Photo-Isomerization of Methyl Vinyl Ketone and Methacrolein in the

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Photo-Isomerization of Methyl Vinyl Ketone and Methacrolein in the Troposphere: A Theoretical Investigation of Ground State Reaction Pathways Sui So, Uta Wille, and Gabriel da Silva ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00066 • Publication Date (Web): 13 Jun 2018 Downloaded from http://pubs.acs.org on June 15, 2018

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ACS Earth and Space Chemistry

Photo-Isomerization of Methyl Vinyl Ketone and Methacrolein in the Troposphere: A Theoretical Investigation of Ground State Reaction Pathways

Sui So,1 Uta Wille,2 Gabriel da Silva1* 1

Department of Chemical Engineering, The University of Melbourne, Victoria 3010, Australia.

2

School of Chemistry and Bio21 Institute, The University of Melbourne, Victoria 3010, Australia.

* Author to whom correspondence should be addressed. [email protected]

Keywords: Carbonyl compounds; Photo-isomerization; Kinetics; Master Equation; RRKM Theory

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ABSTRACT The ground-state rearrangement and decomposition of methyl vinyl ketone (MVK) and methacrolein (MACR) has been investigated using quantum chemical calculations and RRKM theory/master equation simulations. MVK and MACR absorb actinic radiation at around 380280 nm, and we have identified a number of isomerization pathways with barriers that are accessible from the longer wavelength end of this range (visible/near-UV). Assuming that radiationless transitions dominate, master equation simulations of the reactions on the vibrationally excited ground state potential energy surface predict that isomerization to 2hydroxybutadiene and 1-hydroxymethylallene from MVK, and isomerization to dimethylketene from MACR, are the major tropospheric reaction channels. Despite these processes having low quantum yields, they are prevalent due to the coincidence of high absorption cross sections with significant solar photon fluxes at around 320 - 330 nm, where photo-dissociation does not occur. This work suggests that photo-isomerization may be an important process in the photolysis of these compounds in the troposphere, particularly for MVK, which, in comparison to MACR, has both a shorter lifetime with respect to photolysis and a longer lifetime with respect to reaction with the •OH radical.


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INTRODUCTION Methyl vinyl ketone (MVK, CH3C(O)CH=CH2) and methacrolein (MACR, CH2=C(CH3)CHO) are important volatile organic compounds (VOCs) in the troposphere. They play a significant role in atmospheric chemistry as products from hydrocarbon oxidation,1-3 and they have implications for the formation of secondary organic aerosols.4-7 MVK and MACR are primary oxidation products of isoprene, which is the second most emitted VOC to the troposphere after methane. Mixing ratios of MVK and MACR of up to 2.5 ppbv have been measured in forested areas, across various field campaigns.8-10 The atmospheric consumption of carbonyl compounds, such as MVK and MACR, occurs at daytime primarily through •OH-initiated oxidation. The reaction of •OH with MVK and MACR has been previously studied using relative rate techniques,11-13 flash photolysis resonance fluorescence14 and pulsed laser-induced fluorescence.15 The major products of •OH-initiated oxidation of MVK have been identified as glycolaldehyde, glyoxal, methylglyoxal and formaldehyde,15-18 whereas the •OH-initiated oxidation of MACR yields methylglyoxal, formaldehyde and hydroxyacetone (Scheme 1).19-21 With 298 K rate coefficients for the reaction of •OH with MACR and MVK of about 3.3 × 10-11 and 1.9 × 10-11 cm3 molecule-1 s-1, respectively,16,17 both species are removed rapidly under sunlit conditions, but the tropospheric lifetime of MVK (~ 7 hr) is substantially longer than that of MACR (~ 4 hr).

O O H

H MeOH O

O HO H

CO H

H

O

OH

O H

OH

photo-fragmentation

MVK

reaction with OH

O

H

HO

O H O

photo-induced isomerisation

O O

H

O C

H O

MACR

OH

(from MACR) OH

(from MVK)

C (from MVK)

Scheme 1. Atmospheric daytime reactions of methyl vinyl ketone (MVK) and methacrolein (MACR). 3 ACS Paragon Plus Environment


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Although removal through photolysis is only a minor atmospheric sink for MVK and MACR, current chemical kinetic models do include such photo-degradation pathways.22-25 However, they remain relatively poorly studied. Raber and co-workers have investigated the photolysis of MVK and MACR using pulsed laser-induced fluorescence,26 where they identified propene and ethene as major products, respectively, with carbon monoxide and formaldehyde being common products of both compounds. Minor photolysis products were methanol, formic acid, and acetic acid. Other studies of MVK and MACR photolysis showed similar product formation.15, 27, 28 There are clear indications that the unimolecular reaction pathways in the tropospheric oxidation of MVK and MACR are still not completely known. For example, a Fourier transform infrared (FTIR) study of MVK and MACR photolysis revealed absorptions that could be assigned to a ketene or allene, potentially from photo-isomerization.26 In fact, recent studies provided strong evidence that photo-initiated keto–enol isomerization is an important transformation pathway for carbonyl compounds, for example for the conversion of acetaldehyde to vinyl alcohol.29, 30 The subsequent •OH-initiated oxidation of vinyl alcohol can produce formic acid, formaldehyde and glycolaldehyde.31 In light of this, we were motivated to investigate the possible photo-isomerization of MVK and MACR, particularly since they are characterized by significant actinic absorption due to the conjugated nature of their chromophores. Recent studies have shown that the photo-dissociation of formaldehyde,32, 33 acetone34, 35 and methyl ethyl ketone36 occurs predominantly on the S0 ground state surface, after going through intersystem crossing between the S1 to T1 and then T1 to S0 states. A similar study involving acetaldehyde also showed that the excited state surface is unimportant for atmospheric chemistry.29 Because of the very low efficiency of MVK and MACR photolysis, transition back to the ground state surface must dominate. There is no evidence for fluorescence in these compounds, and transition to the ground state must therefore take place through radiationless processes (internal conversion or intersystem crossing). Thus, it appears that the photoexcitation of MVK and MACR by solar radiation produces a significant amount of vibrationally excited ground state molecules, which can potentially undergo rearrangement or fragmentation processes. In this work, we have studied the ground state dissociation and isomerization of MVK and MACR using quantum chemical calculations. These calculations reveal that a variety of ground state isomerization channels are available with barriers lower than that for photo-dissociation. Competition between the isomerization and dissociation mechanisms is investigated using energy-grained master equation simulations. The models proposed are benchmarked to experiments conducted previously.15 Calculations show that MVK can isomerize to 14 ACS Paragon Plus Environment

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hydroxymethylallene (1-HMA, CH2=C=C(OH)CH3) and 2-hydroxybutadiene (2-HBD, CH2=CHC(OH)=CH2). On the other hand, MACR does not possess an enol form, but can isomerize to dimethylketene (DMK, (CH3)2C=C=O). Both MVK and MACR can also undergo cyclization to form methyloxetene isomers.

METHODS Electronic Structure Calculations All electronic structure calculations were performed using the Gaussian 0937 software package. Reported structures in the MVK and MACR reaction systems were optimized at the M06-2X/631G(2df,p) level of theory,38 with the integration grid of the computations specified to span 75 radial shells and 302 angular points. Wavefunctions were stable for all optimized species. Intrinsic reaction coordinate scans were conducted for transition states with ambiguous structure and inconclusive imaginary frequency using a Hessian-based predictor-corrector algorithm.39 The M06-2X functional was chosen to optimize structures due to its accurate prediction of transition state geometries and reaction barriers at a relatively low cost of calculation.31, 38 The optimized geometries and vibrational frequencies were also utilized in higher level composite G3X-K energy calculations.40 This method combines a series of single point energy calculations from Hartree-Fock, through Møller-Plesset perturbation theory to coupled cluster theory. The final energy incorporates empirical scaling corrections and approximates the CCSD(T) energy with a large basis set. From the CCSD(T) energies the T1 diagnostic was also used to confirm that none of the species suffered from significant multireference character. G3X-K theory was selected as it is particularly designed for thermochemical kinetic analysis, which can reproduce barrier heights with an expected mean accuracy of 0.6 kcal mol-1.40 The master equation calculations reported in the following sections are based on these G3X-K energies. Additional single point M06-2X energy calculations were performed using the aug-ccpVTZ augmented correlation consistent basis set. The mean absolute deviation between the G3X-K and M06-2X/aug-cc-pVTZ model chemistries is 0.9 kcal mol-1, which is consistent with the expected combined error range of around 1 to 2 kcal mol-1.38, 40

RRKM Theory/Master Equation Calculations The isomerization and decomposition of MVK and MACR on their ground state potential energy surfaces was simulated using an energy-grained master equation model, so as to predict 5 ACS Paragon Plus Environment


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product formation after intersystem crossing/internal conversion back to the ground state surface following solar excitation. Simulations were conducted under representative tropospheric conditions of 298 K and 1 atm pressure (N2). All calculations were performed within the MultiWell2016 suite of programs.41-43 Calculations were based on optimized structures, vibrational frequencies and moments of inertia at the M06-2X/6-31G(2df,p) level of theory, with G3X-K energies. The MultiWell model files are provided as Supporting Information. Standard statistical mechanical formulae were used to treat stationary points in the MVK and MACR systems. The internal degrees of freedom were described by harmonic oscillators and the external degrees of freedom were treated as separable active 1D K-rotors and inactive 2D Jrotors. Microcanonical rate coefficients were determined from sums and densities of states using RRKM theory. Quantum mechanical tunneling was incorporated for reactions involving a hydrogen shift using unsymmetrical Eckart barriers, although test simulations showed that tunneling is relatively insignificant for these systems. The grain size of the master equation was set to be 10 cm-1. The energy-grained master equation was solved for energies up to 2,000 cm-1 and the continuum master equation was extended to 200,000 cm-1. Each simulation featured 107 trials to produce reliable statistics for reactions with low yields. Simulations were run for up to 7500 collisions, and assumed shifted thermal energy distributions at 298 K, where internal energy was increased by the energy equivalent to a single absorbed photon of wavelength λ. The Lennard-Jones model was used to estimate the frequency of bath gas collisions. Using additivity calculations employing partial molar volumes,44 the Lennard-Jones parameters σ and ε/kb for MVK and its isomers are in the range of 5.69-5.79 Å and 344-488 K respectively; for MACR and its isomers, σ = 5.69 Å and ε/kb = 382-414 K. Collisional energy transfer with the bath gas was treated using the single exponential down model. The bath gas was approximated to be N2 and the average energy transferred in deactivating collisions (∆Edown) was benchmarked at 40 cm-1 (see below). Barrierless dissociation reactions were treated using the restricted Gorin model, in a similar fashion to Golden and co-workers,45, 46 as described in detail in recent studies.31, 47-49 The equilibrium center of mass distance was estimated by the reduced mass and 1D rotor moment of inertia. A Morse function was used to describe the dissociation potential by fitting the inactive 2D J-rotor of the transition state. High pressure limit rate coefficients of all the reverse recombination reactions were approximated using structurally similar reactions, as listed in Table 1.

Table 1 Approximated high pressure limit rate coefficients of the barrierless recombination reactions.

Reaction

Structurally Similar

k

Ref. 6

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MVK → •CH3 + •COCH=CH2 MVK → CH3C•O + •CH=CH2 2-HBD → CH2CHC(O•)CH2 + •H 1-HCB → cy-CH2C(O)CHCH2- + •H MACR → CH2=CCH3C•O + •H MACR → CH2=C•CH3 + •CHO MACR → CH2=C•CO + •CH3 DMK → C•H2C(CH3)CO + •H DMK → CH3C•CO + •CH3 DMK → (CH3)2C: + CO

Recombination • CH3 + •COCH3 CH3C•O + •CH2CH3 CH3O• + •H CH3O• + •H CH3C•O + •H CH2C•H + •CHO CH2=C•H + •CH3 CH3C•H2 + •H CH2=C•H + •CH3 H2C: + CO

(cm3 molecule-1 s-1) 4.6×10-11 3.0×10-11 1.7×10-10 1.7×10-10 5.5×10-11 3.0×10-11 1.2×10-10 1.7×10-10 1.2×10-10 9.0×10-12

50-52 53 54 54 55 52 56 57 56 58

RESULTS AND DISCUSSION Carbonyl photolysis in the actinic spectrum involves excitation to the first singlet excited state (S1) followed by transitions to the T1 triplet and S0 ground states. For MVK, the relative energies of these three states are illustrated in Figure 1 (a similar diagram for MACR is included as Supporting Information). Included in Figure 1 are the conventional Norrish Type I photolysis products resulting from homolytic bond cleavage: CH2CHC•O + •CH3 and CH3C•O + CH2C•H. These products sit respectively at around 10 and 20 kcal mol-1 above the threshold for excitation to the S1 state. Although dissociation on the ground state takes place with only a thermodynamic barrier (i.e., barrierless in the reverse direction), appreciable barriers are encountered on the triplet surface. Negligible solar radiation with photon wavelengths corresponding to these energies (< 300 nm / > 95 kcal mol-1) reaches Earth’s surface, suggesting that MVK (and MACR) photo-dissociation in the troposphere is predominantly a ground state process. The subsequent section of this manuscript develops detailed ground-state potential energy surfaces for MVK and MACR, in order to shed light on this process. Importantly, a number of new isomerization reactions are identified, with barriers below those for conventional bond homolysis. Following this, master equation models are developed for the isomerization and dissociation of MVK and MACR, which are then used to simulate the photolysis of these molecules. Finally, we estimate the contribution of photo-isomerization to MVK and MACR photolysis in the troposphere.



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Figure 1. Potential energy surface illustrating the Norrish Type I dissociation reactions of MVK on the T1 (red) and S0 (green) surfaces. Energies at 0 K with zero point energy are shown at the G3X-K level of theory, in kcal mol-1. The S1 energy is also indicated in blue, estimated from the experimental absorption spectra.15

Ground State Potential Energy Surfaces A variety of dissociation and isomerization reactions were identified for ground state MVK and MACR, with energies falling within or below the ultraviolet (UV) or visible range. The developed ground state potential energy surfaces for MVK and MACR are shown in Figure 2. Optimized structures of the wells and transition states in the photochemical reactions of MVK and MACR are presented in Figures 3 and 4.


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Figure 2. Potential energy surfaces for the ground state isomerization and decomposition of (a) MVK and (b) MACR used in the master equation simulations. Energies at 0 K with zero point energy are shown at the G3X-K level of theory, in kcal mol-1. The S1 state energies of MVK and MACR, estimated from the experimental absorption spectra,15 are also indicated.



MVK

2-MO

2-HBD

1-HCB

TS1

TS2

TS4

TS5

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1-HMA

CBO

TS3

TS6


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TS7

TS8

TS9

TS10 Figure 3. Optimized structures of MVK, its isomers, and connecting transition states, at the M06-2X/631G(2df,p) level of theory. Displacement vectors of imaginary frequencies are also shown.



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MACR

3-MO

DMK

TS11

TS12

TS13

TS14

TS15

TS16

TS17 Figure 4. Optimized structures of MACR, its isomers, and connecting transition states, at the M06-2X/631G(2df,p) level of theory. Displacement vectors of imaginary frequencies are also shown.


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For MVK, two bond dissociation channels have been included, leading to the products sets •CH3 + CH2CHC•O and CH3C•O + CH2C•H. The former pathway has a barrier of 82.2 kcal mol-1, whereas the latter is higher by 11.4 kcal mol-1. A competitive molecular dissociation channel, to CH2CO + C2H4, is also available with barrier of 79.5 kcal mol-1 (TS1). In addition to the dissociation channels, two tautomerization pathways exist for MVK, which produce the enols 1hydroxymethylallene (1-HMA) and 2-hydroxybutadiene (2-HBD) through 1,3-H shifts via TS3 and TS4 respectively. The formation of 2-HBD through keto-enol tautomerization has a particularly low barrier (63.8 kcal mol-1),59-61 which we attribute to the conjugated structure of the enol product (a similar enol form has recently been reported for pyruvic acid).62 This low barrier, coupled with the relatively small reaction endothermicity (9.4 kcal mol-1) makes 2-HBD a likely candidate for isomerization with subsequent collisional deactivation. A further tautomerization reaction is also available, which directly connects 1-HMA with 2-HBD via TS5, but with a substantially higher barrier. Following its formation, 2-HBD can undergo further rearrangement into 1-hydroxycyclobutene (1-HCB) and then cyclobutanone (CBO), albeit with unfavorable thermodynamics for the first step and a relatively high barrier for the second (TS9; 84.3 kcal mol-1 vs. MVK). It is also possible for 1-HCB to isomerize and decompose via ethylketene (shown in the Supporting Information), although preliminary master equation modelling indicated that these reactions were insignificant, and they were subsequently excluded. There is also a cyclization reaction leading to the formation of 2-methyloxetene (2MO) with a low barrier (TS2, 53.5 kcal mol-1). This reaction, however, is significantly endothermic (27 kcal mol-1), which makes collisional deactivation in high yields unlikely. Finally, a number of dissociation reactions are included for the MVK isomers. 1-HMA and 2-HBD can both eliminate water at energies of around 100 kcal mol-1 above MVK (TS6 and TS7), giving cumulene, CH2CCCH2. Dissociation of the O—H bonds in 2-HBD and 1-HCB is barrierless and requires energies of 93.9 and 98.7 kcal mol-1, respectively. Finally, CBO is known to readily dissociate via a retro-cyclo-addition mechanism to give CH2CO + C2H4, via TS10.63 Importantly, the isomerization reactions to 1-HMA, 2-HBD, and 2-MO all have lower barrier heights (67.0, 63.8 and 53.5 kcal mol-1) than any of the dissociation channels, with their thresholds sitting in the visible light range (427, 448 and 534 nm). The MVK potential energy surface translates to a master equation model with 6 wells, 6 fragmentation products, and 14 reaction channels. For MACR, eight significant dissociation pathways were identified. Three of these are bond homolysis processes, generating CH2C(CH3)C•O + H• (89.4 kcal mol-1), CH2C•CH3 + •CHO (94.6 kcal mol-1) and CH2C•CHO + •CH3 (102.9 kcal mol-1). There are also five molecular dissociation channels. One occurs through a 1,2-hydrogen shift (TS15) producing CH2CHCH3 + CO with a barrier of 86.3 kcal mol-1. Note that a similar reaction channel is not possible in MVK due to the methyl substituent. The other molecular dissociation channels produce the C3H4 isomers allene and propyne, either via triple fragment dissociation of H2 and CO (TS11 and TS12) or via HCHO elimination (TS13 and TS14). In contrast to MVK, MACR cannot undergo keto–enol 13 ACS Paragon Plus Environment


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tautomerization but can transform into dimethylketene (DMK) through a 1,3-hydrogen shift (TS17), which can subsequently dissociate to dimethylcarbene + CO, CH3C•CO + •CH3, and • CH2C(CH3)CO + H•. Again, ring closing to a methyloxetene compound (3-MO) is also available. As for MVK, MACR isomerization pathways not only have lower barriers (71.4 and 57.6 kcal mol1 ) than the dissociation channels but are also in the visible range (400 and 496 nm). The MACR master equation model comprises 3 wells, 11 fragmentation products, and 14 reaction channels.

Model Development Having identified their important ground state reaction pathways, we now turn to developing reaction models of vibrationally excited MVK and MACR on these ground state surfaces, in order to estimate photolysis rates. Gierczak and co-workers investigated the photolysis of MVK and MACR at various wavelengths and pressures15 and concluded that the MVK photolysis quantum yield was both pressure and wavelength dependent but that the MACR photolysis quantum yield was too low to be accurately measured. Equation 1 was used by Gierczak et al. to fit the pressure and wavelength dependence of the MVK photolysis quantum yield (ϕph), where λ is the wavelength in nm and N is the number density in molecule cm-3.15 This is referred to as the GBTMBR model in the following sections.15

=

exp −0.055 − 308 5.5 + 9.2 × 10

(1)

The master equation model of MVK dissociation on the ground state surface proposed here has been benchmarked to the experimental findings of Gierczak and co-workers.15 This modeling approach is based on the assumption that the fate of photo-excited MVK is dominated by transitions back to ground state MVK, carrying excess vibrational energy to undergo dissociation and isomerization. Because of the importance of carbonyl compounds for the chemical transformation processes in the atmosphere, the photochemistry of two important carbonyls, formaldehyde and acetone, has been extensively studied in the last two decades.32, 34, 64, 65 The dissociation of these small carbonyl compounds is dominated by the Norrish Type I reaction,63, 66-68 which proceeds through homolytic cleavage of α C—H and C–C bonds. The reaction mechanism involves potential energy surfaces of both ground and excited states, connected through conical intersections.32 Recent studies have shown that the dissociation of formaldehyde,32, 33 acetone34, 35 and methyl ethyl ketone36 occurs predominantly on the S0 ground state surface, after going through intersystem crossing between the S1 to T1 and then T1 to S0 states. A similar 14 ACS Paragon Plus Environment

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study involving acetaldehyde also showed that the excited state surface is unimportant for atmospheric chemistry.29 Given the very low efficiency of MVK and MACR photolysis, transitions back to the ground state surface must dominate, and they are expected to be radiationless processes (internal conversion and/or intersystem crossing). Therefore, it is reasonable to postulate that the photo-excitation of MVK and MACR by solar radiation produces a significant amount of vibrationally excited ground state molecules. Once the vibrationally excited ground state molecules are formed, they will undergo collisions with the bath gas and transfer energy. Collisional energy transfer between MVK and the bath gas in the ground state model was treated using the single exponential down model. The reactant is assumed to be deactivated during collisions with the bath gas and the energy lost in deactivating collisions is characterized as ΔEdown. The photolysis product yield is expected to increase with decreasing pressure as lower pressure leads to fewer deactivating collisions. Referring to Figure 5(a), the two data points are the experiments conducted at 308 nm by Gierczak et al. (GBTMBR) and the solid line is their proposed model fit.15 The theoretical ground state model of MVK photolysis was run at a range of ΔEdown values, and a selected average energy transferred of 40 cm-1 in deactivating collisions best matches the pressure dependence of the experimental data. Although this is a relatively small ΔEdown value, it reproduces the gradient of the GBTMBR model on the Stern-Volmer plot, as shown in Figure 5(a) with ΔEdown = 40 ± 10 cm-1, indicating that it accurately describes the competition between collisional deactivation and photo-induced dissociation/isomerization. The difference in photolysis quantum yield observed between the theoretical and experimental model at a given pressure is predominantly attributed to the error in the computationally estimated reaction barriers, and will also stem in part from uncertainties in the estimated rate coefficients for the barrierless dissociation steps. The relatively low value of ΔEdown = 40 cm-1 is possibly due to the assumption that photolysis takes place exclusively on the ground state surface. In reality, some fraction of the excited state population will react and/or undergo collisional deactivation on the triplet surface, altering the apparent ΔEdown value and introducing an additional error source. In a recent modeling study of acetaldehyde photolysis, the collisional deactivation of acetaldehyde was benchmarked with ΔEdown = 150 cm-1,29 which is considerably higher than the value determined for MVK photolysis in this study. The discrepancy may also be rationalized by considering the influence of excitation energy to collisional deactivation of the excited molecule,69 where there is typically more energy transferred per collision for more highly excited molecules. The absorption spectrum of acetaldehyde peaks at around 290 nm, compared to MVK at around 330 nm. Since MVK carries lower excitation energy than acetaldehyde, collisional energy transfer in MVK may indeed be less efficient than in acetaldehyde. Moreover, similar values of ΔEdown have been estimated for the dissociation of CH4 to •CH3 and H• at 300 K in helium,70 in which ΔEdown was 15 ACS Paragon Plus Environment


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recently corrected from 100 cm-1 to 25 cm-1. In a study of the reaction between alkylperoxyl radicals and •NO, the benchmarked value of ΔEdown (25 cm-1) was also significantly lower than the expected value to provide the best fit between theoretical model and experimental data.71 The photo-induced dissociation and isomerization of MVK on the ground state surface was also simulated at 650 Torr with ΔEdown = 40 cm-1 for various wavelengths. As depicted in Figure 5(b), the theoretical photolysis quantum yield is plotted as a function of wavelength (solid line) and is compared to the experimental data from GBTMBR (dashed line). Raber and Moortgat (RM) have also studied MVK photolysis at a similar pressure of 760 Torr, and they concluded that the photolysis quantum yield is less than 0.04 when illuminated with a lamp covering wavelengths between 275 and 380 nm; this is illustrated as the horizontal line in Figure 5(b). Overall, the theoretical model agrees with the GBTMBR experimental data (and model fit) to within the experimental uncertainty, although there is an overestimation by the theoretical model below 310 nm. At higher photolysis energies, phosphorescence of excited molecules from the T1 surface back to the S0 surface may occur, which is not considered here. However, the actinic flux falls to negligible levels at around 300 nm and the overestimation is unimportant under atmospheric conditions.


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Pressure (Torr) 0

100

200

300

400

500

600

700

800

40

30

(a)

GBTMBR GBTMBR Model ∆Edown= 30 cm-1

1/φph

∆Edown= 40 cm-1 20

∆Edown= 50 cm

-1

10

0

(b)

0.10 GBTMBR GBTMBR Model Theoretical Model RM (760 Torr)

0.08

φph

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.06 0.04 0.02 0.00 300

310

320

330

340

350

360

Wavelength (nm) Figure 5. Comparison of the MVK photolysis quantum yield (ϕph) from the proposed master equation model to the available experimental data. (a) Stern-Volmer plot showing the pressure dependence of MVK photolysis at 308 nm. (b) ϕph as a function of wavelength using ΔEdown = 40 cm-1 at 650 Torr. The experimental result from Raber and Moortgat (RM) at 760 Torr is also illustrated.

The product quantum yields for the photolysis of MACR predicted by the theoretical model are significantly lower than those for MVK. The theoretical MACR photolysis quantum yield is less than 0.002 for wavelengths between 300-360 nm. This is consistent with the GBTMBR experimental findings where the photolysis yield is very low (less than 0.01) and could not be accurately determined. As a consequence, MACR photolysis could not be benchmarked. Instead, collisional transfer in MACR is assumed to be the same as MVK, with ΔEdown = 40 cm-1.



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The MACR photolysis quantum yield as a function of wavelength at 650 Torr and ΔEdown = 40 cm-1 is provided in the Supporting Information.

Photolysis Dynamics Utilising the master equation models developed in the preceding section, we performed simulations to estimate the yields of photo-products, as well as the branching ratio between dissociation and isomerization, from MVK and MACR photolysis. To reproduce atmospheric conditions, the master equation model was run at 298 K, 760 Torr of N2, ΔEdown = 40 cm-1, and wavelengths between 280-400 nm. The results are presented in Figure 6(a). The formation of stabilized 2-HBD is the dominant process at wavelengths longer than about 310 nm, followed by 1-HMA formation. At very long wavelengths 2-MO formation becomes dominant, but it is never stabilized in significant yields. Formation of the 1-HCB isomer is always a relatively minor channel, and no CBO is predicted to form. The only significant photo-dissociation channel is for formation of CH2CHCO + CH3, which achieves quantum yields of above 0.01 from around 310 nm. The calculated branching ratio between all dissociation and isomerization products is illustrated in Figure 6(b), and it is apparent that dissociation dominates at the shorter, higherenergy, wavelengths (< 310 nm), with isomerization dominating at longer wavelengths.


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Figure 6. Quantum yields (a) and branching ratios (b) for ground state product channels in MVK photolysis, predicted by master equation simulations with excess energy equivalent to 400 to 280 nm.

Simulations using the MACR reaction model were conducted under similar conditions to MVK and the results are depicted in Figure 7(a). The process is dominated by dissociation to CH3CCH3 + CO at wavelengths shorter than ~ 310 nm, with a number of other fragmentation channels providing minor contributions. Isomerization to DMK dominates at wavelengths between about 310 and 350 nm, above which 3-MO is the major product. Significantly, though, the total MACR photolysis quantum yield is considerably lower than that of MVK at all wavelengths. Figure 7(b) shows the branching ratio between dissociation and isomerization, and again dissociation is the key reaction pathway in the UVB range but isomerization is the major contributor at visible to UVA wavelengths.



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Figure 7. Quantum yields (a) and branching ratios (b) for ground state product channels in MACR photolysis, predicted by master equation simulations with excess energy equivalent to 400 to 280 nm.

Atmospheric Implications Previously, photolysis of MVK and MACR was considered to proceed through dissociation pathways only. However, it appears that photo-induced isomerization could also occur in the actinic spectrum and that it may compete with conventional dissociation pathways. It is crucial to evaluate the significance of photo-isomerization of MVK and MACR in the atmosphere to understand the fate of these molecules. This can be investigated by determining the photolysis rate through each pathway. The rate (k, s-1) of a photochemical reaction in the troposphere is a function of the actinic flux (J(λ)), the absorption cross section (σ(λ)) and the quantum yield (ϕ(λ)). It can be determined by the product of these three parameters, integrated with respect to wavelength as shown in Equation 2.

= ! "

(2)


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Figure 8(a) shows the calculated quantum yields for MVK photolysis (ϕ(λ)) between 280-400 nm, taken from the calculations described above. Figure 8(b) shows the absorption cross section (σ(λ)) of MVK measured by Gierczak and co-workers,15 which has a maximum at around 330 nm with an onset at 400 nm. Figure 8(c) illustrates a typical actinic flux (J(λ)) at sea level with zero solar zenith angle.72 The resultant product J(λ)σ(λ)ϕ(λ) for total photolysis, and for the photo-dissociation and photo-isomerization components, is shown in Figure 8(d). The total photolysis rate is determined to be 3.3×10-5 s-1 by integrating the solid line in Figure 8(d) across the wavelength range. The contributions from photo-induced dissociation and isomerization are 22 % and 78 %, respectively. With the calculated total photolysis rate, the expected lifetime of MVK is about 8 hours under sunlit conditions. This is comparable to the lifetime predicted by Gierczak and co-workers of 6-10 hr.15 It is interesting to see that the total J(λ)σ(λ)ϕ(λ) value peaks at around 320 nm, despite quantum yields of only ~ 0.03. This can be explained by the large absorption cross section for MVK at these wavelengths, as well as the significant photon flux, both of which drop off sharply at higher wavelengths where the calculated quantum yields start to increase. Similar to MVK, Figure 9 shows the (a) calculated quantum yields (ϕ(λ)), (b) absorption cross section (σ(λ)), (c) typical actinic flux (J(λ)) and (d) product of J(λ)σ(λ)ϕ(λ) for total photolysis, dissociation, and isomerization channels for MACR between 280-400 nm. The quantum yields are predicted to be significantly lower than those determined for MVK, leading to lower overall photolysis rates. The total photolysis rate is determined to be 1.5×10-6 s-1, with contributions from photo-induced dissociation and isomerization mechanisms being 11 % and 89 % respectively. With the calculated total photolysis rate, the expected photolysis lifetime of MACR is about 192 equivalent hours of sunlight.



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Figure 8. (a) Theoretical MVK photolysis quantum yield, including dissociation, cyclization and hydrogen shift channels, as predicted by the ground state master equation model. (b) MVK absorption cross section. (c) Actinic flux at sea level with zero solar zenith angle. (d) Photolysis rates. 22 ACS Paragon Plus Environment

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Figure 9. (a) Theoretical MACR photolysis quantum yield, including dissociation, cyclization and hydrogen shift channels, as predicted by the ground state master equation model. (b) MACR absorption cross section. (c) Actinic flux at sea level with zero solar zenith angle. (d) Photolysis rates. 23 ACS Paragon Plus Environment


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The results presented here are in agreement with the consensus that MACR photolysis is not a significant tropospheric process, and is unable to compete with the •OH radical sink. For MVK, however, it appears that photo-dissociation and photo-isomerization may both play some role, given the higher photolysis rates and longer lifetime toward •OH when compared to MACR. In this case, photo-isomerization products of MVK, particularly 2-HBD, may need to be included in atmospheric chemical mechanisms. Based on previous investigations on enol oxidation pathways,31, 73 2-HBD is expected to undergo •OH-initiated oxidation producing organic acids, • OH and •HO2. Targeted experimental studies to observe 2-HBD as an MVK photo-product are suggested, focusing on the 320 – 380 nm wavelength range where photo-dissociation is expected to provide less interference.

CONCLUSION This work investigated the ground state rearrangement and decomposition of MVK and MACR. Theoretical calculations showed that a variety of isomerization channels, including tautomerizations and cyclizations, are available with barriers accessible in the visible to near-UV range, coinciding with the onset of absorption in MVK and MACR. Master equation simulations of the reactions on the ground-state surface, following photo-excitation and radiationless transition back to the ground state, predicted that the major reaction channels are isomerization to hydroxymethylallene and hydroxybutadiene for MVK, and dimethylketene for MACR. Despite being formed in low quantum yields, it is predicted that isomerization will dominate over dissociation in the troposphere due to the coincidence of high absorption cross sections and solar photon fluxes at around 320 nm. The results presented here suggest that isomerization may be an important process in the photolysis of MVK and MACR, and targeted experimental searches for their isomerization products are warranted.

SUPPORTING INFORMATION AVAILABLE Potential energy surface illustrating the Norrish Type I dissociation of MACR. Potential energy surface for the formation and dissociation of ethyl ketone. Simulated photolysis yield of MACR from master equation simulation at 298 K and 1 atm. MultiWell master equation model files for MVK and MACR.

ACKNOWLEDGMENTS 24 ACS Paragon Plus Environment

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The authors are grateful to the Australian Research Council for funding through the Discovery Project (DP130100862) and Future Fellowship (FT130101304) schemes.



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TABLE OF CONTENTS GRAPHIC

Quantum mechanical calculations and master equation simulations suggest that methyl vinyl ketone can photo-isomerize in the troposphere via ground-state rearrangements.


Photo-Isomerization of Methyl Vinyl Ketone and Methacrolein in the

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