ARTICLE pubs.acs.org/JPCA
Pressure Dependence of Stabilized Criegee Intermediate Formation from a Sequence of Alkenes Greg T. Drozd* and Neil M. Donahue* Center for Atmospheric Particle Studies, Carnegie Mellon University, 1106 Doherty Hall, 5000 Forbes Ave., Pittsburgh, Pennsylvania, United States ABSTRACT: Ozonolysis is a key reaction in atmospheric chemistry, although important details of the behavior of the ozonolysis intermediates are not known. The key intermediate in ozonolysis, the Criegee intermeiate (CI), is known to quickly isomerize, with the favored unimolecular pathway depending on the relative barriers to isomerization. Stabilized Criegee intermediates (SCI), those with energy below any barriers to isomerization, may result from initial formation with low energy or collisional stabilization of high energy CI. Bimolecular reactions of SCI have been proposed to play a role in OH formation and nucleation of new particles, but unimolecular reactions of SCI may well be too fast for these to be significant. We present measurements of the pressure dependence of SCI formation for a set of alkenes utilizing a hexafluoroacetone scavenger. We studied four alkenes (2,3-dimethyl-2-butene (TME), trans-5-decene, cyclohexene, R-pinene) to characterize how size and cyclization (endo vs exo) affect the stability of Criegee intermediates formed in ozonolysis. SCI yields in ozonolysis were measured in a high pressure flow reactor within a range of 30750 Torr. The linear alkenes show considerable stabilization with trans-5-decene showing 100% stabilization at ∼400 Torr and TME having 65% stabilization at 710 Torr. Extrapolation of the yields for linear alkenes to 0 Torr shows yields significantly above zero, indicating that a fraction of their CI are formed below the barrier to isomerization. CI from endocyclic alkenes show little to no stabilization and appear to have neglible stabilization at 0 Torr. Cyclohexene derived CI showed no stabilization even at 650 Torr, while R-pinene CI had ∼15% stabilization at 740 Torr. Our results show a strong dependence of SCI formation on carbon number; adding just 2 to 3 CI carbons in linear alkenes increases stabilization by a factor of 10. Stabilization for endocyclic alkenes, at atmospheric pressure, begins to occur at a carbon number of 10, with only modest yields of SCI.
’ INTRODUCTION Ozonolysis of alkenes has been extensively studied because of its complex reaction mechanism1 and its central role in atmospheric chemistry,2 but the exact nature of the intermediates controlling ozonolysis is unclear. In particular, the stabilities of two ozonolysis intermediates, the Criegee Intermediate (CI), and a vinylhydroperoxide (VHP) are uncertain (Scheme 1). To react with other molecules, CI must be collisionally stabilized, and after stabilization they must have a long enough thermal lifetime to encounter those molecules. Formation of low vapor pressure products by reaction of stabilized CI (SCI) with reactants such as SO2 or other organics has been proposed as a route to secondary organic aerosol (SOA) and nucleation of new particles, but without more detailed knowledge of SCI formation, this is mainly speculation.27 Recent experiments have shown that the VHP formed in ozonolysis may be subject to considerable stabilization under atmopsheric conditions, but also that the extent of CI stabilization is correspondingly much lower than previously considered.8 Direct measurement of the individual lifetimes of these stabilized intermediates has not been possible, so their effects on critical atmospheric processes remain uncertain. Radical budgets and particulate matter formation in the atmosphere may be strongly affected by the behavior of these intermediates, and further mechanistic details of gas phase ozonolysis are needed to fully understand the impact of alkene ozonolysis on the atmosphere. r 2011 American Chemical Society
Much of our understanding of the ozonolysis mechanism comes indirectly from measurements of stable products, and from computational studies of the ozonolysis potential energy surface. Intermediate stabilization has been overwhelmingly verified using yield measurements for OH and stable products, which are indirect methods of probing intermediate stabilization. Detection of ozonolysis products at low pressure (e10 Torr) via mass spectrometry helped to confirm the Criegee mechanism and indicated SCI formation may give rise to a pressure dependent mechanism.5 Studies of the stoichiometry of alkene loss and SO2 oxidation during alkene ozonolysis clearly showed the importance of intermediate stabilization.9,10 Many scavenger studies have indirectly shown OH formation from alkene ozonolysis, and OH production was more recently directly confirmed.1117 A strong pressure dependence of OH formation has been clearly established from LIF measurements as well as scavenging experiments using NO2.1720 Computational studies have supported OH formation and pressure-dependent behavior in alkene ozonolysis, evaluated the thermodynamics of bimolecular CI reactions, and predicted many details of CI structure.2125 Recently, we used a CI-specific scavenger, a more direct probe of CI stability, to measure the pressure-dependent yields of SCI for Received: January 5, 2011 Revised: March 21, 2011 Published: April 08, 2011 4381
dx.doi.org/10.1021/jp2001089 | J. Phys. Chem. A 2011, 115, 4381–4387
The Journal of Physical Chemistry A
ARTICLE
Scheme 1. Carbonyl Oxide Formation via the Criegee Mechanism
Figure 1. The anti- and syn-conformers of the Criegee Intermediate (left and right respectively). The syn-conformer forms VHP, while the anti-conformer forms dioxirane.
Scheme 2. Reaction Pathways for Carbonyl Oxide Decomposition: Isomerization to an Excited Hydroperoxide (R3a), Which Forms OH (R3b), Ring Closure to Form Dioxirane and Subsequent Products (R4), and Bimolecular Reaction, Such As with Hexafluoroacetone (HFA) Scavenger (R5)
2,3-dimethylbut-2-ene (TME).8 Comparing our results to timedependent LIF measurements of OH yields, we showed that stabilized vinylhydroperoxides (SVHP), in addition to SCI, contribute much of the pressure dependence to OH yields for small alkenes.8 Although it now seems that all of the candidate intermediates do exhibit collisional stabilization to some extent, studies on a range of alkenes are needed to determine the importance, or lack thereof, of stabilized ozonolysis intermediates in the atmosphere. Generalizable effects of structure on CI stability must be determined to accurately account for the impacts of alkene ozonolysis in the atmosphere. Our current focus is how structure affects CI dynamics, and we compare SCI formation from parent alkenes with varying structural aspects: substitution, linear/cyclic structure, and carbon number. The initial mechanistic steps of ozonolysis determine the internal energy of CI immediately after formation and their propensity for stabilization. Ozonolysis begins with formation of an alkene-ozone adduct, the primary ozonide (POZ),1,26,27 which decomposes into a carbonyl þ CI pair (Scheme 1). The exothermicity of ozonolysis reactions leads to CIs with very high internal energies, and reaction of these highly excited intermediates depends on the magnitude of their excess internal energy relative to any barriers to isomerization. Chemically activated CI will isomerize far too rapidly for bimolecular reactions to compete, but SCI can either thermally decompose or react with other reagents. The CI is the critical branching point in the ozonolysis mechanism, because the chemistry of this reactive intermediate will determine the branching ratio between isomerization to VHP (OH formation) and bimolecular reactions. There are at least two ways SCI can form, and both are affected by the structure of the parent alkene. SCI may either be generated with insufficient internal energy to decompose (“born cold”
nascent SCI) or result from collisional stabilization of chemically activated CI. Nascent SCI can form because the energy partitioning between the CI and the carbonyl co-product includes a sizable amount as translational motion and carbonyl co-product internal motion. In the gas phase, the rate of stabilizing collisions governs CI stability, so the role of stabilized intermediates will depend on pressure. In addition, the carbon number influences stability by allowing internally partitioned energy to be stored in nonreactive modes, thus increasing the CI lifetime and easing collisional stabilization. Two major unimolecular pathways exist for CI, and the structural aspects of substitution and conformation determine which of these are available in a given CI. The first involves isomerization to unsaturated vinyl hydroperoxides (VHP) and their subsequent decomposition to OH.5,22 The second involves a dioxirane intermediate, which likely also proceeds to form an OH radical as a secondary (∼ 15%) product via “hot acid” decomposition (Scheme 2).5,17 The relative importance of these two pathways appears to depend on CI conformation. A syn conformation has the terminal CI oxygen facing an alkyl group, and the anti conformation has this oxygen facing a hydrogen (Figure 1). Two main electronic configurations contribute to CI electronic structure: a zwitterionic form with a positive central oxygen and negative terminal oxygen and a biradical form with an unpaired electron at both the central carbon and terminal oxygen. In the zwitterionic form the carbonoxygen bond has double-bond character, and this resonance structure leads to a high barrier to conversion between syn- and anti-conformers.21,23 VHP formation requires a syn-CI conformation to allow internal hydrogen-atom abstraction; this occurs for ketooxides and syn-aldehyde oxides. Aldehyde-oxides with an anti conformation will almost exclusively form dioxirane by cyclization of the CI (COO) functional group. Gas phase bimolecular reactions of SCI are known to occur. Water presents the most important reaction partner for CI in the atmosphere and is known to react with CI to form hydroperoxides and organic acids.28,29 Molecules with carbonyl groups react with CI via 13 dipolar cycloaddition to form secondary ozonides, and reaction of CI with NO2 or SO2 forms the corresponding ketone and NO3 or SO3, respectively.20,9 The rate constants for reaction between CI and molecules with carbonyl functionality is about 100 times that of the reaction between CI and water, but much larger differences in atmospheric abundance make reaction with water the most likely.16 In this work, we present measurements of SCI formation using a hexafluoroacetone (HFA) scavenger to directly probe SCI formation. HFA is a highly specific scavenger and this reaction unambiguously produces a characteristic secondary ozonide.30 Our goal is to further characterize SCI formation and reactivity, with an emphasis on the relationship between CI structure and stabilization. To this end, we studied 4 alkenes with related structures: 2,3-dimethyl-2-butene (tetramethylethylene, TME), trans-5-decene, cyclohexene, and R-pinene. This group can be 4382
dx.doi.org/10.1021/jp2001089 |J. Phys. Chem. A 2011, 115, 4381–4387
The Journal of Physical Chemistry A
ARTICLE
divided into pairs of C6 and C10 alkenes with different stucture and pairs of linear/branched and cyclic alkenes with different carbon number. In a recent work,8 we explored the identity of a succession of intermediates formed during the ozonolysis of highly symmetric TME; here we compare SCI yields from several alkenes to determine the role structure plays in CI stabilization.
’ EXPERIMENTAL SECTION All experiments were conducted in the Carnegie Mellon high pressure flow system (HPFS), with conditions similar to that described in Drozd et al.8 Alkenes were exposed to ozone in the presence of a CI scavenger (HFA) to determine the fraction of scavengable CI. HFA is highly selective toward SCI, producing a fluorinated secondary ozonide (SOZ); it is unreactive toward OH radical, greatly simplifying SCI yield measurements. Each reacted alkene has stoichiometric loss of ozone and production of CI, and each scavenged CI has stoichiometric loss of HFA and production of SOZ. The fraction of scavenged CI is thus equal to either the ratio of HFA loss to ozone loss (LHFA/LO3) or the ratio of SOZ production to ozone loss (PSOZ/LO3). Using FTIR spectroscopy, we monitored changes in column absorption across the mid-IR due to ozonolysis and subsequent scavenging. Difference spectra were generated via reaction modulation spectroscopy (RMS), with alternating alkene flow, and they directly show the losses of both HFA and ozone.8,31 Bulk flow in the HPFS was provided by a nitrogen carrier gas at 30 to 120 slpm (depending on HPFS pressure) regulated by a 200 slpm mass flow controller (MFC). Bulk flow velocities ranged from 1020 (cm s1), which gave total reaction times of about 59 s. HFA and ozone were mixed with the bulk flow just prior to the flow tube entrance. HFA concentrations were in the range of 250350 ppm, to give a time scale for scavenging CI of e1 ms.30,32 Ozone was generated by passing 5 slpm of oxygen through a corona discharge cell (Pacific Ozone model L11). Alkenes used in this study were 2,3-dimethyl-2-butene (98%, Fisher), cyclohexene (97.5%, Fisher), R-pinene (99%, Aldrich), and trans-5-decene (99%,Aldrich). Each alkene was evaporated into a carrier nitrogen flow that was injected into the bulk flow via a capillary injector33 to give concentrations of ∼10 ppm, or e10% of the ozone and HFA concentrations. The concentrations employed in this work are well above ambient levels, but the goal of this work is to probe the inherent energy distributions of ozonolysis intermediates, which are not related to concentration. To ensure complete scavenging by HFA, it was verified that, for each alkene at each pressure, an increase in HFA concentration did not produce an increase in SOZ formation or HFA loss. SCI yields are calculated as the ratio of either SOZ production or HFA loss to ozone loss (PSOZ/LO3 or LHFA/LO3). All of the SOZ produced have a common set of peaks at approximately 985, 1130, and 1245 cm1 . The TME-HFA secondary ozonide peak at ∼1130 cm1 is well separated from other products and can be calibrated to HFA loss, allowing the SCI yield to be calculated as PSOZ/LO3.8 This peak is not well separated from other species for the larger alkenes, so these SOZ yields are calculated as LHFA/LO3. HFA loss in scavenging experiments was determined using spectral subtraction of the low-resolution (16 cm1) carbonyl region peak in experiments without scavenging and a low-resolution reference spectrum for the nearby 1800 cm1 HFA peak.
Figure 2. Distinctive spectral features of SOZ formed from reaction with 3 CI and HFA. The SOZ spectra from ozonolysis of TME (green), trans-5-decene (black), and R-pinene (red) all include peaks at ∼985, ∼ 1140, and ∼1250 cm1. SOZ spectra are determined from the difference of RMS spectra with and without HFA scavenging.
’ RESULTS First, we show that the RMS spectra for all of the alkenes studied have similar features and, via TME, agree with previous spectra of SOZ from scavenging CI with HFA. Next, the pressure dependence of each alkene is shown to highlight the effects of structure on the yield of SCI. SOZ Spectra. The RMS spectra from ozonolysis with HFA scavenging for three alkenes are shown in Figure 2, in the range of 9001300 cm1 . Spectra for ozonolysis of TME, R-pinene, and trans-5-decene are shown in green, red, and black, respectively, with a set of three common features highlighted at 980, 1140, and 1240 cm1 . The spectra have all been normalized to the peak for the TME-HFA secondary ozonide at 1240 cm1 . These peaks were correlated to the normal motions of the TME-HFA secondary ozonide using DFT calculations (B3LYP;6-31G2d2p). The peak at 980 cm1 is a ring-stretching mode involving the central carbon from HFA, which has no analogous motion in HFA. At 1130 cm1, there is a ring breathing mode incorporating the CCC wagging mode from HFA, but heavily red-shifted. The peak at 1240 cm1 is essentially the symmetric CC stretch from HFA slightly red-shifted. Relative intensities of these three peaks match well across these alkenes, and decrease moving toward longer wavelengths. In a recent publication,8 we showed the TME-HFA secondary ozonide spectra obtained here matches exactly with previous results from Horie et al.30 The 1140 cm1 peak is also present in the trans-2-butene SOZ, though slightly shifted to 1128 cm1 .34 The consistent presence of spectral similarities that are well correlated to spectral features of HFA is a clear indication of SOZ formation from HFA scavenging. SCI Formation. For the alkenes studied, increasing the carbon number increases stabilization, and endocyclic alkenes have much lower or completely suppressed stabilization compared to a linear alkene with the same carbon number. SCI yields for 4 alkenes are shown in Figure 3. The top row shows results from C6 alkenes; the bottom row, C10 alkenes. The left column shows results from linear/branched alkenes; the right column, cyclic alkenes. For TME, the pressure dependence of YSCI is essentially linear in the pressure range of our measurements (35710 Torr), with a zero-pressure intercept of ∼15%. At zero-pressure, there is no possibility of collisional stabilization and the stability of CI directly reflects the initial energy distribution after POZ 4383
dx.doi.org/10.1021/jp2001089 |J. Phys. Chem. A 2011, 115, 4381–4387
The Journal of Physical Chemistry A
ARTICLE
Figure 3. SCI yields vs pressure for 4 alkenes. Stabilization increases with carbon number. Cyclic alkenes show very little or no stabilization, because the energy of POZ decomposition is deposited only into internal energy and not into translational motion. (a) TME: a linear trend with an intercept of ∼15%; (b) trans-5-decene: a clear falloff curve with 100% stabilization by 420 Torr; (c) cyclohexene: no stabilization; and (d) R-pinene: ∼15% stabilization at 740 Torr.
decomposition. Formation of YSCI at zero-pressure thus indicates that the initial CI energy distribution extends below the barrier to isomerization. The zero pressure intercept in a plot of YSCI vs pressure thus defines the fraction of CI initially formed with energy less than the barrier to isomerization. Further discussion of the TME yields can be found in Drozd et al.8 YSCI for trans-5decene is more strongly pressure dependent than TME and may have a very different zero pressure intercept. YSCI for trans-5decene approaches unity much more steeply than TME, showing full stabilization around 400 Torr. The trend in YSCI for trans-5decene is not clearly linear as in the case of TME. From unimolecular reaction theory, we do expect this trend to become linear at sufficiently low pressure, but this linear region must begin near 70 Torr or below. Because there is no clear linear range in our data, extrapolation to a pressure of zero is difficult; from our measurements, we can only determine that the intercept for trans-5-decene falls between 0 and 35%. Stabilization of the CI from cyclohexene was not observed at either 540 or 660 Torr; neither loss of HFA nor the peaks characteristic of SOZ formation were observed. Yields for R-pinene were greater than zero but much smaller than for its C10 counterpart, trans-5decene. At 740 Torr, YSCI for R-pinene is ∼15%, decreasing to 5% at 110 Torr. The decreasing trend in these low yields suggests that the zero-pressure intercept for R-pinene is zero.
’ DISCUSSION Two factors have the greatest effects on CI stabilization: the internal energy distribution and size (carbon number) of the nascent CI. The energy of POZ decomposition is either deposited into a single species, in the case of endocyclic alkenes, or two fragments, in the case of linear or exocyclic alkenes. Distributing energy into two fragments has multiple effects, but its main effect is to decrease CI internal energy by releasing energy into the co-product and into external motion from fragment recoil. A
second key result of releasing energy into translation is to greatly broaden the CI energy distribution by accessing the continuum energy regime. A greater carbon number facilitates CI stabilization by increasing the number of vibrational states for distribution of excess vibrational energy. Distributing the energy of POZ decomposition over a large manifold of nonreactive vibrational modes (e.g., CC stretches) increases CI stability by reducing the statistical partitioning of energy in the critical reactive mode. These effects directly translate to terms in the microcanonical RRK rate constant: E E0 s 1 ð1Þ kðEÞRRK ¼ ν E The linear or cyclic structure of alkenes affects the excess energy available to CI, (E E0), while carbon number affects the molecular complexity or number of vibrational modes, s. These two main factors determining YSCI are exemplified by the yields for the four alkenes shown in Figure 3. Horizontal comparisons involve moving the double bond to an endocyclic position, and, the differences in YSCI are mainly due to changing the magnitude of CI internal energy. In both cases, TME vs cyclohexene and trans-5-decene vs R-pinene, there is a dramatic decrease in YSCI . As discussed above, all of the energy of POZ decomposition is converted to CI internal energy for endocyclic alkenes; the carbonyl and carbonyl-O-oxide are tethered together as a single species with no energy going to fragment recoil. Comparing the rows in Figure 3, one sees the effect of increasing carbon number for alkenes with similar structure with respect to cyclization. The trans-5-decene CI has two more carbons than that of TME, and the R-pinene CI has four more carbons than the cyclohexene CI. For each pair of alkenes, the C10 alkene has a dramatic increase in YSCI, and this is due to the greater number of available vibrational modes for statistically distributing internal energy. TME has 50% stabilization (M1/2) at 500 Torr, and the 4384
dx.doi.org/10.1021/jp2001089 |J. Phys. Chem. A 2011, 115, 4381–4387
The Journal of Physical Chemistry A M1/2 for trans-5-decene lies much lower, near 50 Torr. These linear alkene data show that adding 2 carbons to the CI shifts the pressure dependence downward by roughly 1 decade. While we measure no stabilization for cyclohexene, R-pinene clearly shows a small extent of stabilization, so the onset of atmospheric-pressure stabilization for cyclic alkenes occurs at a carbon number of about 10. The strong dependence on carbon number we measure for linear alkenes has also been predicted by master equation calculations. Kroll et al. found a similar trend in increased stabilization per carbon added for linear alkenes: increasing the chain length by 2 decreases M1/2 by roughly a factor of 10.18 Calculations for cyclic alkenes show a factor of 10 increase in CI stabilization, which is indicated by intramolecular SOZ formation, per 4 added carbons.39 With roughly 60% of the reaction energy for linear alkenes going to fragment recoil or coproduct internal energy, it is reasonable that cyclic alkenes, which partition no reaction energy to fragment recoil, require a greater increase in carbon number for similar stabilization. Calculations for syn-acetaldehyde-oxide at 1 atm show 70% stabilization,35 and this is nearly the same as our YSCI measurements at 1 atm for TME, which only forms syn-CI. The agreement between our measurements and master equation calculations for syn-CI from linear alkenes implies roughly 40 kJ mol1 of excess energy, a barrier to VHP isomerization of about 16 kJ mol1, and average collisional deactiviation, (ÆEdownæ), of 300500 cm1. There have been several studies involving indirect measurements of SCI yields or the influences of SCI scavengers on OH production, and a wide range of effects were found. Water vapor was not seen to have an effect on OH yields for several alkenes, and these results indicate that either SCI þ H2O produces an unstable hydroperoxide or the isomerization to VHP is much faster than reaction with water.16,28,32 Results from Kuwata et al. indicate that only OH yields from anti-CI are likely to be influenced by water vapor because syn-CI are slow to react with water, although this has not been generalized to other scavengers.35 HFA may be estimated to react with CI roughly 3 orders of magnitude faster than H2O, and is not known to be syn/anti sensitive.30,32 The increased reactivity and particularly the apparent lack of selectivity of HFA can explain our increased YSCI ; water is not nearly as effective a scavenger and should only reflect a portion of the anti-CI formed in ozonolysis. Marston et al.32 also used SO2, butanone, and acetic acid scavengers to cover a wide range of reactivities with SCI and did not see any effect on OH yields from 2-methylbut-2-ene ozonolysis. Our TME yields agree with the OH yields of Presto et al. who used NO2 to scavenge OH while preventing OH production from SCI.20 1-YSCI was shown to closely follow these OH yields, indicating that NO2 scavenging of SCI does indeed affect OH yields. It appears that the SCI yield from 2-methylbut-2-ene, expected to be less than that of TME, is small enough that tracer measurements do not clearly show any effects on OH yields due to SCI scavenging. Our results at 740 Torr agree with scavenging measurements of the OH yields for R-pinene, where humidity was seen to affect OH yields by ∼15%.36 In that study, and also that of Wegner et al.,37 OH yields increased with relative humidity, implying anti-SCI from R-pinene may react to produce additional OH. All of these results are consistent with the direct LIF measurements of OH by Kroll et al., which show significant pressure dependent stabilization of intermediates on the path to OH formation and subsequent thermal production of OH.1719 Finally, although we see no evidence of
ARTICLE
SCI formation from cyclohexene ozonolysis, secondary ozonides have been reported from this system, suggesting SCI formation, although without any quantitative measure of the yield.38 In addition to the major effects of energy distribution and size, substitution also plays a role in CI stabilization. Substitution has several effects on CI dynamics, changing the available reaction pathways and the shape of the PES for each. A disubstituted CI has only a syn configuration with hydrogens readily available for abstraction, while a monosubstituted CI has both syn and anti configurations.(Figure 1) Calculations show that a large energy barrier (∼35 kcal/mol) due to partial zwitterionic character and the resulting π CO bonding should prevent conversion between these conformers.23 The syn-conformers mainly isomerize to VHP; anti-conformers, to dioxirane. A recent computational study including estimates of tunneling effects has shown that unimolecular reaction rates are fairly similar for syn- vs anticonformation, with anti-CI isomerization to dioxirane faster than syn-CI isomerization to VHP.35 This same study found that the bimolecular reactivity of CI can strongly depend on conformation. The rate of syn-CI isomerization was calculated to be 3 orders of magnitude faster than reaction with water at its saturation concentration, while anti-CI isomerization occurs at rate similar to the reaction with water. Alkyl substitution will also alter the relative energies of the CI and the isomerization transition states, thus changing the well depth and the barrier height for CI decomposition. For intermediates with high excess energy, preferentially lowering the well depth vs the barrier height will lead to slower unimolecular reaction. The energy of the CI, due to CI zwitterionic character, should be much more sensitive to electron donation from alkyl substituents than either transition state. Isolating the individual effects of these energy changes is difficult, but it is likely that substitution will lead to slower CI isomerization rates. Our results for trans-5-decene and TME indicate that increasing substitution plays a minor role in aiding stabilization compared to increasing carbon number, because the roughly 1 decade shift per 2 added CI carbons is maintained in comparing YSCI for TME and trans-5-decene. So while substitution may have some secondary effects on the time scale of CI stabilization, its main effect may instead be to affect conformation and hence bimolecular reaction rates. The importance of VHP stabilization can be inferred by comparing prompt OH yields and our YSCI . While quantitative comparisons are difficult due to the uncertainty of VHP:dioxirane branching in trans-5-decene, some comparisons can be made to the prompt OH yields of Kroll et al. These LIF OH yield measurements are “prompt” LIF OH at 10 ms, and the pressure dependence of this signal indicates the total collisional stabilization into stable wells along the reaction coordinate leading to OH formation. Results from Chuong et al. show that POZ from trans5-decene, which is only a C10 alkene, is not expected to be stabilized to any significant extent,39 so any influence of POZ stabilization is unlikely. Similar to the results for TME,8 our YSCI are well below the YOH{LIF}, indicating a stabilized intermediate between the SCI and OH along the reaction coordinate. Extrapolating YOH{LIF} to higher pressures, both YSCI and YOH{LIF} reach unity at about 400 Torr. VHP stabilization appears to be about equal to CI stabilization at 30 Torr, much lower than for TME due to the increase in carbon number.8 Because ozonolysis of trans-5-decene forms both anti- and syn-CI, isomerization forms both dioxirane and VHP. While it is certain that some of the difference between our YSCI and YOH{LIF} is due to VHP 4385
dx.doi.org/10.1021/jp2001089 |J. Phys. Chem. A 2011, 115, 4381–4387
The Journal of Physical Chemistry A stabilization, the exact amount will depend on the extent of dioxirane formation.
’ ATMOSPHERIC IMPLICATIONS The modest stabilization for even C10 endocyclic alkenes is potentially quite significant for atmospheric chemistry, where reactions of SCI from monoterpenes are sometimes invoked as important to the formation of secondary organic aerosol. For example, SCI þ water reactions have been proposed as potential explanations for the observed humidity dependence of pinonic and pinic acid yields from the ozonolysis of R-pinene.40 Our findings do not directly rule this out, as the molar yields of these acids are very low and as a consequence even a 15% SCI yield could lead to large fractional enhancements in the acid formation. However, explaining the bulk SOA yields with this modest effect is much more difficult, and it is impossible when considering cyclohexene SOA.41 Also, computational results showing low syn-CI reactivity and our large YSCI for TME combined with pressure independent OH yields in tracer studies are beginning to suggest conformer specific CI reactivity, and this may strongly affect the availability of SCI for bimolecular reactions. Consequently, these results cast doubt on the role of SCI in organic aerosol formation from endocyclic monoterpenes while leaving the possibility open for a more significant role in the larger sesquiterpenes. A similar argument applies to the implications of our measured YSCI for OH formation; the low stabilization of CI from C10 endocyclic alkenes should prevent any significant effects of bimolecular reactions on their OH production. ’ CONCLUSIONS We have presented yields of stabilized Criegee intermediates for four alkenes: TME, trans-5-decene, cyclohexene, and R-pinene . Our results support the trends in stabilization due to increasing carbon number as calculated by Choung et al.39 and Kroll et al.17 A 2- to 3-carbon increase in linear alkenes and a 5-carbon increase in endocyclic alkenes increases stabilization 10-fold, i.e., decreases M1/2 by a factor of 10. These results are also consistent with the approximate partitioning of 60% of the energy from POZ decomposition to fragment recoil of the CI and the carbonyl coproduct. Subsitution seems to have a much smaller effect compared to increasing carbon number. The disubstituted and monosubsituted CI from TME and trans-5-decene exhibit the same trend in stabilization as measured for CI from trans-2-butene and trans-3-hexene, which are both monosubstituted. Endocyclic alkenes have little to no stabilization even for the C10, R-pinene . The lack of stabilization for the cyclohexene and R-pinene derived CI can be explained by the energy partitioning upon POZ decomposition. For the endocyclic alkenes, all of the energy of POZ decomposition, plus essentially all of the energy of POZ formation from the initial ozonolysis, is converted to internal excitation of the single product. Energy is not deposited into external motion or excitation of a carbonyl coproduct, because the co-product is instead a distant moiety on the same product molecule. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected];
[email protected].
ARTICLE
’ ACKNOWLEDGMENT This work was supported by Grant CHE-0717532 from the National Science Foundation. ’ REFERENCES (1) Criegee, R. Angew. Chem., Int. Ed. Engl. 1975, 14, 745–752. (2) Hallquist, M.; Wenger, J. C.; Baltensperger, U.; Rudich, Y.; Simpson, D.; Claeys, M.; Dommen, J.; Donahue, N. M.; George, C.; Goldstein, A. H.; Hamilton, J. F.; Herrmann, H.; Hoffmann, T.; Iinuma, Y.; Jang, M.; Jenkin, M. E.; Jimenez, J. L.; Kiendler-Scharr, A.; Maenhaut, W.; McFiggans, G.; Mentel, Th. F.; Monod, A.; Prev^ot, A. S. H.; Seinfeld, J. H.; Surratt, J. D.; Szmigielski, R.; Wildt, J. Atm. Chem. Phys. 2009, 9, 5155–5236. (3) Paulson, S. E.; Orlando, J. J. Geophys. Res. Lett. 1996, 23, 3727–3730. (4) Neeb, P.; Sauer, F.; Horie, O.; Moortgat, G. Atmos. Environ. 1997, 31, 1417. (5) Martinez, R. I.; Herron, J. T.; Huie, R. E. J. Am. Chem. Soc. 1981, 103, 3807–3820. (6) Crehuet, R.; Anglada, J.; Bofill, J. Chem.—Eur. J. 2001, 7, 2227–2235. (7) Bonn, B.; Schuster, G.; Moortgat, G. K. J. Phys. Chem. A 2002, 106, 2869–2881. (8) Drozd, G. T.; Donahue, N. M. J. Phys. Chem. A 115, 161–166. (9) Hatakeyama, S.; Kobayashi, H.; Akimoto, H. J. Phys. Chem. 1984, 88, 4736–4739. (10) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P.; Hurley, M. D. J. Phys. Chem. 1987, 91, 941–946. (11) Atkinson, R.; Aschmann, S. M.; Arey, J.; Shorees, B. J. Geophys. Res. 1992, 97, 6065–6073. (12) Atkinson, R.; Aschmann, S. M. Environ. Sci. Technol. 1993, 27, 1357–1363. (13) Chew, A. A.; Atkinson, R. J. Geophys. Res. 1996, 101, 28649–28653. (14) Paulson, S. E.; Fenske, J. D.; Sen, A. D.; Callahan, T. W. J. Phys. Chem. A 1999, 103, 2050–2059. (15) Neeb, P.; Moortgat, G. K. J. Phys. Chem. A 1999, 103, 9003–9012. (16) Johnson, D.; Lewin, A. G.; Marston, G. J. Phys. Chem. A 2001, 105, 2933–2935. (17) Kroll, J. H.; Clarke, J. S.; Donahue, N. M.; Anderson, J. G. J. Phys. Chem. A 2001, 105, 1554–1560. (18) Kroll, J. H.; Sahay, S. R.; Anderson, J. G.; Demerjian, K. L.; Donahue, N. M. J. Phys. Chem. A 2001, 105, 4446–4457. (19) Kroll, J. H.; Hanisco, T. F.; Donahue, N. M.; Demerjian, K. L.; Anderson, J. G. Geophys. Res. Lett. 2001, 28, 3863–3866. (20) Presto, A.; Donahue, N. J. Phys. Chem. A 2004, 108, 9096–9104. (21) Cremer, D. J. Am. Chem. Soc. 1979, 101, 7199–7205. (22) Gutbrod, R.; Kraka, E.; Schindler, R. N.; Cremer, D. J. Am. Chem. Soc. 1997, 119, 7330–7342. (23) Anglada, J. M.; Bofill, J. M.; Olivella, S.; Sole, A. J. Am. Chem. Soc. 1996, 118, 4636–4647. (24) Anglada, J.; Aplincourt, P.; Bofill, J.; Cremer, D. ChemPhysChem 2002, 3, 215. (25) Olzmann, M.; Kraka, E.; Cremer, D.; Gutbrod, R.; Andersson, S. J. Phys. Chem. A 1997, 101, 9421–9429. (26) Epstein, S. A.; Donahue, N. M. J. Phys. Chem. A 2008, 112, 13535–13541. (27) Epstein, S. A.; Donahue, N. M. J.Phys. Chem. A 2010, 119, 7330–7342. (28) Hasson, A. S.; Chung, M. Y.; Kuwata, K. T.; Converse, A. D.; Krohn, D.; Paulson, S. E. J. Phys. Chem. A 2003, 107, 6176–6182. (29) Docherty, K. S.; Ziemann, P. J. Aerosol Sci. Technol. 2003, 37, 877–891. (30) Horie, O.; Sch€afer, C.; Moortgat, G. K. Int. J. Chem. Kin. 1999, 31, 261–269. 4386
dx.doi.org/10.1021/jp2001089 |J. Phys. Chem. A 2011, 115, 4381–4387
The Journal of Physical Chemistry A
ARTICLE
(31) Donahue, N. M.; Demerjian, K. L.; Anderson, J. G. J. Phys. Chem. 1996, 100, 17855–17861. (32) Johnson, D.; Marston, G. Chem. Soc. Rev. 2008, 37, 699–716. (33) Lambe, A. T.; Zhang, J.; Sage, A. M.; Donahue, N. M. Environ. Sci. Technol. 2007, 41, 2357–2363. (34) Fenske, J. D.; Kuwata, K. T.; Houk, K. N.; Paulson, S. E. J. Phys. Chem. A 2000, 104, 7246–7254. (35) Kuwata, K. T.; Hermes, M. R.; Carlson, M. J.; Zogg, C. K. J. Phys. Chem. A 2010, 114, 9192–9204. (36) Tillmann, R.; Hallquist, M.; Jonsson, Å. M.; Kiendler-Scharr, A.; Saathoff, H.; Iinuma, Y.; Mentel, T. F. Atmos. Chem. Phys. Disc 2010, 10, 3129–3172. (37) Wegener, R.; Brauers, T.; Koppmann, R.; Rodríguez Bares, S.; Rohrer, F.; Tillmann, R.; Wahner, A.; Hansel, A.; Wisthaler, A. J. Geophys. Res. 2007, 112, 1611–1614. (38) Nøjgaard, J.; Nørgaard, A.; Wolkoff, P. Atmos. Environ. 2007, 41, 8345–8354. (39) Chuong, B.; Zhang, J.; Donahue, N. M. J. Am. Chem. Soc. 2004, 126, 12363–12373. (40) Ma, Y.; Russell, A. T.; Marston, G. Phys. Chem. Chem. Phys. 2008, 10, 4294–4312. (41) Keywood, M. D.; Varutbangkul, V.; Bahreini, R.; Flagan, R. C.; Seinfeld, J. H. Environ. Sci. Technol. 2004, 38, 4157–4164.
4387
dx.doi.org/10.1021/jp2001089 |J. Phys. Chem. A 2011, 115, 4381–4387
