Experimental and theoretical investigation on the OH + CH3C(O)CH3

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Experimental and theoretical investigation on the OH + CHC(O)CH reaction at interstellar temperatures (T=11.7-64.4 K) 3

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Sergio Blazquez, Daniel González, Alberto García-Sáez, Maria Antiñolo, Astrid Bergeat, Francoise Caralp, Raphael Mereau, Andre Canosa, Bernabe Ballesteros, Jose Albaladejo, and Elena Jimenez ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.9b00144 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 12, 2019

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ACS Earth and Space Chemistry

Experimental and theoretical investigation on the OH + CH3C(O)CH3 reaction at interstellar temperatures (T=11.7-64.4 K) Sergio Blázquez1, Daniel González1, Alberto García-Sáez1, María Antiñolo2, Astrid Bergeat3,*, Françoise Caralp3, Raphaël Mereau3, André Canosa4, Bernabé Ballesteros,1,2 José Albaladejo,1,2 Elena Jiménez1,2,* 1. Departamento de Química Física. Facultad de Ciencias y Tecnologías Químicas. Universidad de Castilla-La Mancha, Avda. Camilo José Cela, 1B. 13071 Ciudad Real, Spain. 2. Instituto de Investigación en Combustión y Contaminación Atmosférica (ICCA). Universidad de Castilla-La Mancha, Camino de Moledores s/n. 13071 Ciudad Real, Spain. 3. Univ. Bordeaux, CNRS, Bordeaux INP, ISM, UMR 5255, F-33405 Talence, France. 4. Univ Rennes, CNRS, IPR (Institut de Physique de Rennes) - UMR 6251, F-35000 Rennes, France

ACS Earth and Space Chemistry Special issue on Complex Organic Molecules (COMs) in StarForming Regions Edited by Eric Herbst

Corresponding Authors * Phone: +34 926 29 53 00, e-mail: [email protected]; Phone:+33 5 4000 6341, e-mail: [email protected].

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Abstract. The rate coefficient, k(T), for the gas-phase reaction between OH radicals and acetone CH3C(O)CH3, has been measured using the pulsed CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) technique (T = 11.7-64.4 K). The temperature dependence of k(T = 10-300 K) has also been computed using a RRKM-Master equation analysis after partial revision of the potential energy surface. In agreement with previous studies we found that the reaction proceeds via initial formation of two pre-reactive complexes both leading to H2O + CH3C(O)CH2 by H-abstraction tunneling. The experimental k(T) was found to increase as temperature was lowered. The measured values have been found to be several orders of magnitude higher than k(300 K). This trend is reproduced by calculations, with a special good agreement with experiments below 25 K. The effect of total gas density on k(T) has been explored. Experimentally, no pressure dependence of k(20 K) and k(64 K) was observed, while k(50 K) at the largest gas density 4.47×1017 cm-3 is twice higher than the average values found at lower densities. The computed k(T) is also reported for 103 cm-3 of He (representative of the interstellar medium). The predicted rate coefficients at 10 K surround the experimental value which appears to be very close to the low pressure regime prevailing in the interstellar medium. For gas-phase model chemistry of interstellar molecular clouds, we suggest using the calculated value of 1.8×10-10 cm3 molecule-1 s-1 at 10 K and the reaction products are water and CH3C(O)CH2 radicals. Keywords: Gas-phase kinetics, interstellar molecules, CRESU technique, ultra-low temperatures


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1. Introduction The mechanism of formation of many complex organic molecules (COMs) detected in starforming regions of the interstellar medium (ISM), such as cold dense molecular clouds (T10100 K), is not fully known. In particular, acetone (CH3C(O)CH3) has been detected in 1987 in Sagittarius B2 molecular cloud and Combes et al.1 proposed that its formation in this cold environment starts with the radiative association reaction of CH3+ and acetaldehyde (CH3CHO) forming CH3C(O)CH4+ followed by the dissociative recombination of CH3C(O)CH4+. However, Herbst et al.2 suggested that this large saturated COM is not easily formed in the gas phase at 10 K and the current mechanisms proposed to explain the gas phase abundance of interstellar acetone are based on grain mantle chemistry3. In much hotter environments, such as the Earth’s atmosphere and internal combustion and injection diesel engines, CH3C(O)CH3 is also of great importance. In the terrestrial atmosphere, CH3C(O)CH3 is one of the most important trace organic molecules which are emitted mainly from anthropogenic sources. In diesel-biodiesel blends, acetone can be used as oxygenator4 and it can also be an additive in gasoline fuel blends.5 The depletion of CH3C(O)CH3 in the gas-phase involves reactions initiated by the hydroxyl (OH) radicals in the three environments (ISM, Earth’s atmosphere and in high-temperature combustion processes). The kinetics of the gas-phase reaction of CH3C(O)CH3 with OH radicals has been extensively investigated as a function of temperature both experimentally 6– 15

and theoretically,10,12,15–20 but mainly at T>200 K. Only the work of Shannon et al.15 reported

experimental and theoretical rate coefficients down to 79 K and 10 K, respectively. Regarding the products of reaction (1), there are two possible exothermic channels: H-atom abstraction leading to water and acetonyl (CH3C(O)CH2) radical (Reaction 1) and OH-addition to the carbonyl C-atom followed by methyl elimination leading to acetic acid, CH3C(O)OH (Reaction 2). OH + CH3C(O)CH3  H2O + CH3C(O)CH2

(1)

OH + CH3C(O)CH3  CH3 + CH3C(O)OH

(2)

The reaction products (CH3C(O)CH2 and CH3 radicals) have been detected by several groups17,18,21–25 with contradictory conclusions, and also studied by theoretical methods.17–19 A branching ratio for CH3C(O)OH formation was found to be around 50%17,21 at room temperature, while upper limits of less than 10% were found by other authors at room temperature and below.18,22–25 However, the large deuterium isotope effect,9,10,13 the high yields 3 ACS Paragon Plus Environment


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of CH3C(O)CH2 (>90% independent of temperature over the range 242-350 K),23,25 and the lack of a pressure dependence of k(T) indicate that the reaction proceeds by H-atom abstraction.9 At temperatures lower than 242 K, the branching ratio is not experimentally known. The kinetic behavior of the OH + CH3C(O)CH3 reaction is different according to the investigated temperature and pressure ranges. At combustion temperatures (T = 982-1300 K), Vasudevan et al.11 showed that the overall rate coefficient, k(T), exhibits an Arrhenius behavior, i.e. k(T) increases as the temperature increases, while at temperatures of the Earth´s troposphere (down to 199 K), the temperature dependence of k(T) deviates from the Arrhenius behavior.6– 10,12,13,19,21,25

Wollenhaupt et al.8 and Gierczak et al.9 clearly observed a curvature in the

Arrhenius plot between 199 and 395 K. It was an indication that k(T) could go on increasing at T 400 K) to H-abstraction through the formation of a OH-acetone pre-reactive complex and substantial quantum mechanical tunneling at lower temperatures.10,12,18,22,25 This observation was confirmed by Caralp et al.19 who computed the kinetics of the overall rate coefficient for the OH + acetone reaction between 1 mbar and 1 bar in the 200-710 K temperature range. An RRKM treatment that included tunneling, explained and reproduced very well the unusual temperature dependence of the overall rate coefficient observed experimentally. This treatment was applied at the microcanonical level, with chemically activated distribution of entrance species incorporating collisional energy transfer and competition between the redissociation and exit channel via tunneling. Moreover, the analysis confirmed that the principal channel was the formation of acetonyl radical + H2O (reaction 1) via hydrogen-atom abstraction by OH, while the channel leading to acetic acid (reaction 2) was of negligible importance. Reaction 1 takes place via a complex mechanism involving two hydrogen-bonded pre-reactive complexes (henceforth, molecular complex MC1a or MC1b) prior to formation of transition states, TS1a or b. The transition states then evolve via other complexes into acetonyl + H2O. Two channels are characterized: 4 ACS Paragon Plus Environment

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OH + CH3C(O)CH3 → MC1a → TS1a → H2O + CH3C(O)CH2

(1a)

OH + CH3C(O)CH3 → MC1b → TS1b → H2O + CH3C(O)CH2

(1b)

More recently, Shannon et al.15 reported a noticeable pressure dependence of k(T) at 80 K, while the p-dependence observed at 140 K was less pronounced (and more scattered). Using MESMER (Master Equation Solver for Multi Energy well Reactions), Shannon et al. were able to reproduce all the rate coefficients, including the pressure effect at ~80 K, taking into account the stabilisation of the complex by the carrier gas. Further these authors computed k(T) between 10 K and 80 K at a typical gas density of a dark molecular cloud (106 cm-3). For example, the calculated k(10 K) using transmission coefficients from the Eckart or WKB (Wentzel, Kramers, Brillouin) treatment of tunneling was around 6 or 3.5 ×10-11 cm3 molecule-1 s-1, respectively. This reveals that even at the extremely low gas densities of the ISM, the OH+CH3C(O)CH3 reaction would be a potential loss process for these species. No pressure dependence study of k(T) below 80 K has been performed experimentally, yet. The aim of this work is, then, to experimentally investigate the temperature dependence of k(T) between 11.7 K and 64.4 K and the pressure dependence of k(T) around 22 K, 50 K and 64 K. For that purpose, the pulsed CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) technique was employed. These kinetic results are the first determination of k(T) in that temperature range and the first investigation of the pressure dependence of k(T) at temperatures below 80 K. Additionally, another purpose of this work is to theoretically verify the unusual temperature dependence of the overall rate coefficient for reaction (1), as observed experimentally, and to quantitatively validate at very low temperatures the tunneling hypothesis proposed by Caralp et al.19 in the temperature range 200-700 K, taking also into consideration that stabilization into the pre-reactive complexes could occur at the experimental pressures and temperatures. A discussion on the temperature and pressure dependence of k(T) and on the reaction mechanism is presented, as well as calculated results in the ISM conditions.



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2. Experimental part 2.1. Kinetic experiments The CRESU technique and the experimental system available at the University of Castilla-La Mancha in Ciudad Real (Spain) were widely described in previous works.26–32 Here, only a brief description is given below. In the CRESU technique, an isentropic gas expansion through a Laval nozzle from a reservoir to a reaction chamber provokes the cooling of the gas and forms a jet uniform in temperature and total gas density over several tens of centimeters. In the present work, the gas mixture is pulsed by an aerodynamic chopper (rotary disk), which is placed in the divergent part of the Laval nozzle. The rotation frequency of the rotary disk was always 5 Hz, which generates a pulsed flow at 10 Hz when using a rotary disk with two symmetrical apertures or a pulsed flow at 5 Hz when using a rotary disk with one aperture (T = 11.7 K and 13.0 K). Table 1 summarizes the temperatures achieved in the experiments, the total gas density of the jet, and the buffer gas used. The jet temperatures provided by the Laval nozzles used have been characterized in previous works26–32 except for (54.2 ± 0.5) K, which is described in the supporting information. The temperature and gas density characterization of this helium expansion condition was performed by a Pitot tube, consisting in a fast pressure transducer (Kulite model XCQ-062) which measures the impact pressure (Pi) as a function of the distance from the exit of the nozzle. From Pi and the adiabatic expansion coefficient of the bath gas, the spatial evolution of the Mach number (M), the temperature (T) and the total gas density of the jet (n) and the hydrodynamic time (thydro) were calculated. Figure S1 shows the temperature and the total gas density profile at several distances from the exit of the Laval nozzle. The OH radicals were generated in the jet by pulsed laser photolysis (PLP) of gaseous H2O2 at 248 nm using a KrF excimer laser. Gaseous H2O2 was introduced in the reservoir by flowing a portion of the buffer gas through a glass bubbler containing a commercial aqueous solution previously concentrated. The initial concentration of H2O2 in the jet was not determined here, but in recent unpublished experiments on the OH+acetaldehyde reaction at 99.3 K we determined by FTIR spectroscopy that the initial concentration of H2O2 in the jet (P = 1.064 mbar) was around 1012 cm-3.


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The time evolution of OH(X2 radicals in the presence and absence of acetone was monitored by laser induced fluorescence (LIF) ca. 310 nm, which was collected by a filtered photomultiplier tube, PMT (Electron tube, model 9813B). The excitation wavelength (ca. 282 nm) was produced by a KDP crystal in a doubling unit of a dye (Rhodamine 6G) laser (Lambda Physik, model Scanmate) pumped by the second harmonic of a Nd-YAG laser (Continuum, model Surelite). The signal from the PMT was fed into a gated boxcar integration unit (Standford Research System, model SRS250) and the integrated signal was recorded and processed in a computer by a homemade LabView program. Under pseudo-first order conditions ([CH3C(O)CH3]0, [H2O2]0 >> [OH]0), the analysis of the exponential OH decays, after rotational relaxation, yields the pseudo-first order rate coefficient, k’, which is linearly related to acetone concentration. Varying the mass flow rate of a diluted mixture containing a well-known quantity of acetone and maintaining the OH-precursor concentration constant, k(T) was obtained from the slope of the plot of k (or k-k0, where k0 is the rate coefficient in absence of acetone) versus acetone concentration, [CH3C(O)CH3]. An example of the temporal profiles of the LIF signal from OH radicals recorded at 64.1 K in the absence and in the presence of acetone is depicted in Figure 1. Several plots of k-k0 versus [CH3C(O)CH3] in the jet are shown in Figure 2 for various temperatures. As it can be seen, the slope of such plots, i.e. the rate coefficient k(T) increases as the jet temperature decreases. Table 1 summarizes the flow characteristics as well as the main experimental conditions for each individual expansion. The range of [CH3C(O)CH3] (shown in 8th column of Table 1) corresponds to the linear portion of the second-order plots, k' – k'0 versus [CH3C(O)CH3], as exemplified in Figure 2. Beyond these ranges, a downward curvature was observed (see Figure S3 in the SI material), indicating the onset of acetone dimerization which reduces the parent concentration in the flow. This is a usual observation in CRESU experiments and special care is always taken in order to limit the reactant concentration low enough to keep second order plots in the linear regime.

2.2. UV absorption measurement of acetone concentrations In the jet, [CH3C(O)CH3] is calculated from the total gas density, the mixing ratio of acetone in a 20-L storage bulb (f) and the flow rates Fi, as n×f×Facetone/FTotal. The dilution factor f in the storage bulb ranged from 9.9×10-4 to 3.2×10-2 at room temperature. f was regularly 7 ACS Paragon Plus Environment


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checked by UV absorption spectroscopy at 185 nm (radiation from a Hg-Ar lamp) using the Beer-Lambert’s law, as described elsewhere.26,32 In the present study, the total pressure in the 107-cm absorption cell (PT,UV) ranged from 10 to 100 Torr of diluted acetone taken from the storage bulb. The absorption cross section employed was σ185nm = 2.96×10-18 cm2 molecule-1 (Khamaganov et al.33). In Figure S2 plots of the absorbance of acetone at 185 nm versus PT,UV are shown. The difference between [CH3C(O)CH3] in the jet measured from flow rates (see Table 1) and from UV measurements was typically smaller than 5%. This difference is considered in the evaluation of the systematic uncertainties in k(T). Conservatively, we assumed a 10% uncertainty in the acetone concentration.

2.3 Chemicals Buffer gases: He (99.999%, Praxair), N2 (99.999%, Praxair) and Ar (99.999%, Praxair) were used as supplied. Liquid samples of acetone (99.9 %, Sigma Aldrich) were degassed by repeated freeze-pump-thaw cycles prior to its use. Aqueous solution of H2O2 (Sharlab, initially at 50% w/w) was pre-concentrated as described earlier.34

3. Statistical kinetics calculations All details of the procedure were described in Caralp et al.19 and only a brief summary is given here. A schematic energy diagram of the most important part of the PES is shown in Figure 3 and all the parameters used in this RRKM calculations were listed in Caralp et al.19 For the present calculations at low temperature, the microcanonical rate coefficients were evaluated at the E- and J-resolved levels. 3.1. Microcanonical rate coefficients, k(E, J) The microcanonical rate coefficient at a given energy E relative to the ground state of the complex and at a specified total angular momentum J, k(E,J), are calculated for unimolecular redissociation of the MC1a or MC1b complexes to reactants or their isomerization to the adduct, which is the limiting step leading to the H2O + CH3C(O)CH2 products. k(E,J) can be written in a standard RRKM form by:

k(E,J) =

α G * (E,J) h N(E,J)

(E1)


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where G(E,J) is the transition state (TS1a or TS1b) sum of states at given (E, J), N(E, J) is the corresponding complex density of states (MC1a or MC1b),  is the reaction path degeneracy and h is Planck’s constant. For the isomerization reactions that proceed through saddle point, G(E,J) and N(E, J) are calculated from molecular parameters obtained by quantum chemical calculations.19 Species are treated as symmetric tops and the external K-rotor associated with the largest rotational constant, A (all the molecules are prolate tops) is treated as an active degree of freedom, but not an independent or separable rotor. It means that the exact sum or density of states is calculated considering that the rotational energy is Erot(J, K) = Erot(J) + Erot(K) = B J(J+1) + (A-B) K2, where K is restricted to integer values between –J and +J and B is an average rotational constant defined as the square root of the product of the two lowest nearly equal rotational constants. This procedure was adopted to avoid any overcount for the low J values used in our calculations at low temperatures. As these isomerization steps involve H-atom transfer through energy barrier, the semiclassical WKB approximation was used for calculating the tunneling probability. The only modification compared to Caralp et al.’s methodology19 was the complex MC1a well depth: new quantum calculations were performed at the CCSD(T)/6-311++G(2d,2p)//MP2/631G++(d,p). Moreover, as the J-rotational effect modifies the position and the height of the calculated barrier in a very minor way, a rotational energy Er- or J-dependence is introduced into the microcanonical rate coefficient by simply shifting the energy zero in the TSs. 3.2. Microcanonical rate coefficients for redissociation (kreturn) and thermal high-pressure capture rate coefficients (kcapture). The OH + acetone association reactions are the barrierless formations of hydrogenbonded complexes (MC1a and MC1b). They proceed under an effective potential having a maximum, i.e. a J-dependent centrifugal barrier with the TS located at this maximum.35 The capture is supposed to take place with unit probability when the height of the centrifugal barrier is lower than the collision energy. The effective potential was calculated from the minimum energy path (MEP), obtained by calculating energies along the reaction coordinate from r = 0.35 nm to r = 2 nm with a step-size of 0.02 nm at the CCSD(T)/6-311G(d,p)//MP2/6-31G(d,p) level. Since the centrifugal barrier, i.e. the TS location for each E and J couple is far out along the reaction coordinate, only the attractive portion of the long-range part of the potential needs to be considered and is assumed to be central potential of the simple form – C/ rn. Appropriate 9 ACS Paragon Plus Environment


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n and C were obtained from fits of this expression to the attractive part of the MEP’s: n = 3.81 and C = 3.80 × 105 cm-1 Å3.81 and n = 4.28 and C = 2.73 × 105 cm-1 Å4.28 were obtained for reactions 1a and 1b, respectively. A simplified phase space theory (PST) is used and the angular-momentum-conserved rotational-translational sum of states of the fragments is approximated for each value of J by interpolation between “high J” and “low J”.36–38 The sum of states of the transition state, G*(E, J), is obtained by convolution of the density of “internal” (i.e., harmonic vibrational) states and the sum of external rotational-translational states of the fragments. The thermal high-pressure PST redissociation rate coefficient, k∞(J,T), i.e. the Jresolved canonical rate coefficient for redissociation of the MC1 species with a specified angular momentum and at a specified temperature, T, is obtained by averaging the microcanonical rate coefficients over a thermal distribution function: 

k (J ,T ) 

 k ( E, J ) N ( E, J ) exp( E / kT )dE 0



(E2)

 N ( E, J ) exp( E / kT )dE 0

Averaging k∞(J,T) over all J produces the high-pressure limit redissociation rate coefficient, k∞(T) by considering a thermal angular distribution of the complex.36 The corresponding high-pressure limit recombination rate coefficient, that is the capture rate coefficient, kcapture, follows by application of the equilibrium constant. The spin-orbit coupling of OH was taken into account in the electronic degeneracy, but considering that only the ground state, OH(23/2), reacts. According to the study of Nguyen et al. on the OH + methanol reaction,40 the rigid rotor approximation (with separated electronic, vibrational and rotational components) compared to the direct count of the coupled rovibronic states leads to an undercount of the partition function at low temperature. An effect on the dissociation rate to the reactants is also probable (due to the total angular momentum and energy conservations), but cannot be quantified by our method. Thus, the separation of the rotation and the electronic states was kept in this study.


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3.3. Calculated observable rate coefficients: RRKM-Master Equation analysis A RRKM-Master Equation (RRKM-ME) analysis was performed to evaluate the competition between redissociation of the MC complex to reactants, its isomerization via TS1 and collisional energy transfer to the bath gas (see Figure 3). This procedure is described in detail by Caralp et al.19 and Daranlot et al.39 The initial nascent (E, J) distribution function of the energized complex, which is just formed from the association of OH and acetone, is determined by detailed balance assuming thermal energy distribution in the reactants. Two typical energy distributions of MC1a are given in Figure 4 at two temperatures for the most probable J. Figure 4 shows that the chemically activated MC1a has a narrow distribution at the lowest temperature. In the present work, the master equation was constructed using an energy bandwidth of 5 or 10 cm-1. The number of grains, m, and the J maximum value, Jmax, were chosen such that the population of the m-th grain or Jmax contributes negligibly to the rate coefficient at a given temperature. The energy loss in collision down was taken to be equal to 270, 180 and 250 cm-1 for N2, He and Ar bath gas, respectively, and a simple exponential model was chosen for describing the probability of the energy transfer: it was assumed that total internal energy, E, can change through collisions but total angular momentum, J, remains unchanged. Knowledge of the dependence of the second-order collisional energy transfer rate constant on E, J states is typically not available.40,41 The ME calculation produces, at temperature T and for each value of J, the fraction of the MC complex, Bexit, which isomerizes via TS1a or TS1b and will give the products H2O + acetonyl, and the fraction which redissociates, Breturn, by the loose transition state. Two situations can occur: i) If (Bexit + Breturn) = 1, no stabilization of the complex MC through collisional energy transfer in the well is predicted, and the calculated thermal rate coefficient, for a specified J, is the corresponding kcapture multiplied by Bexit. ii) If (Bexit + Breturn) < 1, the stabilized fraction in the MC well is obtained as 1 - (Bexit + Breturn) and the calculated rate coefficient, for a specified J, is the corresponding kcapture multiplied by (1 – Breturn). Averaging over all J produces the observable thermal rate coefficient at the temperature T to be compared with the experimentally measured value.



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In conclusion, the calculated observable rate coefficient, kcalculated(T), corresponds to the experimentally measured rate coefficient of reaction 1, i.e. related to the OH decay kinetics which can lead to the formation of the H2O + acetonyl products, but also to the complex MC1a or MC1b at high pressures.

4. Results and discussion 4.1. Reaction mechanism At room temperature, Gierczak et al.9 reported, based on the measured deuterium isotope effect and the lack of pressure dependence of k(T), that the reaction proceeds by H-atom abstraction and that the rate-determining step involves C-H (or C-D) bond breakage. Yamada et al.10 also concluded that the dominant product for the OH+acetone reaction are CH3C(O)CH2 radicals (channels 1a and 1b) at temperatures between 298 and 832 K, being direct abstraction the most important mechanism above 450 K. Caralp et al.19 performed RRKM calculations based on the potential energy surface calculated by Henon et al.16 with the main features of the molecular mechanism from Yamada et al.10 A very good agreement between calculations and experiments in the temperature range 200-700 K was found, not only for this reaction, but for the isotopic analogue reaction (OH + acetone-d6), perfectly reproducing the temperature dependence of the kinetic isotope effect observed experimentally, without any adjustment of parameters of calculation, except the TS1 barrier heights. Moreover, no pressure dependence of k(T) was perceived between 1.5 and 1013 mbar, as observed experimentally. Caralp et al.19 concluded that the unusual temperature dependence of the overall rate coefficient was mainly due to the competition between the redissociation and exit channel, considering tunneling via the channels 1a and 1b (see Figure 3). However, Shannon et al.14,15 found experimentally a larger negative temperature dependence of k(T) below 150 K, compared to the previous studies above 200 K. Moreover, Shannon et al.15 observed a pressure dependence of k(80 K) between 3×1016 cm-3 and 1.7×1017 cm-3. The measured rate coefficient k(80 K) increased 2.5 times in a pressure change of 5 times, while k(140 K) was found to be almost P-independent between 8×1016 cm-3 and 2.6×1017 cm-3. These authors proposed that the rate determining step in the reaction of OH with acetone at these temperatures was the formation of the stabilized pre-reactive complex evidenced by observation of a pressure dependence. Their MESMER modelling showed that collisional stabilization of the energized complex occurs readily on the experimental timescale, 12 ACS Paragon Plus Environment

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despite the weak binding energies of these complexes and the relatively low total densities of the N2 bath gas used at 80 K and 140 K. They either did not observe experimentally a kinetic isotope effect at 93 K, concluding that tunneling cannot be the dominant mechanism in their experimental temperature range. Indeed, tunneling is significantly slower for a D-atom transfer compared to a H-atom. At 146 K, a change of the rate coefficient with deuteration was measured but close to the experimental uncertainties. To reproduce their experimental rate coefficients along with all those obtained previously between 200 and 700 K, Shannon et al.15 have solved the master equation with MESMER, fitting some parameters: the TS1a barrier height, the complex MC1a well depth energy, the imaginary frequency used in the Eckart model for the tunnelling and the association reaction rate coefficient to form the complex MC1a. The microcanonical rate coefficients kreturn(E) for the dissociation of the complex back to reactants are deduced from an inverse Laplace transform (ILT) of the fitted canonical high pressure limiting rate coefficient, 1.9×10-10 cm3 molecule-1 s-1, neglecting any temperature dependence of this association rate coefficient. Whatever the model, i.e. harmonic oscillator or hindered rotors, the fitted value of the well-depth was likely to be outside the error of the ab initio calculations. To reproduce the pressure dependence of the rate coefficients at 80 K and 140 K, they had to consider a complex well depth, relative to the reactants, of H°0 = 3150 cm-1 (harmonic oscillator model). The pressure dependence of the rate coefficient at ~80 K was well described by the master equation calculations, whereas it was overestimated at 140 K. In the temperature range above 200 K, no pressure dependence were neither observed experimentally nor previously calculated by Caralp et al.19 The limiting steps were the competitions between the isomerization and the redissociation of the complexes: the most important parameters were thus the barrier heights above the OH + acetone energy asymptote and the tunneling effects through the barriers. As these parameters were adjusted and validated in the 200 – 700 K temperature range, we have used them without any modification in these calculations. In the 80-140 K temperature range, an experimental pressure dependence of the rate coefficient was observed by Shannon et al.15 Due to the values of the barrier heights of TS1a and TS1b (see Figure 3), at low temperatures, the observed kinetics reaction should lead to the formations of the products H2O and acetonyl by tunneling through mainly TS1a, which presents the lowest barrier height, but also to the formation of MC1a by stabilization in the well, the MC1b being largely less stabilized according to previous calculations10,16,19 of the well depth. The best-fit value of the MC1a complex well depth proposed by Shannon et al.15 was H°0 = 3150 cm-1, largely higher than our previous value of 1539 cm-1.19 New ab initio 13 ACS Paragon Plus Environment


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calculations on the entrance channel OH + acetone → MC1a (keeping the previous results for MC1b) were thus performed. The well depth considered in our new calculated rate coefficient is 1820 cm-1 (Figure 3). Moreover, a simplified phase space theory (PST) is used to calculate the kreturn(E,J) coefficients, as well as the capture rate coefficient considering a central potential. The capture rate coefficient thus varies slightly with temperature (see Figures 5 and 6) and is close to the value calculated with the equations of Georgievskii and Klippenstein,42 considering a dipole-dipole interaction (10-9 cm3 molecule-1 s-1 at 10 K). The capture rate coefficients for both channels 1a and 1b depicted in Figure 5 differ slightly as we used different parameters for the fits of the attractive part of the MEP’s for the two associations leading to MC1a and MC1b. Note that the calculated kcapture may be overestimated as the potential is considered isotropic in this model. The dramatic inverse temperature dependence of the rate coefficients for temperatures below 200 K, can be explained by the competition between the re-dissociation of the complex MC1a to reactants (kreturn) and its reactivity to the products via tunneling (kexit). Figure 4 shows the chemically nascent (E, J=7 or 18) distribution function, the return microcanonical rate coefficient kreturn(E, J=7 or 18) and the exit microcanonical rate coefficient, kexit(E, J=7 or 18) at 10 or 65 K, respectively for the complex MC1a. At 65 K, kreturn(E,J) are always at least one order of magnitude larger than the kexit(E,J). At 10K, kexit(E,J) in the energy range of the nascent chemically activated MC1a becomes competitive with kreturn(E,J): the observed rate coefficient, i.e. the OH loss, is thus enhanced compared to 65 K. 4.2. Temperature dependence of the rate coefficients, k(T) Table 1 summarizes the rate coefficients k(T) in the temperature and gas density ranges experimentally investigated. Measurements have been carried out in the temperature range 12–65 K, sometimes at several pressures for a given temperature (that is for different Laval nozzles or/and flow conditions). Rate coefficients k(T) are given with a ±1σ statistical error issued from the second-order plots as exemplified in Figure 2. The total uncertainty, however, must include a systematic error due to potential instrumental biases, such as for example deviations from initial calibrations of mass flow controllers or pressure gauges. This directly influences the determination of the gas-phase acetone concentration. To minimize these systematic uncertainties, the acetone concentration was periodically checked by UV absorption spectroscopy as mentioned in section 2.2. We estimate that the systematic error does not exceed 10% of the measured rate coefficient value and, therefore, the total uncertainty in k(T) (error bars in Figures 6 and 7) has been calculated as the square root of the sum of the systematic error 14 ACS Paragon Plus Environment

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and the ±2σ statistical error squared. In Table 1, other errors indicated for the mean temperature, gas density and pressure in the uniform flow are not, strictly speaking, uncertainties, because they reflect the fluctuations of the physical conditions of the supersonic jet along its expansion, hence its geometric profile downstream of the Laval nozzle as illustrated in Figure S1. The calculated observable rate coefficients, kcalculated(T), in the flow conditions (T, P, bath gas) are also presented in Table 1 for a direct comparison with the experimental k(T). kcalculated is the overall OH loss and the calculated branching ratio leading to the products H2O + acetonyl, Bexit, is also reported in Table 1. The contributions of channels 1a and 1b evolve with temperature (see Figure 5). Channel 1b starts to be non-negligible only for T ≥ 250 K. Thus the main channel at low temperature is channel 1a, as expected with the barrier height of TS1a. However, in Table 1 and Figure 6, the reported calculated rate coefficients are always the sum of the rate coefficients of both channels. As shown in Figure 6 and Table 1, the experimental rate coefficient k(T) increases when decreasing temperature in the explored temperature range (11.7-64.4 K) thus extending the trend previously observed by Shannon et al.14,15 at T > 80 K. As the pressure starts to play a role below 150 K (see next section) the calculations shown in Table 1 were performed matching the experimental conditions (T, P) for the different buffer gases used. For those situations for which a mixture of buffer gases was used, an effective down which takes into account the mixture proportions has been considered in the calculations. Normally, a temperature dependence of (T/300 K)0.8 for down is used. However, changing the values for down hardly affects the calculated observable rate coefficients as exemplified in Table 1, where the results for He with down = 30 and 180 cm-1 are reported for some (T, P) conditions. Hence, constant values for down were chosen in the present calculations (see section 3.3). Below 30 K, our calculations are in good agreement with the experimental results (Table 1). At intermediate temperatures (30-65 K), the experimental and computational results deviate to each other by an increasing factor which exceeds one order of magnitude in many situations for T ≥ 50 K. The rate coefficients (kcalculated = k1a + k1b) are lower than the experimental ones by a factor 4 at ~140 K to about 20 at 80 K (Table 1 and Table S2 in the supplementary information). Using a well depth of 3150 cm-1 for MC1a as suggested by Shannon et al.15 allows us to reproduce the experimental data as well as the observed pressure-dependence of k(T) at 140 K and 80 K (see Table S1 and Figures S4 and S5 in the supplementary information). However, calculations performed with the same well depth at 12 and 22 K, are almost 6 times the ones measured in this work (see Table S1 in the supplementary information). A deeper well 15 ACS Paragon Plus Environment


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depth allows to reproduce the experimental data between 50 and 150 K, but fails at 12 and 22 K. Indeed, at low temperatures, any loss of energy of the chemically activated complex in the well starts to play a dominant role: by increasing the well depth, the complex density of states changes and thus, also the competition between kexit, kreturn and the stabilization. All the presented calculations, given in Figures 5 and 6, were thus performed with our ab initio value for channel 1a well depth. It should be noticed that the contributions of channel 1b below 250 K are small compared to channel 1a but not null (Figure 5). Lastly, the calculated rate coefficients above 200 K are the same than those previously calculated by Caralp et al.,19 which were in very good agreement with all the experimental data.6-13 4.3. Pressure dependence of the rate coefficient k(T) When possible, the pressure dependence of the rate coefficient was explored experimentally for several temperatures by using various supersonic flow conditions as shown in Table 1. The results are plotted in Figure 7. The rate coefficients obtained in this work at temperatures around 22 K and 64 K reveal no pressure dependence between 3.37×1016 and 16.7×1016 cm-3, and between 2.24×1016 and 17.4×1016 cm-3, respectively, or bath gas dependence. However, at around 50 K, the rate coefficient at the largest gas density 44.7×1016 cm-3 is twice higher than the average values below that gas density, revealing a potential weak pressure dependence. At first sight this could be somewhat surprising because one would have expected finding a pressure dependence at 64 K as well, since this has been observed by Shannon et al. at 80 K.15 Nevertheless, the onset of the P-dependence regime, experimentally observable, should be around 50 K. Our calculated microcanonical rate coefficients for MC1a are presented in Figure 4. The collisional frequency at 5×1016 cm-3 of N2 is plotted on the same figure and its value is comparable with the exit rate coefficients at 65 K: in fact, the pressure influence becomes to play a role below 150 K, with a maximum effect at 50-60 K. It should be noticed that the collisional efficiency of N2 is usually considered to be around 0.2. Thus this efficiency multiplied by the collisional frequency represents the frequency of energy exchange in the MC1 complex, which is mainly an energy loss. At low temperatures, the entrance energy distribution of the chemically activated complex is thin and any loss of energy will decrease the return flux back to the reactants, increasing the formation of the products by tunneling or the stabilization in the well. The calculated rate coefficients have been reported for two distinct nitrogen densities typical of flow experiments at temperatures up to 300 K (5×1016 and 20×1016 cm-3, respectively) in Figures 5 and 6. The pressure phenomenon can be seen in Figure 5, especially 16 ACS Paragon Plus Environment

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for channel 1b, where any increase of the pressure increases the rate coefficient at low temperatures. Indeed, the well depth of the complex starts to play a role. For example, below 30 K and at the highest pressure of 2×1017 cm-3, the rate coefficient of channel 1b is almost 10% of the rate coefficient of channel 1a. The pressure effect is enhanced as the well depth of MC1b is smaller than MC1a one and the TS1b barrier is higher. As explained previously, our calculated data failed to reproduce the experimental measurements in the 30-140 K range with the MC1a well depth of 1820 cm-1 and consequently the pressure-dependences as well. To reproduce the experimental data, an arbitrary well depth of 3150 cm-1, similar to the one used by Shannon et al.15, was considered (Table S1 in the supplementary information). At 12 K and 22 K, however, agreement between theory and experiments is significantly better with our ab initio value of 1820 cm-1 (Table 1). Calculations (kISM) reflecting the ISM tenuous conditions are also presented in Figures 5 and 6 in the temperature range 10 – 298 K. They have been carried out for a typical helium density of 103 cm-3 and down = 30 cm-1, which is more representative of the He collisional efficiency at the temperatures of ISM. As illustrated in Figure S6 of the supporting information, it should be noted that calculations at 10 K indicate the onset of a pressure effect on k(T) at n > 1016 cm-3 for He and no stabilization producing MC1a or MC1b was found below 5×1016 cm-3, since Bexit is 100% below this density. Thus, the calculated rate coefficients at 103 cm-3, kISM, can be representative of the low-pressure regime and a value of 1.8×10-10 cm3 molecule-1 s-1 has been obtained at 10 K. As expected, the rate coefficients are always the lower ones (Figures 5 and 6). Above 200 K, kISM is in reasonable agreement with data from the literature6–10,12,13,17,43,44 in accordance with the belief that the experimental rate coefficients at T > 200 K are in a pressure independent range. It is interesting to note that kISM significantly increases when the temperature is lowered and becomes very close to the experimental data obtained at 11.7 K with a difference of less than 30% clearly indicating that experiments are very close to the lowpressure regime at the lowest temperature as well. 5. Conclusions and astrophysical implications The rate coefficient for the reaction of OH with acetone was studied experimentally by different groups and techniques between 180 and 800 K.6-13 Above 400 K, the observed temperature dependence can be described with the Arrhenius law. The rate coefficient decreases with the temperature, with a minimum around 200 K and is pressure independent. Between 30 and 150 K, there are only two experimental studies, including the current investigation.15 The rate coefficient drastically increases when the temperature decreases and a pressure dependency 17 ACS Paragon Plus Environment


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is observed for some temperatures. Below 30 K, the rate coefficient seems to reach an upper limit (this experimental study) around (2-3)×10-10 cm3 molecule-1 s-1 for a He density of ~ 6×1016 cm-3. Unfortunately, the pressure dependence at 12 K could not be studied experimentally due to the present apparatus limits. Our theoretical model for this reaction allowed us to reproduce all the experimental results between 200 and 800 K, as well as the reaction of OH with deuterated acetone.19 This agreement validates our treatment of the competition between the dissociation of the complex back to the reactants and its reactivity via tunneling through a barrier to the products, H2O + acetonyl. This model allows us also to reproduce the experimental results between 10 and 30 K. However, the calculated results depend strongly on pressure at T ≤ 150 K and failed to reproduce the experimental data between 30 and 150 K by a factor varying from 4 (at 30 K and 140 K) to 28 (at 64 K and 90 K). An explanation for these differences is not readily apparent. As suggested by Shannon et al.15, increasing the well depth of the complex MC1a improves the agreement between theory and experiment in the temperature range of 50 – 150 K, but moves the gap at the lower temperatures. It seems that the main failure of our model could be related to the pressure effect. One of the following assumptions may be inaccurate: (1) a simple exponential model with conservation of J after collision with the bath gas, (2) the collision frequencies or (3) the average energy loss in a collision. Further experimental and theoretical studies on wider ranges of pressure are clearly desirable. This problem was also observed in the case of the OH + methanol reaction.40,45 Moreover, a more complicated dynamical effect at low temperature in the entrance channel cannot be ruled out, as a roaming mechanism which was found for the OH + methanol reaction.46

As the agreement between our calculations and experimental measurements is good below 25 K, the model presented in this article can be used to predict the rate coefficient for the OH + acetone reaction at a typical gas density of 103 cm-3. Indeed, at the densities of interest for dense interstellar clouds, the energy value of the MC1a well depth has no effect on the calculated rate coefficient and on the branching ratio leading to the products (Figures S4 and S6 in the supplementary information). In conclusion, the recommended rate coefficient for the OH+acetone reaction at 10 K that can be used to model the gas-phase chemistry of the interstellar molecular clouds is 1.8×10-10 cm3 molecule-1 s-1 leading to H2O and CH3C(O)CH2 radicals as products. A full quantum treatment of the entrance channel may refine this rate coefficient. 18 ACS Paragon Plus Environment

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The role of the title reaction in the chemistry of interstellar clouds has been discussed recently by Acharyya et al.47 These authors used a gas-phase network gathering 8670 reactions involving 670 species in order to probe the chemistry of homogeneous dense clouds at 10, 50 and 100 K for a total hydrogen density of 2×104 cm-3. For the OH + CH3C(O)CH3 reaction, they used a rate coefficient of 1.0×10-10, 1.0×10-10 and 0.077×10-10 cm3 molecule-1 s-1 at 10, 50 and 100 K, respectively. They also assume that the products of the reaction are CH3C(O)CH2 and H2O. The choice of these values was issued from extrapolations of the initial measures by Shannon et al.15 or modeling using the MESMER code. The value assumed at 50 K is close to the present experimental results obtained with a density in the range (2-20)×1016 cm-3 (see Table 1), but 30 times higher than our evaluation at 103 cm-3 of He (see Figure 6). At 10 K, the assumed value is about a factor of 2 lower than the one obtained in our investigation (2.5×10-10 and 1.8×10-10 cm3 molecule-1 s-1 experimental and theoretical values, at densities of 7×1016 and 103 cm-3, respectively). Acharyya et al.47 pointed out that the acetone abundance was not affected by the inclusion of the title reaction in the reaction network and that the calculated CH3C(O)CH2 abundance was below the current detection sensitivity of modern telescopes. Increasing the rate coefficient by a factor of about two in their model is unlikely to change their conclusions.

Supporting information The following information can be found in the supporting information: the temperature and gas density profile as a function of the distance (Fig. S1); Beer-Lambert plots (Fig. S2); curvature in the k’-k0’ versus [Acetone] plots (Fig. S3); kcalculated(T) for channel 1a at different gas densities and well depths (Fig. S4); comparison of the pressure dependence of kcalculated(T) with the experimental observations from Shannon et al.14,15 at 80 K and 140 K (Fig. S5) and pressure dependence of the calculated observable rate coefficients at 10 K (Fig. S6). A comparison of the experimental k(T) with kcalculated(T) and the calculated branching ratio leading to the products H2O+acetonyl) is reported in Tables S1 (using a well depth of 3150 cm-1) and S2 (using a well depth of 1820 cm-1). The influence of the uncertainty on T and n on the calculated observable rate coefficient is presented in Table S3 at 21 K.

Acknowledgments All authors from UCLM wish to thank European Research Council project (NANOCOSMOS, SyG-610256) for supporting this work. M. Antiñolo, S. Blázquez and A. García-Sáez would also like to thank UCLM for funding (Plan Propio de Investigación). A. Canosa and A. Bergeat are grateful to the French National programme "Physique et Chimie du Milieu Interstellaire" (PCMI) of CNRS/INSU with INC/INP co-funded by CEA and CNES for constant support. A. Canosa also acknowledges COST Action CM1401 “Our Astro-Chemical History”, UCLM and the CNRS for specific subsidy via the PICS "Cingaz" contract, for funding 19 ACS Paragon Plus Environment


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a research stay at the Department of Physical Chemistry of UCLM during the performance of these experiments. AB and FC thank Wendell Forst for the helps in the RRKM calculations.


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Photolysis of CH3C(O)CH3 at 248 and 266 nm: Pressure and Temperature Dependent Overall Quantum Yields. Phys. Chem. Chem. Phys. 2009, 11 (29), 6173–6181. (34) Jiménez, E.; Lanza, B.; Garzón, A.; Ballesteros, B.; Albaladejo, J. Atmospheric Degradation of 2-Butanol, 2-Methyl-2-Butanol, and 2,3-Dimethyl-2-Butanol: OH Kinetics and UV Absorption Cross Sections. J. Phys. Chem. A 2005, 109 (48), 10903– 10909. (35) Forst, W. Unimolecular Reactions : A Concise Introduction; Cambridge : Cambridge University Press, 2003. (36) Forst, W. Unimolecular Phase Space Theory Rates by Inversion of Angular Momentum-Conserved Partition Function. Phys. Chem. Chem. Phys. 1999, 1 (6), 1283–1291. (37) Forst, W. Approximations for Angular Momentum-Conserved Polyatomic VibrationalRotational Sum and Density of States under a Central Potential. Chem. Phys. Lett. 1996, 262 (5), 539–545. (38) Forst, W. Approximation for Sums and Densities of State of Vibrations Coupled with Symmetric Top K-Rotor. Comput. Chem. 1996, 20 (4), 419–425. (39) Daranlot, J.; Bergeat, A.; Caralp, F.; Caubet, P.; Costes, M.; Forst, W.; Loison, J.-C.; Hickson, K. M. Gas-Phase Kinetics of Hydroxyl Radical Reactions with Alkenes: Experiment and Theory. ChemPhysChem 2010, 11 (18), 4002–4010. (40) Nguyen, T. L.; Ruscic, B.; Stanton, J. F. A Master Equation Simulation for the •OH + CH3OH Reaction. J. Chem. Phys. 2019, 150 (8), 84105. (41) Jasper, A. W.; Miller, J. A. Theoretical Unimolecular Kinetics for CH4 + M ⇄ CH3 + H + M in Eight Baths, M = He, Ne, Ar, Kr, H2, N2, CO, and CH4. J. Phys. Chem. A 2011, 115 (24), 6438–6455. (42) Georgievskii, Y.; Klippenstein, S. J. Long-Range Transition State Theory. J. Chem. Phys. 2005, 122 (19), 194103. (43) Kerr, J. A.; Stocker, D. W. Kinetics of the Reactions of Hydroxyl Radicals with Alkyl Nitrates and with Some Oxygen-Containing Organic Compounds Studied under Simulated Atmospheric Conditions. J. Atmos. Chem. 1986, 4 (2), 253–262. (44) Carr, S.; Shallcross, D. E.; Canosa-Mas, C. E.; Wenger, J. C.; Sidebottom, H. W.; Treacy, J. J.; Wayne, R. P. A Kinetic and Mechanistic Study of the Gas-Phase Reactions of OH Radicals and Cl Atoms with Some Halogenated Acetones and Their Atmospheric Implications. Phys. Chem. Chem. Phys. 2003, 5 (18), 3874–3883. (45) Ocaña, A. J.; Blázquez, S.; Potapov, A.; Ballesteros, B.; Canosa, A.; Antiñolo, M.; Vereecken, L.; Albaladejo, J.; Jiménez, E. Gas-Phase Reactivity of CH 3 OH toward OH at Interstellar Temperatures (11.7–177.5 K): Experimental and Theoretical Study. Phys. Chem. Chem. Phys. 2019, 21 (13), 6942–6957. (46) del Mazo-Sevillano, P.; Aguado, A.; Jiménez, E.; Suleimanov, Y. V.; Roncero, O. Quantum Roaming in the Complex-Forming Mechanism of the Reactions of OH with Formaldehyde and Methanol at Low Temperature and Zero Pressure: A Ring Polymer Molecular Dynamics Approach. J. Phys. Chem. Lett. 2019, 10, 1900–1907. (47) Acharyya, K.; Herbst, E.; Caravan, R. L.; Shannon, R. J.; Blitz, M. A.; Heard, D. E. The Importance of OH Radical–neutral Low Temperature Tunnelling Reactions in Interstellar Clouds Using a New Model. Mol. Phys. 2015, 113 (15–16), 2243–2254.


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Tables Table 1. Gas-phase rate coefficients for the CH3C(O)CH3+OH reaction obtained in this work. Uncertainties ± are only statistical kcalculated is the calculated observable rate coefficient considering channels 1a and 1b and the experimental conditions (temperature, carrier gas and density). kcalculated represent the overall OH loss rate coefficients and Bexit, the branching ratio leading to the products H2O+acetonyl. ~T/ K

T/ K

 12

11.7 ± 0.7

100

6.88  0.62

13.0  0.7

100

6.41  0.55

[CH3C(O)CH3]/ k(T)/ 10-11 kcalculated(T)/ 10-11 1012 cm-3 cm3 molecule-1 s-1 cm3 molecule-1 s-1 0.58 – 5.79 24.5 ± 1.7 33 (42) b 0.11  0.02 1.32 – 7.05 22.7 ± 2.3 26 (34)b 0.12  0.02

21.1 ± 0.6

100

3.37 ± 0.15

0.10  0.01

0.91 – 18.8

13.9 ± 0.5

6 (9) b

100 (100)b

21.7 ± 1.4

100

16.7 ± 1.6

0.51  0.08

2.14 – 21.7

16.1 ± 0.6

14 (24)b

100 (98)b

 36

36.2 ± 1.2

100

17.7 ± 0.9

0.90  0.07

4.77 – 19.1

14.1 ± 0.7

3 (7)b

100 (100)b

 50

50.5 ± 1.6

100

1.50 ± 0.12

0.11  0.01

1.79 – 15.4

9.01 ± 0.25

0.6

100

51.6 ± 1.7

100

4.17 ± 0.35

0.30  0.04

1.21 – 9.38

12.1 ± 0.4

0.9

100

8.33 ± 0.41 19.5 ± 0.3

0.58  0.05 1.42  0.03

7.38 – 29.9 2.61 – 15.2

8.04 ± 0.33 11.8 ± 0.8

1.3 2.4

100 100

44.4 ± 0.6

3.37  0.08

1.60 – 8.93

20.9 ± 1.3

1.4 (4)b

100 (100)b

100 a

2.24 ± 0.15

0.20  0.02

2.09 – 50.3

8.44 ± 0.21

0.3

100

80

4.63 ± 0.27

0.42  0.03

3.72 – 31.6

7.39 ± 0.19

0.5

100

30

17.4 ± 0.3

1.56  0.04

2.27 – 11.7

7.27 ± 0.40

1.2

100

 22

 64

% He % Ar % N2 n / 1016 cm-3

49.9 ± 1.4 52.1 ± 0.5

74

54.2 ± 0.5

100

26 100

64.2 ± 1.7 64.1 ± 1.6 64.4 ± 0.6

a

20 70

P/ mbar

Bexit % 78 (37)b 78 (43)b

(a) continuous flow conditions, (b) considering down = 30 (180) cm-1.


Page 25 of 32

Figures

Residual

0.8 0.4 0.0 -0.4 -0.8 10 Absence of CH3C(O)CH3

9

13

[CH3C(O)CH3] = 2.97 × 10 cm

-3

8

Rotational relaxation

7

ILIF / a.u.

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


6 5 4 3

k'0 = (1801 ± 38) s

-1

2 -1

k' = (4115 ± 33) s

1 0 0

100

200

300

400

500

600

700

t / μs

Figure 1. Examples of the OH LIF temporal profiles at 64.1 K.



a)

2000 1800 1600

k' - k'0 / s

-1

1400 1200 1000 800 600 400

T = (11.7 ± 0.7) K T = (51.6 ± 1.7) K

200 0 0.0

0.2

0.4

0.6

0.8 13

[CH3C(O)CH3] / 10 cm

b)

1.0

1.2

-3

3500 3000

-1

2500

k' - k'0 / s

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

Page 26 of 32

2000 1500 1000 T = (21.1 ± 0.6) K T = (64.1 ± 1.6) K

500 0 0.0

0.5

1.0

1.5

2.0

2.5 13

[CH3C(O)CH3] / 10 cm

3.0

3.5

-3

Figure 2. Examples of the k’-k0’ versus concentration plots for a) 11.7 K and 51.6 K and b) 21.1 K and 64.1 K. Uncertainties in acetone concentration are conservatively considered as ±10%, while for k’-k0’ the error bars represent the combined standard deviations obtained from the fit of the LIF OH decay in the presence and absence of acetone.


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Figure 3. Schematic energy diagram of the most important part of the potential energy surface for the OH+acetone reaction illustrating the kinetics model used (the star * indicates an energized complex). Values are from Caralp et al.19 except for energies of MC1a derived from this work. Energies are in cm-1 relative to the reactants energy.



Page 28 of 32

Figure 4. Entrance energy distribution P(E) in black solid line and microcanonical rate coefficients k(E, J) for unimolecular redissociation of MC1a to reactants (kreturn) or its isomerization, with tunneling, to the adduct (kexit) through TS1a at the most probable total angular momentum J. (N2) is the collision frequency for a N2 density of 5×1016 cm-3. Upper panel: T = 10 K most probable J = 7 and lower panel: T = 65 K most probable J = 18. Energies in cm-1 are relative to the ground state of the MC1a complex.


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Figure 5. Capture rate coefficients for channels 1a and 1b, kcapture(T), and calculated observable rate coefficients, kcalculated(T) for each channel at a gas density of 103 cm-3 of He (solid line) with down = 30 cm-1, 5×1016 cm-3 of N2 (dashed line) and 2×1017 cm-3 of N2 (dotted line).



2

10

T < 200 K 1

This work, experiments This work, computed kcapture(T)

10

3

This work, computed kISM(T) @ [He] = 10 cm 16

This work, computed k(T) @ [N2]= 5×10 cm

0

10 -1 -1

This work, computed k(T) @ [N2]= 2×10

17

-3

-3

cm

-3

Shannon et al. (2014) Shannon et al. (2010) -1

10

3

cm molecule s

-10

k(T)/ 10

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

Page 30 of 32

-2

10

-3

10

T-dependence at T ≥ 200 K

Room temperature measurements

Wallington et al. (1987) Le Calvé et al. (1998) Wollenhaupt et al. (2000) Yamada et al. (2003) Gierczak et al. (2003) Raff et al. (2005) Davis et al. (2005)

-4

10

Kerr and Stocker (1986) Vasvári et al. (2001) Carr et al. (2003)

-5

10

0

100

200

300

400

500

T/ K

Figure 6. Experimental and theoretical rate coefficients for reaction (1) as a function of temperature between 10 K and 500 K.


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Figure 7. Rate coefficients for the reaction of OH with CH3C(O)CH3 as a function of gas density for a temperature around a) 21 K; b) 50 K and c) 64 K.



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Experimental and theoretical investigation on the OH + CH3C(O)CH3

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