214
J. Phys. Chem. 1996, 100, 214-219
Alkoxy Radical Isomerization in the OH Radical-Initiated Reactions of C4-C8 n-Alkanes Eric S. C. Kwok,† Janet Arey,*,‡ and Roger Atkinson*,‡ Statewide Air Pollution Research Center, UniVersity of California, RiVerside, California 92521 ReceiVed: July 19, 1995; In Final Form: October 3, 1995X
Isomerizations of the alkoxy radicals formed from the OH radical reactions with n-butane through n-octane and n-pentane-d12 through n-octane-d18 have been studied in the presence of NO at 296 ( 2 K and 740 Torr total pressure of air. In addition to carbonyls of the same carbon number as the alkane precursors, the previously predicted δ-hydroxycarbonyls were detected by direct air sampling atmospheric pressure ionization triple quadrupole mass spectrometry. The formation yields of carbonyl compounds containing the same number of carbons as the parent n-alkane decreased with increasing carbon number in the n-alkane, while the hydroxycarbonyl/carbonyl formation yield ratios increased markedly from n-butane through n-octane, in agreement with previous theoretical predictions.
Introduction
SCHEME 1
The alkanes constitute an important class of volatile organic compounds present in urban atmospheres.1-3 In the troposphere, the dominant loss process of alkanes is by reaction with the hydroxyl (OH) radical,4-8 resulting in calculated lifetimes for the C4-C8 n-alkanes of ∼2-6 days.9 In the presence of NO, the OH radical-initiated reactions of alkanes lead to the formation of alkoxy (RO•) radicals10
OH + RH f H2O + R˙ M
R˙ + O2 98 RO˙ 2 M
. RO2 + NO
(1) (2)
RONO2
(3a)
. RO + NO2
(3b)
Alkoxy radicals are key intermediates in the photooxidations of most organic compounds in the troposphere,10 and the fates of these radicals determine the products formed. In the troposphere, reactions of alkoxy radicals with NO and NO2 are of minor importance,10 and alkoxy radicals react with O2, unimolecularly decompose, or isomerize via a 1,5-H shift involving a six-member transition state.10 For example, the predicted reactions of the 1-butoxy radical (the simplest alkoxy radical that can undergo isomerization via a six-member transition state) in the presence of NO are shown in Scheme 1, with the δ-hydroxyalkyl radical reacting further to form the δ-hydroxycarbonyl, 4-hydroxybutanal.10 Isomerization of the alkoxy radical formed after the first isomerization (HOCH2CH2CH2CH2O˙ in Scheme 1) is predicted to be more rapid than the first isomerization,10 and hence reaction of this alkoxy radical with O2 is expected to be of minor importance.10 To date, few data exist concerning the isomerization of alkoxy radicals formed from alkanes.11-17 Apart from the studies of Do´be´ et al.15 and Eberhard et al.16 in which radical trap and derivatization techniques were used to identify and quantify the products formed after isomerization of the 2-pentoxy15 and 2- and 3-hexoxy16 radicals, respectively, the importance of alkoxy radical isomerization has been inferred from analyses of the products arising from the competing decomposition and O2 reaction pathways.11-14 These data indicate that * Authors to whom correspondence should be addressed. † Present address: Environmental Chemistry Division, Brookhaven National Laboratory, Upton, NY 11973. ‡ Also Department of Soil and Environmental Sciences, University of California, Riverside, CA 92521. X Abstract published in AdVance ACS Abstracts, December 1, 1995.
0022-3654/96/20100-0214$12.00/0
alkoxy radical decomposition and reaction with O2 become less important with increasing carbon number of the precursor n-alkane from n-butane to n-hexane.10,11 In this study we have investigated the isomerization of alkoxy radicals formed from the OH radical-initiated reactions of n-butane through n-octane and of n-pentane-d12 through noctane-d18 in air using an atmospheric pressure ionization mass spectrometer. Experimental Section Atmospheric Pressure Ionization Tandem Mass Spectrometer (API-MS/MS). Experiments were carried out in a ∼6500 L all-Teflon chamber interfaced to a PE SCIEX API III MS/MS direct air sampling, atmospheric pressure ionization (API) triple quadrupole mass spectrometer. The Teflon chamber is equipped with two parallel banks of blacklamps for irradiation, and all experiments were carried out at 296 ( 2 K and 740 Torr total pressure of purified air, or N2-O2 mixtures, at ∼5% relative humidity. Hydroxyl radicals were generated by the photolysis of methyl nitrite (CH3ONO) in air at wavelengths >300 nm,18 and NO was added to the reactant mixtures to suppress the formation of O3 and hence of NO3 radicals.18 The ionization region of the PE SCIEX API III MS/MS atmospheric pressure ionization triple quadrupole mass spectrometer is a point-to-plane design, with the point electrode being a discharge needle and the plane being part of the MS atmospheric pressure-to-vacuum interface flange.19 Sample introduction from the Teflon reaction chamber into the ion source was through a 25 mm diameter × ∼75 cm length Pyrex © 1996 American Chemical Society
OH Radical-Initiated Reactions of C4-C8 n-Alkanes
J. Phys. Chem., Vol. 100, No. 1, 1996 215
tube, at a flow rate of ∼18-22 L min-1 provided by a suction pump, with the collapsible Teflon chamber allowing sampling without dilution of the chamber contents. Under positive ion API conditions, protonated water hydrates (H3O+(H2O)n) (n ) ∼3-6 at 298 K and ∼5% relative humidity)20 generated by the corona discharge in the chamber diluent gas are responsible for the protonation of analytes,
H3O+(H2O)n + M f MH+(H2O)m + (n - m + 1)H2O
(4)
where M is the neutral analyte of interest. Ions are drawn by an electric potential from the ion source through the sampling orifice into the mass-analyzing first quadrupole (Q1) or third quadrupole (Q3). Neutral molecules and particles are prevented from entering the orifice by a counter flow of high purity nitrogen (“interface curtain gas”), and as a result of the declustering action of the curtain gas on the hydrated ions, the ions which are mass-analyzed are mainly protonated molecular ions ([M+H]+). The sensitivity of API-MS to analyte M is determined by the stability of MH+(H2O)m and the water vapor concentration in the samples,21 and the observed lack of a [M+H]+ signal for the n-alkanes in the API mass spectra of n-alkane-air mixtures indicates that hydrates of [alkane+H]+ were not stable under our instrumental conditions. The mass spectrometer was operated in the positive ion mode throughout these experiments. Both MS (scanning) and MS/ MS with collision-activated dissociation (CAD) were used to analyze the reactant and reaction mixtures. In the MS mode, mass spectra (each the sum of 10 scans) of the reactants and the reacted mixtures were obtained by the first quadrupole (Q1), with the second and third quadrupoles (Q2 and Q3) being operated in the “total-ion” radio-frequency-only mode. Each scan was acquired over the range 10-360 u using a step size of 0.2 u and a dwell time of 10 ms. The curtain gas flow (CG), orifice voltage (OR), and rod-offset voltage of the radiofrequency quadrupole (R0) of the API-MS used during sample analysis were adjusted to minimize the fragmentation of ions in the vacuum side of the orifice.22 While it was not possible to eliminate fragmentation prior to Q1, the system could be optimized to favor protonated molecular ions [M+H]+ and/or protonated dimer ions [M2+H]+. The quadrupole power supplies were adjusted so that unit resolution (50% valley definition) and 0.7 u peak width (at 70% peak height) were achieved, and the channel electron multiple (CEM) was operated in the pulse-counting mode. The protonated dimer ion of diisopropyl methylphosphonate (DIMP) was used for optimizing the instrument parameters CG, OR, R0, and CEM. Mass calibration and quadrupole power supply optimization were performed on ions at 29, 181, and 361 u, corresponding to [N2H]+, [DIMP+H]+, and [(DIMP)2+H]+, respectively. Under our instrumental conditions, C4-C8 carbonyls gave strong protonated molecular ions [M+H]+. Butanones, pentanones, hexanones, heptanones, and octanones were quantified by external calibration of the signal intensity of the [M+H]+ ions at 73, 87, 101, 115, and 129 u, respectively, to those of standards of 2-butanone, 3-pentanone, 3-hexanone, 4-heptanone, and 3-octanone, respectively, and assuming identical molar response for all ketone isomers with the same carbon number. The C4-C8 carbonyls chosen for calibration were those observed11-17 or expected10 to be formed from the respective C4-C8 alkane in highest yield, and the carbonyl concentrations were verified by gas chromatography with flame ionization detection (GC-FID). The API-MS response is reported to be influenced mainly by the structural groups present rather than by the number of carbon atoms.23 For example, the C4-C8 carbonyl compounds are expected to have similar molar responses, and from duplicate calibrations at carbonyl concentrations of (3.4-4.6) × 1011 molecules cm-3, measured molar responses of the ketones (normalized to 3-pentanone ) 1.00) were as follows: 2-butanone, 0.63; 3-pentanone, 1.00; 3-hex-
anone, 1.24; 4-heptanone, 0.76; 3-octanone, 1.78, with uncertainties in the absolute and relative response factors of (∼2030% (two standard deviations). The deuterated carbonyl compounds formed from the perdeuterated n-alkane reactions were quantified by assuming an identical molar response as the corresponding nondeuterated carbonyls under our instrumental conditions. Under our experimental conditions, alcohols and the hydroxycarbonyls 4-hydroxy-4-methyl-2-pentanone and 5-hydroxy-2-pentanone gave strong ion peaks at [M+H-H2O]+ in addition to generally weaker [M+H]+ and [M-H]+ ion peaks. The δ-hydroxycarbonyls (including 5-hydroxy-2-pentanone which is the only δ-hydroxycarbonyl commercially available) could not be collected and/or analyzed by GC using our sampling and analysis procedures. Accordingly, calibrations of the API-MS response factors for δ-hydroxycarbonyls were not carried out, and we therefore assumed that the relative response of C4-C8 hydroxycarbonyls compared to the carbonyls with the same carbon number was a constant for our API-MS analyses. Based on our carbonyl results where the molar response factors varied over a range of ∼3, our yield data must be viewed as semiquantitative. Two MS/MS modes of the API MS were employed in this work. In the “daughter” ion scan mode, the precursor ion was selected by Q1 and the fragment ions were scanned by Q3 after CAD in Q2. In the “parent ion” scan mode, the fragment ions were selected by Q3 and matched to their precursors by scanning Q1. Ultrapure argon was used as the collision gas in an open, radio-frequency-only, Q2 collision cell. The CAD spectrum of an analyte (sum of 60-200 scans) was acquired using a step size of 0.2 u and a dwell time of 10 ms. Optimization of MS/ MS instrumental parameters was performed by multiple reaction monitoring,24 and the instrument sensitivity was enhanced by reducing the resolution of the mass filtering quadrupole Q1. Experimental Conditions. The initial reactant concentrations (in molecules cm-3 units) were CH3ONO, NO, and n-alkane, (2.4-4.8) × 1013 each, with the initial concentrations of CH3ONO, NO, and n-alkane being equal in each experiment. Irradiations were carried out at the maximum light intensity for 2.3-7.8 min, corresponding to 23 ( 4% and 10 ( 3% consumption of the n-alkanes and the deuterated n-alkanes, respectively. Because the API-MS instrument was insensitive to the alkanes, they were analyzed during the experiments by GC-FID.17 The concentrations of the carbonyls employed in the API-MS calibrations were also measured by GC-FID.25,26 Chemicals. The chemicals used, and their stated purities, were as follows: n-heptane-d16 (99+% D), n-hexane, (99+%), n-hexane-d14 (99% D), n-octane (99+%), n-octane-d18 (98+% D), n-pentane (99+%), 4-hydroxy-4-methyl-2-pentanone (99%), 2-butanone (99+%), 2-hexanone (99+%), 3-hexanone (98%), 2-heptanone (98%), 3-heptanone (98%), 4-heptanone (98%), 2-octanone (98%), 3-octanone (98+%), and 4-methyl-2-pentanol (99%), Aldrich Chemical Company; n-pentane-d12 (98% D), n-pentanol-d12 (98% D), and n-hexanol-d13 (98% D), Cambridge Isotope Laboratories; n-heptane (Reagent Grade), Mallinckrodt; 2-pentanone and 3-pentanone (both Reagent Grade), Matheson Coleman and Bell; 4-octanone (98%), Pfaltz and Bauer; 5-hydroxy-2-pentanone (96+%), TCI America; n-butane (>99.0%) and NO (>99.0%), Matheson Gas Products. Methyl nitrite was prepared and stored as described previously.18 Results and Discussion CH3ONO-NO-n-alkane-air irradiations were carried out with analyses by GC-FID (for the n-alkane reactants) and APIMS, and Figure 1 shows the positive ion API mass spectra obtained after the reactions. As noted above, no API-MS signals from the alkanes were observed. H atom abstraction by the OH radical occurs at the various C-H bonds in the n-alkanes.10 Hence, by reactions 1-3 and reactions analogous to those shown in Scheme 1 for the 1-butoxy radical, isomeric carbonyl products
216 J. Phys. Chem., Vol. 100, No. 1, 1996
Figure 1. Positive API mass spectra of irradiated CH3ONO-NO-nalkane-air mixtures for n-butane, n-pentane, n-hexane, n-heptane, and n-octane. The [M+H]+, [M-H]+, and [M+H-H2O]+ peaks of the δ-hydroxycarbonyls are grouped by a solid line.
having the same number of carbons as the n-alkane reactant are formed. Previous studies using gas chromatography and Fourier transform infrared spectroscopy have identified and quantified butanal and 2-butanone from n-butane,12-14 2- and 3-pentanone from n-pentane,11,17 and 2- and 3-hexanone from n-hexane.16,27 Carbonyl products containing the same number of carbon atoms as the alkane precursor were observed by APIMS as their [M+H]+ ion peaks at 73, 87, 101, 115, and 129 u for the n-butane, n-pentane, n-hexane, n-heptane, and n-octane reactions, respectively (Figure 1), with the carbonyl [M+H]+ ion peaks from the n-hexane, n-heptane, and n-octane reactions being small. Identifications of carbonyl products of the same carbon number as the n-alkanes studied were carried out by matching the CAD spectra of the carbonyl [M+H]+ ions in the reaction mixtures (see, for example, Figure 2A) with those of 2-butanone, 2- and 3-pentanone, 2- and 3-hexanone, 2-, 3-, and 4-heptanone, and 2-, 3-, and 4-octanone. However, isomer-specific carbonyl compound identification was impossible because the CAD spectra of isomeric ketones and aldehydes (for example, of 2and 3-pentanone and of butanal and 2-butanone) gave similar fragment ions. (It should be noted that fragmentation occurring in the API source region generally results in fragment ions which are also observed in the CAD spectrum of the [M+H]+ ion.17,28) The signal intensity of the [M+H]+ ion peaks shown in Figure 1, therefore, represents the total of the isomeric carbonyls formed from the OH radical reaction with an n-alkane. The yields of total carbonyl compounds (ketones, except for the n-butane reaction which also includes a small amount of butanal12-14) obtained in this and previous13,16,17,27 work are given in Table 1. Based on the literature29 and estimated30,31 OH radical reaction rate constants for the n-alkanes studied and their ketone products, secondary reactions of the C4-C8 ketones with the OH radical were calculated to be e25%. As seen in Table 1, our carbonyl compound formation yields are consistent with the available literature data and decrease markedly with
Kwok et al.
Figure 2. CAD spectra of the [M+H]+ ions of (A) pentanones and (B) C5 hydroxycarbonyl(s) from the reaction of the OH radical with n-pentane and (C) authentic 5-hydroxy-2-pentanone.
TABLE 1: Measured Formation Yields of Carbonyls Containing the Same Number of Carbon Atoms as the Alkane Precursor from the OH Radical-Initiated Reactions of n-Alkanes in the Presence of NO at 296 ( 2 K and Atmospheric Pressure, Together with Room Temperature Literature Data alkane n-butane n-pentane n-pentane-d12 n-hexane
n-hexane-d14 n-heptane n-heptane-d16 n-octane n-octane-d18
yield
reference
0.7a 0.54b 0.25a 0.30 ( 0.03 0.18a 0.033a 0.035 ( 0.019c 0.077 ( 0.024d 0.060 ( 0.005e 0.15a 0.011a 0.011a 0.007a 0.014a
this work 10, 12-14 this work 17 this work this work 16 16 27 this work this work this work this work this work
a Estimated overall uncertainty is a factor of ∼2. b Calculated from the rate constant ratios for H atom abstraction from CH2 and CH3 groups,10 alkyl nitrate formation from the RO2 + NO reactions,10 and alkoxy radical decomposition, isomerization, and reaction with O2.12-14 c From the OH radical-initated reaction of n-hexane. d From NO-air irradiations of 2- and 3-hexyl nitrite. e Obtained from irradiated CH3ONO-NO-n-hexane-air (or O2-N2) mixtures at 296 ( 2 K with GC-FID analyses as described by Atkinson et al.17 The measured 3-hexanone formation yields, corrected for secondary reactions, were 0.060 ( 0.005 in air and 0.130 ( 0.012 in 75% O2 + 25% N2. 2-Hexanone was not identified or quantified in air, but observed with a formation yield of 0.010 ( 0.006 in 75% O2 + 25% N2.
increasing carbon number of the n-alkane. The estimated ketone yields from the OH radical reactions with n-pentane-d12 through n-octane-d18 are also given in Table 1 and again decrease with increasing carbon number of the deuterated n-alkane. The molecular weights of the expected δ-hydroxycarbonyl-
OH Radical-Initiated Reactions of C4-C8 n-Alkanes
J. Phys. Chem., Vol. 100, No. 1, 1996 217 The formation of C5 δ-hydroxycarbonyls from the n-pentane reaction was confirmed by the presence of an ion peak at 205 u ([M2+H]+) when the mass spectrometer was tuned to induce dimer formation, and fragment ions corresponding to the C5 δ-hydroxycarbonyl peaks at 101 and 85 u were also observed at 203 u ([M2-H]+) and 187 u ([M2+H-H2O]+), respectively. Because of the successful induction of dimer formation from C5 hydroxycarbonyl(s), whenever possible dimer formation was also used to confirm the identity of the protonated molecular ion peaks of the reaction products. Table 2 summarizes the ion peaks observed for the δ-hydroxycarbonyls and carbonyls of the same carbon number as the alkane precursor, and their corresponding dimers, from the n-alkane reactions. Ions corresponding to the C5 through C7 δ-hydroxycarbonyl dimers and their fragments were observed in the n-pentane through nheptane reactions (Table 2). For the n-butane reaction, only an ion peak at 159 u corresponding to the [M2+H-H2O]+ ion was observed in the API-MS of the reaction mixture, with the lack of [M2+H]+ and [M2-H]+ ions probably resulting from the low formation yield of 4-hydroxybutanal10-14 and the preferential loss of a water molecule after its protonation under our instrumental conditions. For n-octane, the signal intensities of ions corresponding to the C8 hydroxycarbonyl dimer and its fragments were too low to be distinguished from the background. Figure 2B shows the CAD spectrum of the C5 δ-hydroxycarbonyl(s) formed in the n-pentane reaction. This CAD spectrum is in excellent agreement with the CAD spectrum of an authentic sample of 5-hydroxy-2-pentanone (Figure 2, bottom). For the 1- and 2-pentoxy radicals formed in the n-pentane reaction (with formation of the 2-pentoxy radical dominating over formation of the 1-pentoxy radical10,29), isomerization reactions analogous to that shown in Scheme 1 are expected to yield 4-hydroxypentanal and 5-hydroxy-2pentanone, respectively.10 4-Hydroxypentanal is predicted to be formed in significantly lower yield than 5-hydroxy-2pentanone.10,17,29 Furthermore, 4-hydroxypentanal is calculated to be significantly more reactive than 5-hydroxy-2-pentanone.31 Hence, the CAD spectrum shown in Figure 2B is expected to be that of the sum of these two C5 δ-hydroxycarbonyls, but being mainly 5-hydroxy-2-pentanone. It is of interest to note that the presence of an intense ion signal at 43 u corresponds to the expected [CH3C(O)]+ fragment ion from 5-hydroxy-2pentanone, suggesting that the cyclization reaction of 1,4hydroxycarbonyls34 does not occur to a major extent in the gas phase.
Figure 3. Positive API mass spectra of authentic standards of 4-methyl2-pentanol [MW ) 102] (top), n-pentanol-d12 [MW ) 100 (prior to D/H exchange)] (middle), and n-hexanol-d13 [MW ) 115] (bottom).
(s) formed from the C4-C8 alkoxy radicals by Scheme 1 and analogous reaction schemes are 88 u (C4), 102 u (C5), 116 u (C6), 130 u (C7), and 144 u (C8). The product ion peaks observed at 89, 103, 117, 131, and 145 u, respectively (Figure 1), suggest that these are the [M+H]+ ions of the C4-C8 hydroxycarbonyls. Based on the observation of strong [M+HH2O]+ ion peaks from alcohols and hydroxycarbonyls (see, for example, Figures 3 and 2, respectively), the 71, 85, 99, 113, and 127 u ion peaks observed in the n-butane through n-octane reactions, respectively (Figure 1), are attributed to the [M+HH2O]+ ions of the δ-hydroxycarbonyls. A “parent ion” scan showed that the 101 u peak observed in the n-pentane reaction originated from the [M+H]+ ion at 103 u, by loss of H2. As shown in Figure 1, [M-H]+ ions were observed at 87, 101, 115, 129, and 143 u for the n-butane, n-pentane, n-hexane, n-heptane, and n-octane reactions, respectively. Nitrogen clusters [(N2)n(H3O)]+ and [(N2)mH]+ (where n g 0 and m g 1) and water clusters [(H2O)n(H3O)]+ (where n g 0) may be formed in the API interface,32,33 resulting in peaks at 19, 29, 37, 47, 55, 57, 73, 75, 85, and 103 u. The presence of these background peaks in the API mass spectra of the reaction mixtures complicates the identification of products whose protonated ions have the same molecular weight as the nitrogen and water clusters. For example, in the n-pentane reaction the [M+H]+ peak of C5 δ-hydroxycarbonyls at 103 u may also include a contribution from the [(N2)3H3O+] cluster.
In contrast to the n-butane reaction, reactions of the OH radical with n-pentane, n-hexane, n-heptane, and n-octane form more than one alkoxy radical capable of undergoing isomerization via a six-member transition state. Isomeric δ-hydroxycarbonyls are therefore expected to be formed from the n-pentane (see above), n-hexane, n-heptane, and n-octane reactions,10 appearing as single [M+H]+ and [M+H-H2O]+
TABLE 2: Summary of API-MS Ions for Hydroxycarbonyls, Carbonyls, and Their Corresponding Dimers in the Mass Spectra of the Irradiated CH3ONO-NO-n-Alkane-Air Mixtures hydroxycarbonyl
carbonyl
alkane
[M+H]
[M-H]
[M+H-H2O]
[M2+H]
[M2-H]
[M2+H-H2O]
[M+H]
[M2+H]
n-butane n-pentane n-hexane n-heptane n-octane
89 103 117 131 145
87 101 115 129 143
71 85 99 113 127
a 205 233 261 a
a 203 231 259 a
159 187 215 243 a
73 87 101 115 129
145 173 201 229 257
a
See text.
218 J. Phys. Chem., Vol. 100, No. 1, 1996
Kwok et al. TABLE 3: Summary of API-MS Ions for Hydroxycarbonyls and Carbonyls in the Mass Spectra of the Irradiated CH3ONO-NO-Deuterated n-Alkane-Air Mixtures alkane
[M+H]
n-pentane-d12 n-hexane-d14 n-heptane-d16 n-octane-d18
112 128 144 160
hydroxycarbonyl [M-D] [M+H-H2O]a 109 125 141 157
94 110 126 142
carbonyl [M+H] 97 113 129 145
a The [M+H-HDO]+ and [M+H-D2O]+ ion peaks were also observed at 1 and 2 u lower, respectively.
Figure 4. Positive API mass spectra of irradiated CH3ONO-NOdeuterated n-alkane-air mixtures for n-pentane-d12, n-hexane-d14, n-heptane-d16, and n-octane-d18. The [M+H]+, [M-D]+, and [M+HH2O]+ peaks of the δ-hydroxycarbonyls are grouped by a solid line.
peaks in the API mass spectra of the reaction mixtures. In addition, for the gC6 n-alkanes smaller alkoxy radicals formed from decomposition of the initially formed alkoxy radicals can lead to δ-hydroxycarbonyl(s) containing fewer carbon atoms than the precursor alkane. For example, the n-hexane reaction gave weak product ion peaks at 89, 87, and 71 u, and CAD spectra showed that these ion peaks were identical to those of the 4-hydroxybutanal product observed from the OH radical reaction with n-butane. The formation of 4-hydroxybutanal from the n-hexane reaction can arise from decomposition of the 2-hexoxy radical16 to form acetaldehyde plus the 1-butyl radical, with the subsequent reactions of the 1-butyl radical leading to 4-hydroxybutanal (Scheme 1). As can be seen from inspection of Figure 1, the API-MS spectra of the n-heptane and n-octane reactions gave ion peaks corresponding to the formation of the C4 and C5 and C4-C6 δ-hydroxycarbonyls, respectively. The aldehydes arising from the decompositions of the Cn alkoxy radicals formed from the Cn alkanes were also observed by API-MS. The positive ion API mass spectra of the OH radical reactions with n-pentane-d12, n-hexane-d14, n-heptane-d16, and n-octaned18 are shown in Figure 4. The [M+H]+ ion peak of the fully deuterated δ-hydroxycarbonyl product would occur at 113, 129, 145, and 161 u for the n-pentane-d12 through n-octane-d18 reactions, respectively. However, the previously observed rapid exchange of the D atom of the OD group with water vapor in the chamber to form the OH group35,36 leads to protonated deuterated δ-hydroxycarbonyl peaks appearing at 1 mass unit lower than expected (formation of, for example, [CD3C(O)CD2CD2CD2OH)+H]+ instead of [(CD3C(O)CD2CD2CD2OD)+H]+). That D/H exchange in the OD group occurred under our experimental conditions was demonstrated for npentanol-d12, with the [M+H]+ ion appearing at 100 u (Figure 3). Furthermore, that D/H exchange occurs only at the OD group was confirmed by the occurrence of the [M+H]+ peak
of n-hexanol-d13, CD3(CD2)4CD2OH, at 116 u (Figure 3). The ion peaks at 112, 128, 144, and 160 u from the n-pentane-d12 through n-octane-d18 reactions, respectively, are therefore assigned to protonated C5 hydroxycarbonyl-d9, C6 hydroxycarbonyl-d11, C7 hydroxycarbonyl-d13, and C8 hydroxycarbonyld15, respectively. These assignments are further supported by the occurrence of the corresponding [M+H-H2O]+ peaks at 94, 110, 126, and 142 u, respectively, and of [M+H-HDO]+ and [M+H-D2O]+ ion peaks at 1 and 2 u lower, respectively (Figure 4). Analogous to the situation for the nondeuterated n-alkane reactions, apart from the n-heptane reaction, the [M+H-H2O]+ ion was the major ion of the δ-hydroxycarbonyls observed in the spectra of the reactions of the deuterated n-alkanes (Figure 4) (for example, [CD3C(O)CD2CD2CD2]+ in the case of the n-pentane-d12 reaction), and a [M-D]+ peak (for example, [CD3C(O)CD2CD2CDOH]+ in the case of the n-pentane-d12 reaction) was also observed at 109, 125, 141, and 157 u from the n-pentane-d12 through n-octane-d18 reactions, respectively. Thus, despite optimizing the instrument to preserve the molecular ions of interest, the majority of the δ-hydroxycarbonyl molecules preferentially fragmented by loss of a water molecule after protonation. The ion assignments described above for the deuterated n-alkane reactions are summarized in Table 3. Alkoxy Radical Isomerization versus Reaction with Oxygen. Based on an empirical estimation method for assessing the relative importance of alkoxy radical decomposition versus reaction with O2,10 the reaction with oxygen in 1 atm of air at room temperature is predicted to be more important than decomposition for the alkoxy radicals formed from the n-alkanes studied here. The present study allows the relative importance of alkoxy radical isomerization versus oxygen reaction to be semiquantitatively assessed for the C4-C8 n-alkane photooxidations. A calculated (C5 hydroxycarbonyl/C5 carbonyl compound) formation yield ratio of 1.3 for the n-pentane reaction is derived from the pentanone formation yield data and the rate constant ratios kdecomp/kO2 and kisomer/kO2 for the decomposition, isomerization, and reaction with O2 of the 1-, 2-, and 3-pentoxy radicals reported by Atkinson et al.17 Assuming that the response of a Cn hydroxycarbonyl relative to that of a Cn carbonyl compound was a constant, the sum of the signal intensities of the [M+H]+, [M-H]+ and [M+H-H2O]+ ion peaks of the hydroxycarbonyls and the signal intensities of the carbonyl [M+H]+ ion peaks allow the approximate hydroxycarbonyl/carbonyl formation yield ratios given in Table 4 to be obtained. As seen from Table 4 and Figure 1, the hydroxycarbonyl/carbonyl yield ratio increases markedly with carbon number of the n-alkane, with carbonyl compound formation dominating for the n-butane reaction and hydroxycarbonyl formation dominating for the n-hexane through n-octane reactions. The corresponding hydroxycarbonyl/carbonyl yield ratios for the OH radical reactions of the deuterated alkanes were also calculated from the API mass spectra obtained after the reaction of n-pentane-d12 through n-octane-d18 and are given in Table 4. For the deuterated hydroxycarbonyls, the signal intensities of the [M+H]+, [M-D]+, [M+H-H2O]+, [M+H-HDO]+, and
OH Radical-Initiated Reactions of C4-C8 n-Alkanes TABLE 4: Hydroxycarbonyl/Carbonyl Formation Yield Ratios For the OH Radical-Initated Reactions of n-Butane through n-Octane in the Presence of NO hydroxycarbonyl/carbonyl ratio n-alkane n-butane n-pentane n-pentane-d12 n-hexane n-hexane-d14 n-heptane n-heptane-d16 n-octane n-octane-d18
this worka
calculatedb
0.04 0.14c 1.3 0.8 6.5 2.2 14 15 50 52
0.18 1.3 12 35 60
a Measured ion count ratios converted to a yield ratio using the calculated value for the (C5 hydroxycarbonyl/C5 carbonyl compound) formation yield ratio from the n-pentane reaction of 1.317 (see text). b Calculated using the product yield and rate constant data given in refs 10, 12-14, and 17. c Corrected for reaction of 4-hydroxybutanal with the OH radical (see text).
[M+H-D2O]+ ions were summed. The hydroxycarbonyl/ carbonyl yield ratios in Table 4 show no consistent deuterium isotope effect. Scheme 1 shows that both the reaction with O2 and the isomerization pathway involve H (or D) atom abstraction from a C-H (or C-D) bond, and a significant deuterium isotope effect on the hydroxycarbonyl/carbonyl yield ratios is not expected, consistent with our data. While the hydroxycarbonyls are calculated to be somewhat more reactive than the corresponding ketones toward the OH radical,30,31 the relative hydroxycarbonyl/carbonyl ratios obtained from our data are expected to parallel the formation yield ratios corrected for secondary reactions, except for the n-butane reaction where the 4-hydroxybutanal is estimated31 to be markedly more reactive (by a factor of ∼25) than 2-butanone10,29,30 (the major carbonyl formed). A 4-hydroxybutanal/ (butanone + butanal) formation ratio obtained from the APIMS ion peak intensities, approximately corrected for secondary reactions, is also given in Table 4. It is clearly apparent from the data given in Table 4, as well as those presented in Figures 1 and 4, that the formation of hydroxycarbonyls from the alkoxy radical isomerization reactions increases in importance with increasing carbon number in the n-alkane series and that δ-hydroxycarbonyls are important products of the atmospheric photooxidations of n-alkanes in the troposphere and are, in fact, the dominant products for gC5 n-alkanes. Acknowledgment. The authors gratefully acknowledge support of this research by the U.S. Environmental Protection Agency (through Cooperative Agreement CR-821787-01-0; Marcia C. Dodge, Project Officer) and the California Air Resources Board (through Contract A032-067; Bart B. Croes, Project Monitor) and thank the National Science Foundation (Grant ATM-9015361) and the University of California, Riverside, for funding the purchase of the PE SCIEX API III MS/ MS instrument. Although the research in this article has been supported by the U.S. Environmental Protection Agency and the California Air Resources Board, it has not been subjected to agency reviews and therefore does not necessarily reflect the
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