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Products of the gas-phase reactions of cis-3-hexen-1-ol with OH radicals and O3
Atmospheric Environment Vol. 31, No. 21, pp. 3551-3560, 1997 © 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 1352-2310/97 $1%00 + 0.00
Pergamon PII:
S1352-2310(97)00205-7
P R O D U C T S O F T H E GAS-PHASE R E A C T I O N S O F cis-3-HEXEN-1-OL W I T H O H RADICALS A N D 03 SARA M. ASCHMANN, YONGHUI SHU,i" JANET AREY*t:~ and ROGER ATKINSON*I'~:§ Statewide Air Pollution Research Center, University of California, Riverside,CA 92521, U.S.A. (First received 19 November 1996 and in final form 27 March 1997. Published Au#ust 1997) Abstract--Products of the gas-phase reactions of OH radicals (in the presence of NO) and 03 with the biogenicemi:;sioncis-3-hexen-l-ol have been investigated using gas chromatography with flame ionization detection (GC-FID), combined gas chromatography-mass spectrometry (GC-MS), gas chromatography with Fourier transform infrared spectroscopy (GC-FTIR), and direct air sampling, atmospheric pressure ionization tandem mass spectrometry (API-MS). Propanal and 3-hydroxypropanal were identified and quantified from the OH radical and 03 radical reactions, with respectiveformation yields of 0.746 + 0.067 and 0.48-0.24 +0.48 from the OH radical reaction and 0.493 + 0.075 and 0.33-o:16 +o 33 from the 03 reaction. In addition, a hydroxynitrate of molecular weight 179 [CH3CH2CH(OH)CH(ONO2)CH2CHzOH and/or CH3CH2CH(ONO2)CH(OH)CH2CH2OH] and a hydroxycarbonyl of molecular weight 132, expected to be CHaCH2CH(OH)CH(OH)CH2CHO, were observed from the OH radical reaction by API-MS analyses. The reaction mechanisms are discussed. © 1997 Elsevier ScienceLtd. Key word index: cis-3-Hexen-l-ol, hydroxyl radical, ozone, atmospheric reactions, reaction products.
INTRODUCTION cis-3-Hexen-l-ol [CH3CH~CH =CHCH2CH2OH], leaf alcohol, is an oxygenate emitted into the atmosphere from certain vegetation (Schulting et al., 1980; Buttery et al., 1982~ 1985; Ohta, 1984; Isidorov et al., 1985; Lwande and Bentley, 1987; Arey et al., 1991; K6nig et al., 1995), and the physiological function of its release in response to plant wounding has recently been described (Sharkey, 1996). In the troposphere, cis-3-hexen-l-ol re~Lcts with OH radicals, NO3 radicals and 03 (Gros]ean et al., 1993a; Atkinson et al., 1995a), with a calculated lifetime due to these reactions of a few hour,,; (Atkinson et al., 1995a). Selected products have been identified and measured from the NOx-air photooxidation of cis-3-hexen-l-ol (Grosjean et al., 1993b) and from the 03 reaction (Grosjean et al., 1993c; Atkinson et al., 1995a). Specifically, propanal was observed as a major product of the NOx-air photooxidation (Grosjean et al., 1993b), and propanal and OH radicals were observed from the 03 reaction with reported yields of 0.59 _+ 0.12
*Authors to whom correspondence should be addressed. tAlso Environmental Toxicology Graduate Program. :~AlsoDepartment of Soil and Environmental Sciences University of California, Riverside, CA 92521, U.S.A. §Also Department of Chemistry, University of California, Riverside, CA 92521, U.S.A.
(Grosjean et al., 1993c) and 0.26-+°:~ (Atkinson et al., 1995a), respectively. In this work, we have carried out more extensive studies of the products formed from the gas-phase reactions of OH radicals and 03 with cis-3-hexen-1ol, including analyses of the reaction products by atmospheric pressure ionization mass spectrometry.
EXPERIMENTAL All experiments were carried out in 6500-7900 t' Teflon chambers, each equipped with two parallel banks of blacklamps for irradiation and with a Teflon-coatedfan to ensure rapid mixing of reactants during their introduction into the chamber. Products were analyzed from the OH radical and 03 reactions by gas chromatography with flame ionization detection(GC-FID) and combinedgas chromatography-mass spectrometry (GC-MS). Because expected hydroxycarbonyl products did not appear to be directly amenable to gas chromatography, in one set of experiments trimethylsilyl derivatives of hydroxy-compounds were formed prior to the GC-FID and GC-MS analyses.Atmospheric pressure ionization tandem mass spectrometry (API-MS) was also used to identify products from the OH radical-initiated reaction. Separate sets of experimentswere carried out for these differing types of analyses. Hydroxyl radicals were generated by the photolysis of methyl nitrite (CHaONO) in air at wavelengths >300 nm (Atkinson et al., 1981) and NO was added to the reactant mixtures to suppress the formation of 03 and of NO3 radicals. The initial CHaONO, NO and cis-3-bexen-l-ol concentrations were 2.4x 1014, 2.4x 1014, and (2.43-5.30)x 1013 moleculecm-3, respectively, for the experiments with
3551
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S.M. ASCHMANN et al.
GC-FID analyses, and (2.4-4.8)x 1013 molecule cm -3 each for the experiments with API-MS analyses. Irradiations were carried out at 20% of the maximum light intensity for 1-3.5 min for the experiments with GC without derivatization (resulting in 21-60% reaction of the initial cis-3hexen-l-ol), 3-8 min for the experiments with derivatization (resulting in 57-90% reaction of the initial cis-3-hexen1-ol), and 1-1.5 min for the experiments with API-MS analyses. For the experiments with GC analyses without derivatization, the 0 3 reactions were carried out in the presence of sufficient cyclohexane to scavenge > 95% of the OH radicals formed from the 03 reaction with cis-3-hexen-l-ol (Atkinson et al., 1995a), with initial cis-3-hexen-l-ol and cyclohexane concentrations of (4.61-5.03) x 1013 and (1.8-2.1) x 10 t6 molecule cm-a, respectively. Four additions of 50 c m 3 volume of 03 in 0 2 diluent (each addition corresponding to an initial 0 3 concentration of ~6 x 1012 molecule c m - 3 in the chamber) were made to the chamber during an experiment, resulting in 9-56% reaction of the initial cis-3-hexen-l-ol. For the experiments involving product derivatization with GC, cyclohexane was not added to the reactant mixtures, and the initial cis-3-hexen-l-ol concentrations were (5.08-5.39) x l0 t3 moleculecm -3, with 4 additions of 50 cm 3 volume of 03 in 0 2 diluent being added to the chamber during an experiment and resulting in 38-72% reaction of the initial cis-3-hexen-l-ol. The concentrations of cis-3-hexen-1-ol and propanal were measured by GC-FID. For the analysis of cis-3-hexen-l-ol, 100 cm 3 volume gas samples were collected from the chamber onto Tenax-TA solid adsorbent, with subsequent thermal desorption at ~225°C onto a 30 m DB-5 or DB-1701 megabore column held at 0°C and then temperature programmed to 200°C at 8°C min- 1. Propanal was analyzed on the 30 m DB-1701 megabore column using the same procedure as used for cis-3-hexen-l-ol. Samples were also collected onto Tenax-TA solid adsorbent for thermal desorption and analysis by GC-MS using a 50 m DB-5MS fused silica capillary column in a Hewlett-Packard (HP) 5890 GC interfaced to a HP 5970 mass selective detector operated in the scanning mode. For the experiments with product derivatization and GC analyses, anisole was added to the chamber after the single irradiation period for the OH radical-initiated experiments and, because anisole does not react with O a, initially in the 03 reactions to act as an internal standard. The anisole concentrations in the chamber [(0.32-1.6) x 1013 molecule cm- 3] were measured by GC-FID using thermal desorption with the same 30 m DB-5 megabore column used to analyze for cis-3-hexen-1-ol. Approximately 10-12 E volume samples were collected from the chamber onto Tenax-TA solid adsorbent, with subsequent elution by diethyl ether. GC-FID and GC-MS analyses were conducted on these samples (after concentration from ~ 5 ml to ~ 0.5 ml) prior to and after treatment with a N-trimethylsilylimidazole-
N,O-bis(trimethylsilyl)trifluoroacetamide-trimethylchlorosilane (1 : 0.5 : 0.005) mixture, using a 30 m DB-5MS fused silica megabore column in an HP 5890 GC for the GC-FID analyses and a 50 m DB-5MS column in an HP 5890 GC interfaced to a HP 5971A mass selective detector operated in the scanning mode for the GC-MS analyses. The concentration of the internal standard anisole was therefore measured before and after treatment with the silylating reagent, while the concentrations of cis-3-hexen-l-ol were measured before and after derivatization as cis-3-hexen-l-ol and as the trimethylsilyl-derivative, respectively, and the concentrations of any hydroxy-product were measured after derivatization as the trimethylsilyl-derivative. The measured anisole concentrations in the chamber and after sample collection (before and after treatment with the silylating reagent) allowed the cis-3-hexen-l-ol and hydroxy-product concentrations measured after sample collection to be placed on an absolute basis as regards their concentrations in the
chamber. The GC-FID response factors for the trimethylsilyl-derivatives of the products and of cis-3-hexen-l-ol were calculated relative to that for anisole using their Effective Carbon Numbers (ECNs) (Scanlon and Willis, 1985). On a relative basis, the measured GC-FID response factors for cis-3-hexen-l-ol and anisole agreed well with the relative response factors calculated using their ECNs. Gas samples were also collected onto Tenax-TA solid adsorbent for analysis by GC-FTIR using a 30 m DB-5 column in a HP 5890 GC interfaced to a HP 5965B FTIR detector. For the API-MS analyses, a 6500{ Teflon chamber was interfaced to a PE SCIEX API III MS/MS instrument via a 25 mm diameter x 75 cm length Pyrex tube, with a sampling flow rate of ~20 E min- 1. The operation of the API-MS in the MS (scanning) and MS/MS [with collision activated dissociation (CAD)] modes has been described elsewhere (Kwok et al., 1995, 1996a,b; Atkinson et al., 1995b). 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 (Kwok et al., 1995, 1996ab). The positive ion mode was used in all these APIMS and API-MS/MS analyses, with protonated water hydrates ( H a O + ( H 2 0 ) n ) generated by the corona discharge in the chamber diluent gas being responsible for the protonation of analytes, HaO+(HzO), + M --. MH+(H20),, + (n - m + 1)H20 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 were mass-analyzed were mainly protonated molecular ions ([M + H] +) or, because the instrument was tuned to minimize fragmentation, dimers and trimers (Kwok and Arey, 1997). The chemicals used, and their stated purities, were: cyclohexane (high-purity solvent grade), American Burdick and Jackson; cis-3-hexen-l-ol (98%) and propanal (99 + %), Aldrich Chemical Company; N-trimethylsilylimidazole, N,O-bis(trimethylsilyl)-trifluoroacetamide and trimethylchlorosilane, Pierce Chemical; and NO (~>99.0%), Matheson Gas Products. Methyl nitrite was prepared and stored as described previously (Atkinson et al., 1981), and 0 3 was generated as needed using a Welsbach T-408 ozone generator.
RESULTS
G C - F I D analyses (without derivatization) GC-FID
and
GC-MS
analyses
of irradiated
CHaONO-NO-cis-3-hexen-l-ol-air and of reacted O3-cis-3-hexen-l-ol-cyclohexane (in excess)-air mixtures showed the formation of propanal, based on matching the G C retention times a n d the mass spectrum with those of an authentic standard. In agreement with our previous study (Atkinson et al., 1995a), cyclohexanone and cyciohexanol were also observed in the O3 reaction, showing the formation of O H radicals from the reaction of O3 with cis-3-hexen-l-ol. In the O H radical reactions, the measured concentrations of p r o p a n a l were corrected to take into account secondary reactions with the O H radical (Atkinson et al., 1982), using rate constants for the reactions of the O H radical with cis-3-hexen-l-ol and p r o p a n a l at
Products of gas-phase reactions room temperature: of 1.08 x 10-lo cm 3 molecules -1 (Atkinson et al., 1995a) and 1.96x 10-11cm 3 molecule-1 s- ~ (Atkinson, 1994), respectively. These corrections to the measured propanal concentrations for the effects of secondary reactions, which increase with the extent of reaction, were always ~<10%. The amounts of propanal formed (corrected for secondary reactions in the O1-I radical reaction as noted above) are plotted against the amounts of cis-3-hexen-l-ol reacted with the OH radical and with 03 in Fig. 1. The formation yields of propanal from these reac-
EACTION
~~o.za/ 0.4
7,
0
tions, obtained by least-squares analysis of the data shown in Fig. 1, are given in Table 1. In the OH radical reaction, an additional product peak was observed by GC-FID which eluted shortly before the cis-3-hexen-l-ol. The area of this GC peak increased linearly with the extent of reaction and, assuming a GC-FID response factor identical to that for cis-3-hexen-l-ol ( + 2 5 % ) , this product had a formation yield of 0.13 _ 0.05. Unfortunately, the GC-MS spectrum of this product showed no obvious molecular ion and this product could therefore not be identified. GC analysis with derivatization
1.6
~
3553
,
I 1
i
I
,
2
I 3 x 1 0 la
-A [cis,<~-HEXEN-1431.] molecule cm "3
Fig. 1. Plots of the amounts of propanal formed (corrected for reaction with the OH radical for the OH radical reaction) against the amounts of c/s-3-hexen-1-olreacted with the OH radical in the presence of NO and with O3 in the presence of sufficient cyclohexan~to scavenge > 95% of the OH radicals formed. The propanal data from the OH radical reaction have been displaced vertically by 2x 1012molecule cm-3 for clarity.
GC-FID and GC-MS analyses of the diethyl ether eluates of the ~ 10-12 ~ gas samples collected onto Tenax-TA solid adsorbent showed the presence of cis-3-hexen-l-ol and the internal standard anisole. Treatment of the diethyl ether solution with the silylating reagent led to the conversion of cis-3hexen-l-ol to its trimethylsilyl-derivative and to the observation of the trimethylsilyl-derivative of a reaction product. The mass spectra of the trimethylsilyl-derivatives of cis-3-hexen-l-ol and the reaction product showed high-mass ions at 157 and 131 u, respectively. Because trimethylsilyl-derivatives of alcohols, MOH, generally have [MOSi(CH3)3-15] + high-mass peaks (as observed at 157 u for the cis-3hexen-l-ol derivative, for example), the trimethylsilyl-derivative of the hydroxy-containing reaction product therefore has a molecular weight of 146 and the reaction product a molecular weight of 74. GC-FTIR spectra of the trimethylsilyl derivative formed from the OH radical and 03 reaction products were identical and exhibited IR absorption bands at 2719 and 2818cm -1 [aldehydic C-H stretch] and at 1743 c m - 1 [C =O stretch], indicating the presence of an aldehydic -CHO group. The reaction product is therefore identified as 3-hydroxypropanal,
Table 1. Products observed from the gas-phase reactions of OH radicals and 03 with cis-3-hexen-l-ol at 296 + 2 K and 740 Torr total pressure of air Reaction with Product
OH radicalsa
03
Propanal
0.746 _ 0.067b
0.493 _ 0.075b'c 0.59 + 0.12¢'d +0.33 0.33-o.16
3-Hydroxypropanal CH3CH2CIq[(OH)CH(ONO2)CH2CH2OH and/ or CHaCH2CH(ONO2)CH(OH)CH2CH2OH CH3CH2CH(OH)CH(OH)CH2CHO OH radical
+0.4-8
0.48-0.24 observede observede
+O,13f
0.26-o.o9
~In the presence of NO. Product yields are corrected for secondary reactions with the OH radical (see text). blndicated errors are two least-squares standard deviations combined with estimated uncertainties in the GC-FID response factors for propanal and cis-3-hexen-l-ol of + 5% each. tin the presence of sufficientcyclohexane to scavenge > 95% of the OH radicals formed. dFrom Grosjean et al. (1993c). eFrom API-MS analyses (see text). fFrom Atldnson et al. (1995a).
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S.M. ASCHMANN et al.
HOCH2-CH2CHO, the expected co-product to the propanal observed and quantified by GC-FID (see above). The concentrations of anisole, cis-3-hexen-l-ol and 3-hydroxypropanal were quantified by GC-FID during the experiments. Anisole and eis-3-hexen-l-ol were analyzed without derivatization, cis-3-Hexen-1ol and 3-hydroxypropanal were analyzed after derivatization, with the cis-3-hexen-l-ol concentrations measured before and after derivatization being used to assess the derivatization efficiency. Data were obtained from seven OH radical reactions (1 data point per experiment) and three 03 reactions (each experiment resulting in 3 data points per experiment of which seven data points were free from interfering GC peaks). In two of the OH radical reactions, only partial derivatization of the cis-3-hexen-l-ol was achieved and, as shown during certain of these OH radical and 03 reactions, this resulted in incomplete derivatization of 3-hydroxypropanal, and these data were therefore not used. For the OH radical reactions, in all cases the (cis-3hexen-l-ol/anisole) concentration ratios measured in the diethyl ether solution (before and after derivatization, adding the derivatized and non-derivatized cis3-hexen-l-ol concentrations for those analyses with non-complete derivatization) were in good agreement with the thermal desorption/GC-FID analyses, with the ratios being 0.94 +__0.10 (range 0.81-1.12) before derivatization and 0.93 +_ 0.18 (range 0.74-1.21) after derivatization, where the indicated errors are one standard deviation. The measured 3-hydroxypropanal concentrations were corrected for secondary reactions with the OH radical, with the multiplicative correction factors (Atkinson et al., 1982) ranging from 1.17 to 1.30 for the five experiments with complete 3-hydroxypropanal data. Within the appreciable scatter, the corrected 3-hydroxypropanal concentrations increased linearly with the amount of cis-3-hexen-l-ol reacted, and a least-squares analysis led to a 3-hydroxypropanal formation yield of 0.48. The five individual 3-hydroxypropanal formation yields ranged from 0.31 to 0.72, with an average of 0.45. Taking the estimated overall uncertainties into account, we cite a 3-hydroxypropanal formation yield of 0.48_+oo~24. 4s For the O3 reactions, in all cases the (cis-3-hexen-1ol/anisole) concentration ratios measured in the diethyl ether solution (before and after derivatization) were in good agreement with the thermal desorption/GC-FID analyses, with the ratios being 1.06 + 0.05 (range 1.02-1.16) before derivatization and 1.12 +__0.09 (range 1.03-1.30) after derivatization, where the indicated errors are one standard deviation. Within the scatter, the 3-hydroxypropanal concentrations increased linearly with the amount of cis-3-hexen-l-ol reacted, and a least-squares analysis led to a 3-hydroxypropanal formation yield of 0.33. The seven individual 3-hydroxypropanal formation yields ranged from 0.19 to 0.53, with an average of 0.32. Taking the estimated overall uncertainties into account, we cite a 3-hydroxypropanal formation yield of 0.33-+°1136 a.
OH radical reactions with A P I - M S analysis For the API-MS instrumental conditions employed, the API-MS spectrum of cis-3-hexen-l-ol (molecular weight 100, designated as Ms for starting compound) prior to reaction (Fig. 2, bottom) showed little evidence of the starting material in monomer form, but strong dimer and trimer ion peaks were present at 201 ([Ms + Ms + H] +) and 301 u ([Ms + Ms + Ms + HI+), respectively. API-MS spectra of irradiated CH3ONO-NO-cis-3-hexen-l-ol-air mixtures showed the presence of several additional ion peaks (Fig. 2, top), most notably at 280, 359 and 380 u. Compounds such as cis-3-hexen-l-ol which readily form homodimers will also form heterodimers, in this case with the reaction products as suggested by the difference of 100 u between the strong 380 and 280 u ion peaks. Because there is also an ion peak at 180 u, this suggests that the peak at 280 u is a heterodimer [Ms + Mpl + HI +, where the product, Mpl, has a molecular weight of 179 u and contains a nitrogen atom in the molecule. Figure 3 shows the APIMS/MS CAD daughter ion spectrum (top) and parent ion spectrum (bottom) of the 180 u IMp1 + H I + ion, and these spectra confirm that the 180 u ion peak is [M + H] ÷ of a molecular weight 179 u species. The API-MS/MS parent ion spectrum shows that the higher mass peaks which give rise to the 180 u peak can be explained a s heterodimers at 254 (IMp1 + Mp2 + H]+) and 280 u([Mp~ + Ms + H]+), a homodimer at 359u (IMp1 + Mp~ + HI+), and a heterotrimer at 380u ([Mpa + Ms + Ms + H]+), where Mp2 refers to a second product. The fragmentation pattern of the API-MS/MS CAD daughter ion spectrum of the 180 u ion with its large number of fragment ion peaks (Fig. 3, top) rules out the possibility of this ion being a dimer ion, because dimer ions mainly fragment to yield one or both of the protonated monomers under our instrumental conditions (Kwok and Arey, 1997). The API-MS/MS CAD daughter ion spectrum of the 180 u ion peak shows a [ M + H-H20] + fragment ion at 162 u together with a strong fragment ion at 46 u ([NO2] +) (Fig. 3, top), consistent with this product being an hydroxynitrate of molecular weight 179. The heterodimer ion at 254 u discussed above suggests a second product Mp2 of molecular weight 74 u. The appearance of peaks at 175 and 275 u in the API-MS/MS parent ion spectrum of the relatively minor 75 u ion peak confirms the presence of this product, with the API-MS/MS CAD daughter ion spectrum of the 275 u ion peak showing that this ion corresponds to [Mp2 + Ms + Ms + H] +. Finally, the API-MS/MS CAD daughter ion spectrum of the 75 u ion peak, which again shows a [ M + H-H20] + fragment ion, is consistent with the 3-hydroxypropanal product quantified by GC-FID after derivatization. API-MS/MS parent ion and daughter ion spectra were carried out on the remaining unassigned and less intense product ion peaks (Fig. 2, top), with the
Products of gas-phase reactions
3555 280
201
301
75
380
27~
359
25 115
254
175
v
# E
o
i
i
J
331
301
ID
75
201
25
d-..:_
0
50
100
150
200 m/z
250
300
350
.[
,, 400
Fig. 2. Representative API-MS spectra ofa CH3ONO-NO-cis-3-hexen-l-ol-airmixture prior to (bottom) and after (top) irradiation. The starting material, cis-3-hexen-l-ol, has been designated as Ms (100 u) in the text and the products identified have been designated as Mpl = 179 u, Mv2 = 74 u and Mp3 = 132 u in the text. interpretation being aided by consideration of the possible first-generation products of the OH radicalinitiated reaction of cis-3-hexen-1-ol in the presence of NO (Atkinson, 19£4, 1997). Propanal, quantified by G C - F I D (see above), was observed as a weak 59 u ion peak and the API-MS/MS daughter ion spectrum was consistent with that of an authentic standard. The numerous API-MS/MS parent ion and daughter ion spectra obtained revealed potential products with molecular weights of 130, 132 and 148.
Hydroxy compounds have previously been shown to give a strong [M + H - H 2 0 ] + fragment ion in the API-MS under our instrumental conditions (Atkinson et al., 1995b; Kwok et al., 1996a). API-MS/MS parent ion spectra of the 115 u ion peak indicated that this ion was a fragment of the 133 and 180 u ion peaks. The API-MS/MS daughter ion spectrum of the 133 u ion peak (Fig. 4, top) shows a strong [M + HH 2 0 ] + fragment ion at 115 u, indicative of a hydroxy-compound, and the API-MS/MS parent ion
3556
S.M. ASCHMANN
et al.
46
100
75
50
73
88 99 115
25 180 A v
162
=-
20
c•
i
40
i
100 280
8'0
°~,~ ID ¢r
120
140
160
180
75
180
50
359
25
38O
254~ 0
.
175
2(~0
225
.
250
~
.
275
L,, A. , k 300 325
j,
,1 i
350
t ~. . . .
375
,
400
m/z Fig. 3. API-MS/MS daughter ion (top) and parent ion (bottom) spectra of the 180 u ion peak. spectrum (Fig. 4, bottom) shows a water cluster ion at 151 u and heterodimer and heterotrimer ions at 233 ([Mp3 + Ms + HI÷), 312 (Mp3 + Mpl + HI+), and 333 u ([Mpa + Ms + Ms + H] ÷), together with a very weak self-dimer at 265 u ([Mp3 + Mp3 -I- HI+). These data provide evidence for the formation of a hydroxyproduct of molecular weight 132 (Mp3 = 132 u). API-MS/MS parent ion spectra of the 231 and 331 u ion peaks observed in Fig. 2 (top) showed that these ion peaks are fragments (-H20) of somewhat weaker 249 and 349 u ion peaks, respectively. Similarly, an API-MS/MS parent ion spectrum of the weak 131 u ion peak indicated that it was a fragment
of a 149 u ion peak. An API-MS/MS daughter ion spectrum of the very weak 149 u ion peak showed a fragment at 131 u ([M + H-H20] +) as well as a strong fragment at 75 u, suggesting that the 149 u ion peak was mainly a [Mp2 + M p 2 + H] ÷ dimer ion of 3-hydroxypropanal. Our API-MS data therefore indicate the formation of a hydroxynitrate(s) of molecular weight 179 and a hydroxy-compound(s) of molecular weight 132 in addition to propanal and 3-hydroxypropanal, with the possibility of the formation of a hydroxy-compound(s) of molecular weight 148 as a very minor product.
Products of gas-phase reactions
3557
115
100
133 75
50 75
25 45 57 69711
J
._z,
¢n 0
.~_ 100
20 -
40
60
80
100
120
14.0
133
n" 75
50 23,3
25-
l
151
312
l 150
265 .lI ~ ' 200
250
l 300
333 350
40'0
m/z Fig. 4. API-MS/MS daughter ion (top) and parent ion (bottom) spectra of the 133 u ion peak.
DISCUSSION
CHsCH2CH(OH)CHCH2CH2OH. Under tropospheric conditions, these fl-hydroxyalkyl radicals rapidly and solely add 02 (Atkinson, 1994) and in the presence of NO react to form the corresponding fl-hydroxyalkoxy radical plus NO2 or the
OH radical reaction The OH radical reaction is expected to proceed largely ( ~ 94.%) by initial addition of the OH radical to the > C = C < bond (Atkinson, 1994; Atkinson et al., 1995a), leading to the two fl-hydroxyalkyl radicals CHaCH2CHCH(OH)CH2CHzOH and
M
OH + CHsCH2CH=CHCH2CHzOH~ CHsCH2(~HCH(OH)CH2CHzOHand CH3CH2CH(OH)CHCH2CH2OH
3558
S.M. ASCHMANN et al.
fl-hydroxynitrate. For example, M
CH3CH2(~HCH(OH)CH2CH2OH + 02 ~ CH3CH2CH(O0)CH(OH)CH2CH2OH CH3CH2CH(OO)CH(OH)CH2CH2OH + NO
and similarly for the CHaCH2CH(OH)CHCH2CH2OH radical. The fl-hydroxyalkoxy radicals CHaCH2CH(O)CH(OH)CH2CH2OH and CHaCH2CH(OH)CH(O)CH2CH2OH can decompose, isomerize or react with 02 (Grosjean et al., 1993b; Atkinson, 1994, 1997). The potential reactions of the CH3CH2CH(O)CH(OH)CH2CH2OH radical are shown in Reaction Scheme 1, leading to the formation of propanal plus 3-hydroxypropanal from the decomposition reaction, 3,4-dihydroxyhexanal [CH3CH2CH(OH)CH(OH)CH2CHO] from the isomerization reaction, and 1,3-dihydroxy-4-hexanone [CH3CH2C(O)CH(OH)CHECH2OH] from the 02 reaction.
CH3CH2CH(())CH(OH)CH2CH20H 02 ~
CH3CHICH(0)CH(OH)CH2CH2OH + NO2 F t.... CH3CH2CH(ONOz)CH(OH)CH2CH:OH
radicals dominate to lead to propanal plus 3-hydroxypropanal, and that the only other significant reaction pathway is the isomerization of the CHaCH2CH(O)CH(OH)CH2CHEOH radical to form 3,4-dihydroxyhexanal. Our API-MS observation of a nitrate of molecular weight 179 is consistent with the formation of the fl-hydroxynitrates CH3CHECH(OH)CH(ONO2)CH2CHEOH and/or CHaCH2CH(ONO2)CH(OH)CHECH2OH from the reactions of the fl-hydroxyalkyl peroxy radicals with NO, and our observation of a hydroxy-product of molecular weight 132 is consistent with the predicted formation of 3,4-dihydroxhexanal I-CHaCH2CH(OH)CH(OH)CHECHO]. These two products probably account for
CH3CH2C(O)CH(OH)CH2CH2OH+ HO2
I decomposition ~ e r i z a t i o n CH3CH2CHO + HOCH2CH2CHOH
HOCH2CH2CHO + HO2
CH3CH2CH(OH)CH(OH)CH2CHOH
CH3CH2CH(OH)CH(OH)CH2CHO+ HO2
Reaction Scheme 1. The CH3CH2CH(OH)CH(O)CH2CH2OH radical reacts by analogous reaction pathways, as shown in Reaction Scheme 2, to form propanal plus 3-hydroxypropanal from the decomposition reaction, 1,4,6-trihydroxy-3-hexanone [HOCH2CH2CH(OH)C(O)CH2CH2OH] from the isomerization reaction, and 1,4-dihydroxy-3-hexanone [CH3CH2CH(OH)C(O)CH2CH2OH-I from the 0 2 reaction. Atkinson (1997) has used the literature data concerning the tropospheric reactions of alkoxy and fl-hydroxyalkoxy radicals to calculate the rate constants for the decomposition, isomerization and 02 reactions. These estimation methods lead to calculated reaction rates for decomposition, isomerization and reaction with 02 at 298 K and 760 Torr total pressure of air of 1.1 x 108, 8.7x 106 and 4.1 x 10as -1, respectively, for the CHaCH2CH(O)CH(OH)CH2CH2OH radical, and 8.0 × 107, 2.0 x 105 and 4.1 x 104s -1, respectively, for the CHaCH2CH(OH)CH(O)CH2CH2OH radical. These estimates predict that decomposition of both fl-hydroxyalkoxy
the 25% of the products other than propanal plus 3-hydroxypropanal. 03 Reaction The reaction of 03 with cis-3-hexen-l-ol proceeds by initial addition of 03 to the > C = C < bond to form a primary ozonide which decomposes to (propanal plus [HOCH2CH2CHOO]*) or to (3-hydroxypropanal plus [CH3CH2CHO0]*). The reactions of the biradicals are not well understood, but lead in part to the formation of OH radicals (Atkinson and Aschmann, 1993; Atkinson et al., 1995a, c). Our observation of propanal and 3-hydroxypropanal with formation yields of 0.493 + 0.075 and ~"~J-O.16~ t~+o.33 respectively, is reasonably consistent with the expectation that the sum of the formation yields of propanal and 3-hydroxypropanal should be unity, unless additional formation of the "primary" carbonyls arise from reactions of the [CH3CH2t~HO0]* and [HOCH2CH2CHO0]* biradicals (Atkinson, 1994; Atkinson et al., 1995c). Furthermore, our present - -
Products of gas-phase reactions
CH:3CH2CH(OH)CH(O)CH2CH2OH
02
~
I decomposition HOCH2CH2CHO + CtI3CH2CHOH
CH3CH2CHO + HO2
3559
CH3CH2CH(OH)C(O)CH2CH2OH + HO2 erization (~H2CH2CH(OH)CH(OH)CH2CH2OH
"OOCH2CH2CH(O~CH(O~CH2CH2OH NO - - ~
NO2
°OCH2CH2CH(OI-I)CH(OH)CH2CH2OH ~ isomerization HOCH2CH2CH(OI~(OH)CH2CH2OH
HOCH2CH2CH(OH)C(O)CH2CH2OH + HO2 Reaction Scheme 2.
propanal formation yield of 0.493 + 0.075 is in agreement within the experimental uncertainties with the propanal yield of 0 . 5 9 _ 0.12 previously measured by Grosjean et al. (1993c) as the 2,4-dinitrophenylhydrazone derivative and also in the presence of an OH scavenger. While we quantified 3hydroxypropanal in the absence of an OH radical scavenger, the relatively low OH radical formation yield of -,~ 0.26 from the 03 reaction with cis-3hexen-l-ol (Atkinson et al., 1995a) suggests that the 3-hydroxypropana] formation yield will not be drastically different in the presence or absence of an OH radical scavenger.
CONCLUSIONS The major products of the gas-phase reactions of the OH radical and 03 with cis-3-hexen-l-ol are propanal and 3-hydroxypropanal. Further reactions of these two products in the presence of N O x will lead to the formation of peroxypropionyl nitrate (Grosjean et al., 1993b) and tile hydroxy-analog HOCH2CH2C(O)OONO2. Additionally and for the first time, direct evidence for the formation of dihydroxynitrate(s) and 3,4-dihydroxyhexanal from the OH radical-initiated reaction of cis-3-hexen-l-ol has been obtained by API-MS/MS analyses. Acknowledgements--The authors gratefully thank the National Science Foundation for support of this research through Grant No. ATM-9414036,and thank the National Science Foundation (Grant No. ATM-9015361) and the University of California, Riverside,for funds for the purchase of the PE SCIEX API III MS/MS instrument.
REFERENCES
Arey, J., Winer, A. M., Atkinson, R., Aschmann, S. M., Long, W. D. and Morrison, C. L. (1991) The emission of (Z)-3hexen-1-ol,(Z)-3-hexenylacetateand other oxygenatedhydrocarbons from agricultural plant species. Atmospheric Environment 25A, 1063-1075. Atkinson, R. (1994) Gas-phase tropospheric chemistry of organic compounds. Journal of Physical Chemistry Ref. Data Monograph 2, 1-216. Atkinson, R. (1997) Atmospheric reactions of alkoxy and fl-hydroxyalkoxyradicals. International Journal of Chemical Kinetics 29, 99-111. Atkinson, R., Carter, W. P. L., Winer, A. M. and Pitts, J. N. Jr. (1981) An experimental protocol for the determination of OH radical rate constants with organics using methyl nitrite photolysis as an OH radical source. Journal of the Air Pollutution Control Association 31, 1090-1092. Atkinson, R., Aschmann, S. M., Carter, W. P. L., Winer, A. M. and Pitts, J. N. Jr. (1982) Alkyl nitrate formation from the NOx air photooxidations of C2-C8 n-alkanes. Journal of Physical Chemistry 86, 4563-4569. Atkinson, R. and Aschmann, S. M. (1993) OH radical production from the gas-phase reactions of O3 with a series of alkenes under atmospheric conditions. Environmental Science and Technology 27, 1357 1363. Atkinson, R., Arey, J., Aschmann, S. M., Corchnoy, S. B. and Shu, Y. (1995a) Rate constants for the gas-phase reactions of cis-3-hexen-l-ol, cis-3-hexenylacetate, trans-2-hexenal, and linalool with OH and NO 3 radicals and 03 at 296 ± 2 K, and OH radical formation yields from the 03 reactions. International Journal of Chemical Kinetics 27, 941-955. Atkinson, R., Kwok, E. S. C., Arey, J. and Aschmann, S. M. (1995b) Reactions of alkoxy radicals in the atmosphere. Faraday Discussions of the Chemical Society 100, 23-37. Atkinson, R., Tuazon, E. C. and Aschmann, S. M. (1995c) Products of the gas-phase reactions of Oa with alkenes. Environment Science and Technology 29, 1860-1866.
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Buttery, R. G., Ling, L. C. and Wellso, S. G. (1982) Oat leaf volatiles: possible insect attractants. Journal of Agricultural and Food Chemistry 30, 791-792. Buttery, R. G., Xu, C. and Ling, L. C. (1985) Volatile components of wheat leaves (and stems): possible insect attractants. Journal Agriculural and Food Chemistry 33, 115-117. Grosjean, D., Grosjean, E. and Williams, E. L II (1993a) Rate constants for the gas-phase reactions of ozone with unsaturated alcohols, esters, and carbonyls. International Journal of Chemical Kinetics 25, 783-794. Grosjean, D., Williams, E. L. II and Grosjean, E. (1993b) A biogenic precursor of peroxypropionyl nitrate: Atmospheric oxidation of cis-3-hexen-l-ol. Environmental Science and Technology 27, 979-981. Grosjean, D., Grosjean, E. and Williams, E. L. II (1993c) Atmospheric chemistry of unsaturated alcohols. Environmental Science and Technology 27, 2478-2485. Isidorov, V. A., Zenkevich, I. G. and Ioffe, B. V. (1985) Volatile organic compounds in the atmosphere of forests. Atmospheric Environment 19, 1-8. K/Snig, G., Brunda, M., Puxbaum, H., Hewitt, C. N., Duckham, S. C. and Rudolph, J. (1995) Relative contribution of oxygenated hydrocarbons to the total biogenic VOC emissions of selected mid-European agricultural and natural plant species. Atmospheric Environment 29, 861-874. Kwok, E. S. C., Atkinson, R. and Arey, J. (1995) Observation of hydroxycarbonyls from the OH radical-initiated reac-
tion of isoprene. Environmental Science and Technology 29, 2467-2469. Kwok, E. S. C., Arey, J. and Atkinson, R. (1996a) Alkoxy radical isomerization in the OH radical-initiated reactions of C4-C 8 n-alkanes. Journal of Physical Chemistry 100, 214-219. Kwok, E. S. C., Atkinson, R. and Arey, J. (1996b) Isomerization of fl-hydroxyalkoxy radicals formed from the OH radical-initiated reactions of C4-C8 1-alkenes. Environmental Science and Technology 30, 1048-1052. Kwok, E. S. C. and Arey, J. (1997) Dimer formation in the atmospheric pressure ionization tandem mass spectrometer (API-MS/MS) for product studies of atmospherically important reactions of hydrocarbons (in preparation). Lwande, W. and Bentley, M. D. (1987) Volatiles of Sorghum bicolor seedlings. Journal of Natural Products 50, 950-952. Ohta K. (1984) Emission of hexenol from higher plants. Geochemical Journal 18, 135-141. Scanlon, J. T. and Willis, D. E. (1985) Calculation of flame ionization detector relative response factors using the effective carbon number concept. Journal of Chromatographic Science 23, 333-340. Schulting, F. L., Meyer, G. M. and van Aalst, R. M. (1980) Emissie van koolwaterstoffen door vegetatie en de bijdrage aan de luchtverontreiniging in Nederland. TNO Rapport CMP 80/16, Delft, The Netherlands, November 1. Sharkey, T. D. (1996) Emission of low molecular mass hydrocarbons from plants. Trends in Plant Science 1, 78-82.
Pergamon PII:
S1352-2310(97)00205-7
P R O D U C T S O F T H E GAS-PHASE R E A C T I O N S O F cis-3-HEXEN-1-OL W I T H O H RADICALS A N D 03 SARA M. ASCHMANN, YONGHUI SHU,i" JANET AREY*t:~ and ROGER ATKINSON*I'~:§ Statewide Air Pollution Research Center, University of California, Riverside,CA 92521, U.S.A. (First received 19 November 1996 and in final form 27 March 1997. Published Au#ust 1997) Abstract--Products of the gas-phase reactions of OH radicals (in the presence of NO) and 03 with the biogenicemi:;sioncis-3-hexen-l-ol have been investigated using gas chromatography with flame ionization detection (GC-FID), combined gas chromatography-mass spectrometry (GC-MS), gas chromatography with Fourier transform infrared spectroscopy (GC-FTIR), and direct air sampling, atmospheric pressure ionization tandem mass spectrometry (API-MS). Propanal and 3-hydroxypropanal were identified and quantified from the OH radical and 03 radical reactions, with respectiveformation yields of 0.746 + 0.067 and 0.48-0.24 +0.48 from the OH radical reaction and 0.493 + 0.075 and 0.33-o:16 +o 33 from the 03 reaction. In addition, a hydroxynitrate of molecular weight 179 [CH3CH2CH(OH)CH(ONO2)CH2CHzOH and/or CH3CH2CH(ONO2)CH(OH)CH2CH2OH] and a hydroxycarbonyl of molecular weight 132, expected to be CHaCH2CH(OH)CH(OH)CH2CHO, were observed from the OH radical reaction by API-MS analyses. The reaction mechanisms are discussed. © 1997 Elsevier ScienceLtd. Key word index: cis-3-Hexen-l-ol, hydroxyl radical, ozone, atmospheric reactions, reaction products.
INTRODUCTION cis-3-Hexen-l-ol [CH3CH~CH =CHCH2CH2OH], leaf alcohol, is an oxygenate emitted into the atmosphere from certain vegetation (Schulting et al., 1980; Buttery et al., 1982~ 1985; Ohta, 1984; Isidorov et al., 1985; Lwande and Bentley, 1987; Arey et al., 1991; K6nig et al., 1995), and the physiological function of its release in response to plant wounding has recently been described (Sharkey, 1996). In the troposphere, cis-3-hexen-l-ol re~Lcts with OH radicals, NO3 radicals and 03 (Gros]ean et al., 1993a; Atkinson et al., 1995a), with a calculated lifetime due to these reactions of a few hour,,; (Atkinson et al., 1995a). Selected products have been identified and measured from the NOx-air photooxidation of cis-3-hexen-l-ol (Grosjean et al., 1993b) and from the 03 reaction (Grosjean et al., 1993c; Atkinson et al., 1995a). Specifically, propanal was observed as a major product of the NOx-air photooxidation (Grosjean et al., 1993b), and propanal and OH radicals were observed from the 03 reaction with reported yields of 0.59 _+ 0.12
*Authors to whom correspondence should be addressed. tAlso Environmental Toxicology Graduate Program. :~AlsoDepartment of Soil and Environmental Sciences University of California, Riverside, CA 92521, U.S.A. §Also Department of Chemistry, University of California, Riverside, CA 92521, U.S.A.
(Grosjean et al., 1993c) and 0.26-+°:~ (Atkinson et al., 1995a), respectively. In this work, we have carried out more extensive studies of the products formed from the gas-phase reactions of OH radicals and 03 with cis-3-hexen-1ol, including analyses of the reaction products by atmospheric pressure ionization mass spectrometry.
EXPERIMENTAL All experiments were carried out in 6500-7900 t' Teflon chambers, each equipped with two parallel banks of blacklamps for irradiation and with a Teflon-coatedfan to ensure rapid mixing of reactants during their introduction into the chamber. Products were analyzed from the OH radical and 03 reactions by gas chromatography with flame ionization detection(GC-FID) and combinedgas chromatography-mass spectrometry (GC-MS). Because expected hydroxycarbonyl products did not appear to be directly amenable to gas chromatography, in one set of experiments trimethylsilyl derivatives of hydroxy-compounds were formed prior to the GC-FID and GC-MS analyses.Atmospheric pressure ionization tandem mass spectrometry (API-MS) was also used to identify products from the OH radical-initiated reaction. Separate sets of experimentswere carried out for these differing types of analyses. Hydroxyl radicals were generated by the photolysis of methyl nitrite (CHaONO) in air at wavelengths >300 nm (Atkinson et al., 1981) and NO was added to the reactant mixtures to suppress the formation of 03 and of NO3 radicals. The initial CHaONO, NO and cis-3-bexen-l-ol concentrations were 2.4x 1014, 2.4x 1014, and (2.43-5.30)x 1013 moleculecm-3, respectively, for the experiments with
3551
3552
S.M. ASCHMANN et al.
GC-FID analyses, and (2.4-4.8)x 1013 molecule cm -3 each for the experiments with API-MS analyses. Irradiations were carried out at 20% of the maximum light intensity for 1-3.5 min for the experiments with GC without derivatization (resulting in 21-60% reaction of the initial cis-3hexen-l-ol), 3-8 min for the experiments with derivatization (resulting in 57-90% reaction of the initial cis-3-hexen1-ol), and 1-1.5 min for the experiments with API-MS analyses. For the experiments with GC analyses without derivatization, the 0 3 reactions were carried out in the presence of sufficient cyclohexane to scavenge > 95% of the OH radicals formed from the 03 reaction with cis-3-hexen-l-ol (Atkinson et al., 1995a), with initial cis-3-hexen-l-ol and cyclohexane concentrations of (4.61-5.03) x 1013 and (1.8-2.1) x 10 t6 molecule cm-a, respectively. Four additions of 50 c m 3 volume of 03 in 0 2 diluent (each addition corresponding to an initial 0 3 concentration of ~6 x 1012 molecule c m - 3 in the chamber) were made to the chamber during an experiment, resulting in 9-56% reaction of the initial cis-3-hexen-l-ol. For the experiments involving product derivatization with GC, cyclohexane was not added to the reactant mixtures, and the initial cis-3-hexen-l-ol concentrations were (5.08-5.39) x l0 t3 moleculecm -3, with 4 additions of 50 cm 3 volume of 03 in 0 2 diluent being added to the chamber during an experiment and resulting in 38-72% reaction of the initial cis-3-hexen-l-ol. The concentrations of cis-3-hexen-1-ol and propanal were measured by GC-FID. For the analysis of cis-3-hexen-l-ol, 100 cm 3 volume gas samples were collected from the chamber onto Tenax-TA solid adsorbent, with subsequent thermal desorption at ~225°C onto a 30 m DB-5 or DB-1701 megabore column held at 0°C and then temperature programmed to 200°C at 8°C min- 1. Propanal was analyzed on the 30 m DB-1701 megabore column using the same procedure as used for cis-3-hexen-l-ol. Samples were also collected onto Tenax-TA solid adsorbent for thermal desorption and analysis by GC-MS using a 50 m DB-5MS fused silica capillary column in a Hewlett-Packard (HP) 5890 GC interfaced to a HP 5970 mass selective detector operated in the scanning mode. For the experiments with product derivatization and GC analyses, anisole was added to the chamber after the single irradiation period for the OH radical-initiated experiments and, because anisole does not react with O a, initially in the 03 reactions to act as an internal standard. The anisole concentrations in the chamber [(0.32-1.6) x 1013 molecule cm- 3] were measured by GC-FID using thermal desorption with the same 30 m DB-5 megabore column used to analyze for cis-3-hexen-1-ol. Approximately 10-12 E volume samples were collected from the chamber onto Tenax-TA solid adsorbent, with subsequent elution by diethyl ether. GC-FID and GC-MS analyses were conducted on these samples (after concentration from ~ 5 ml to ~ 0.5 ml) prior to and after treatment with a N-trimethylsilylimidazole-
N,O-bis(trimethylsilyl)trifluoroacetamide-trimethylchlorosilane (1 : 0.5 : 0.005) mixture, using a 30 m DB-5MS fused silica megabore column in an HP 5890 GC for the GC-FID analyses and a 50 m DB-5MS column in an HP 5890 GC interfaced to a HP 5971A mass selective detector operated in the scanning mode for the GC-MS analyses. The concentration of the internal standard anisole was therefore measured before and after treatment with the silylating reagent, while the concentrations of cis-3-hexen-l-ol were measured before and after derivatization as cis-3-hexen-l-ol and as the trimethylsilyl-derivative, respectively, and the concentrations of any hydroxy-product were measured after derivatization as the trimethylsilyl-derivative. The measured anisole concentrations in the chamber and after sample collection (before and after treatment with the silylating reagent) allowed the cis-3-hexen-l-ol and hydroxy-product concentrations measured after sample collection to be placed on an absolute basis as regards their concentrations in the
chamber. The GC-FID response factors for the trimethylsilyl-derivatives of the products and of cis-3-hexen-l-ol were calculated relative to that for anisole using their Effective Carbon Numbers (ECNs) (Scanlon and Willis, 1985). On a relative basis, the measured GC-FID response factors for cis-3-hexen-l-ol and anisole agreed well with the relative response factors calculated using their ECNs. Gas samples were also collected onto Tenax-TA solid adsorbent for analysis by GC-FTIR using a 30 m DB-5 column in a HP 5890 GC interfaced to a HP 5965B FTIR detector. For the API-MS analyses, a 6500{ Teflon chamber was interfaced to a PE SCIEX API III MS/MS instrument via a 25 mm diameter x 75 cm length Pyrex tube, with a sampling flow rate of ~20 E min- 1. The operation of the API-MS in the MS (scanning) and MS/MS [with collision activated dissociation (CAD)] modes has been described elsewhere (Kwok et al., 1995, 1996a,b; Atkinson et al., 1995b). 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 (Kwok et al., 1995, 1996ab). The positive ion mode was used in all these APIMS and API-MS/MS analyses, with protonated water hydrates ( H a O + ( H 2 0 ) n ) generated by the corona discharge in the chamber diluent gas being responsible for the protonation of analytes, HaO+(HzO), + M --. MH+(H20),, + (n - m + 1)H20 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 were mass-analyzed were mainly protonated molecular ions ([M + H] +) or, because the instrument was tuned to minimize fragmentation, dimers and trimers (Kwok and Arey, 1997). The chemicals used, and their stated purities, were: cyclohexane (high-purity solvent grade), American Burdick and Jackson; cis-3-hexen-l-ol (98%) and propanal (99 + %), Aldrich Chemical Company; N-trimethylsilylimidazole, N,O-bis(trimethylsilyl)-trifluoroacetamide and trimethylchlorosilane, Pierce Chemical; and NO (~>99.0%), Matheson Gas Products. Methyl nitrite was prepared and stored as described previously (Atkinson et al., 1981), and 0 3 was generated as needed using a Welsbach T-408 ozone generator.
RESULTS
G C - F I D analyses (without derivatization) GC-FID
and
GC-MS
analyses
of irradiated
CHaONO-NO-cis-3-hexen-l-ol-air and of reacted O3-cis-3-hexen-l-ol-cyclohexane (in excess)-air mixtures showed the formation of propanal, based on matching the G C retention times a n d the mass spectrum with those of an authentic standard. In agreement with our previous study (Atkinson et al., 1995a), cyclohexanone and cyciohexanol were also observed in the O3 reaction, showing the formation of O H radicals from the reaction of O3 with cis-3-hexen-l-ol. In the O H radical reactions, the measured concentrations of p r o p a n a l were corrected to take into account secondary reactions with the O H radical (Atkinson et al., 1982), using rate constants for the reactions of the O H radical with cis-3-hexen-l-ol and p r o p a n a l at
Products of gas-phase reactions room temperature: of 1.08 x 10-lo cm 3 molecules -1 (Atkinson et al., 1995a) and 1.96x 10-11cm 3 molecule-1 s- ~ (Atkinson, 1994), respectively. These corrections to the measured propanal concentrations for the effects of secondary reactions, which increase with the extent of reaction, were always ~<10%. The amounts of propanal formed (corrected for secondary reactions in the O1-I radical reaction as noted above) are plotted against the amounts of cis-3-hexen-l-ol reacted with the OH radical and with 03 in Fig. 1. The formation yields of propanal from these reac-
EACTION
~~o.za/ 0.4
7,
0
tions, obtained by least-squares analysis of the data shown in Fig. 1, are given in Table 1. In the OH radical reaction, an additional product peak was observed by GC-FID which eluted shortly before the cis-3-hexen-l-ol. The area of this GC peak increased linearly with the extent of reaction and, assuming a GC-FID response factor identical to that for cis-3-hexen-l-ol ( + 2 5 % ) , this product had a formation yield of 0.13 _ 0.05. Unfortunately, the GC-MS spectrum of this product showed no obvious molecular ion and this product could therefore not be identified. GC analysis with derivatization
1.6
~
3553
,
I 1
i
I
,
2
I 3 x 1 0 la
-A [cis,<~-HEXEN-1431.] molecule cm "3
Fig. 1. Plots of the amounts of propanal formed (corrected for reaction with the OH radical for the OH radical reaction) against the amounts of c/s-3-hexen-1-olreacted with the OH radical in the presence of NO and with O3 in the presence of sufficient cyclohexan~to scavenge > 95% of the OH radicals formed. The propanal data from the OH radical reaction have been displaced vertically by 2x 1012molecule cm-3 for clarity.
GC-FID and GC-MS analyses of the diethyl ether eluates of the ~ 10-12 ~ gas samples collected onto Tenax-TA solid adsorbent showed the presence of cis-3-hexen-l-ol and the internal standard anisole. Treatment of the diethyl ether solution with the silylating reagent led to the conversion of cis-3hexen-l-ol to its trimethylsilyl-derivative and to the observation of the trimethylsilyl-derivative of a reaction product. The mass spectra of the trimethylsilyl-derivatives of cis-3-hexen-l-ol and the reaction product showed high-mass ions at 157 and 131 u, respectively. Because trimethylsilyl-derivatives of alcohols, MOH, generally have [MOSi(CH3)3-15] + high-mass peaks (as observed at 157 u for the cis-3hexen-l-ol derivative, for example), the trimethylsilyl-derivative of the hydroxy-containing reaction product therefore has a molecular weight of 146 and the reaction product a molecular weight of 74. GC-FTIR spectra of the trimethylsilyl derivative formed from the OH radical and 03 reaction products were identical and exhibited IR absorption bands at 2719 and 2818cm -1 [aldehydic C-H stretch] and at 1743 c m - 1 [C =O stretch], indicating the presence of an aldehydic -CHO group. The reaction product is therefore identified as 3-hydroxypropanal,
Table 1. Products observed from the gas-phase reactions of OH radicals and 03 with cis-3-hexen-l-ol at 296 + 2 K and 740 Torr total pressure of air Reaction with Product
OH radicalsa
03
Propanal
0.746 _ 0.067b
0.493 _ 0.075b'c 0.59 + 0.12¢'d +0.33 0.33-o.16
3-Hydroxypropanal CH3CH2CIq[(OH)CH(ONO2)CH2CH2OH and/ or CHaCH2CH(ONO2)CH(OH)CH2CH2OH CH3CH2CH(OH)CH(OH)CH2CHO OH radical
+0.4-8
0.48-0.24 observede observede
+O,13f
0.26-o.o9
~In the presence of NO. Product yields are corrected for secondary reactions with the OH radical (see text). blndicated errors are two least-squares standard deviations combined with estimated uncertainties in the GC-FID response factors for propanal and cis-3-hexen-l-ol of + 5% each. tin the presence of sufficientcyclohexane to scavenge > 95% of the OH radicals formed. dFrom Grosjean et al. (1993c). eFrom API-MS analyses (see text). fFrom Atldnson et al. (1995a).
3554
S.M. ASCHMANN et al.
HOCH2-CH2CHO, the expected co-product to the propanal observed and quantified by GC-FID (see above). The concentrations of anisole, cis-3-hexen-l-ol and 3-hydroxypropanal were quantified by GC-FID during the experiments. Anisole and eis-3-hexen-l-ol were analyzed without derivatization, cis-3-Hexen-1ol and 3-hydroxypropanal were analyzed after derivatization, with the cis-3-hexen-l-ol concentrations measured before and after derivatization being used to assess the derivatization efficiency. Data were obtained from seven OH radical reactions (1 data point per experiment) and three 03 reactions (each experiment resulting in 3 data points per experiment of which seven data points were free from interfering GC peaks). In two of the OH radical reactions, only partial derivatization of the cis-3-hexen-l-ol was achieved and, as shown during certain of these OH radical and 03 reactions, this resulted in incomplete derivatization of 3-hydroxypropanal, and these data were therefore not used. For the OH radical reactions, in all cases the (cis-3hexen-l-ol/anisole) concentration ratios measured in the diethyl ether solution (before and after derivatization, adding the derivatized and non-derivatized cis3-hexen-l-ol concentrations for those analyses with non-complete derivatization) were in good agreement with the thermal desorption/GC-FID analyses, with the ratios being 0.94 +__0.10 (range 0.81-1.12) before derivatization and 0.93 +_ 0.18 (range 0.74-1.21) after derivatization, where the indicated errors are one standard deviation. The measured 3-hydroxypropanal concentrations were corrected for secondary reactions with the OH radical, with the multiplicative correction factors (Atkinson et al., 1982) ranging from 1.17 to 1.30 for the five experiments with complete 3-hydroxypropanal data. Within the appreciable scatter, the corrected 3-hydroxypropanal concentrations increased linearly with the amount of cis-3-hexen-l-ol reacted, and a least-squares analysis led to a 3-hydroxypropanal formation yield of 0.48. The five individual 3-hydroxypropanal formation yields ranged from 0.31 to 0.72, with an average of 0.45. Taking the estimated overall uncertainties into account, we cite a 3-hydroxypropanal formation yield of 0.48_+oo~24. 4s For the O3 reactions, in all cases the (cis-3-hexen-1ol/anisole) concentration ratios measured in the diethyl ether solution (before and after derivatization) were in good agreement with the thermal desorption/GC-FID analyses, with the ratios being 1.06 + 0.05 (range 1.02-1.16) before derivatization and 1.12 +__0.09 (range 1.03-1.30) after derivatization, where the indicated errors are one standard deviation. Within the scatter, the 3-hydroxypropanal concentrations increased linearly with the amount of cis-3-hexen-l-ol reacted, and a least-squares analysis led to a 3-hydroxypropanal formation yield of 0.33. The seven individual 3-hydroxypropanal formation yields ranged from 0.19 to 0.53, with an average of 0.32. Taking the estimated overall uncertainties into account, we cite a 3-hydroxypropanal formation yield of 0.33-+°1136 a.
OH radical reactions with A P I - M S analysis For the API-MS instrumental conditions employed, the API-MS spectrum of cis-3-hexen-l-ol (molecular weight 100, designated as Ms for starting compound) prior to reaction (Fig. 2, bottom) showed little evidence of the starting material in monomer form, but strong dimer and trimer ion peaks were present at 201 ([Ms + Ms + H] +) and 301 u ([Ms + Ms + Ms + HI+), respectively. API-MS spectra of irradiated CH3ONO-NO-cis-3-hexen-l-ol-air mixtures showed the presence of several additional ion peaks (Fig. 2, top), most notably at 280, 359 and 380 u. Compounds such as cis-3-hexen-l-ol which readily form homodimers will also form heterodimers, in this case with the reaction products as suggested by the difference of 100 u between the strong 380 and 280 u ion peaks. Because there is also an ion peak at 180 u, this suggests that the peak at 280 u is a heterodimer [Ms + Mpl + HI +, where the product, Mpl, has a molecular weight of 179 u and contains a nitrogen atom in the molecule. Figure 3 shows the APIMS/MS CAD daughter ion spectrum (top) and parent ion spectrum (bottom) of the 180 u IMp1 + H I + ion, and these spectra confirm that the 180 u ion peak is [M + H] ÷ of a molecular weight 179 u species. The API-MS/MS parent ion spectrum shows that the higher mass peaks which give rise to the 180 u peak can be explained a s heterodimers at 254 (IMp1 + Mp2 + H]+) and 280 u([Mp~ + Ms + H]+), a homodimer at 359u (IMp1 + Mp~ + HI+), and a heterotrimer at 380u ([Mpa + Ms + Ms + H]+), where Mp2 refers to a second product. The fragmentation pattern of the API-MS/MS CAD daughter ion spectrum of the 180 u ion with its large number of fragment ion peaks (Fig. 3, top) rules out the possibility of this ion being a dimer ion, because dimer ions mainly fragment to yield one or both of the protonated monomers under our instrumental conditions (Kwok and Arey, 1997). The API-MS/MS CAD daughter ion spectrum of the 180 u ion peak shows a [ M + H-H20] + fragment ion at 162 u together with a strong fragment ion at 46 u ([NO2] +) (Fig. 3, top), consistent with this product being an hydroxynitrate of molecular weight 179. The heterodimer ion at 254 u discussed above suggests a second product Mp2 of molecular weight 74 u. The appearance of peaks at 175 and 275 u in the API-MS/MS parent ion spectrum of the relatively minor 75 u ion peak confirms the presence of this product, with the API-MS/MS CAD daughter ion spectrum of the 275 u ion peak showing that this ion corresponds to [Mp2 + Ms + Ms + H] +. Finally, the API-MS/MS CAD daughter ion spectrum of the 75 u ion peak, which again shows a [ M + H-H20] + fragment ion, is consistent with the 3-hydroxypropanal product quantified by GC-FID after derivatization. API-MS/MS parent ion and daughter ion spectra were carried out on the remaining unassigned and less intense product ion peaks (Fig. 2, top), with the
Products of gas-phase reactions
3555 280
201
301
75
380
27~
359
25 115
254
175
v
# E
o
i
i
J
331
301
ID
75
201
25
d-..:_
0
50
100
150
200 m/z
250
300
350
.[
,, 400
Fig. 2. Representative API-MS spectra ofa CH3ONO-NO-cis-3-hexen-l-ol-airmixture prior to (bottom) and after (top) irradiation. The starting material, cis-3-hexen-l-ol, has been designated as Ms (100 u) in the text and the products identified have been designated as Mpl = 179 u, Mv2 = 74 u and Mp3 = 132 u in the text. interpretation being aided by consideration of the possible first-generation products of the OH radicalinitiated reaction of cis-3-hexen-1-ol in the presence of NO (Atkinson, 19£4, 1997). Propanal, quantified by G C - F I D (see above), was observed as a weak 59 u ion peak and the API-MS/MS daughter ion spectrum was consistent with that of an authentic standard. The numerous API-MS/MS parent ion and daughter ion spectra obtained revealed potential products with molecular weights of 130, 132 and 148.
Hydroxy compounds have previously been shown to give a strong [M + H - H 2 0 ] + fragment ion in the API-MS under our instrumental conditions (Atkinson et al., 1995b; Kwok et al., 1996a). API-MS/MS parent ion spectra of the 115 u ion peak indicated that this ion was a fragment of the 133 and 180 u ion peaks. The API-MS/MS daughter ion spectrum of the 133 u ion peak (Fig. 4, top) shows a strong [M + HH 2 0 ] + fragment ion at 115 u, indicative of a hydroxy-compound, and the API-MS/MS parent ion
3556
S.M. ASCHMANN
et al.
46
100
75
50
73
88 99 115
25 180 A v
162
=-
20
c•
i
40
i
100 280
8'0
°~,~ ID ¢r
120
140
160
180
75
180
50
359
25
38O
254~ 0
.
175
2(~0
225
.
250
~
.
275
L,, A. , k 300 325
j,
,1 i
350
t ~. . . .
375
,
400
m/z Fig. 3. API-MS/MS daughter ion (top) and parent ion (bottom) spectra of the 180 u ion peak. spectrum (Fig. 4, bottom) shows a water cluster ion at 151 u and heterodimer and heterotrimer ions at 233 ([Mp3 + Ms + HI÷), 312 (Mp3 + Mpl + HI+), and 333 u ([Mpa + Ms + Ms + H] ÷), together with a very weak self-dimer at 265 u ([Mp3 + Mp3 -I- HI+). These data provide evidence for the formation of a hydroxyproduct of molecular weight 132 (Mp3 = 132 u). API-MS/MS parent ion spectra of the 231 and 331 u ion peaks observed in Fig. 2 (top) showed that these ion peaks are fragments (-H20) of somewhat weaker 249 and 349 u ion peaks, respectively. Similarly, an API-MS/MS parent ion spectrum of the weak 131 u ion peak indicated that it was a fragment
of a 149 u ion peak. An API-MS/MS daughter ion spectrum of the very weak 149 u ion peak showed a fragment at 131 u ([M + H-H20] +) as well as a strong fragment at 75 u, suggesting that the 149 u ion peak was mainly a [Mp2 + M p 2 + H] ÷ dimer ion of 3-hydroxypropanal. Our API-MS data therefore indicate the formation of a hydroxynitrate(s) of molecular weight 179 and a hydroxy-compound(s) of molecular weight 132 in addition to propanal and 3-hydroxypropanal, with the possibility of the formation of a hydroxy-compound(s) of molecular weight 148 as a very minor product.
Products of gas-phase reactions
3557
115
100
133 75
50 75
25 45 57 69711
J
._z,
¢n 0
.~_ 100
20 -
40
60
80
100
120
14.0
133
n" 75
50 23,3
25-
l
151
312
l 150
265 .lI ~ ' 200
250
l 300
333 350
40'0
m/z Fig. 4. API-MS/MS daughter ion (top) and parent ion (bottom) spectra of the 133 u ion peak.
DISCUSSION
CHsCH2CH(OH)CHCH2CH2OH. Under tropospheric conditions, these fl-hydroxyalkyl radicals rapidly and solely add 02 (Atkinson, 1994) and in the presence of NO react to form the corresponding fl-hydroxyalkoxy radical plus NO2 or the
OH radical reaction The OH radical reaction is expected to proceed largely ( ~ 94.%) by initial addition of the OH radical to the > C = C < bond (Atkinson, 1994; Atkinson et al., 1995a), leading to the two fl-hydroxyalkyl radicals CHaCH2CHCH(OH)CH2CHzOH and
M
OH + CHsCH2CH=CHCH2CHzOH~ CHsCH2(~HCH(OH)CH2CHzOHand CH3CH2CH(OH)CHCH2CH2OH
3558
S.M. ASCHMANN et al.
fl-hydroxynitrate. For example, M
CH3CH2(~HCH(OH)CH2CH2OH + 02 ~ CH3CH2CH(O0)CH(OH)CH2CH2OH CH3CH2CH(OO)CH(OH)CH2CH2OH + NO
and similarly for the CHaCH2CH(OH)CHCH2CH2OH radical. The fl-hydroxyalkoxy radicals CHaCH2CH(O)CH(OH)CH2CH2OH and CHaCH2CH(OH)CH(O)CH2CH2OH can decompose, isomerize or react with 02 (Grosjean et al., 1993b; Atkinson, 1994, 1997). The potential reactions of the CH3CH2CH(O)CH(OH)CH2CH2OH radical are shown in Reaction Scheme 1, leading to the formation of propanal plus 3-hydroxypropanal from the decomposition reaction, 3,4-dihydroxyhexanal [CH3CH2CH(OH)CH(OH)CH2CHO] from the isomerization reaction, and 1,3-dihydroxy-4-hexanone [CH3CH2C(O)CH(OH)CHECH2OH] from the 02 reaction.
CH3CH2CH(())CH(OH)CH2CH20H 02 ~
CH3CHICH(0)CH(OH)CH2CH2OH + NO2 F t.... CH3CH2CH(ONOz)CH(OH)CH2CH:OH
radicals dominate to lead to propanal plus 3-hydroxypropanal, and that the only other significant reaction pathway is the isomerization of the CHaCH2CH(O)CH(OH)CH2CHEOH radical to form 3,4-dihydroxyhexanal. Our API-MS observation of a nitrate of molecular weight 179 is consistent with the formation of the fl-hydroxynitrates CH3CHECH(OH)CH(ONO2)CH2CHEOH and/or CHaCH2CH(ONO2)CH(OH)CHECH2OH from the reactions of the fl-hydroxyalkyl peroxy radicals with NO, and our observation of a hydroxy-product of molecular weight 132 is consistent with the predicted formation of 3,4-dihydroxhexanal I-CHaCH2CH(OH)CH(OH)CHECHO]. These two products probably account for
CH3CH2C(O)CH(OH)CH2CH2OH+ HO2
I decomposition ~ e r i z a t i o n CH3CH2CHO + HOCH2CH2CHOH
HOCH2CH2CHO + HO2
CH3CH2CH(OH)CH(OH)CH2CHOH
CH3CH2CH(OH)CH(OH)CH2CHO+ HO2
Reaction Scheme 1. The CH3CH2CH(OH)CH(O)CH2CH2OH radical reacts by analogous reaction pathways, as shown in Reaction Scheme 2, to form propanal plus 3-hydroxypropanal from the decomposition reaction, 1,4,6-trihydroxy-3-hexanone [HOCH2CH2CH(OH)C(O)CH2CH2OH] from the isomerization reaction, and 1,4-dihydroxy-3-hexanone [CH3CH2CH(OH)C(O)CH2CH2OH-I from the 0 2 reaction. Atkinson (1997) has used the literature data concerning the tropospheric reactions of alkoxy and fl-hydroxyalkoxy radicals to calculate the rate constants for the decomposition, isomerization and 02 reactions. These estimation methods lead to calculated reaction rates for decomposition, isomerization and reaction with 02 at 298 K and 760 Torr total pressure of air of 1.1 x 108, 8.7x 106 and 4.1 x 10as -1, respectively, for the CHaCH2CH(O)CH(OH)CH2CH2OH radical, and 8.0 × 107, 2.0 x 105 and 4.1 x 104s -1, respectively, for the CHaCH2CH(OH)CH(O)CH2CH2OH radical. These estimates predict that decomposition of both fl-hydroxyalkoxy
the 25% of the products other than propanal plus 3-hydroxypropanal. 03 Reaction The reaction of 03 with cis-3-hexen-l-ol proceeds by initial addition of 03 to the > C = C < bond to form a primary ozonide which decomposes to (propanal plus [HOCH2CH2CHOO]*) or to (3-hydroxypropanal plus [CH3CH2CHO0]*). The reactions of the biradicals are not well understood, but lead in part to the formation of OH radicals (Atkinson and Aschmann, 1993; Atkinson et al., 1995a, c). Our observation of propanal and 3-hydroxypropanal with formation yields of 0.493 + 0.075 and ~"~J-O.16~ t~+o.33 respectively, is reasonably consistent with the expectation that the sum of the formation yields of propanal and 3-hydroxypropanal should be unity, unless additional formation of the "primary" carbonyls arise from reactions of the [CH3CH2t~HO0]* and [HOCH2CH2CHO0]* biradicals (Atkinson, 1994; Atkinson et al., 1995c). Furthermore, our present - -
Products of gas-phase reactions
CH:3CH2CH(OH)CH(O)CH2CH2OH
02
~
I decomposition HOCH2CH2CHO + CtI3CH2CHOH
CH3CH2CHO + HO2
3559
CH3CH2CH(OH)C(O)CH2CH2OH + HO2 erization (~H2CH2CH(OH)CH(OH)CH2CH2OH
"OOCH2CH2CH(O~CH(O~CH2CH2OH NO - - ~
NO2
°OCH2CH2CH(OI-I)CH(OH)CH2CH2OH ~ isomerization HOCH2CH2CH(OI~(OH)CH2CH2OH
HOCH2CH2CH(OH)C(O)CH2CH2OH + HO2 Reaction Scheme 2.
propanal formation yield of 0.493 + 0.075 is in agreement within the experimental uncertainties with the propanal yield of 0 . 5 9 _ 0.12 previously measured by Grosjean et al. (1993c) as the 2,4-dinitrophenylhydrazone derivative and also in the presence of an OH scavenger. While we quantified 3hydroxypropanal in the absence of an OH radical scavenger, the relatively low OH radical formation yield of -,~ 0.26 from the 03 reaction with cis-3hexen-l-ol (Atkinson et al., 1995a) suggests that the 3-hydroxypropana] formation yield will not be drastically different in the presence or absence of an OH radical scavenger.
CONCLUSIONS The major products of the gas-phase reactions of the OH radical and 03 with cis-3-hexen-l-ol are propanal and 3-hydroxypropanal. Further reactions of these two products in the presence of N O x will lead to the formation of peroxypropionyl nitrate (Grosjean et al., 1993b) and tile hydroxy-analog HOCH2CH2C(O)OONO2. Additionally and for the first time, direct evidence for the formation of dihydroxynitrate(s) and 3,4-dihydroxyhexanal from the OH radical-initiated reaction of cis-3-hexen-l-ol has been obtained by API-MS/MS analyses. Acknowledgements--The authors gratefully thank the National Science Foundation for support of this research through Grant No. ATM-9414036,and thank the National Science Foundation (Grant No. ATM-9015361) and the University of California, Riverside,for funds for the purchase of the PE SCIEX API III MS/MS instrument.
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tion of isoprene. Environmental Science and Technology 29, 2467-2469. Kwok, E. S. C., Arey, J. and Atkinson, R. (1996a) Alkoxy radical isomerization in the OH radical-initiated reactions of C4-C 8 n-alkanes. Journal of Physical Chemistry 100, 214-219. Kwok, E. S. C., Atkinson, R. and Arey, J. (1996b) Isomerization of fl-hydroxyalkoxy radicals formed from the OH radical-initiated reactions of C4-C8 1-alkenes. Environmental Science and Technology 30, 1048-1052. Kwok, E. S. C. and Arey, J. (1997) Dimer formation in the atmospheric pressure ionization tandem mass spectrometer (API-MS/MS) for product studies of atmospherically important reactions of hydrocarbons (in preparation). Lwande, W. and Bentley, M. D. (1987) Volatiles of Sorghum bicolor seedlings. Journal of Natural Products 50, 950-952. Ohta K. (1984) Emission of hexenol from higher plants. Geochemical Journal 18, 135-141. Scanlon, J. T. and Willis, D. E. (1985) Calculation of flame ionization detector relative response factors using the effective carbon number concept. Journal of Chromatographic Science 23, 333-340. Schulting, F. L., Meyer, G. M. and van Aalst, R. M. (1980) Emissie van koolwaterstoffen door vegetatie en de bijdrage aan de luchtverontreiniging in Nederland. TNO Rapport CMP 80/16, Delft, The Netherlands, November 1. Sharkey, T. D. (1996) Emission of low molecular mass hydrocarbons from plants. Trends in Plant Science 1, 78-82.