD3O profile. As expected, at t > 25 ms, the measured decay of CD3O2 and the simulated decay of CD3O show the same first-order rate constant (slope). The leastsquares fit routine returned values of k10a ) (9.6 ( 1.3) × 10-13 cm3 molecule-1 s-1, k17a ) (2.7 ( 0.4) × 10-12 cm3 molecule-1 s-1 and k20 ) 11 s-1 (the wall loss rate of CD3O) for this particular data set, in satisfactory agreement with the independent LIF investigations. The error limits are 95% confidence limits as returned by the fit routine. Upon scaling the CD3O2 concentration to either 4 × 1011 or 6 × 1011 molecules cm-3, changes in the fitted values of 1-2% for k10a and k17a and ca. 7% for k20 were observed. This indicates that the decays are controlled to a large extent by the NO3 concentrations and that
Figure 6. Measured decay of CD3O2 in the presence of 2.88 × 1013 NO3 (stars), and in the absence of NO3 (circles). The solid lines are first-order fits to the CD3O2 profiles (when NO3 ) 0) and numerical fits (when NO3 present). The lower solid line is the simulated concentration profile of CD3O.
TABLE 2: Facsimile Reaction Schemea reaction
rate constant
no.b
NO3 + CD3O2 f CD3O + O2 + NO2 NO3 + CD3O f CD3O2 + NO2 CD3O2 + NO3 f products (not CD3O) CD3O + NO3 f products (not CD3O2) CD3O + O2 f DCDO + DO2 CD3O + wall DO2 + NO3 f OD + NO2 + O2 OD + NO3 f DO2 + NO2 OD + NO2 + M f DNO3 + M OD + HNO3 f NO3 + HDO CD3O + CD3O f products CD3O + CD3O2 f products
varied varied varied varied 1.9 × 10-15c varied 3.5 × 10-12 2.3 × 10-11 1.80 × 10-12 1.15 × 10-13 7 × 10-12 1 × 10-11
10a 17a 10b 17b 19
21 22
a All rate constants are for the non-deuterated analogues and were taken from ref 10 except for the reactions of CD3O with itself31 and with CD3O2.32 The units are cm3 molecule-1 s-1. Rate constants for termolecular reactions refer to 1.7 Torr total pressure of He. b No. refers to the number of the reaction as listed in the text. c Also 0.6 × 10-15 cm3 molecule-1 s-1 used (see text).
non-first-order processes (reactions 21 and 22) are relatively unimportant. A more satisfactory method of extracting the rate constant data is to analyze a complete set of decay curves simultaneously for which the initial CD3O2 concentration was held fixed, i.e., a fit to the complete set of data displayed in Figure 3. In this way, experiments with low [NO3]i, which have better sensitivity to the approach to equilibrium but a large contribution to the CD3O2 decay rate Via CD3O wall loss (and reaction with O2), are combined with those with high [NO3]i, which reduces the relative contribution of CD3O wall loss to the decay but have poorer sensitivity to the rate constants k10a and k17a. Prerequisite to this are data sets obtained in experiments in which the mass spectrometer and the CD3O2 source were very stable over the course of the measurement of a set of decays, about 3-4 hours. We therefore analyze only one set of decays obtained when the above criteria were best fulfilled. Scenario 1. In scenario 1, as above, the rate constants for reactions 10a and 17a were allowed to vary as was the total [NO3]-independent loss rate of CD3O. Reaction 10b was fixed at 2 × 10-13 cm3 molecule-1 s-1. The results of this data fit are shown as dotted lines in Figure 7. The fit returned values of k10a ) (1.0 ( 0.1) × 10-12 cm3 molecule-1 s-1, k17a ) (3.8 ( 0.4) × 10-12 cm3 molecule-1 s-1, and k20 ([NO3]-independent wall loss of CD3O) ) 24 ( 8 s-1. The dotted lines are the result of assuming that CD3O reacts
Reactions of CD3O2 with NO and NO3 at 298 K
J. Phys. Chem., Vol. 100, No. 45, 1996 17851
Figure 7. Multifile fits to the experimental data using Facsimile. In this case the varied parameters were k10a, k17a, and k20. k10b was held at 2.0 × 10-13 cm3 molecule-1 s-1. This corresponds to scenario 1 (see Discussion).
Figure 8. As Figure 7 but with k10a, k17a, and k17b varied. This corresponds to scenario 2 (see Discussion).
with O2 to form DO2 and DCDO at 1.9 × 10-15 cm3 molecule-1 s-1. The solid lines were obtained when this rate constant is reduced to 0.6 × 10-15 cm3 molecule-1 s-1. The difference in the solid and dotted line modeled decays are due to a slight (not measurable) change in NO3 concentration during the course of a decay. This is caused in part by DO2 formation (in reaction 19), and subsequent reaction of DO2 with NO3 (see Table 2). The kinetic data from this fit are k10a ) (9.7 ( 0.8) × 10-13 cm3 molecule-1 s-1, k17a ) (3.8 ( 0.5) × 10-12 cm3 molecule-1 s-1, and k20 (total [NO3]-independent loss of CD3O) ) 35 ( 7 s-1. The reduction in the rate constant for the reaction between CD3O and O2 is thus converted into a higher CD3O wall loss rate necessary to fit the data. We emphasize at this point that, although the decay of CD3O2 was modeled by a direct reaction with NO3 that did not regenerate CD3O2, we cannot discriminate between this and a reaction between CD3O and NO3 that does not regenerate CD3O2 as long as the pseudoequilibrium between CD3O and CD3O2 is rapid. Scenario 2. We now therefore examine the scenario in which CD3O reacts with NO3 to form stable products. Possible products13 are
CD3O + NO3 f CD2O + DNO3
(17b)
CD3O + NO3 f CD3O2NO2
(17c)
In our first simulation (using the same data set) we reproduced the decay out of equilibrium by letting CD3O2 react with NO3
Figure 9. Plots of first-order decay rates of CH3O2 (solid circles) and CD3O2 (open squares) against [NO]i. The slope gives the bimolecular rate constants k8 ) 7.49 × 10-12 and k9 ) 8.59 × 10-12 cm3 molecule-1 s-1 for CH3O2 and CD3O2, respectively.
(and the wall with a known rate constant) and non-[NO3]dependent terms (wall loss of CD3O) which accounted for the observed total decay rate. We carry this wall loss rate of CD3O into our next scenario in which the decay is determined by loss processes of CD3O only. Here, k10a, k17a, and k17b are fitted. The resultant fit to the data is shown in Figure 8. The rate constants obtained are k10a ) (1.28 ( 0.1) × 10-12 cm3 molecule-1 s-1, k17a ) (3.3 ( 0.5) × 10-12 cm3 molecule-1 s-1, and k17b ) (4.8 ( 1.4) × 10-13 cm3 molecule-1 s-1. When the rate constant for CD3O + O2 is reduced to 0.6 × 10-15 cm3 molecule-1 s-1, these values become k10a ) (1.29 ( 0.1) × 10-12, k17a ) (3.75 ( 0.6) × 10-12, and k17b ) (5.6 ( 0.3) × 10-13 cm3 molecule-1 s-1. The best determined parameter is k17b as, due to the multifile fitting procedure, this is largely independent of our backextrapolated initial [CD3O2]i and wall losses of CD3O and is determined by the experimental parts of the decays in Figure 3. We have said that the slope of the straight line in Figure 4 corresponds to either reaction of CD3O2 with NO3 with a rate constant of 2.0 × 10-13 cm3 molecule-1 s-1 or loss of CD3O by reaction with NO3 with a rate constant of 5.6 × 10-13 cm3 molecule-1 s-1 × [CD3O2]eqm/[CD3O]eqm. Thus
[CD3O2]eqm/[CD3O]eqm ) 5.6 × 10-13/2.0 × 10-13 ) 2.8 The value of 2.8 is thus also the ratio of the rate constants for k10a/k17a and is internally consistent with k10a ) (1.29 ( 0.1) × 10-12 cm3 molecule s-1 and k17a ) (3.75 ( 0.6) × 10-12 cm3 molecule s-1 derived above. 2. Reaction of CD3O2 and CH3O2 with NO. The kinetics of the reaction between methylperoxy and NO was measured by monitoring the decay of CD3O2 or CH3O2 in the presence of a large excess of NO. Figure 5 displays a series of exponential decays in the presence of various initial amounts of NO. The concentration of NO is determined by its partial flow rate and the total pressure in the flow tube and is accurate to within 5%. The rate constants, derived from plots of pseudofirst-order decay rate (corrected for wall loss and diffusion using eq ii) against [NO] as shown in Figure 9, are found to be (7.49 ( 0.4) × 10-12 and (8.59 ( 0.4) × 10-12 cm3 molecule-1 s-1 for CH3O2 and CD3O2, respectively (errors are statistical 95% confidence limits). Discussion 1. CD3O2 + NO3. We have presented rate constants for the reactions 10a, and 17a along with 10b or 17b based on
17852 J. Phys. Chem., Vol. 100, No. 45, 1996
Helleis et al.
different assumptions made in the numerical modeling of the kinetic data. The effect of not accurately knowing the rate constant for the reaction of CD3O with O2 was shown to be relatively unimportant (changes in k10a of ca. 4% when values of 1.9 × 10-15 or 0.6 × 10-15 cm3 molecule-1 s-1 are assumed for this reaction) as it is coupled with wall losses. Two sets of rate constants are thus obtained, one in which the observed decay of CD3O2 is due to reaction 10b or its equivalent and one in which it is due to reaction 17b or its equivalent. The two sets of rate constants (in units of cm3 molecule-1 s-1) are
scenario 1 scenario 2
1012k10a
1013k10b
1012k17a
1013k17b
1.0 ( 0.1 1.3 ( 0.1
2.0 ( 0.1
3.8 ( 0.5 3.7 ( 0.6
5.6 ( 0.3
where scenario 1 refers to modeling the decay of CD3O2 by a direct reaction with NO3 which does not generate CD3O and scenario 2 to the case where the loss of CD3O2 was caused by first-order loss of CD3O and the quasi-equilibrium k10a/k17a. Our data does not allow us to conclude whether reaction 10b or 17b, or indeed both, contribute to the observed decay of CD3O2 from an equilibrium concentration. It is apparent, however, that the rate constants obtained for the title reaction and for the reaction of CD3O with NO3 do not differ greatly from scenario 1 to scenario 2. This is because the values of k10a and k17a are to a large extent established by the rapid change in concentration of CD3O2 from the extrapolated initial concentration to our first measured data point. The observation of reaction products such as DCDO (10b,17b), DONO (10b), DNO3 (17b), or CD3O2NO2 (17c) would shed light on this issue. Unfortunately, a search for these products was inconclusive due to strong interferences in the mass spectrum. Kukui et al.24 detected DCDO formation by LIF and assigned it to reaction 17b. They conclude that the reaction between CD3O2 and NO3 generates CD3O with a branching ratio in excess of 90%. We also draw analogy to the reaction of HO2 with NO3 and find that the chain propagating channel (formation of OH) dominates, i.e., the reaction HO2 + NO3 f HNO3 + O2, which is the equivalent of CD3O2 + NO3 f DNO3 + DCDO + O2, is only minor. In addition, CH3O shows a high propensity to transfer a H atom to form stable products in its reactions with radicals, i.e., reactions 21, 23, and 24:
CH3O + ClO f HCHO + HOCl
(23)
CH3O + NO f HCHO + HNO
(24)
Because of these considerations we prefer scenario 2 to scenario 1. We note, however, that our data does not indicate which stable product channel in the reaction between CD3O and NO3 is taking place. Possible reaction channels and their enthalpies for reaction 10 and 17 are listed by Crowley et al.11 and Dae¨le et al.,13 respectively. We now compare the scenario 2 results of this study with previous investigations of the title reaction that have employed both normal and deuterated methylperoxy radicals. As already mentioned, the previous work by Crowley et al. suffered from potential systematic errors due to the unexpected presence of a dark source of NO311,12 which may explain the rather high rate constant obtained in this work. The experiments of Crowley et al. avoided the quasi-equilibrium of CH3O and CH3O2 as observed in this work by using several Torr of O2 to scavenge the CH3O radical. Despite this, the results were obtained by modeling a complex reaction scheme and monitoring only the NO3 radical. In view of this and the fact that the results for
k10a overlap within the error limits, the results of Crowley et al. and the present study can be considered to show broad agreement. Two more recent studies of the title reaction have also utilized flow-tube experiments, coupled with LIF detection of the CH3O product, and were blind to CH3O2. Both studies report a quasiequilibrium between CH3O and CH3O2 and therefore required numerical modeling of the data. Despite this, the consensus regarding the rate constant for the title reaction seems to be good with values of k10a ) (1.2 ( 0.6) × 10-12 cm3 molecule-1 s-1 13 and (1.0 ( 0.6) × 10-12 cm3 molecule-1 s-1 14 comparing well with our result of (1.3 ( 0.2) × 10-12 cm3 molecule-1 s-1. However, the LIF studies did not consider the reaction of CH3O with NO3 to form stable products, i.e., (17b) and (17c). This is in contrast to our indirect observations of a [NO3]dependent decay of CD3O and to those of a combined mass spectrometry/LIF investigation of the CD3O/CD3O2/NO3 system which yielded ca. 40% branching into channel 17b.24 In addition, the rate constants derived for reaction 17a are different with values of k17 ) (1.8 ( 0.5) × 10-12 cm3 molecule-1 s-1 (Dae¨le et al.13) and k17 ) (2.3 ( 0.7) × 10-12 cm3 molecule-1 s-1 (Biggs et al.14). Our value is (3.7 ( 0.8) × 10-12 cm3 molecule-1 s-1. The available data from the literature (rate constants in units of cm3 molecule-1 s-1) is given below:
this work Crowley11 Dae¨le13 Biggs14 Kukui24
1012k10a
1012k17a
1013k17b
1.3 ( 0.2 2.3 ( 0.7 1.2 ( 0.6 1.0 ( 0.6 3.1 ( 0.5
3.7 ( 0.8
5.6 ( 1.0
1.8 ( 0.5 2.3 ( 0.7 1.7 ( 0.4
15 ( 4
The error limits in the present study are conservative and include both the errors returned by the fit routine and a propagation of estimated errors in a flow tube experiment. The rate constants of Dae¨le et al.13 are averaged from fits to single data sets which show large variability from one experiment to the next. A comparison between single experiments (their Table 4) indicates variations of a factor of 3 or more for both rate constants, e.g., values of between 0.77 × 10-12 and 2.79 × 10-12 cm3 molecule-1 s-1 were obtained for k17. The data of Biggs et al. were also analyzed by fitting to the CH3O profile from a single experiment. The parameter measured is the ratio k10/k17. This varied between 0.3 and 0.58 but was considered to be well determined. We note that the data of Biggs et al. could only be fitted by assuming large and variable (over more than a factor of 3) wall-loss rate constants for the CH3O radical, which is unexpected and disagrees with the constant wall-loss rate measured by Dae¨le et al. of ca. 30 s-1. It is clear that the convergence in the values of k10a may be somewhat fortuitous. This is confirmed by the data of Kukui et al.24 One might expect that the simultaneous measurement of both CD3O and CD3O2 would give the best pair of rate constants for reactions 10a and 17a. However, both the absolute values of k10a and k17a and ratio k17/k10 differ greatly. The latter of these should be a well-determined number. The source of the discrepancy is uncertain, though it may be related to the rather large fluctuations in the CD3O2 signal of Kukui et al. which result in a poorly defined kinetic profile. We do not expect that the rate constants for the oxygen atom transfer reactions of CD3O2 and CD3O with NO3 will differ from those of CH3O2/CH3O (see below) and rule this out as a source of any discrepancy between the deuterated and nondeuterated studies. However, the reaction between CD3O and NO3 to give DNO3 and DCDO (17b) is a D-atom abstraction.
Reactions of CD3O2 with NO and NO3 at 298 K We would expect that the rate constant for the equivalent H-atom abstraction is larger than that obtained for k17b in this work. We suggest that some of the “wall loss” simulated by Biggs et al. was in fact due to reaction 17b. As mentioned already, this could be confirmed by the combined LIF/MS study.24 Clearly, none of the experimental determinations of the reaction between methylperoxy radical and NO3 can be described as definitive. The advantage offered by our multifit approach is that [NO3]-independent losses of CD3O are separated from the reaction of CD3O with NO3 to regenerate CD3O2 or to form stable products. A disadvantage is that our rate constants k10a and k17a are strongly dependent on an accurately back-extrapolated initial concentration of CD3O2 as our first measurement point is often taken shortly before or after equilibrium has been reached. This disadvantage is shared by all low-pressure flow-tube studies in which the minimum reaction time is defined by the rate of mixing of the gas flows that separately carry the reactants into the region of the flow tube where the reaction takes place. Concentrations at zeroreaction time are therefore carried over from experiments in which the reactant in excess is removed. This applies not only to our results but also to those of Dae¨le et al.13 and Biggs et al.14 However, in the LIF studies where the chemistry is initiated by mixing CH3O and NO3 to make the CH3O2, the back reaction is then the slower of the two reactions that establish the equilibrium, and the approach to equilibrium may resemble an exponential decay (of CH3O) over two or more measurement points (see Figure 3 of Dae¨le et al.13). In lieu of the uncertainties associated with the rate constant for the reaction between CH3O2 and NO3, we have used a simple chemical box model to assess the sensitivity of the predicted nighttime concentrations of HO2 and CH3O2 to this parameter. The model is essentially identical to that of Platt et al.4 in which the NO3 radical, generated in the reaction between NO2 and O3, reacts with CH3SCH3 (DMS) to generate CH3O2 and was initialized with the Platt et al. initial concentrations (O3 ) 40 ppb, DMS ) 0.5 ppb, HCHO ) 2 ppb, CH4 ) 1.7 ppm, NO2 varied as in Figure 10, CO between 0.1 and 1 ppm and covaried with NO2). We modeled the production of HO2 and CH3O2 over an 8 h night using rate constants of 2.3 × 10-12 cm3 molecule-1 s-1 for CH3O2 + NO3 (as in Platt et al.) and obtained a similar dependence of HO2 and CH3O2 on [NO2]i, with concentrations of about 5 × 106 molecules cm-3 HO2 and 1 × 108 molecules cm-3 CH3O2 when [NO2]i ) 109 molecules cm-3 (40 ppt). These results are summarized in Figure 10. When the reaction between CH3O2 and NO3 is fully neglected from the reaction scheme, almost identical results are obtained for [NO2]i e 40 ppt. At higher [NO2]i slightly higher concentrations of CH3O2 are predicted; i.e., reaction of CH3O2 with NO3 is a sink of CH3O2. The HO2 concentration is slightly lower when reaction 5 is omitted. This indicates that if CH3O2 radicals do not react with NO3 they can still undergo self-reaction ([NO]i ) 0) to form alkoxy radicals, which are the source of HO2. The stoichiometry of HO2 formation from methylperoxy self-reaction is about 0.3, which explains the decrease of predicted [HO2] at higher [NO2]i when reaction 5 is omitted. The production rate of OH radicals is lowered by about 20%. It is clear that a reaction between NO3 and CH3O2 has only a moderate influence on the concentration of peroxy radicals generated in this simple model. 2. CD3O2/CH3O2 + NO. Our measured room temperature rate constants for CH3O2 and CD3O2 with NO are (7.5 ( 1.0) × 10-12 and (8.6 ( 1.0) × 10-12 cm3 molecule-1 s-1,
J. Phys. Chem., Vol. 100, No. 45, 1996 17853
Figure 10. Nighttime peroxy radical formation as a function of [NO2]. The solid lines were calculated with k5 ) 2.3 × 10-12 cm3 molecule-1 s-1 and the dotted lines with the rate constant for reaction between NO3 and CH3O2 set to zero.
respectively. The error limits are derived Via a propagation of errors analysis26 and include error associated with the subtraction of the residual under the 47 and 50 amu masses.21 The result for CH3O2 + NO is in good agreement with other flow tube/ mass spectrometer measurements of the rate constant using chemical ionization ((7.5 ( 1.3) × 10-12 cm3 molecule-1 s-1 27), electron bombardment ionization ((8.0 ( 2.0) × 10-12 and (8.6 ( 2.0) × 10-12 cm3 molecule-1 s-1 28) and with the presently recommended value 7.7 × 10-12 cm3 molecule-1 s-1.10 The data on the CH3O2 reaction has recently been summarized and we refer the reader to ref 27 for a discussion of this. Our result for CD3O2 + NO is, within the error limits, indistinguishable from that for CH3O2. The only other measurement of CD3O2 + NO is that of Masaki et al.29 who conducted flash photolysis experiments with photoionization/mass spectrometer detection of CD3O2 and found k9 ) (10.9 ( 1.3) × 10-12 cm3 molecule-1 s-1. They also determined the rate constant for k8 and, although their absolute value for CH3O2 is higher than all previous measurements, the relative reactivity (k8/k9) was found to be close to unity k8 ) (11.2 ( 1.4) × 10-12 cm3 molecule-1 s-1). This is to be expected as the reaction between methylperoxy and NO proceeds Via a simple oxygen atom transfer mechanism and reactions of this nature are not expected to show a large kinetic isotope effect upon deuteration. Another example of this is seen in the reactions of HO2 and DO2 with NO which have similar rate constants.30 We are therefore confident that our rate constant measurements for CD3O2 with NO3 can be applied to the CH3O2 atmospheric reaction. Conclusions The rate constants for the reactions of CD3O2 with NO and NO3 were found to be (8.6 ( 0.1) × 10-12 and (1.3 ( 0.2) × 10-12 cm3 molecule-1 s-1, respectively. A rate constant due to the reaction between CH3O2 and NO was also obtained and is in good agreement with previous investigations (k8 ) (7.5 ( 1.0) × 10-12 cm3 molecule-1 s-1). The data for CD3O2 shows reasonable agreement with recent measurements using flow tube methods though this agreement may be fortuitous and further measurements of this rate constant using different techniques are desirable. As far as modeling of nighttime NO3/RO2 chemistry is concerned, the results of our simple box model suggest that the present kinetic data for the CH3O2 + NO3 reaction is sufficiently accurate.
17854 J. Phys. Chem., Vol. 100, No. 45, 1996 Acknowledgment. We gratefully acknowledge funding from the CEC within the environmental program (LABVOC). References and Notes (1) Wayne, R. P.; Barnes, I.; Biggs, P.; Burrows, J. P.; Canosa-Mas, C. E.; Hjorth, J.; Le Bras, G.; Moortgat, G. K.; Perner, D.; Restelli, G.; Sidebottom, H. Atmos. EnViron. 1991, 25A, 1. (2) Lightfoot, P. D.; Cox, R. A.; Crowley, J. N.; Destriau, M.; Hayman, G. D.; Jenkin, M. E.; Moortgat, G. K.; Zabel, F. Atmos. EnViron. 1992, 26A, 1805. (3) Wallington, T. J.; Dagaut, P.; Kurylo, M. J. Chem. ReV. 1992, 92, 667. (4) Platt, U.; Le Bras, G.; Poulet, G.; Burrows, J. P.; Moortgat, G. K. Nature 1990, 348, 147. (5) Mihelcic, D.; Klemp, D., Mu¨sgen, P.; Pa¨tz, H. W.; Volz-Thomas, A. J. Atmos. Chem. 1993, 16, 313. (6) Mellouki, A.; Le Bras, G.; Poulet, G. J. Phys. Chem. 1988, 92, 2229. (7) Hall, I. W.; Wayne, R. P.; Cox, R. A.; Jenkin, M. E.; Hayman, G. D. J. Phys. Chem. 1988, 92, 5049. (8) Becker, E.; Rahman, M. M.; Schindler, R. N. Ber. Bunsenges. Phys. Chem. 1992, 96, 776. (9) Mellouki, A., Talukdar, R. K., Bopegedera, M. R. P., Howard, C. J. Int. J. Chem. Kinet. 1993, 25, 25. (10) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina, M. J.; Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling, JPL Publication 94-26, Pasadena, 1994. (11) Crowley, J. N.; Burrows, J. P.; Moortgat, G. K.; Poulet, G.; Le Bras, G. Int. J. Chem. Kinetics 1990, 22, 673. (12) Crowley, J. N.; Burrows, J. P.; Moortgat, G. K.; Poulet, G.; Le Bras, G. Int. J. Chem. Kinetics 1993, 25, 795. (13) Dae¨le, V.; Laverdet, G.; Le Bras, G.; Poulet, G. J. Phys. Chem. 1995, 99, 1470.
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