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Role of Proton Tunneling and Metal Free Organocatalysis in Decomposition of Methanediol: A Theoretical Study Manoj Kumar, Josep M. Anglada, and Joseph S. Francisco J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 17 May 2017 Downloaded from http://pubs.acs.org on May 29, 2017

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The Journal of Physical Chemistry

Role of Proton Tunneling and Metal Free Organocatalysis in Decomposition of Methanediol: A Theoretical Study Manoj Kumar,1 Josep M. Anglada2 and Joseph S. Francisco1,*

1

2

Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE, USA 68588

Departament de Química Biològica i Modelització Molecular, c/Jordi Girona 18, E08034 Barcelona, Spain.

ABSTRACT

Canonical variational transition state theory rate calculations have been performed to assess the fate of methanediol in the troposphere. The calculations suggest that proton tunneling plays a very important role in the gas-phase decomposition of methanediol as it enhances the rate of the reaction by 1-9 orders of magnitude in the tropospherically relevant temperature range of 200-300 K. The effect of proton tunneling is greatest at 200 K; the rate constant is enhanced up to 9 orders of magnitude. This is in stark contrast to previous calculations suggesting that tunneling would not play any role in the alkanediol decomposition under typical laboratory and interstellar conditions. Furthermore, the results imply that though water is the most dominant trace component of troposphere, formic acid and hydroperoxyl radical, which are relatively less abundant, outcompete water in catalyzing the decomposition. Methanediol may also catalyze its own decomposition below 280 K. However, this autocatalytic pathway turns out to be less effective than the water-catalyzed one. These results may play a crucial role in improving our understanding of alkanediol chemistry, which has a broad appeal beyond troposphere.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

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ACS Paragon Plus Environment


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I. INTRODUCTION

Geminal alkanediols (referred to as diols from now on) contain two hydroxyl functionalities bound to the same carbon atom with methanediol being the simplest geminal diol. Diols impact various fields such as atmosphere, aqueous-phase chemistry, industry and interstellar medium. In the atmosphere, diols are believed to play a role in the aerosol growth.1,2 Methanediol is also of considerable interest to the field of astrochemistry. It is thought to be formed on the surface of interstellar grains by the UV radiation or cosmic ray processing of ice mantles.3,4 Recent theoretical calculations suggest that the hydration of formaldehyde (HCHO) could be the most probable mechanism for the diol formation in these mantles.1,2 The reaction between OH and CH2OH radicals is another pathway for the diol formation in astrophysical ices.5 Diols are also important industrial intermediates in the manufacturing of resins, plastics, adhesives and many other commercial products. In aqueous phase, diols are spontaneously formed by the hydration of aldehydes at ambient conditions.6 Analytical experiments reveal that in a 5% by weight aqueous solution of formaldehyde, approximately 80% methanediol is present.6,7 However, the calculations of Matubayasi et al suggested that in hot water liquid above 200˚C and at a fixed density of 1.0 g/cm3, the chemical equilibrium is dominantly shifted in favor of HCHO.8 More recently, the Raman experiment of Hanoune et al.9 demonstrated the apparition of HCHO under its non-hydrated form at temperatures as low as 90˚C, over which the amount of HCHO progressively increases. The diols are capable of polymerizing into different poly(oxymethylene)glycols

HO(CH2O)nH,

depending

upon

the

temperature,

concentration and pH of the solution.10 Methanediol has also been used as the prototypical compound for the study of the anomeric effect.11-14 Despite their broad profile, high-resolution spectroscopic characterization of diols has not been achieved so far. This is because diols are difficult to isolate under typical gas-phase laboratory conditions. Evidently, there is only limited experimental information available on the structure of diols. Lugez et al. studied methanediol under

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matrix-isolation conditions where infrared spectroscopy was used to monitor the products of the reaction of methanol with O(1D).15 The photolysis of ozone produces O(1D), which then reacts with methanol. The possibility of the O(1D) insertion into the CH bond of methanol is supported by recent theoretical calculations.16 Methanediol in the gas-phase was also formed by the reaction between O(1D) and isotopically enriched methanol.17 The experimental elusiveness of methanediol in the gas-phase makes theoretical calculations an important tool to study the diol chemistry under tropospheric and terrestrial conditions. The decomposition of diols leads to either aldehydes or carboxylic acids. Previous computational studies suggest that the former pathway is usually preferred.18,19 Kent et al. investigated the thermodynamic and kinetic stability of methanediol under the laboratory and interstellar conditions using used the fourthorder Møller-Plesset, coupled cluster and infinite-pressure Rice–Ramsperger–Kassel– Marcus (RRKM) rate calculations.20 The calculated barrier for the unimolecular decomposition of methanediol at 300 K was 42.8 and 44.6 kcal/mol at the MP4/ccpVTZ//MP2/cc-pVDZ and CCSD(T)/cc-pVTZ//MP2/cc-pVDZ levels of theory, respectively. The calculated infinite-pressure RRKM rate for the uncatalyzed decomposition was 3.6 × 10-20 s-1 at 300 K, implying gas-phase kinetic stability for laboratory and hot core conditions. Several research groups have performed calculations to examine the stability of methanediol in different tropospheric conditions. Böhm et al. examined the gas-phase dehydration of methanediol with and without water catalyst.21 The MP2/631G**-calculations show that a single water molecule lowers the decomposition barrier by 16.5 kcal/mol. Kramer et al. probed the role of water catalysis in the vibrational overtone-induced dehydration reaction of methanediol.22 Quantum chemistry calculations indicated that the presence of one and two catalytic water molecules lowers the barrier relative to the uncatalyzed thermal reaction by 17.6 and 22.5 kcal/mol, respectively. However, the reactive dynamics simulations of the methanediol···water complexes revealed a strong delayed threshold effect in the quantum yield for the dehydration reaction, implying that water catalysis does not play

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any role in the overtone-induced photoreaction. Recently, Inaba carried out molecular dynamics simulations and Gaussian 4 method-based quantum chemical calculations to examine the decomposition of methanediol in aqueous phase.23 The uncatalyzed reaction has a barrier of 46 kcal/mol. The quantum mechanical tunneling is found to play an important role in the decomposition reaction with effect being more pronounced at low temperatures; the rate constant at 300 K is enhanced by a factor of 1600 at room temperature upon inclusion of tunneling corrections. As seen in previous theoretical studies, water catalysis causes an appreciable lowering in the barrier for the thermal decomposition; the lowest barrier height of 19 kcal/mol is reported for the three water-cluster-catalyzed reaction when two additional water molecules form hydrogen bonds with methanediol. We have recently examined the gas-phase decomposition of the two simplest diols, methanediol and ethanediol in the presence of various potential catalysts.18 The calculations suggest that though the dehydration of diols with or without water catalyst involves large barriers, organic acids (formic acid and acetic acid), inorganic acids (nitric acid and sulfuric acid), and hydroperoxyl radical catalytically influence the reaction to such an extent that the dehydration reaction either involve significantly reduced barriers or essentially become barrierless. The diols contain hydroxyl groups and their dimers have high binding energies. As a result, a noticeable fraction of the gas-phase dehydration of diols may be self-driven. Despite being extensively explored, several fundamental questions about diol chemistry remain to be answered. For example, Kent et al. suggested that there is an intramolecular hydrogen bond in methanediol and thus, tunneling will not play any role in the reaction under typical laboratory and interstellar conditions.20 However, there is a significant amount of literature suggesting that quantum mechanical tunneling of atoms plays a very important role in a variety of reactions like hydrogen atom transfer, proton transfer and proton-coupled electron transfer in molecules and enzymes,24,25,26 oxidative additions and reductive eliminations in organometallic complexes,27,28,29 sigmatropic rearrangements,30,31 isomerization of hydroxycarbenes,32 surface reactions involving hydrogen addition in heterogeneous catalysis33,34 and space,35,36 heavy

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deuteration of small molecules in space,37,38 rotation of hydrogen-bonded water complexes,39,40 and hydrogen transfer processes in atmosphere.41,42 Criegee intermediates and 3,5 pyridinedicarboxylic acid are the two latest systems that involve intramolecular hydrogen bonding and tunneling makes an important contribution towards the overall rate of the reaction. Using NMR spin-lattice and relaxometry data, Frantsuzov et al. have recently shown the proton tunnelling effects in one of the shortest known N-H···O hydrogen bonds in a single crystal of 3,5 pyridinedicarboxylic acid at low temperature.43 The unimolecular decomposition of Criegee intermediates, which could be a significant source of hydroxyl radical in troposphere, is also found to be significantly influenced by tunneling.41 Clearly, the gas-phase decomposition of methanediol requires a detailed and careful reinvestigation. Another important issue concerning the diol chemistry is the role of acids and hydroperoxyl radical in their decomposition in the troposphere. We recently showed using theoretical calculations that these species may catalyse the diol decomposition.18 However, these conclusions were based on the dehydration barrier lowering caused by these acids and hydroperoxy radical, and it remains to be seen whether these catalysts would be present in appreciable amounts in troposphere to make any meaningful contribution towards the methanediol decomposition. Clearly, the rates of the diol dehydration need to be calculated taking into account the tropospheric concentrations of these species and compared with the water-catalyzed reaction to fully assess the implications of these new pathways for the diol decomposition. In this work, we have performed ab initio electronic structure and rate constant calculations to quantify the impact of proton tunneling, and organic acid and hydroperoxyl radical catalysis, and self-catalysis on the gas-phase decomposition of methanediol. The results suggest that organic acid and hydroperoxyl radical are more efficient than water in decomposing methanediol under tropospheric conditions and must be included in the atmospheric models. Though methanediol is less effective catalyst than water towards its own decomposition, it may still catalyze its own decomposition below 280 K.

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II. COMPUTATIONAL DETAILS All quantum mechanical calculations reported in this work were performed using Gaussian0944 software package for the study of the electronic structure and for the calculation of the thermodinamic properties at standard temperature (298.15 K) and pressure (1 atm). The decomposition of methanediol in the presence of water (H2O), formic acid (HCOOH), hydroperoxyl radical (HO2) and methanediol itself, that are viable catalysts in the troposphere, has been examined. There are two possible conformations for a methanediol dimer that can participate in its dehydration (Scheme 1). We have considered both in this work. The geometries of all the stationary points on the potential energy surfaces for the methanediol decomposition were fully optimized using the M06-2X45 density functional theory method and the augmented correlationconsistent triplet zeta basis set, aug-cc-pVTZ46 (M06-2X/aug-cc-pVTZ), and subsequent normal-mode vibrational frequency analyses were performed to ascertain that the stable minima have all positive vibrational frequencies and that the transition states have only single imaginary frequency. Intrinsic reaction coordinate calculations (IRC) have been also carried out to ensure that a given transition state connects with the desired reactant and product. The energetics of the decomposition reaction was further refined by performing single point energy calculations with the coupled cluster single and double substitution method with a perturbative treatment of triple excitation (CCSD(T))47 and the aug-cc-pVTZ basis set. These calculations have been carried out at geometries optimized at level. For all the reactions, the M06-2X/aug-cc-pVTZ calculated vibrational frequencies were used to estimate the zero-point and thermal energy corrections for the reactants, products, transition states, and intermediates. Note that we have used unscaled harmonic vibrational frequencies in our calculations. The structural coordinates, unscaled harmonic vibrational frequencies and rotational constants of the reactant, catalysts, reaction complexes, product complexes, transition states, and products are provided in the Supporting Information.

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Scheme 1. Two different conformations of a methanediol dimer. All rate constant calculations were performed with POLYRATE,48 version 2015. The rate constants are calculated using variational transition state theory with multidimensional tunneling. Specifically, canonical variational transition state theory49,50 (CVT) is used with a transmission coefficient κ calculated by the small curvature tunneling51 (SCT) approximation; the resulting rate constant is labeled (CVT+SCT). We have used energies obtained at CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level of theory, and partition functions computed at M06-2X/aug-cc-pVTZ level of theory for estimating the rate constants for unimolecular and bimolecular dehydration of methanediol. Table 1. CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ calculated energetics for the unimolecular and bimolecular gas-phase decomposition of methanediol at 298 K and 1 atm. The zero-point-corrected electronic energies and thermally corrected enthalpy and free energies for key species are given in kcal/mol units. Reaction

∆E CH2(OH)2 CH2(OH)2 + H2O CH2(OH)2 + HCOOH CH2(OH)2 + HO2 a CH2(OH)2 + CH2(OH)2 b CH2(OH)2 + CH2(OH)2 a

-3.3 -9.4 -8.1 -10.0 -4.3

Int1 ∆H -3.8 -9.5 -8.5 -10.1 -4.2

∆G

∆E

4.9 0.4 1.4 0.1 5.9

44.7 23.9 7.6 10.2 26.5 21.5

TS ∆H 44.6 22.3 6.8 9.1 26.1 20.7

∆G

∆E

44.8 33.1 18.9 20.8 37.5 32.9

6.8 -0.3 -5.1 -3.8 3.7 -1.5

Int2 ∆H 8.3 0.3 -4.2 -3.1 6.0 -0.5

P

∆G

∆E

∆H

4.8 6.6 3.9 4.5 10.9 8.0

8.3 8.3 8.3 8.3 16.6 8.3

10.1 10.1 10.1 10.1 20.2 10.1

CH2(OH)2+CH2(OH)2 corresponds to a bimolecular reaction in which all four hydroxyl groups are hydrogen bonded in the prereaction complex. This is a non-catalytic reaction because both CH2(OH)2 molecules decompose. b CH2(OH)2+CH2(OH)2 corresponds to a bimolecular reaction in which three of four hydroxyl groups are hydrogen bonded in the prereaction complex.

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∆G -0.3 -0.3 -0.3 -0.3 -0.6 -0.3


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III. RESULTS AND DISCUSSION Unimolecular Dehydration. In the first step, we analysed the unimolecular or uncatalyzed dehydration of methanediol. The uncatalyzed reaction has a very large energy barrier of 44.7 kcal/mol and an endothermicity of 8.3 kcal/mol at the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level of theory (Figure 1 and Table 1). See Figure S1 for the M06-2X/aug-cc-pVTZ calculated reaction profile, and our r calculated energy barrier heights and reaction energies are in excellent agreement with the previous estimates.18,20-23 The rate constant has been calculated according equation 1 k(CVT + SCT ) = κ

k B T QGT ( s*) e h Q Reactant

−V ( s *) k BT

(1)

In this equation s* is the free energy maximum along the reaction path at temperature T, Q Reactant is the partition function of the reactant, Q GT(s*) is the generalized transition state partition function and V(s*) is the potential energy and κ is the tunneling parameter. At 300 K, the calculated CVT rate constant is 1.3 x 10-20 s-1. Upon inclusion of quantum mechanical tunneling, the reaction rate changes by more than an order of magnitude, i.e., k(CVT+SCT) = 6.4 x 10-19 s-1 at 300 K. The magnitude of tunneling is found to be dramatically enhanced at lower temperatures (Figure 2 and Table 2). At 250 K, the tunnelling increases the rate constant for decomposition by 3 orders of magnitude, i.e., k(CVT) = 3.5 x 10-27 s-1; k(CVT+SCT) = 4.3 x 10-24 s-1. At 200 K, tunneling accounts for nearly 9 orders of magnitude rate enhancement, i.e., k(CVT) = 4.8 x 10-37 s-1; k(CVT+SCT) = 7.2 x 10-28 s-1. Though the experimental data on the gas-phase decomposition of methanediol is still lacking, there is theoretical data available for comparison. According to the RRKM calculations of Kent et al.,20 the upper bound for the unimolecular decomposition rate of methanediol is 10-22-10-18 s-1 at 300 K. It is important to point out here that Kent et al. never calculated the rate constants taking into account the effect of proton tunneling. Instead, they assumed that diols have a short intramolecular hydrogen bond and is therefore, tunneling effects will not be significant. Their calculated RRKM rate constant for the uncatalyzed decomposition at 300 K is 3.6 x 10-20 8


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s-1 at CCSD(T)/cc-pVTZ level. This value matches closely with our calculated CVT value of 1.3 x 10-20 s-1. Clearly, our theoretical method reproduces previous results, provided tunneling effects are ignored. Overall, our calculations suggest that proton tunneling plays a very important role in the diol decomposition especially at lower temperatures and must be taken into account in the atmospheric diol models.

Figure 1. CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ calculated reaction profiles for the uncatalyzed and catalysed gas-phase decomposition of methanediol at 298.15 K and 1 atm. The zero point-corrected electronic energies are given in kcal/mol units. Catalysts considered here are water, formic acid, hydroperoxyl radical and methanediol itself. Methanediol can possibly interact with itself via two (D1) and one (D2) hydroxyl functional groups. We have considered both possibilities here.

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Table 2. Calculated rate constants (k) and tunneling factors (κ) for the uncatalyzed gasphase decomposition of methanediol at various temperatures. The rate constants have been calculated using transition state theory (TST), and canonical variational transition theory (CVT) with and without small curvature tunneling (SCT) correction.

k (s-1)

Temperature (K)

200 210 220 230 240 250 260 270 280 290 298.15 300

Bimolecular

CVT

CVT+SCT

4.8 x 10-37 1.1 x 10-34 1.5 x 10-32 1.3 x 10-30 7.9 x 10-29 3.5 x 10-27 1.1 x 10-25 2.9 x 10-24 5.9 x 10-23 9.6 x 10-22 8.2 x 10-21 1.3 x 10-20

7.2 x 10-28 2.3 x 10-27 7.7 x 10-27 3.7 x 10-26 3.4 x 10-25 4.3 x 10-24 5.6 x 10-23 6.8 x 10-22 7.4 x 10-21 7.3 x 10-20 4.3 x 10-19 6.4 x 10-19

κ 1.5 x 109 2.1 x 107 5.3 x 105 2.9 x 104 4.3 x 103 1.2 x 103 4.9 x 102 2.3 x 102 1.3 x 102 7.6 x 101 5.3 x 101 4.9 x 101

Dehydration. We next examined the effect of various

atmospherically important catalysts on the methanediol decomposition. Catalysts considered in this work were H2O, HCOOH, HO2, and methanediol itself. All the bimolecular reactions are mediated by prereaction complexes (Int1) and postreaction complexes (Int2). Though the uncatalyzed reaction is endothermic, the formation of Int2 in bimolecular reactions is exothermic except for the non-catalytic D1 dimer involving decomposition (Figure 1). The D1-mediated self-hydration is 16.6 kcal/mol endothermic

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Figure 2. Arrhenius plots of the CVT and CVT+SCT thermal rate constants for the gasphase decomposition of methanediol with and without water catalyst from 200 to 300 K. and the formation of Int2 is 3.7 kcal/mol endothermic because both monomers decompose during the reaction that result in the weakly-bound hydrogen-bonded complexes. H2O and methanediol lower the barrier of the reaction by 17.5 and 18.9 kcal/mol, respectively. HCOOH and the HO2 radical produce greater catalytic effect in the reaction; the barriers for HCOOH and the HO2 reactions are 10.2 and 9.0 kcal/mol lower than that for the H2O-catalyzed reaction. The ability of catalysts to significantly lower the barrier for methanediol decomposition suggests that they may potentially play an important role in the tropospheric budget of carbonyl compounds. Therefore, we next evaluated the possible tropospheric impact of the bimolecular methanediol decompositions by comparing their rate constants to that for the uncatalyzed one. According to the reaction profiles shown in Figure 1, the gas-phase reaction between methanediol and a catalyst follows a two-

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step mechanism as described by eq 2, where the pre-reactive complex Int1 is in equilibrium with the methanediol and catalyst (Cat), and the reaction proceeds through the unimolecular decomposition of this complex.

(2)

In the steady state approximation, the forward rate constant can be approximated by k k

TS

=

k

1 k =K k 2 eq 2 −1

Q

(3)

Complex K = e eq Q Q CH2(OH)2 Cat

− (E

−E ) C R RT

where Keq is the equilibrium constant for the formation

of the Int1 complex. the various Q denote the partition functions of the reactants and, the pre reactive complex.and k2 is the unimolecular rate constant for its decomposition as given in equation 2, In order to make a more direct comparison between the uncatalized and catalyzed reaction we can define an effective rate constant for the methanediol decomposition due to the reactions with a catalyst according to keff = kTS·[Cat] (4) Where [Cat] is the catalyst concentration in troposphere. In these equations, the equilibrium constant has been evaluated from the relative energy obtained at CCSD(T)/aug-cc-pVTZ level of theory and partition functions computed at M06-2X/aug-cc-pVTZ level of theory, whereas for k2, we have carried out variational transition state theory calculations employing energies obtained at CCSD(T)/aug-cc-pVTZ level of theory and partition functions computed at M06-2X/augcc-pVTZ level of theory. Quantum effects on the reaction dynamics were computed, using the SCT51 approximation. For the concentration [Cat], we have used [H2O] ~ 6.1 x 1017 molecules/cm3, [HCOOH] ~ 5 x 1010 molecules/cm3, [HO2] ~ 1.1 x 109 and [methanediol] ~ 4.6 x 1010. These values were adopted from a recent study by

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Vereecken et al.,52 in which they have summarized concentrations of various species in different environments based on previous literature estimates. Note here that these concentrations were obtained near the surface in several geographic regions where the temperatures are relatively high (typically >280 K) and atmospheric concentrations of the minor species are also relatively high. The results of the kinetic study are collected in Tables 2, 3 and S1-S2, and Figures 2-4. The calculated Keq for various methanediol⋅⋅catalyst complexes are shown in Figure 3 and Table S1. Catalysts that form more stable double hydrogen-bonded complexes with methanediol (e.g., HCOOH, HO2 radical, and methanediol itself) have larger Keq at all temperatures considered. This is consistent with the large binding energies predicted for these complexes. Temperature has a noticeable effect on the calculated Keq; the Keq for such catalysts is enhanced by 2-4 orders of magnitude in going from 300 K to 200 K.

Figure 3. CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ calculated equilibrium constants for various methanediol••catalyst complexes from 200 to 300 K. Catalyst considered here are water, formic acid, hydroperoxyl radical and methanediol itself.

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The rate constant calculations suggest that tunneling plays an important role in the H2O-catalyzed and autocatalyzed reactions as it enhances the rate by 1-2 orders of magnitude (Figures 2 and 4, and Tables 3 and S2). At 300 K, the k2(CVT) for the H2Ocatalyzed reaction is 1.3 x 10-8 s-1, which is enhanced to 8.0 x 10-8 s-1 upon inclusion of quantum mechanical tunneling. At 200 K, the tunneling-induced rate constant enhancement is relatively larger; k2(CVT+SCT) = 4.6 x 10-16 s-1, 2.7 x 102 times larger than kTS(CVT) = 1.7 x 10-18 s-1. Similar proton tunneling-induced rate constant enhancements for the self-catalyzed reaction are calculated; k2(CVT+SCT) at 300 K = 6.2 x 10-7 s-1, 5.93 times larger than k2(CVT) = 1.1 x 10-7 s-1, and k2(CVT+SCT) at 200 K = 5.1 x 10-15 s-1, 2 orders of magnitude larger than k2(CVT) = 4.1 x 10-17 s-1. However, the other bimolecular reactions are not appreciably impacted by tunneling in the tropospherically relevant temperature range.

Figure 4. Arrhenius plots of the CVT+SCT thermal rate constants for the gas-phase decomposition of methanediol with and without various catalysts from 200 to 300 K.

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Table 3. Calculated effective rate constants (keff) and tunneling factors (κ) for the uncatalyzed and catalyzed gas-phase decomposition of methanediol at different temperatures.

Temperature (K)

uncatalyzed -1

200 210 220 230 240 250 260 270 280 290 298.15 300 a

keff (s ) -28 7.2 x 10 -27 2.3 x 10 -27 7.7 x 10 -26 3.7 x 10 -25 3.4 x 10 -24 4.3 x 10 -23 5.6 x 10 -22 6.8 x 10 -21 7.4 x 10 -20 7.3 x 10 -19 4.3 x 10 -19 6.4 x 10

κ

H2O -1

9

1.5 x 10 7 2.1 x 10 5 5.3 x 10 4 2.9 x 10 3 4.3 x 10 3 1.2 x 10 2 4.9 x 10 2 2.3 x 10 2 1.3 x 10 1 7.6 x 10 1 5.3 x 10 1 4.9 x 10


HCOOH

κ 268.2 122.2 65.5 39.6 26.2 18.6 14.0 10.9 8.8 7.4 6.5 6.3

-1


a

HO2

κ 2.9 2.5 2.2 2.1 1.9 1.8 1.7 1.6 1.5 1.5 1.5 1.4

-1


κ 5.3 4.4 3.8 3.3 2.9 2.3 2.1 2.0 1.9 1.8 1.7 1.7

b

CH2(OH)2 -1


κ 3.5 2.9 2.6 2.3 2.1 1.9 1.8 1.7 1.6 1.5 1.5 1.4

We next examined the effective reaction rate constants (keff = k2Keq[cat]) for the bimolecular decompositions of methanediol taking into account the concentration of various catalysts. This provides quantitative insight into the effect of various catalysts on the methanediol decomposition. The k2 values corrected for proton-tunneling have been used in calculating the keff values. The estimated keff for H2O-, HCOOH-, and HO2mediated decompositions are several orders of magnitude larger compared to the uncatalyzed reaction at all temperatures considered. At 300 K, the keff for the H2Ocatalyzed decomposition of methanediol is 1.0 x 10-12 s-1, representing an enhancement in the decomposition rate constant by a factor of 1.6 x 106 as compared to the uncatalyzed decomposition (keff = 6.4 x 10-19 s-1). For the HCOOH-catalyzed reaction, keff at 300 K = 1.8 x 10-10 s-1, an increase of a factor of 2.8 x 108 over the uncatalyzed reaction (Table 1), and more importantly, 2 orders of magnitude over the H2O-catalyzed

15

-1


CH2(OH)2+CH2(OH)2 corresponds to a bimolecular reaction in which all four hydroxyl groups are hydrogen bonded in the prereaction complex. This is a non-catalytic reaction because both CH2(OH)2 molecules decompose. b CH2(OH)2+CH2(OH)2 corresponds to a bimolecular reaction in which three of four hydroxyl groups are hydrogen bonded in the prereaction complex.


CH2(OH)2

κ 124.1 65.2 39.9 26.8 19.3 14.6 11.5 9.4 7.9 6.8 6.0 5.9


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

reaction. At lower temperature, the catalytic effect of HCOOH was significantly higher than that of H2O. For example, at 200 K, the keff for HCOOH reaction was 5.0 x 10-13 s-1, which was 5 x 106 larger than that for the H2O reaction (keff = 1.0 x 10-19 s-1). Clearly, the HCOOH-catalyzed decomposition is faster than the water-catalyzed one under tropospheric conditions despite the fact that the HCOOH levels are 7 orders of magnitude lower than [H2O]. For the HO2 reaction, keff at 300 K = 2.3 x 10-13 s-1, which reflects a rate constant enhancement of 3.6 x 105 over the uncatalyzed reaction. The HO2 reaction was ~103 slower than the HCOOH reaction and ~101 slower than the H2O reaction at 300 K. However, at 270 K or lower temperatures, the rate of the HO2 reaction outcompetes that of the H2O reaction. At 200 K, the keff for the HO2 reaction was 1.3 x 10-16 s-1, reflecting 3 orders of magnitude enhancement over the H2O reaction. Overall, these results suggest that carboxylic acids and HO2 radical in the troposphere may play larger role than water in the diol decomposition especially at lower temperatures. Recent measurements over Boreal and tropical forests indicating that the actual formic acid levels are 2-3 times higher than commonly perceived,53 further support the larger role of organic acids in determining the fate of atmospheric chemistries under certain conditions. Carboxylic acids have recently also shown to influence the fate of Criegee intermediate,54,55 an important tropospheric species that is formed in the olefin ozonolysis. The keff for the self-catalyzed methanediol decomposition is lower than the H2O reaction at all temperatures, but becomes larger than that for the uncatalyzed reaction at 270 K or lower temperatures. At 200 K, the keff for the self-catalyzed reaction is 2.4 x 10-26 s-1, 33 times larger than the uncatalyzed reaction (keff = 7.2 x 10-28 s-1). This suggests that though the autocatalytic decomposition is going to be insignificant in troposphere where water or organic acids are present in significant amounts, but could play a role in certain conditions where the diols are present in large quantities.

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V. CONCLUSION In summary, canonical variational transition state theory has been used to gain insight into the tropospheric decomposition of methanediol. The results suggest that quantum mechanical tunneling plays a very important role in the reaction especially at atmospherically relevant low temperatures of 200-300 K. The results further suggest that formic acid and hydroperoxyl radical catalysis are thermodynamically and kinetically more efficient than water catalysis in impacting the alkanediol decomposition. The chemical equilibrium between formaldehyde and methanediol, which is dominantly shifted in favour of methanediol in aqueous-phase, would be shifted towards formaldehyde in troposphere, and diols in acidic environments would contribute towards the budget of carbonyl compounds. Considering that diols play versatile roles in various fields, these results may help in improving our fundamental understanding of diol chemistry.

ASSOCIATED CONTENT Supporting Information Available: Tables containing calculated equilibrium constants for various methanediol complexes, and rate constants (without tunneling correction) for the bimolecular dehydration of methanediol, and harmonic frequencies, rotational constants and geometries of all the optimized structures presented in this work. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGEMENT JMA thanks the Spanish Secretaria de Estado de Investigación, Desarrollo e Innovación (CTQ2014-59768-P), the Generalitat de Catalunya (Grant 2014SGR139), the Spanish Salvador de Madariaga grant (PRX15/00139) and the Fulbright for financial support. We thank Holland computing center of the University of Nebraska-Lincoln for providing computing resources.

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