Gas-Phase Kinetics of Hydroxyl Radical Reactions with C3H6 and

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Gas-Phase Kinetics of Hydroxyl Radical Reactions with C3H6 and C4H8: Product Branching Ratios and OH Addition Site-Specificity Jean-Christophe Loison,* Julien Daranlot, Astrid Bergeat, Franc¸oise Caralp, Raphae¨l Mereau, and Kevin M. Hickson Institut des Sciences Mole´culaires, CNRS UMR 5255, UniVersite´ Bordeaux I, 351 Cours de la Libe´ration, 33405 Talence cedex, France ReceiVed: August 1, 2010; ReVised Manuscript ReceiVed: NoVember 12, 2010

Products of the reaction of OH radicals with propene, trans-2-butene, and 1-butene have been investigated in a fast flow reactor, coupled with time-of-flight mass spectrometry, at pressures between 0.8 and 3.0 Torr. The product determination includes H atom abstraction channels as well as site-specific OH addition. The OH radicals are produced by the H + NO2 f OH + NO reaction or by the F + H2O f OH + HF reaction, the H or F atoms being produced in a microwave discharge. The gas mixture is ionized using single photon ionization (SPI at 10.54 eV), and products are detected using time-of-flight mass spectrometry (TOF-MS). The H atom abstraction channels are measured to be C4 alkenes at 300 K.3,4 H atom abstraction is a minor channel at room temperature and only becomes the dominant channel above 700 K.5,6 In the temperature range 300-800 K, where both abstraction and addition occur, experimental7,8 and recent theoretical studies9-15 seem to indicate than the adduct may decompose to bimolecular products such as vinyl alcohol or aldehydes. Some alkenes (such as terpenes) seem to react however more rapidly with OH radicals than predicted from structure-activity relationships (SARs).16-18 This extra reactivity could be attributed to the H atom abstraction channel due to the weakness of the allylic C-H bond in the β position of the double bond, a weakness due to resonance stabilization of the allyl radical product. Usual SARs for H-abstraction, such as the one established by Atkinson et al.,16,19,20 do not take into consideration the resonance stabilization effect and, consequently, strongly underestimate the H atom abstraction process.3,21,22 As the alkyl radical reactivity is different from OH-alkene adduct reactivity, it is important to know the exact amount of alkyl radicals formed by this abstraction. Moreover, because of the large variety of VOCs emitted in the atmosphere, it will be difficult to study the reactions for each compound separately in laboratory experiments or by theoretical work, and the lack of experimental data for H atom abstraction prevent us from establishing a reliable SAR of the H atom abstraction as already indicated by Peeters et al.17 Few scattered measurement of H atom abstraction for the OH + propene and OH + 1-butene reactions exist. Biermann et al.3 reported, using a photoionization mass spec* Corresponding author. E-mail: [email protected].

trometer technique, an abstraction fraction of 20 ( 6% at 2 Torr for the OH + propene reaction. For the OH + propene and OH + 1-butene reactions, Hoyermann and Sievert8 proposed an upper limit of 10%, and Atkinson et al.23 suggested a low H abstraction fraction, this channel being lower than 10%, which may in fact be totally negligible. Another critical aspect of the reaction of OH radicals with alkenes is the site-specific addition of OH radicals to the double bond. The SAR of Atkinson16-18 considers the double bond as a single entity without differentiation on which carbon OH will bond. This idea is based on the theoretical first step of the reaction leading to a T-shaped van der Waals complex.9,11,13-15 The kinetics of these reactions is well-reproduced by an outer TS for reactant complex formation and an inner TS for the addition. However, considering the double bond as a single entity does not yield information on the relative importance of the different addition sites. In many cases, the subsequent degradation mechanism for asymmetric alkenes depends on the specific site of OH addition, and accurate modeling of the chemical atmosphere requires that the quantity of each contribution is known. Recently, Peeters et al.24 have developed an interesting new site-specific SAR based on the experimental global rate constant of OH reactions with alkenes and dienes. They also compare their predicted site-specific rate constant with experimental measurements with a multistage flow reactor technique in combination with molecular beam sampling mass spectrometry. They determined the primary hydroxy adduct distributions of the reaction of OH radicals with several alkenes by using the different fragmentation patterns of the site-specific adduct ions. However, there remain real uncertainties due to ion fragmentation, and comparisons with previous measurements (associated with considerable uncertainties) for the OH + propene25,26 and OH + 1-butene8,26 reactions are not entirely concordant. Moreover, the relationship between the SAR and the theoretical calculations is unclear, as the main experimental channel is the addition channel leading to terminal OH adduct R-•CH-CH2OH formation, even if the more stable adduct is

10.1021/jp107217w  2010 American Chemical Society Published on Web 12/08/2010

Gas-Phase Kinetics of Hydroxyl Radical Reactions the R-CH(OH)-•CH2 one.9-11,14,27 These results suggest that the key mechanism for the overall rate coefficient and its T-dependence is the interplay between the inner and outer TS. The uncertainties in the calculated barrier heights lead to the difficulty of predicting theoretically the site specific branching ratio9-11,14,28,29 as well as the branching ratio of the various bimolecular products.9-11 The purpose of the present study is to measure H-atom abstraction branching ratios, site-specific OH addition, and the lower limit value of bimolecular product formation coming from adduct decomposition at 300 K using a fast flow reactor coupled to mass spectrometry with single photon ionization to detect all products simultaneously present in the chemical system. To identify the site of OH addition, we used the difference of the fragmentation pattern of each site-specific adduct ion using 10.54 eV photoionization (instead of the 30 eV electron beam ionization method used by Peeters et al.24) using deuterated compounds associated with ab initio calculations of stationary points of the ions potential energy surfaces. Reactions with deuterated compounds also allow us to quantify the formation of bimolecular products at 300 K and 0.8-2.4 Torr. II. Experimental Section The experimental setup used in this study has been detailed elsewhere,30 and only a brief description is thus given. It is comprised of a fast flow reactor coupled to a time of flight mass spectrometer (R. M. Jordan Co., D850 Reflectron). The fast flow reactor consists of a main tube and an injector. The main tube is a 24 mm internal diameter/65 cm long quartz tube, inside which is mounted a sliding movable injector which is a 6 mm internal diameter/90 cm long glass tube with a showerhead mixer. OH radicals are produced by the H + NO2 f OH + NO reaction or by the F + H2O f OH + HF reaction. H or F atoms are produced in a microwave discharge at 2450 MHz (Sairem GMP 03 KSM) in a mixture of 1% H2 in He (Linde) or 1% F2 in He (Linde) introduced into the 24 mm main reactor 50 cm before the reaction zone. Typically, the OH concentration ranges from 1 to 4 × 1012 molecules · cm-3, this concentration being determined either by NO2 titration of H atom produced in the discharge (monitoring H atom by REMPI and NO2 and NO by VUV single photoionization) or directly by electron beam ionization.31-33 Alkenes (propene, AlphaGaz 2.5; 1-butene, Linde 2.0; and 2-trans-butene, Linde 2.0) are premixed in a secondary flow of He gas in large excess with respect to the OH concentration (typically [alkenes] ) 1-10 × 1014 molecules · cm-3) and are introduced through the injector tube. Gas flow rates were measured with carefully calibrated Tylan FC-2900 mass flow controllers. The pressure in the flow reactor was measured using an Edwards capacitance manometer. Typical flow velocities ranged from 7 to 31 m s-1. The total exiting gas was pumped away through a two-stage Edwards primary pump (300 m3 · h-1). The interface between the reactor and the mass spectrometer consisted of a differentially pumped orifice-skimmer combination. The reacting gases at 0.8-2.4 Torr total pressure were expanded through a 1.0 mm diameter homemade Kel-F skimmer (a small cone with a 2 cm base and a 2 cm height with a sampling orifice of 1 mm at the end of the cone) into a region where the background pressure is maintained at 1 × 10-3 Torr using a Alcatel T550 turbo pump (450 L · s-1 for He). The centerline portion of the expanded jet passed through a 2.1 mm skimmer (Beam Dynamics) aperture into the ionization chamber, which was maintained at less than 10-5 Torr by a Turbo pump (Varian 550 L · s-1 for He). Ionized particles were extracted and

J. Phys. Chem. A, Vol. 114, No. 51, 2010 13327 sent to the detector through a 820 mm reflectron, maintained at the pressure of 1 × 10-7 Torr by a Turbo pump (Varian 150 L · s-1 for He). Ions were detected through microchannel plate detectors (MCP: C-726; 40 mm active area). A mass resolution R50% of m/z ) 1300 (measured at m/z ) 30) was achieved. Electrical signals produced by ion detection were acquired by a numerical oscilloscope (TDS 3000 from Tektronix) and were sent to a microcomputer. The ion signals were corrected for the mass discrimination effect34 that depletes the concentrations of light mass molecules relative to heavier mass molecules. The mass discrimination effects have been carefully calibrated for our molecular beam TOFMS system using atoms and molecules ionized with an electron beam at 50 eV or using molecules photoionized at 10.54 eV with known photoionization cross sections. For ion fragmentation, the detection efficiency of microchannel plate ion detectors has been taken to be proportional to the speed of the ions,35,36 i.e., proportional to 1/
M as the kinetic energy is constant (791 eV in our case). Ionization Method. The photons [117.6 nm (10.54 eV)] used in this study are generated by tripling a UV laser beam in a rare gas (35 Torr of Xe). To avoid residual photons at the 353 nm wavelength participating in the ionization process, we located the tripling cell 120 cm from the ionization region (in an evacuated side arm) and used the difference of the dispersion coefficient of the collimating MgF2 lens (f ) 75 mm at 121.6 nm) to disperse the 353 nm beam and minimize its effect. By pumping the tripling medium (leading to a complete disappearance of the ion signal) we ruled out the multiphoton ionization with 353 nm alone. By varying the position of the quartz lens (f ) 150 mm) as well as the Xe pressure in the tripling cell, i.e., through varying the focal point of the VUV and residual UV beam as well as the VUV intensity, we checked that the small amount of residual UV photons do not interfere with the ionization process as the ratios of ion signals are constant for all of the various conditions. To estimate the branching ratio, we have to evaluate the relative photoionization cross section of the various products. Several semiempirical models37,38 exist to estimate the ionization cross section of polyatomic molecules. These models are based on the additivity concept, addition of the photoionization cross section of specific fragments (CH2, CH3...) or specific bonds (C-C, CdC...). These models give satisfactory estimation of the absolute ionization cross section for various organic molecules for ionizing radiation at 11.8, 16.7, or >18 eV. However, these models have to be updated for 10.54 eV ionization. Moreover, with these models, the ionization cross section increases with the number of atoms in the molecules, which is not experimentally verified for photoionization, leading to the ground state of the ions. We prefer to use the model of Koizumi,39 neglecting autoionization and using the excitation cross section of the hydrogen atom to the continuum {6.3 × exp[-0.148(hν - 13.6 eV)] Mb},40,41 to estimate the photoionization cross section. This model works well for ionization involving bonding or nonbonding electrons for which the corresponding excited Rydberg states lead mainly to dissociation. This is the case for alkanes, alkenes, alkynes, and alcohols,39 but also for radicals for which the absolute ionization cross section is known: methyl,42,43 vinyl,44 propargyl,44 allyl,45 2-propenyl,45 and phenyl radicals.46 Photoionization cross sections show stepwise increases at each ionization energy (IE corresponding to the various electronic states of the ions), and when IEs exist in small energy intervals, or with wide Franck-Condon overlap, the steps overlap with one another, and in consequence, the photoionization cross section monotoni-

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cally increases like for alkanes39 and also vinyl44 and phenyl radicals.46 We estimate the Franck-Condon overlaps with a linear progression having a slope equal to 6.3/(2(IEvertical IEadiabatic)) in Mb. The ionization cross section is then

σionization (Mb) ) 6.3 × exp[-0.148(hν - IEadiabatic)] × hν - IEadiabatic 2(IEvertical - IEadiabatic) when IEadiabatic < hν < IEadiabatic + 2(IEvertical - IEadiabatic) and

σionization (Mb) ) 6.3 × exp[-0.148(hν - IEadiabatic)] when hν > IEadiabatic + 2(IEvertical - IEadiabatic). In this work, the adiabatic IEs are calculated at the G3B3 level and vertical IEs with the electron propagator theory (EPT) using a cc-pVTZ basis set at the geometry obtained at the B3LYP/cc-pVTZ level. All of these calculations were performed using the GAUSSIAN 03 package.47 The calculated IE and the estimation of the photoionization cross section at 10.54 eV of the various radicals studied in this work are presented in Table

TABLE 1: Experimental and Theoretical Adiabatic Ionization Energies (IEad) and Vertical Ionization Energies (IEvert) for the First Two Singlet and Triplet Statesa σionization (Mb) At 10.54 eV IE exp

IEad (G3B3)

EPT (IEvert) state 1, state 2

C 3H 6 f 2C3H6+ 2 C 3H 5 f 1C3H5+ f 3C3H5+ 2 CH3-•CH-CH2OH f 1C3H6 + 2OH• f 1CH3-+CH-CH2OH f 3CH3-+CH-CH2OH 2 CH3-CH(OH)-•CH2 f 1C3H6 + 2OH• f 1CH3-CH(OH)-CH2+ f 3CH3-CH(OH)-CH2+ f 1CH3-C(OH)-CH3+ 1 C3H5NO f 2C3H5• + NO f 1C3H5NO+ f 1C3H5+ + NO f 1C3H5 + NO+ 1 CH3-CH(NO)-CH2OH f CH3-•CH-CH2OH + NO f 1CH3-CH(NO)-CH2OH+ f 1CH3-CH-CH2OH+ + NO f 2CH3-•CH-CH2OH + NO+ 1 CH3-CH(OH)-CH2NO f 2CH3-CH(OH)-CH2• + NO f 2CH3-CH(OH)-CH2NO+ f 1CH3-CH(OH)-CH2+ + NO f 2CH3-CH(OH)-CH2 + NO+ f 1CH3-HC)OH+ + 2•CH2NO f 2CH3-H•C-OH + CH2NO+ 1 CH3- CH2-CH)CH2 f 2C4H8+ 2 CH3-CH2-•CH-CH2OH f 1CH3-CH2-CH)CH2 + 2OH• f 1CH3-CH2-CH-CH2OH+ f 3CH3-CH2-CH-CH2OH+ 2 CH3-CH2-C(OH)H-•CH2 f 1CH3-CH2-CH)CH2 + 2OH• f 1CH3-CH2-CH(OH)-CH2+ f 3CH3-CH2-CH(OH)- CH2+ f 1CH3-CH2-C(OH)-CH3+ 2 CH3-•CH-CHdCH2 f 1CH3-CH-CHdCH2+ f 3CH3-CH-CHdCH2+ 1 CH3- CHdCH-CH3 f 2C4H8+ 2 CH3- •CH-CH(OH)-CH3 f 1CH3-CH)CH-CH3 + 2OH• f 1CH3-CH-CH(OH)-CH3+ f 3CH3-CH-CH(OH)-CH3+ f 1CH3-CH2-COH-CH3+ a

exp 11 ( 1

1

9.7364

9.77

9.86, 12.43

8.16 10.53

8.17, 11.57 10.65, 12.98

48

6.0 ( 1.245

calcd 11 5.5 6.5

1.14 7.46 10.33

8.04, 11.03 10.89, 12.11 7

1.17 no minimum 10.20 5.69 1.13 8.23 (B3LYP)* 9.29 10.32 1.74 8.92 9.20 10.93 1.78 9.08 no minimum 10.97 9.31 11.22

8.59, 11.04 10.82, 12.14

9.85, 12.61

9.34, 10.99

9.25, 11.15

9.5565 9.64

9.78, 11.97

1.02 7.31 10.25

8.16, 10.94 10.79, 11.95

10 ( 139,49

11 7

8 1.08 no minimum 9.83 5.76

8.90, 10.82 10.61, 11.88

7.68 10.20

7.60, 11.10 10.18, 12.55

9.17

9.26, 12.03

9

9.1064

12 ( 139

11 7

1.19 no minimum 10.05 5.87

7.93, 10.80 10.68, 11.73

Experimental and estimated (see text) photoionization cross sections at a photon energy equal to 10.54 eV.

Gas-Phase Kinetics of Hydroxyl Radical Reactions

Figure 1. Relative traces of the ion signal as a function of the distance d in the reactor for the OH + C3H6 reaction: (b) m/z ) 15 (CH3+), (O) m/z ) 29 (HCO+ and C2H5+), (9) m/z ) 31 (H2COH+), (*) m/z ) 44 (C2H4O+), (0) m/z ) 58 (C3H5OH+), (]) m/z ) 59 (C3H6OH+). The OH concentration was equal to 1.6 × 1012 molecules · cm-3 and the C3H6 concentration was equal to 3.8 × 1013 molecules · cm-3.

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Figure 2. Typical medium sensitivity VUV (10.54 eV) single photon ionization mass spectra of propene + OH (top), propene alone (middle), and the corresponding difference spectrum (bottom). The difference spectrum has been multiplied by 2.5 and the mass peak around m/z ) 42.04 (C3H6+) was overloaded.

1 alongside the experimental photoionization cross section of propene,48 butenes,39,49 and C3H5.45 It is worth noting that the calculated photoionization cross section of propene, butene, but also C3H5 at 10.54 eV are very close to the experimental values. Uncertainties for the photoionization cross section are estimated to be 30%, but uncertainties for the ratio of photoionization cross section between the two OH adducts of the same alkenes are estimated to be only 20% as the various ionization energy vslues are very similar. III. Result and Discussion The product distributions from the reaction of OH radicals with propene or butenes were studied by adding the individual alkenes together with the main He flow containing OH radicals. The alkene concentrations as well as the product concentrations were monitored by their mass spectra to follow the progress of the reactions. The alkenes were introduced in large excess with respect to the OH concentration to avoid secondary reactions with OH, and mass peaks representing reactants and products were monitored in experiments conducted for various initial conditions to evaluate secondary reactions. For each reaction studied in this work (including reactions with NO) we performed kinetic studies of product appearance to measure the effect of secondary reactions. Typical traces of the ion signals as a function of distance d in the reactor for the OH + C3H6 reaction are presented in Figure 1. The solid and dashed line are simulations with a very simple mechanism: OH + C3H6 f C3H6OH (k ) 2.63 × 10-11 cm3 · molecules-1 · s-1) and C3H6OH + C3H6OH f (C3H6OH)2 (k ) 1.2 × 10-11 cm3 · molecule-1 · s-1). The solid lines correspond to ions originating from the OH + C3H6 f C3H6OH reaction only and the dashed lines correspond to the total signal. Secondary reactions play a minor role except for M ) 44 m/z (C2H4O+ ) C3H6OH+ CH3) and M ) 59 m/z (C2H5OH+ ) C3H6OH+ - H), for which a fraction of the signal results from (C3H6OH)2+ fragmentation. OH + Propene Reaction. Typical medium sensitivity VUV (10.54 eV) single photon ionization (SPI) mass spectra of

Figure 3. Typical high sensitivity VUV (10.54 eV) single photon ionization difference mass spectra of OH + C3H6 (top), OD + CH3-CHdCH2 (second from the top), OH + CD3-CHdCH2 (third from the top), OH + CH3-CDdCH2 (forth from the top), and OH + CH3-CHdCD2 (bottom). Some mass peaks (mainly C3H6+ and corresponding deuterated compounds) were overloaded.

propene + OH and propene alone are shown in Figure 2, alongside the corresponding difference spectrum. The C3H6+ molecular ion at m/z ) 42.05 is overloaded. We can notice that, except for the C3H5+ ion, there is no fragment coming from C3H6+ fragmentation which superimposes on the ionized products of the OH + C3H6 reaction. To identify the origin of the ions, we performed a kinetic study of the reaction OH + CH3-CHdCH2 using various deuterated isomers. Typical high sensitivity VUV (10.54 eV) SPI mass difference spectra of the reactions are shown in Figure 3 for conditions where secondary reactions are low (OH coming from the F + H2O reaction with the OH concentration equal to 2 × 1012 molecules · cm-3). The spectra shown in the Figure 3 are obtained from the following reactions going from top to bottom: OH + CH3-CHdCH2, OD + CH3-CHdCH2, OH + CD3-CHdCH2, OH + CH3-CDdCH2, and OH + CH3-CHdCD2, respectively (the OD is coming from the F + D2O reaction). A close-up of the m/z ) 29-33 region is presented in Figure 4. Our mass spectral

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Figure 4. Close-up of the m/z ) 28-34 region of typical high sensitivity VUV (10.54 eV) single photon ionization mass spectra of OH + C3H6+ (top), OD + CH3-CHdCH2 (second from the top), OH + CD3-CHdCH2 (third from the top), OH + CH3-CDdCH2 (forth from the top), and OH + CH3-CHdCD2 (bottom).

Figure 5. Single photon ionization mass spectrum of OH + C3H6 around m/z ) 29.

resolution of 1300 at m/z ) 29 (see Figure 5) allows us to assign the various peaks. First we have to identify the origins of the various ions, whether they come from fragmentation or from the ionization of products, the most critical case being the CH3+ ion. Theoretical ab initio calculations9-11 show that the H3C-CH(OH)-•CH2 adduct can evolve through a late exit barrier leading to H or CH3 elimination, the most favorable exit channel being CH3 formation. A simplified schematic representation of the potential energy surface (PES) of the OH + propene reaction is shown in Figure 6. In a first possible channel the H3C-CH(OH)-•CH2 adduct can directly lose a CH3 fragment through a tight transition state calculated, with three different methods, at -1.7 kcal/mol at the PMP2/augcc-PVQZ//MP2/cc-PVTZ level,9 +2.6 kcal/mol at the RQCISD(T)/cc-pV∞Z//B3LYP/6-311++G(d,p) level,11 and +6.5 kcal/mol at the CCSD(T)/cc-pVDZ//B3LYP/cc-pVTZ level10 with respect to the OH + propene energy. The second CH3 production channel comes from H3C-CH(OH)-• CH2 f H3C-CH(O•)-CH3 f CH3 + CH3CHO. The H3C-CH(OH)-•CH2 f H3C-CH(O•)-CH3 isomerization goes through a transition state calculated, with the same three

Loison et al. different methods, at +0.8,9 +3.0,11 and +5.0 kcal/mol10 with respect to the OH + propene entrance level (the H3C-CH(O•)-CH3 f CH3 + CH3CHO dissociation goes through a loose transition state located well below the OH + propene entrance level). Theoretical (ab initio/RRKM)9-11 product branching ratio calculations on the OH + C3H6 system give no CH3 production at 300 K but only around 500 K. However, as the theoretical energy of the transition states are scattered, we cannot completely rule out the possibility of CH3 formation from theoretical work. We then used our experimental study of the reactions of OH with deuterated propene and the reaction of propene with OD and to elucidate the origin of the ions. The mass spectrum from the OH + CD3-CHdCH2 reaction (the third spectrum from the top of Figure 3) is incompatible with direct CH3 production, as in this case it should produce only CD3 and not CH3. Direct CH3 production is also incompatible with the mass spectra obtained from the OH + CH3-CDdCH2 and OH + CH3-CHdCD2 reactions (the two bottom spectra of Figure 3), as in both cases they should have only CH3 production and neither CH2D nor CHD2. To assess the importance of indirect CH3 production after H migration from the OH group, we studied the OD + C3H6 reaction. The mass spectrum (second from the top in Figure 3) shows no CH2D production through the H3C-CH(OD)-•CH2 f H3C-CH(O•)-CH2D f CH3 + CH2D-CHO and H3C-CHO + CH2D mechanism. However, the TS for D-migration will be higher in energy than the corresponding TS for Hmigration due to a smaller ZPE correction between the reactants and the TS, and D will also tunnel through the barrier less than H. So we cannot completely rule out a small yield of CH3 production for the nondeuterated case. If it exists, the experimental CH3 production is low (only by tunneling) and our experimental data are in good agreement with the calculated branching ratios at 300 K.9,11 As the transition states leading to H atom production from the two adducts are found to be higher in energy than those leading to CH3 production and also entropically less favorable, we rule out H atom production: the calculated rate constants for H atom production9,11 are lower than the rate constants for CH3 production. So there are likely no possible dissociation channels at 300 K and 1-3 Torr for the two adducts H3C-CH(OH)-•CH2 and H3C-•CH-CH2OH. To determine the H atom abstraction and the OH addition site branching ratios we need to elucidate the different fragmentation patterns of each of the adduct ions. At 10.54 eV energy, photoionization occurs not only by ejection of the lone electron leading to the allyl cation (H2C+-CHdCH2) or β-hydroxy cations in a singlet state (corresponding to the first IE) but also by ejection of one electron of the double bond or one electron of the lowest lone pair of the oxygen of the H3C-CH(OH)-•CH2 and H3C-•CH-CH2OH adducts leading to the first triplet state and to the second singlet state of the allyl ion and to the C3H6OH+ ions (corresponding to the second and third IEs with close values). The second singlet states can also be produced by intersystem crossing (ISC) from the first triplet state, as the second singlet states and the first triplet states are very close in energy. The calculated adiabatic and vertical IEs are presented in Table 1. The adiabatic transition toward the second singlet state of the ions is estimated to be close to 10.2 and 10.5 eV by using the difference between the vertical and adiabatic transitions toward the first singlet state of the ions. Abstraction Channel. The H atom abstraction channel leads to C3H5 + H2O. As the allyl ion (H2C+-CHdCH2) can only

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Figure 6. Schematic representation of the potential energy surface (PES) of the OH + propene reaction.

lose an hydrogen, the branching ratio for the abstraction channel is measured by the ratio of the amount of C3H4+ and C3H5+ to the sum of all the ions. The propene ion dissociates losing H atoms, leading also to the C3H4+ and C3H5+ ions. As propene is in large excess with regard to OH, the C3H4+ and C3H5+ ions coming from propene ion dissociation are two high intensity mass peaks, which inevitably lead to large uncertainties in the amount of C3H4+ and C3H5+ ions coming from H abstraction product channels. Nevertheless, the H atom abstraction branching ratio is always found to be inferior to 2% and is likely to be less than 1%. Addition Channels. To elucidate the various fragmentation routes we need precise theoretical studies of the different stationary points relevant not only to the first singlet and triplet states but also to the second singlet state, which requires a considerable amount of work beyond the scope of this paper. Instead, we use the various ion fragmentation patterns produced from OH radical reactions with deuterated propene (and the OD + propene reaction) to attribute the origin of the ions. Nevertheless, we also perform theoretical studies of the different stationary points relevant to the first singlet state and to the first triplet state of the ions fragmentation (calculated at the B3LYP/aug-cc-pVTZ level and at the G3B3 level) to get a global picture of this complex system. Detailed discussion of fragments’ identification is provided in Supporting Information and only a summary is given here: The C3H6OH+ parent ion is coming from the 1-1H3CCH-CH2OH+ ion and from the 1-1H3C-CH(OH)-CH2+ ion. The C3H5OH+ ion is coming from slow dissociation of the 3 H3C-CH-CH2OH+ ion. The C3H6O+, H2COH+, C2H5+, and HCO+ fragments are coming from the 2-1H3C-CH-CH2OH+ ion, this state being partly produced through ISC from 3H3C-CH-CH2OH+. The CH2CHOH+ fragment is coming from quick dissociation of the 3H3C-CH(OH)-CH2+ ion. The CH3+ fragment is coming from the 2-1H3C-CHOH-CH2+ ion. With the estimated ionization cross section of the H3C-•CH-CH2OH and H3C-CH(OH)-CH2• adducts given in Table 1, the ratio of fragments abundance leads to 30 ( 19% of H3C-CH(OH)-•CH2 adduct formation and 70 ( 19% of H3C-•CH-CH2OH adduct formation for the OH + propene reaction. The errors bars (95% confidence interval) reflect the

statistical uncertainties of the measurement, the uncertainties in ion fragmentation, and the estimated uncertainties in the ratio of photoionization cross sections. Another way to measure abstraction branching ratios as well as site specific studies of OH addition is to react the products of the reaction with NO in great excess. Hydroxyl50 and alkyl51,52 radicals react with NO to form adducts, the H abstraction from alkyl to form HNO playing a role only at high temperature.52 A typical VUV (10.54 eV) SPI mass difference spectrum of propene + OH in the presence of NO molecules is shown in Figure 7 as well as a SPI mass difference spectrum of propene + OH for comparison. We can see a great simplification of the fragmentation with two main mass peaks: the mass peak of the adduct C3H6OH+ and the mass peak of C2H5O+. We should notice that the parent ions, C3H6OHNO+ and C3H5NO+, are never observed. Thermodynamic data for C3H6OHNO and C3H5NO are summarized in Table 1. The bond energy of the H3C-CH(NO)-CH2OH+ ion is very low, calculated to be equal to 0.42 eV at the G3B3 level. Moreover, the vertical ionization is above the dissociation limit, leading to a complete fragmentation of H3C-CH(NO)-CH2OH+ to H3C-CH-CH2OH+ + NO, in a similar manner as for C2H5O2+.53 The case of H3C-CH(OH)-CH2• + NO is quite different because the H3C-CH(OH)-CH2+ ion does not present a minimum in its first singlet state but isomerizes without a barrier toward the H3C-C(OH)-CH3+ ion. When the radical is bound with NO, H3C-CH(OH)-CH2NO+ cannot isomerize anymore, and the H3C-CH(OH)-CH2+ + NO fragmentation is not the most exothermic route. Moreover, the direct formation of H3C-C(OH)-CH3+ + NO requires a tight concerted transition state likely to be very high in energy. Instead, the H3C-CH(OH)-CH2NO+ ion dissociates without a barrier, leading to the very stable H3C-HCdOH+ ion associated with the •CH2NO radical. As we observe very few further fragmentations, the energy released during the C3H6OHNO+ fragmentation leads to ions below their dissociation limit. We obtain then very simple mass spectra, the C3H6OH+ mass peak coming from the OH addition on the final carbon atom and the C2H5O+ mass peak coming from OH addition on the central carbon atom. The ratio of the mass peaks gives directly the site specific branching ratio of 27 ( 16% of the H3C-CH(OH)-•CH2 adduct and 73 ( 16% of the H3C-•CH-CH2OH adduct for the OH + propene reaction. The errors bars (95% confidence interval) are mainly

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Loison et al.

Figure 7. Typical high sensitivity VUV (10.54 eV) single photon ionization difference mass spectra of OH + C3H6 (top) and OH + C3H6 + NO (bottom).

due to the uncertainty in ratio of the ionization cross sections. The very good agreement between our two determinations gives some confidence in our results, leading to a site branching ratio of 28 ( 16% for the H3C-CH(OH)-•CH2 adduct and 72 ( 16% for the H3C-•CH-CH2OH adduct for the OH + propene reaction. Our results, as well as previous measurement, are summarized in Table 2. Our result is in good agreement with the Cvetanovich estimation of 35% for H3C-CH(OH)-•CH2 formation25 and is in relatively good agreement with the approximate branching ratio obtained by Feltham et al.26 in lowtemperature Ar matrices and estimated to be 50% for H3C-CH(OH)-•CH2 formation. We can also evaluate the H atom abstraction branching ratio using the NO reaction. Indeed, as seen for C3H6OHNO, the first ionic state of C3H5NO is unstable and will lead to C3H5+ + NO. In the case of the OH + propene reaction, the C3H5+ signal is hardly above the noise and we get the same evaluation as previously: a H atom abstraction branching ratio of less than 2%. OH +1-Butene Reaction. Typical difference mass spectra from the OH + 1-butene reaction and OH + 1-butene reaction in the presence of NO molecules are presented Figure 8. We consider that the ions come from C4H8OH+ fragmentation and are not ionized products of the reaction, as the OH + 1-butene reaction produces only C4H8OH• for temperatures below 800 K,8,24,54 with the only exception being the abstraction product. Due to the various possible isomerizations in the ion fragmentation of the products coming from the OH + 1-butene reaction, we did not try to make as extensive a study as we did for the OH + propene reaction. However, we determined the origin of the various ions considering that only H atoms can migrate before fragmentation. The mass spectra from the OH + 1-butene reaction shows quite different fragmentation channels than proposed by Peeters

et al.,24 suggesting that the proposed dominant fragmentation routes are ionization energy dependent. The main fragment is the 2C3H6+ ion associated with 2H2•COH production, accessible from the triplet surface. This kind of fragmentation does not appear in the OH + propene and OH + trans-2-butene reactions because it will produce 2C2H4+, which has a higher ionization energy. Considering that only H can migrate before fragmentation, the C3H6+, C3H7+, and H2COH+ fragments come without ambiguity from the CH3-CH2-CH-CH2OH+ ion, and the C2H5+ fragment is similar to CH3+ production in the case of the OH + propene reaction and is likely coming from the CH3-CH2-CH(OH)-CH2+ ion. The other fragments, including CH3+ and C2H6+, are more ambiguous. With the estimated ionization cross section given in Table 1, the ratio of fragments abundance leads to 28 ( 18% of H3C-H2C-CHOH-•CH2 adduct and 72 ( 18% of H3C-H2C-•CH-CH2OH adduct for the OH + 1-butene reaction considering that the CH3+ fragment is coming from CH3-CH2-CH(OH)-CH2+ and from CH3-CH2-CH-CH2OH+ fragmentation in the same proportion. The errors bars (95% confidence interval) reflect the statistical uncertainties of the measurement, the uncertainties in the fragments attribution and the estimated uncertainties in the ionization cross sections. As for the OH + propene reaction, another way to measure the site specificity of OH addition is to add NO to the system. A typical VUV (10.54 eV) SPI mass difference spectrum of 1-butene + OH + NO is shown in Figure 8. As for the OH + propene + NO reaction, we can see a simplification of the fragmentation with two main mass peaks: the mass peak of the adduct C3H6OH+ and the mass peak of C2H5O+. We should notice that the parent ions, C4H8OHNO+ and C4H7NO+, are never observed and also that secondary fragmentation occurs, as we have a substantial amount of smaller ions. By comparison with H3C-CH(NO)-CH2OH+

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TABLE 2: Site-Specific Addition and Abstraction Branching Ratio at Room Temperature reaction

addition site specificity (%)

technique

ref

OH + C3H6 2

2

CH3-•CH-CH2OH

CH3-CH(OH)-•CH2

OH + 1-C4H8 2

2

CH3-CH2-•CH-CH2OH

CH3-CH2-CH(OH)-•CH2

OH + C3H6 OH + 1-C4H8

OH + trans-2-C4H8

72 ( 15

experimental

this work

65 50 87 28 ( 15

experimental experimental SAR experimental

Cvetanovic25 Feltham et al.26 Peeters et al.24 this work

35 50 13

experimental experimental SAR

Cvetanovic25 Feltham et al.26 Peeters et al.24

71 ( 16

experimental

this work

85 ( 10 70 87 29 ( 16

experimental experimental SAR experimental

Peeters et al.24 Feltham et al.26 Peeters et al.24 this work

15 ( 10 30 13 abstraction