Products of the Gas-Phase Reactions of Linalool ... - ACS Publications

Linalool reacts rapidly in the gas phase in the troposphere with OH radicals, NO3 radicals, and O3, with a calculated lifetime due to these reactions ...
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Environ. Sci. Technol. 1997, 31, 896-904

Products of the Gas-Phase Reactions of Linalool with OH Radicals, NO3 Radicals, and O3 YONGHUI SHU,† ERIC S. C. KWOK,‡ ERNESTO C. TUAZON, R O G E R A T K I N S O N , * ,§,| A N D J A N E T A R E Y * ,§ Statewide Air Pollution Research Center, University of California, Riverside, California 92521

Linalool [(CH3)2CdCHCH2CH2C(CH3)(OH)CHdCH2] is a terpene derivative emitted from vegetation, including orange blossoms and certain trees and vegetation in the Mediterranean area. Linalool reacts rapidly in the gas phase in the troposphere with OH radicals, NO3 radicals, and O3, with a calculated lifetime due to these reactions of ∼1 h or less. The products of these gas-phase reactions have been studied in ∼6500-7900-L Teflon chambers using gas chromatography, in situ Fourier transform infrared absorption spectroscopy, and direct air sampling atmospheric pressure ionization tandem mass spectrometry. The products identified and their formation yields are as follows: from the OH radical reaction, acetone, 0.505 ( 0.047; 6-methyl-5hepten-2-one, 0.068 ( 0.006; 4-hydroxy-4-methyl-5-hexen1-al (or its cyclized isomer), 0.46 ( 0.11; from the NO3 radical reaction, acetone, 0.225 ( 0.052; 4-hydroxy-4-methyl5-hexen-1-al (or its cyclized isomer), 0.191 ( 0.051; and a non-quantified nitrooxycarbonyl; from the O3 reaction, acetone, 0.211 ( 0.024; 4-hydroxy-4-methyl-5-hexen-1al (or its cyclized isomer), 0.85 ( 0.14; 5-ethenyldihydro-5methyl-2(3H)-furanone, 0.126 ( 0.025; and HCHO, 0.36 ( 0.06. The formation routes to these products and the reaction mechanisms are discussed. Despite the complexity of linalool, a C10-hydroxydiene, the reaction products observed and quantified account for a significant fraction of the carbon reacted (especially for the OH radical and O3 reactions), with the carbon balances being 53 ( 8% for the OH radical reaction in the presence of NO, 20 ( 4% (plus the non-quantified, but anticipated to be major, nitrooxycarbonyl) for the NO3 radical reaction, and 78 ( 10% for the O3 reaction.

Introduction Linalool [(CH3)2CdCHCH2CH2C(CH3)(OH)CHdCH2; 3,7-dimethyl-1,6-octadien-3-ol] is a terpene emitted into the atmosphere from certain vegetation (1-7), including orange blossoms (1), the pine Pinus pinea in Italy (3-7), and other plant species common to Italy (3, 5) and Austria (2). In the troposphere, linalool reacts with OH radicals, NO3 radicals, * Authors to whom correspondence should be addressed. Phone: 909-787-4191; fax: 909-787-5004. † Environmental Toxicology Graduate Program. ‡ Present address: Environmental Toxicology Program, University of California, Riverside, CA 92521. § Also at the Department of Soil and Environmental Sciences, University of California, Riverside, CA 92521. | Also at Department of Chemistry, University of California, Riverside, CA 92521.

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and O3 (8) and has a calculated lifetime due to these gasphase reactions of ∼1 h or less (8). A measurement of the formation yield of OH radicals from the reaction of linalool with O3 has been made (8), and products have recently been reported from linalool-NOx-air irradiations (9). In this work, we have investigated the products of the gas-phase reactions of linalool individually with OH radicals, NO3 radicals, and O3. Surprisingly, in view of the generally poor overall carbon balance obtained for the reactions of O3 with alkenes (10, 11) and monoterpenes (10, 12), an almost complete carbon balance is obtained from the O3 reaction with linalool.

Experimental Section The majority of the experiments were carried out in ∼65006700-L all-Teflon chambers at 296 ( 2 K and 740 Torr total pressure of purified air at ∼5% relative humidity, with each chamber being equipped with two parallel banks of black lamps for irradiation. One of these chambers was interfaced to a PE SCIEX API III MS/MS direct air sampling, atmospheric pressure ionization (API) tandem mass spectrometer (1315). Experiments to determine the yield of HCHO from the O3 reaction with linalool were carried out in a 5800-L evacuable, Teflon-coated chamber fitted with an in situ multiple reflection optical system interfaced to a Nicolet 7199 Fourier transform infrared (FTIR) absorption spectrometer (11) at 298 K and 740 Torr total pressure of dry (300 nm (16), and NO was added to the reactant mixtures to suppress the formation of O3 and, hence, NO3 radicals (16). The initial CH3ONO, NO, and linalool concentrations were 2.4 × 1014, 2.4 × 1014, and 2.4 × 1013 molecule cm-3, respectively, and irradiations were carried out at 20% of the maximum light intensity for 1.2-3.3 min, resulting in reaction of up to 57% of the initially present linalool. NO3 radicals were generated by the thermal decomposition of N2O5 (17) in the presence of NO2 (18). The initial linalool and NO2 concentrations were 2.4 × 1013 and (8.6-30) × 1013 molecule cm-3, respectively, and 3-4 additions of N2O5 [at initial concentrations in the chamber of (0.52-1.2) × 1013 molecule cm-3] were made to the chamber during an experiment. The O3 reactions were carried out in the presence of sufficient cyclohexane to scavenge >95% of the OH radicals formed (8). The initial linalool and cyclohexane concentrations were 2.4 × 1013 and (1.3-3.6) × 1016 molecule cm-3, respectively, and 3-4 additions of O3 in O2 diluent (each addition corresponding to an initial O3 concentration in the chamber of ∼5 × 1012 molecule cm-3) were made to the chamber during an experiment. Samples of 100-500 cm3 volume were collected on Tenax-TA solid adsorbent for thermal desorption and product analysis by gas chromatography with flame ionization detection (GC-FID) and GC with mass spectrometry (GCMS) or FTIR (GC-FTIR) detection. Additional OH radical and O3 reactions were carried out from which 10-12-L volume samples were collected onto Tenax cartridges, eluted with diethyl ether, and derivatized to form the trimethylsilyl ethers from products containing a hydroxyl group (19). In preliminary experiments, N-trimethylsilylimidazole was used for derivatization, and in subsequent experiments a solution of 10% N-trimethylsilylimidazole/5% N,O-bis(trimethylsilyl)trifluoroacetamide/ 0.05% trimethylchlorosilane in dichloromethane was em-

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ployed. 1,4-Dichlorobenzene was added into the chamber prior to reaction to serve as an essentially inert internal standard. GC-FID response factors for trimethylsilyl derivatives were calculated relative to that for the internal standard using their effective carbon numbers (ECNs) (20). Linalool and selected products were quantified during these experiments by GC-FID. Gas samples of 100 cm3 volume were collected from the chamber onto Tenax-TA cartridges, which were thermally desorbed at ∼200 °C onto a 30 m DB-5MS megabore column in a Hewlett Packard (HP) 5710 GC, held at -20 °C and then temperature programmed to 200 °C at 8 °C min-1. GC-FID response factors for linalool, acetone, 6-methyl-5-hepten-2-one, and methyl vinyl ketone were measured as described previously (8, 11, 21). On a relative basis, the GC-FID response factors for acetone, linalool, 6-methyl-5-hepten-2-one, and methyl vinyl ketone agreed with ECNs as calculated by Scanlon and Willis (20) to within (7%, indicating that the calibration factors were accurate and that the sample collection and thermal desorption procedures were quantitative. The GC-FID response factors for other products not commercially available were then calculated using their ECNs (20). Gas samples of 100-500 cm3 volume were collected from the chamber onto Tenax-TA cartridges for product analyses by GC-MS and GC-FTIR. GC-MS analyses with electron impact ionization (EI) were carried out using a 60 m DB-5MS fused silica capillary column in a HP 5890 GC interfaced to a HP 5970 mass selective detector operated in the scanning mode, while analyses by positive ion chemical ionization (CI) with methane as the reagent gas used a similar column in a HP 5890 GC interfaced to a HP 5971 mass selective detector and HP G1072A CI detector. The GC-FTIR analyses were carried out with a 30 m DB-5MS capillary column in a HP 5890 GC interfaced to a HP 5965B FTIR detector. Teflon Chamber Experiments with API-MS/MS Analyses. Reactions of linalool with OH radicals (in the presence of NO), NO3 radicals, and O3 were carried out as described above. For the OH radical reactions, the initial linalool, CH3ONO, and NO concentrations were 2.4 × 1013 molecule cm-3 each, and irradiations were carried out for 2 min at 20% of the maximum light intensity. For the NO3 radical reactions, the initial linalool and NO2 concentrations were 2.4 × 1013 and 4.8 × 1013 molecule cm-3, respectively, and 1-2 additions of N2O5 [each addition corresponding to an initial N2O5 concentration in the chamber of (1.2-3.0) × 1013 molecule cm-3] were added to the chamber during an experiment. For the O3 experiments, the initial linalool and cyclohexane (when added) concentrations were 2.4 × 1013 and 2 × 1016 molecule cm-3, respectively, and 1-2 additions of O3 in O2 diluent [each addition corresponding to an initial O3 concentration in the chamber of (2.5-5) × 1012 molecule cm-3] were made to the chamber during an experiment. The 6500-L Teflon chamber was interfaced to a PE SCIEX API III MS/MS direct air sampling, atmospheric pressure ionization tandem mass spectrometer (API-MS) via a 25 mm diameter × 75 cm length Pyrex tube, with a sampling flow rate of 19-24 L min-1. All experiments were carried out in the positive ion mode (13-15). The operation of the APIMS in the MS (scanning) and MS/MS [with collision activated dissociation (CAD)] modes is described elsewhere (13-15). Use of the MS/MS mode with CAD allows the “daughter ion” or “parent ion” spectrum of a given ion peak observed in the MS scanning mode to be obtained (13-15). In the positive ion mode, as used in these API-MS and API-MS/MS analyses, protonated water hydrates (H3O+(H2O)n) [n ∼3-6 at 298 K and ∼5% relative humidity (22)] 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 (1)

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 or third quadrupole. Neutral molecules and particles are prevented from entering the orifice by a flow of high-purity nitrogen (curtain gas), and as a result of the declustering action of the curtain gas on the hydrated ions, the ions that are mass-analyzed are mainly protonated molecular ions ([M + H]+). Experiments in a 5800-L Evacuable Chamber with FTIR Analyses. Three experiments were carried out in this chamber to measure the formation yield of HCHO from the reaction of O3 with linalool. The initial linalool and cyclohexane concentrations were (1.19-1.27) × 1014 and 2.5 × 1016 molecule cm-3, respectively, and 1-3 additions of O3 in O2 diluent (each addition corresponding to an initial O3 concentration in the chamber of ∼2.5 × 1013 molecule cm-3) were made to the chamber during an experiment. The concentrations of linalool and HCHO were measured during these experiments by FTIR absorption spectroscopy (11), using a path length of 57.7 m and a spectral resolution (full width at half maximum) of 0.7 cm-1, with each FTIR spectrum being comprised of 64 averaged scans acquired over a 1.8 min total measurement time. Chemicals. The chemicals used and their stated purity levels were as follows: cyclohexane (high-purity solvent grade), American Burdick and Jackson; acetone (HPLC grade), Fisher Scientific; linalool (97%) and 6-methyl-5-hepten-2one (99%), Aldrich Chemical Company; N-trimethylsilylimidazole, N,O-bis(trimethylsilyl)trifluoroacetamide, and trimethylchlorosilane, Pierce Chemical Company; and NO (g99.0%), Matheson Gas Products. Methyl nitrite and N2O5 were prepared as described previously (16, 17) and stored at 77 K under vacuum. O3 was generated as needed using a Welsbach T-408 ozone generator. NO2 was prepared by reacting NO with an excess of O2 just prior to use.

Results Reaction with O3. GC-MS, GC-FID, and GC-FTIR analyses of reacted O3-linalool mixtures in the presence of sufficient cyclohexane to scavenge >95% of the OH radicals formed (8) showed the formation of three products. Acetone was identified on the basis of its GC retention times and MS and IR spectra. A second product from this reaction was identified from MS (23) and IR (24) library searches as 5-ethenyldihydro5-methyl-2(3H)-furanone (4-methyl-4-vinyl-γ-butyrolactone) [product III in the structures shown below].

The third, and major, product was observed as a nonbaseline resolved double peak with identical IR spectra as well as CI and EI mass spectra (obtained from GC-FTIR and GC-MS analyses, respectively) as shown in Figure 1A-C.

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The EI-MS and CI-MS spectra both have a highest mass ion peak at 127 amu. The IR spectrum shows absorption bands at 3645 cm-1, attributed to an O-H stretch, 3096 cm-1 (vinyl C-H stretch), 2715 and 2812 cm-1 (aldehydic C-H stretch), and a relatively weak CdO stretch absorption at 1742 cm-1. GC-MS (EI) analyses of the ether eluate of 10-12-L volume samples collected on Tenax cartridges before and after treatment with N-trimethylsilylimidazole showed that this third reaction product reacted to form a trimethylsilyl derivative giving two resolved peaks with identical mass spectra, with a highest mass peak at 185 amu. We observed that the highest mass peak of trimethylsilyl derivatives, MOSi(CH3)3, of hydroxy compounds (several alcohols as well as the hydroxycarbonyls 4-methyl-4-hydroxy-2-pentanone and 5-hydroxy-2-pentanone) in their EI-MS spectra are the [MOSi(CH3)3 - CH3]+ ions, as also observed previously (25). The 185 amu highest mass peak of the third product therefore corresponds to a product of molecular weight of 128 amu. This is consistent with the large m/z 113 fragment ion in the EI spectra of the underivatized sample being loss of a methyl group from the molecular ion (Figure 1C). The m/z 127 and m/z 111 ions in the CI mass spectra (Figure 1B) can then be interpreted as [M + H - H2]+ and [M + H - H2O]+ ions, respectively. GC-FTIR spectra of the two GC peaks following derivatization were identical and showed no evidence for a CdO stretch at ∼1700-1750 cm-1. Therefore, our IR and MS data are consistent with the third reaction product being CH2dCHC(CH3)(OH)CH2CH2CHO (4-hydroxy-4-methyl-5hexen-1-al) (product I) and/or its cyclized form 2-ethenyl2-methyl-5-hydroxytetrahydrofuran (product II), with partial resolution of the enantiomers prior to derivatization and complete resolution on the GC column as the trimethylsilyl ethers. The lack of a CdO stretch in the IR spectra of the trimethylsilyl derivatives suggest that they are present in the cyclic form.

(2) FIGURE 1. Infrared (IR) spectrum (A), chemical ionization mass spectrum (B), and electron impact ionization mass spectrum (C) of the major product from the reaction of O3 with linalool. The API-MS spectrum of a reacted O3-linalool-cyclohexane-air mixture after subtraction of the pre-reaction APIMS spectrum (linalool has major ion peaks at 155 amu, [M + H]+, and 137 amu, [M + H - H2O]+) is shown in Figure 2A, with dominant ion peaks at 111, 93, and 55 amu. API-MS/ MS parent ion scans showed that the majority of the API-MS ion peaks observed were fragment ions from the relatively weak 129 amu product ion peak, and an API-MS/MS CAD daughter ion spectrum of the 129 amu peak is shown in Figure 3. It has been observed that fragmentation occurring in the API source region generally results in ions that are also observed in the CAD spectra of the [M + H]+ ion (13-15). Therefore, the CAD spectrum shown in Figure 3 shows that the 129 amu ion peak and its fragmentation ions account for the majority of the major ion peaks observed in the API-MS spectrum of the O3 reaction with linalool (Figure 2A). The API-MS/MS data show that the dominant product of the O3 reaction with linalool has ion peaks at 129 amu (weak) and 111 amu, attributed to the [M + H]+ and [M + H - H2O]+ ion peaks of a product of molecular weight 128 containing an -OH group, consistent with our identification of the product as I and/or II, based on our GC-MS and GC-FTIR data (see above). The concentrations of acetone, I (or II), and III were measured by GC-FID from the reactions of O3 with linalool in the presence of cyclohexane. Products I (or II) and III are expected to react with O3 (10). The rate constants for the reaction of O3 with these products are expected to be similar

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to that for the reaction of O3 with propene, of 9.7 × 10-18 cm3 molecule-1 s-1 at 296 K (10) [a factor of ∼50 lower than the rate constant for the reaction of O3 with linalool, of 4.3 × 10-16 cm3 molecule-1 s-1 (8)]. The measured concentrations of I (or II) and III were therefore corrected for secondary reactions with O3 (26). The multiplicative factors, F, to take into account secondary reactions of the products with O3 increase with the rate constant ratio k(O3 + product)/k(O3 + linalool) and with the extent of reaction (26). The correction factors F were 95% of the OH radicals formed.

FIGURE 2. API-MS spectra of the products of the gas-phase reactions of linalool with (A) O3 (in the presence of sufficient cyclohexane to scavenge >95% of the OH radicals formed) and (B) OH radicals (in the presence of NO).

FIGURE 3. API-MS/MS spectrum of the 129 amu ion peak observed in the API-MS spectrum of the products formed from the reaction of O3 with linalool in the presence of cyclohexane (and shown in Figure 2A). Reaction with the OH Radical. GC-MS, GC-FID, and GC-FTIR analyses of irradiated CH3ONO-NO-linalool-air mixtures showed the formation of three major products. Acetone and 6-methyl-5-hepten-2-one [(CH3)2CdCHCH2CH2C(O)CH3] were identified by matching GC retention times and MS and IR spectra with those of the authentic standards. The third product had identical IR and MS spectra to the major product observed in the O3 reaction (see above) and identified as I (or II). Furthermore, collection of 10-12-L volume samples followed by derivatization gave an identical trimethylsilyl derivative as observed in the O3 reaction.

The API-MS spectrum of an irradiated CH3ONO-NOlinalool-air mixture (subtracting the pre-reaction API-MS spectrum of linalool) is shown in Figure 2B, and it is evident that this API-MS spectrum is similar to that obtained from the reaction of O3 with linalool (Figure 2A). API-MS/MS CAD spectra showed the presence of the identical 129 amu ion peak and its fragment ions as in the O3 reaction (Figure 3), consistent with the identification of a major product as I (or II). The concentrations of acetone, 6-methyl-5-hepten-2-one, I (or II), and linalool were measured during CH3ONO-NOlinalool-air irradiations. Because the products can also react with the OH radical, the measured concentrations of acetone, 6-methyl-5-hepten-2-one, and I (or II) were corrected for secondary reactions with the OH radical (26). Rate constants at room temperature (in units of 10-12 cm3 molecule-1 s-1) of linalool, 159 (8); acetone, 0.215 (10); 6-methyl-5-hepten2-one, 157 (27); and I (or II), 35-73 were used to calculate the factors F [the estimated rate constant for I is 5.3 × 10-11 cm3 molecule-1 s-1, and likely upper and lower limit estimates for the cyclic isomer II are 7.3 × 10-11 and 3.5 × 10-11 cm3 molecule-1 s-1, respectively, depending on the substituent factor of the -O- group in the cyclic ether ring (28)]. The multiplicative factors F calculated were CdC< bond of the (CH3)2CdCH- group (10). Based on the expected reactions of the resulting β-nitrooxyalkyl peroxy radicals (CH3)2C(OO˙ )CH(ONO2)CH2CH2C(CH3)(OH)CHdCH2 and (CH3)2C(ONO2)CH(OO˙ )CH2CH2C(CH3)(OH)CHdCH2 and of the corresponding β-nitrooxyalkoxy radicals (CH3)2C(O˙ )CH(ONO2)CH2CH2C(CH3)(OH)CHdCH2 (which can decompose or isomerize) and (CH3)2C(ONO2)CH(O˙ )CH2CH2C(CH3)(OH)CHdCH2 (which can decompose or react with O2), the likely products include acetone, I, and (CH3)2C(ONO2)C(O)CH2CH2C(CH3)(OH)CHdCH2. We observed acetone and I by GC analyses, with similar yields of 0.225 ( 0.052 and 0.191 ( 0.051, respectively, and tentatively observed the nitrooxycarbonyl by API-MS analyses (Figure 8B). We believe, based on the API-MS analyses which showed only the ion peaks associated with the nitrooxycarbonyl, that the nitrooxycarbonyl (CH3)2C(ONO2)C(O)CH2CH2C(CH3)(OH)CHdCH2 is the major product of the gas-phase NO3 radical

reaction with linalool, at least under the experimental conditions employed here. Interestingly, despite the complexity of linalool, a C10hydroxydiene, the reaction products observed and quantified account for a significant fraction of the carbon reacted (especially for the OH radical and O3 reactions), with the carbon balances being 53 ( 8% for the OH radical reaction in the presence of NO, 20 ( 4% (plus the non-quantified, but anticipated to be major, nitrooxycarbonyl) for the NO3 radical reaction, and 78 ( 10% for the O3 reaction.

Acknowledgments The authors gratefully thank the U.S. Environmental Protection Agency, Office of Research and Development (Assistance Agreement R8176161) and the National Science Foundation (Grant ATM-9414036) for supporting this research. The National Science Foundation (Grant ATM-9015361) and the University of California, Riverside, are also thanked for funds to purchase the PE SCIEX API MS/MS instrument. While this work has been supported in part by the U.S Environmental Protection Agency, it has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred.

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Received for review July 26, 1996. Revised manuscript received November 11, 1996. Accepted November 13, 1996.X ES960651O X

Abstract published in Advance ACS Abstracts, February 1, 1997.