Isomerization of â-HydroxyalkoxyRadicals Formed from the OHRadical-Initiated Reactions ofC4-C8 1-AlkenesE R I C S . C . K W O K , †

R O G E R A T K I N S O N , * , ‡ A N DJ A N E T A R E Y * , ‡

Statewide Air Pollution Research Center, University ofCalifornia, Riverside, California 92521

The dihydroxycarbonyl products of the isomerizationreactions of the â-hydroxyalkoxy radicals formedafter OH radical addition to the 1-alkenes 1-butenethrough 1-octene have been observed by direct airsampling atmospheric pressure ionization mass spec-trometry (API-MS). The experimental data obtainedprovide direct evidence for â-hydroxyalkoxy radicalisomerization via a six-member transition state. Ourdata show that the importance of â-hydroxyalkoxyradical isomerization in the OH radical-initiated reactionsof the 1-alkenes increases with carbon number,being unimportant for 1-butene and dominating for1-heptene and 1-octene.

IntroductionThe alkenes present in ambient air (1-3) originate fromboth anthropogenic and biogenic sources. In the tropo-sphere, alkenes react with OH radicals, NO3 radicals, andO3 (4), with the daytime OH radical reaction often domi-nating as their removal process (5, 6). While the kineticsof the OH radical reactions are reasonably well understoodfor a large number of alkenes (4, 7), only for a few alkeneshave product studies been conducted at room temperatureand 1 atm of air (4, 8-17). The gas-phase reactions of theOH radical with alkenes proceed mainly by initial additionto the >CdC< bond(s) to form a â-hydroxyalkyl radical (4,7), which then rapidly adds O2 to form the correspondingâ-hydroxyalkyl peroxy radical (4). For example, for 1-pen-tene:

In the presence of NO, the â-hydroxyalkyl peroxy radicalsreact with NO to form either the â-hydroxynitrate or theâ-hydroxyalkoxy radical plus NO2 (4, 18, 19):

The formation of â-hydroxynitrates from reaction 2a hasbeen observed from the OH radical-initiated reactions ofpropene (18) and cis-2-butene (19) in the presence of NO,and rate constant ratios k2a/k2b for the â-hydroxyalkyl peroxyradicals formed from propene and cis-2-butene of∼0.015-0.017 and 0.037 ( 0.009, respectively, were measured (18,19). Reactions of the â-hydroxyalkoxy radicals with NOand NO2 are of minor importance in the troposphere (4),and the â-hydroxyalkoxy radicals react with O2, decompose,or isomerize by a 1,5-H shift through a six-membertransition state (4). The reactions predicted (4) for the CH3-CH2CH2CH(OH)CH2O• radical formed after internal OHradical addition to 1-pentene, leading to “first-generation”products, are shown in Scheme 1.

Previous studies have shown at room temperatureand 1 atm of air that the HOCH2CH2O• radical formedafter OH radical addition to ethene both reacts with O2

and decomposes (9), while the â-hydroxyalkoxy radicalsformed from propene, 1-butene, and trans-2-butene pre-dominantly decompose (8, 10). However, the formationyields of the carbonyls RCHO + HCHO from the OH radicalreactions with the 1-alkenes (RCHdCH2) 1-pentene through1-octene decrease markedly with increasing carbon numberof the alkene (17), suggesting that isomerization of theintermediate â-hydroxyalkoxy radicals occurs. To date, theproducts predicted to be formed from these â-hydroxy-alkoxy radical isomerization reactions have not beenobserved.

In this study, we have used a direct air samplingatmospheric pressure ionization mass spectrometer (API-MS) to investigate the occurrence of isomerization ofâ-hydroxyalkoxy radicals formed in the OH radical-initiatedreactions of 1-butene through 1-octene in the presence ofNO at 296 ( 2 K and atmospheric pressure.

* Authors to whom correspondence should be addressed; fax: (909)787-5004.

† Present address: Environmental Chemistry Division, BrookhavenNational Laboratory, Upton, NY 11973.

‡ Also at the Department of Soil and Environmental Sciences,University of California, Riverside, CA 92521.

OH + CH3CH2CH2CH CH2

CH3CH2CH2CHCH2OH and CH3CH2CH2CH(OH)CH2 (1)

CH3CH2CH2CH(OO)CH2OH CH3CH2CH2CH(OH)CH2OO

..

O2

. .

O2

SCHEME 1

(2a)

(2b)

CH3CH2CH2CH(OH)CH2OO + NO

CH3CH2CH2CH(OH)CH2ONO2

CH3CH2CH2CH(OH)CH2O + NO2

.

.

M

Environ. Sci. Technol. 1996, 30, 1048-1052

1048 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 3, 1996 0013-936X/96/0930-1048$12.00/0 1996 American Chemical Society

Experimental SectionThe experimental methods used were similar to thosedescribed previously (20, 21). Reactions were carried outat 296 ( 2 K and 740 Torr total pressure of purified air orO2-N2 mixtures at ∼5% relative humidity in a ∼6500-Lall-Teflon chamber equipped with blacklamps for irradia-tion. Hydroxyl radicals were generated by the photolysisof methyl nitrite (CH3ONO) in air at wavelengths >300 nm(22), and NO was added to the reactant mixtures to suppressthe formation of O3 and hence of NO3 radicals (22). Theinitial CH3ONO, NO, and 1-alkene concentrations weremaintained equal in all experiments and were (2.4-4.8) ×1013 molecule cm-3 each. Irradiations of CH3ONO-NO-1-alkene-air (or N2 + O2) mixtures were carried out at 20%of the maximum light intensity for 2-5 min, resulting in23 ( 6% reaction of the initially present 1-alkene.

The reaction mixtures were analyzed with a PE SCIEXAPI III MS/MS direct air sampling atmospheric pressureionization tandem mass spectrometer interfaced to thecollapsible Teflon chamber via a 25 mm diameter× 75 cmlength Pyrex tube. All experiments were conducted in thepositive ion mode and with a sample flow rate of ∼22 Lmin-1. Under positive API conditions, a corona dischargein the sample stream from the chamber containing watervapor at∼5% relative humidity generates hydronium ion-water clusters, H3O+(H2O)n, which are responsible for theprotonation of analytes (23):

where M is the neutral analyte of interest. Ions are drawnby an electric potential from the ion source through thesampling orifice into the mass analyzing first quadrupole(Q1) or third quadrupole. Neutral molecules and particlesare prevented from entering the orifice by a counterflowof ultra pure nitrogen (“curtain”) gas. The [M + H]+ ionsmass analyzed are the result of the declustering action ofthe curtain gas, with the sensitivity of the API-MS to theanalyte M being determined by the stability of MH+(H2O)m

(23). It is not possible to eliminate fragmentation prior toQ1, but the system can be optimized to favor protonatedmolecular ions [M + H]+ and/or protonated dimer ions[M2 + H]+. The MS (scanning) mode was used to providemass spectra of the reactant and reacted mixtures. Daugh-ter ion and parent ion spectra of a given ion peak observedin the MS scanning mode were obtained in the MS/MSmode [with collision activated dissociation (CAD)] toprovide structural information and information concerningthe origin of an ion peak (i.e., whether it is a molecular ionpeak or a fragment ion peak) (21). For additional productconfirmation in certain experiments, the API-MS instrumentwas tuned to optimize dimer formation (20, 21). Becausethe API-MS instrument was relatively insensitive to thereactant alkenes in these experiments, the concentrationsof the alkenes were measured during the experiments bygas chromatography with flame ionization detection (GC-FID) (17).

The chemicals used and their stated purities were asfollows: 1-pentene (99%), 1-hexene (99%), 1-heptene (99%),1-octene (98%), propanal (99+%), butanal [n-butyralde-hyde] (99%), pentanal (99%), hexanal (99%), and heptanal(95%), Aldrich Chemical Company; 1-butene (>99%) andNO (g99.0%), Matheson Gas Products. Methyl nitrite was

prepared as described previously (22) and was stored at 77K under vacuum.

Results and DiscussionFigure 1 shows the API mass spectra of irradiated CH3-ONO-NO-1-alkene-air mixtures for 1-butene through1-octene. Spectra taken prior to irradiation showed nosignificant ion peaks from the alkene reactants. Previousstudies using GC-FID and in situ Fourier transform infrared(FTIR) absorption spectroscopy identified Cn-1 aldehydesand formaldehyde as products of the OH radical reactionswith these Cn 1-alkenes (10, 17). Carbonyl compounds givestrong protonated molecular ions ([M + H]+) under ourinstrumental conditions (20, 21) and, therefore, the ionpeaks observed at 59, 73, 87, 101, and 115 u (unified atomicmass unit) from the 1-butene, 1-pentene, 1-hexene, 1-hep-tene, and 1-octene reactions, respectively, were attributedto propanal, butanal, pentanal, hexanal, and heptanal,respectively. API-MS/MS CAD spectra of the 59, 73, 87,101, and 115 u ion peaks matched CAD spectra of the [M+ H]+ ion peaks of authentic standards of the correspondingaldehydes, confirming these assignments. Furthermore,the assumption that the ion peaks at the masses listed aboveare protonated molecular ion peaks and not fragment ionswas confirmed by operating the API-MS instrument under

H3O+(H2O)n + M f MH+(H2O)m + (n - m + 1)H2O(3)

FIGURE 1. API-MS spectra of irradiated CH3ONO-NO-1-alkene-air mixtures. The [M + H]+ ion peaks of the Cn-1 aldehyde productsformed from the Cn 1-alkenes are noted and shown by an arrow, andthe [M - H]+ and [M + H - H2O]+ ion peaks of the Cn-dihydroxycarbonyls are noted and shown by the connected verticallines (the [M + H]+ ion peaks of the dihydroxycarbonyls are tooweak to be readily observed in this figure).

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conditions favoring dimer formation and observing ionpeaks corresponding to the aldehyde dimer in the OHradical reactions with 1-pentene through 1-octene. Theabsence of an [M2 + H]+ ion of propanal from the 1-butenereaction is consistent with the lack of dimer formation fromauthentic samples of propanal in the chamber analyzedunder these instrumental conditions.

Although standards of â-hydroxycarbonyls and of di-hydroxycarbonyls are generally not available, we haveobserved strong [M + H - H2O]+ fragment ions from all thecompounds containing an alcohol functional group thatwe have examined by API-MS. These include authenticstandards of several C4-C8 alcohols, the δ-hydroxycarbonyl5-hydroxy-2-pentanone, and 4-hydroxy-4-methyl-2-pen-tanone, as well as the hydroxycarbonyls formed from theOH radical reactions with the C4-C8 n-alkanes (20, 21). Inaddition to the strong [M + H - H2O]+ ion peaks in ourAPI-MS spectra of alcohols and hydroxycarbonyls, weak[M + H]+ and [M - H]+ ion peaks were also observed (20,21).

In addition to the ion peaks corresponding to theformation of the aldehyde RCHO from the alkeneRCHdCH2, API-MS spectra of irradiated CH3ONO-NO-1-alkene-air mixtures showed the presence of prominenthigher mass ion peaks at 87, 101, 115, 129, and 143 u from1-butene, 1-pentene, 1-hexene, 1-heptene, and 1-octene,respectively (Figure 1). The molecular weights of theexpected dihydroxycarbonyls formed after isomerizationof the â-hydroxyalkoxy radicals formed from 1-butenethrough 1-octene are 104 (C4), 118 (C5), 132 (C6), 146 (C7),and 160 u (C8), suggesting that the observed prominantpeaks are the [M + H - H2O]+ ions of these dihydroxy-carbonyls. API-MS/MS daughter ion spectra of the weakion peaks observed at 119, 133, 147, and 161 u from the1-pentene through 1-octene reactions (Figure 1) as well asAPI-MS/MS parent ion spectra of the [M + H - H2O]+ ionsfrom these reactions were consistent with this interpreta-tion. The ion peaks at 117, 131, 145, and 159 u for the1-pentene through 1-octene reactions, respectively (seeFigure 1), are attributed to the [M - H]+ ions of thedihydroxycarbonyls.

As shown in Figure 2 for the 1-pentene reaction, theAPI-MS/MS CAD spectra of the 119, 133, 147, and 161 u [M+ H]+ ion peaks from the 1-pentene through 1-octenereactions can be rationalized by the expected fragmentationpattern of the dihydroxycarbonyls, with large peaks cor-responding to [M + H - H2O]+ and [M + H-2H2O]+ ionsbeing consistent with fragmentation of the diol under these

conditions. Ion peaks corresponding to [M + H-CO]+ and[M + H - H2O-CO]+ fragments were also observed in theseCAD spectra of the dihydroxycarbonyls (Figure 2).

Figure 3 shows the API mass spectra obtained from thereaction of the OH radical with 1-octene at low (∼4%),normal (21%), and high (∼80%) O2 levels, with 19 ( 4%1-octene reacted. The ion signals are interpreted asfollows: the signal at 115 u ([M + H]+ for heptanal) resultingfrom the decomposition pathway; the signals at 161, 159,and 143 u (C8-dihydroxycarbonyl) resulting from theisomerization pathway; and the signal at 127 u could be the[M + H - H2O]+ of the C8-hydroxycarbonyl (expected tobe the dominant peak of the hydroxycarbonyl API massspectrum) resulting from the O2 reaction. Our data fromtwo independent sets of experiments (one set being shownin Figure 3) showed that the integrated signal intensitiesof the 127 u ion peak relative to the integrated signalintensities of the 159, 143, or 115 u ion peaks varied byfactors of e1.8. Furthermore, the integrated signal intensi-ties of the 115 u ion peak relative to those of the 143 u ionpeak also varied by a factor of <1.8. All of the above changesin ion peak relative intensities are estimated to be withinthe reproducibility of replicate experiments. Therefore, itis concluded that varying the oxygen concentration by afactor of 20 had no significant effect on the observedproducts. The formation of heptanal by decomposition ofthe intermediate â-hydroxyalkoxy radical (Scheme 1) doesnot involve O2 in the initial, rate-determining step. Thus,formation of the ion peaks at 161, 159, and 143 u also cannotinvolve O2 in the rate-determining step(s), consistent with

FIGURE 2. API-MS/MS CAD spectrum of the 119 u [M + H]+ ion ofthe C5-dihydroxycarbonyl formed from the OH radical-initiatedreaction of 1-pentene.

FIGURE 3. API-MS spectra of irradiated CH3ONO-NO-1-octene-N2-O2 mixtures at 4% (upper), 21% (middle), and 80% (lower) O2

levels. The [M + H]+ ion peak of heptanal is at 115, and the [M +H]+, [M - H]+, and [M + H - H2O]+ ion peaks of the C8-dihydroxycarbonyls are at 161, 159, and 143 u, respectively.

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their formation from the isomerization reaction of theâ-hydroxyalkoxy radical.

As noted above, under our instrumental conditions, thedominant ion peak observed from hydroxycarbonyls is the[M + H - H2O]+ ion (20, 21). The low signal intensities ofthe ion peaks at 71, 85, 99, 113, and 127 u for the 1-butenethrough 1-octene reactions, respectively (Figure 1), whichcould be the [M + H - H2O]+ ions of Cn-hydroxycarbonylsformed from the reaction of the â-hydroxyalkoxy radicalswith O2 (Scheme 1), indicate that reaction of the â-hy-droxyalkoxy radicals with O2 is a minor pathway for these1-alkenes under atmospheric conditions. In particular, theweak 127 u ion peak observed in the 1-octene reaction(Figure 3) shows that the O2 reaction of the â-hydroxyalkoxyradicals is of minor importance even at O2 levels a factorof 4 higher than in the ambient atmosphere. Therefore,any contribution of the [M - H]+ ion of the hydroxycar-bonyls to the [M + H - H2O]+ ion peak of the dihydroxy-carbonyls is expected to be negligible.

Based on the calculated heat of reactions for the O2

reactions and decomposition (24) and using the empiricalestimation method proposed by Atkinson (4), decomposi-tion of the â-hydroxyalkoxy radicals formed after OH radicaladdition to the 1-alkenes 1-butene through 1-octene isestimated to dominate over their reactions with oxygen,consistent with our API-MS data. The present work allowsthe relative importance of isomerization versus decom-position of these â-hydroxyalkoxy radicals to be assessed.From the signal intensities of the dihydroxycarbonyl (sumof the [M + H]+, [M - H]+, and [M + H - H2O]+ ion peaks)and aldehyde [M + H]+ ion peaks and assuming that theresponses of Cn-dihydroxycarbonyls relative to Cn-1-aldehydes were constant, approximate relative dihydroxy-carbonyl/aldehyde yield ratios are obtained. These dihy-droxycarbonyl/aldehyde yield ratios are given in Table 1,relative to a dihydroxycarbonyl/aldehyde ratio of 1.0 forthe 1-hexene reaction based on the pentanal molar forma-tion yield of 0.46 ( 0.07 (17) and assuming that isomerizationof the â-hydroxyhexoxy radicals leading to C6-dihydroxy-carbonyls accounts for the majority of the rest of the reactionproducts. Our previously measured aldehyde (RCHO)formation yields from the reactions of the OH radical withthe RCHdCH2 1-alkenes are also given in Table 1, and veryapproximate dihydroxycarbonyl yields are derived (sec-ondary reactions of the carbonyls and dihydroxycarbonyls

under our reaction conditions are calculated to be e25%).Table 1 clearly shows, as also evident from Figure 1, thatthe formation of dihydroxycarbonyls increases markedlywith increasing carbon number of the 1-alkene and thatdecomposition of the â-hydroxyalkoxy radicals formed fromthe 1-butene and 1-pentene reactions dominates whileisomerization of the â-hydroxyalkoxy radicals formed fromthe 1-heptene and 1-octene reactions dominates. Thisobserved increase in importance of â-hydroxyalkoxy radicalisomerization reaction in going from 1-butene to 1-octeneis expected, since for the 1-butene reaction only the CH3-CH2CH(OH)CH2O• radical can isomerize by a six-membertransition state (by H-atom abstraction from the CH3 group),for the 1-pentene reaction isomerization of the CH3CH2-CH2CH(OH)CH2O• radical occurs by H-atom abstractionfrom a CH2 group and isomerization of the CH3CH2CH2-CH(O•)CH2OH radical occurs by H-atom abstraction froma CH3 group, and for the 1-hexene, 1-heptene, and 1-octenereactions both of the â-hydroxyalkoxy radicals can undergoisomerization by the more rapid H-atom abstraction fromCH2 groups (4, 20). The formation of dihydroxycarbonylsfrom the gC5 1-alkenes and the subsequent atmosphericfate of these dihydroxycarbonyls must now be taken intoaccount.

AcknowledgmentsThe authors gratefully thank the U.S. EnvironmentalProtection Agency for supporting this research throughCooperative Agreement CR-821787-01-0 (Dr. Marcia C.Dodge, Project Officer) and thank the National ScienceFoundation (Grant ATM-9015361) and the University ofCalifornia, Riverside, for funds for the purchase of the PESCIEX API III MS/MS instrument. While this research hasbeen supported by the U.S. Environmental ProtectionAgency, it has not been subjected to Agency review and,therefore, does not necessarily reflect the views of theAgency, and no official endorsement should be inferred.

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TABLE 1

Estimated Dihydroxycarbonyl/Aldehyde Yield Ratiosfor Reactions of OH Radical with C4-C8 1-Alkenes(RCHdCH2) in the Presence of NO, Together withAldehyde (RCHO) Formation Yields and EstimatedDihydroxycarbonyl Yields

1-alkene RCHO yieldadihydroxycarbonyl/

aldehyde ratiobdihydroxycarbonyl

yieldb

1-butene 0.94 ( 0.12 0.04 0.041-pentene 0.73 ( 0.09 0.2 0.151-hexene 0.46 ( 0.07 1.0 0.461-heptene 0.30 ( 0.04 3 0.91-octene 0.21 ( 0.03 3 0.6

a From refs 10 and 17. b The measured relative dihydroxycarbonyl/aldehyde yield ratios are placed relative to a ratio of 1.0 for the 1-hexenereaction, based on the pentanal formation yield and assuming thatdihydroxycarbonyl formation accounts for the majority of the remainderof the reaction products (see text).

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(16) Hakola, H.; Arey, J.; Aschmann, S. M.; Atkinson, R. J. Atmos.Chem. 1994, 18, 75-102.

(17) Atkinson, R.; Tuazon, E. C.; Aschmann, S. M. Environ. Sci. Technol.1995, 29, 1674-1680.

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(19) Muthuramu, K.; Shepson, P. B.; O’Brien, J. M. Environ. Sci.Technol. 1993, 27, 1117-1124.

(20) Atkinson, R.; Kwok, E. S. C.; Arey, J.; Aschmann, S. M. Dicuss.Faraday Soc., in press.

(21) Kwok, E. S. C.; Arey, J.; Atkinson, R. J. Phys. Chem., in press.(22) Atkinson, R.; Carter, W. P. L.; Winer, A. M.; Pitts, J. N., Jr. J. Air

Pollut. Control Assoc. 1981, 31, 1090-1092.

(23) Sunner, J.; Nicol, G.; Kebarle, P. Anal. Chem. 1988, 60, 1300-1307.

(24) National Institute for Standards and Technology StandardReference Database 25. Structure and Properties, Version 2.0;Stein, S. E., Gaithersburg, MD, 1994.

Received for review August 10, 1995. Revised manuscriptreceived October 24, 1995. Accepted October 25, 1995.X

ES950584I

X Abstract published in Advance ACS Abstracts, January 1, 1996.

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