HâX (X = H, CH3, CH2F, CHF2, CF3, and SiH3) Bond Activation by

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H-X (X=H, CH, CHF, CHF, CF, and SiH) Bond Activation by Criegee Intermediates: A Theoretical Perspective Manoj Kumar, and Joseph S. Francisco J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b10535 • Publication Date (Web): 21 Nov 2017 Downloaded from http://pubs.acs.org on November 22, 2017

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The Journal of Physical Chemistry

H-X (X=H, CH3, CH2F, CHF2, CF3, and SiH3) Bond Activation by Criegee Intermediates: A Theoretical Perspective

Manoj Kumar1 and Joseph S. Francisco1,*

1Department of Chemistry, University of Nebraska-Lincoln,

639 North 12 Street, Lincoln, Nebraska 68588, United States th

ABSTRACT

Using theoretical calculations and Born Oppenheimer molecular dynamics simulations, it is shown here that Criegee intermediate, which is principally produced in the olefin ozonolysis, can activate H-X (X = H, CH3, CH2F, CHF2, CF3, and SiH3) under mild conditions, a reaction that has long been known for transition metals. The zwitter ionic electronic structure of Criegee intermediate makes it an interesting metal-free system for activating enthalpically strong small molecules such as H2, methane, silanes, and boranes. The calculated barriers for the H2 or SiH4 reactions of CH2OO are significantly lower than those for the CH4 or its fluorinated analogue reactions. The distortion-interaction energy model is found to be successful in explaining the differential reactivity of Criegee intermediate towards activating the various H-X bonds. The canonical transition state theory calculations suggest that the CH2OO-H2 reaction is 9-11 orders of magnitude faster than the CH2OO-CH4 reaction over the 200-300 K temperature range. Considering

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that the laboratory synthesis of Criegee intermediate is now feasible, these findings may open up new vistas in the metal-free activation of small molecules.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Phone: 402-472-6262

INTRODUCTION The small molecule activation has garnered significant interest over the years. One such molecule is dihydrogen (H2), which is not only considered to be the fuel of the future, but is also a fundamental component in several industrial and biological processes.1 Most of systems known to activate H2 under ambient conditions involve transition metal-centered chemistry, where the bond activation is promoted by interaction of the s-bonding orbital of H2 with a vacant d orbital on the metal and through a back-donation from an occupied metal d orbital to the empty s* orbital of H2. Despite the fact that the transition metalcatalyzed hydrogen utilization is of tremendous economic and practical value, there are nonetheless certain limitations, for example many precious metals, such as platinum, can be environmentally unfriendly and challenging or economically nonviable to synthesize. Toxicity is another concern associated with transition metal complexes. This has led to

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significant interest in metal-free chemistries that do not require transition metals to activate and deliver hydrogen to substrates of interest.2 Main group element based compounds that cleave hydrogen are less common, but a growing body of literature,2 beginning with the seminal discoveries of Power and coworkers,3 suggest that main group elements can also cleave H2 via low-energy pathways. Power and co-workers have reported addition of H2 to digermanes and more recently, Stephan and co-workers have reported heterolytic splitting of H2 by phosphineborane species.4 These systems combine sterically hindered Lewis acid and Lewis base moieties and can be considered the analog of heterolytic cleavage of H2 at metal centers. More, recently, Bertrand and co-workers have also demonstrated that certain carbenes can activate H2 without a separate Lewis acid partner under mild conditions.5 Despite these recent advances, there exists hardly any metal free system that can activate a variety of H-X (X = H, CH3, CH2F, CHF2, CF3, and SiH3) bonds. Criegee intermediates are zwitter ionic/biradical species that mediate the ozonealkene reactions.6 Recently, the direct detection of the two simplest Criegee intermediates in gas-phase by Taatjes and co-workers has opened up new avenues for directly studying the reaction kinetics of uni- and bimolecular Criegee reactions.7,8 The presence of nucleophilic terminal oxygen and electrophilic central carbon in the Criegee intermediates (Figure 1) make them an interesting class of metal-free systems that can activate a variety of H-X bonds. The nucleophicity of terminal Criegee oxygen has been

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Figure 1. M06-2X/aug-cc-pVTZ calculated electrostati potential maps for CH2OO (extreme left), anti-CH3CHOO (middle), and syn-CH3CHOO (extreme right). The red and blue represents the positive and negative charges, respectively. shown to promote the hydrogen transfer chemistry in methyl-substituted Criegee intermediates, i.e., the tautomerization barriers for these systems were 30% lower compared to the conventional keto-enol systems.9,10 Though the potential of Criegee intermediate in the context of tropospheric chemistry is extensively explored, its plausible role in the small molecule activation has never been examined before.

COMPUTATIONAL DETAILS Electronic Structure Calculations. With a view to get an insight into the factors that dictate the activation of H-X bonds by Criegee intermediates, the electronic structure calculations on the reactions of Criegee intermediates with various small molecules including H2, CH4, CH3F, CH2F2, CHF3, and SiH4, are presented here. All quantum chemical calculations reported in this work were performed using Gaussian0911 software. In the first step, we examined the gas-phase reactions of the simpest Criegee intermediate, CH2OO with various HX coreactants (X = H, CH3, CH2F, CHF2, CF3, and SiH3):

CH2OO + H-X → (X)C(H2)OOH

(1)

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In the 2nd step, we investigated the gas-phase reactions of eleven different Criegee intermediates with H2: (R1)(R2)COO + H2 → (R1)(R2)C(H)OOH → (R1)(R2)CO + H2O

(2)

Various Criegee intermediates considered in the latter step are shown in Scheme 1.

O R2

C

O

R1

R1=Cl;R2=H R1=CH3;R2=CH3 R1=CH3;R2=H R1 = R2 = H R1=H;R2=CH3 R1=H;R2=Cl R1=H;R2=CF3 R1=H;R2=F R1=Cl;R2=Cl

Scheme 1. Various Criegee intermediates considered in this work.

The (R1)(R2)C(H)OOH adduct forming step of the (R1)(R2)COO + H2 reaction was also examined in the presence of an additional water molecule. The equilibrium and transition state structures were fully optimized using the M06-2X12 level of density functional theory and the augmented correlation-consistent triplet zeta basis set, aug-ccpVTZ.13 This level of theory has been previously shown to perform well for the hydrogen atom transfer-based unimolecular and bimolecular Criegee reactions.14 The presence of zero or single imaginary frequency was used to identify all the stationary points as minima or transition states. The authenticitiy of prereaction and postreaction complexes involved in these reactions was verified by performing intrinsic reaction coordinate calculations. The electrostatic potential maps for the C1 and C2 Criegee intermediates as

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well as the frontier orbitals of the transition state for the CH2OO + H2 reaction were also analyzed to gain deeper insights into the reaction mechanism. The natural bond orbital analysis was carried out to calculate the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the transition state for the CH2OO + H2 reaction.

Theoretical Kinetics Calculations. Computational kinetics calculations were carried out using programs in the Multiwell Program Suite.15-17 The rate constants for the bimolecular gas-phase reactions of (R1)(R2)COO Criegee intermediates were calculated using canonical transition state theory, in which tunneling corrections based on the 1-D unsymmetrical Eckart barrier were included. The rate constants were calculated assuming that the reaction follows a two-step mechanism as described by eq 3, where the prereactive complex Int is in equilibrium with (R1)(R2)COO and H-X, and the reaction 1

proceeds through the unimolecular isomerization of this complex. (3) In the steady state approximation, the forward rate constant can be approximated by k "# =

%& %'&

k ( = K +, k ( (4)

where K +, =

-(/& )(/1 )233∙∙56 7(/& )(/1 )233 756

'(92 '9/ ) /:

e

is the equilibrium constant for the formation of the

Int complex. The various Q denote the partition functions of the reactants and, the pre 1

reaction complex and k is the unimolecular rate constant for its isomerization as given in 2

equation 5, ‡

k ( = G𝐿 ×

>? "

‡ -:A

@ -(/& )(/1 )233∙∙56

‡ '(D9B ) C? :

e

(5)

6


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where G is the quantum mechanical tunneling correction and L is the reaction path ‡

degeneracy. The factor k is Boltzmann’s constant, DEE‡ is the zero-point corrected energy B

‡ difference between the transition state and the prereaction complex. 𝑄"# and

𝑄(G& )(G1 )HII∙∙JK are the total partition functions for the transition state and the reaction complex, respectively, referenced to their respective zero-point energies. 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 were used for estimating the rate constants for all Criegee reactions studied here. Nonlinear reactants and transition states were approximated as symmetric tops. Each symmetric top possesses a degenerate two-dimensional rotation (the 2-D adiabatic “j-rotor”) and a onedimensional rotor (the “k-rotor”). For nonsymmetric molecules, the rotational constants for the j-rotor is approximated as the geometric mean of the two rotational constants that are most similar, in the conventional manner. The k-rotor is assumed to exchange energy 18

freely with the vibrational modes, but it also obeys the requirements that the quantum number K is limited to the range from –J to +J and that all rotational energies are ³ 0.

Ab Initio Molecular Dynamics Simulations. In the final step, we performed Born Oppenheimer molecular dynamics (BOMD) simulations to gain further insights into the mechanism of CH3OOH production from the CH2OO + H2 reaction in the absence and presence of one water molecule. The BOMD simulations were started from the transition states for the gas-phase CH2OO + H2 and CH2OO + H2 + H2O reactions, respectively. The BOMD simulations were performed based on a density functional theory (DFT) method implemented in the CP2K19 code. The dimension of the simulation box is x = 30 Å, y =

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30 Å, z = 30 Å, which is large enough to neglect interactions between adjacent periodic images of the system. In the BOMD simulations, the DFT method, in which the exchange and correlation interaction is treated with the Becke-Lee-Yang-Parr (BLYP) functional20,21 was used. The Grimme’s dispersion correction method is applied to account for the weak dispersion interaction22,23. A double-ζ Gaussian basis set combined with an auxiliary basis set and the Goedecker-Teter-Hutter (GTH) norm-conserved pseudopotentials were adopted to treat the valence electrons and the core electrons, respectively24,25. An energy cutoff of 280 Rydberg was set for the plane-wave basis set and 40 Rydberg for the Gaussian basis set. The BOMD simulations were carried out in the constant volume and temperature (NVT) ensemble, with the Nose-Hoover chain method for controlling the temperature (300 K) of the system. The integration step was set as 0.2 fs, which had been proven to achieve sufficient energy conservation for Criegee systems.26

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Figure 2. CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ calculated reaction profiles for the gas-phase reaction between the simplest Criegee intermediate, CH2OO and H-X (X = H, CH3, CH2F, CHF2, CF3, and SiH3). The energies are referenced to separated reactants and are given in kcal/mol units at 298.15 K and 1 atm. RESULTS AND DISCUSSION The CH2OO-HX Addition Reaction. Among all HX coreactants, the highest barriers for the reactions of CH4 and its flouro-substituted analogues are calculated, i.e., the CH2OO-CH4 reaction has an effective barrier of 26.0 kcal/mol whereas the CH2OOCH3F, CH2OO-CH2F2, and CH2OO-CHF3 reactions have higher effective barriers of 27.8, 31.5, and 29.6 kcal/mol, respectively, relative to their corresponding prereaction complexes. The CH2OO-SiH4 reaction has a relatively lower barrier of 14.9 kcal/mol. The barrier for the CH2OO-H2 reaction is 13.4 kcal/mol, which is even lower than that for the CH2OO-SiH4 reaction. These barrier trends cannot be solely explained on the basis of

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bond energies of H-H, H-SiH3, and H-CH3, i.e., the experimental bond energies for H-H, H-SiH3, and H-CH3 are 104.2, 90.3, and 104.8 kcal/mol,27 respectively, which implies that H2 and CH4 reactions should have similar barriers, whereas the SiH4 reaction should have the lowest barrier, which, however, is not the case. The most dramatic aspect of these results is the very high barrier predicted for C-H bond activation of CH4. H2 with comparable bond energy has a barrier that is ~49% lower than that for CH4. Clearly, the thermodynamics for the CH2OO-HX reactions is controlled by other factors. Similar barrier trends have been noticed in the heterolytic activation of H-H, H-SiH3, H-BH2.PH3, and

H-CH3

by

highly

electrophilic,

coordinatively

unsaturated,

16-electron

[Ru(P(OH)3(Ph2PCH2CH2PPh2)2][OTf]2.28 A better understanding of the factors that impact the H-X activation by Criegee intermediate is, thus, of broad appeal.

Table 1. M06-2X/aug-cc-pVTZ calculated barrier heights, distortion and interaction energies for reactions of CH2OO with H-X. The zero-point-corrected electronic energy values are given in kcal/mol units at 298.15 K and 1 atm.

X H SiH3 CH3 CH2F CHF2 CF3

∆E‡ 12.9 15.6 24.2 26.4 31.2 28.9

∆Ed‡ 13.2 20.1 30.5 37.5 45.9 53.1

∆Ei‡ -0.3 -4.5 -6.3 -11.1 -14.7 -24.2

Distortion-Interaction Model Analysis. To gain fundamental insights into these reactivity trends, we next examined the energetics of these reactions within the

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framework of the distortion-interaction model29. According to this model, the activation energy (∆E‡) for a given bimolecular reaction can be decomposed into the distortion energy (∆Ed‡) and the energy of interaction (∆Ei‡) between the distorted reactants in the transition state. For these CH2OO-H-X reactions, the distortion energy is the energy required to distort CH2OO and H-X in the prereaction complexes into the geometries they have at the transition state without allowing any interaction between them. The activation energy or barrier height in this model, ∆E‡, is then given by ∆E‡ = ∆Ed‡ + ∆Ei‡, where E is the ZPE un-corrected electronic energy. Table 1 contains the M06-2X/aug-cc-pVTZ calculated barrier heights, transition state distortion, and transition state interaction energies, for the CH2OO-HX reactions. We find that both the calculated distortion energy and interaction energy increase in going from H2 → SiH4 → CH4 → CHF3. However, the increase in distortion energy outcompetes the increase in interaction energy along the series and becomes the crucial factor in determining the barrier of the reaction. Interestingly, there is a direct correlation between the barrier height and the distortion energy, i.e., larger the distortion energy, the larger the barrier for a given CH2OO-HX reaction. Figure 3 is a plot of ∆Ed‡ versus ∆E‡ for the seven reactions studied. This linear correlation was quite surprising, since many reactions give no general correlation between rates and distortion energies,29 and indicates that a large barrier is the result of a large distortion energy at the transition state. The distortion energy involves simultaneous geometric and electronic change and is different from Marcus’s reorganization energy, which relates to the energy of the electronic structure of the product in the reactant geometry, rather than the transition state geometry, where the distortion energy is measured. Distortion energy is typically neglected in qualitative frontier molecular orbital

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treatments and is independent of thermodynamic effects. Electron withdrawing groups substituted on CH4 simultaneously increase the barrier height and transition state

Figure 3. Plot of M06-2X/aug-cc-pVTZ calculated total distortion energies versus barrier heights for the gas-phase reaction of the simplest Criegee intermediate, CH2OO with H-X (X = H, CH3, SiH3, CH2F, CHF2, and CF3). The energies are given in kcal/mol units at 298.15 K and 1 atm. distortion energy. The calculated barrier heights for various HX span a wide range of 12.9-31.2 kcal/mol, which supports the generality of the ∆E‡ = f(∆Ed‡) relationship for the bimolecular Criegee addition reactions. This explains why the CH2OO-H2 reaction has lower barrier than CH2OO-SiH4 or CH2OO-CH4. Overall, the distortion-interaction model provides a novel mechanistic framework for understanding the H-X bond activation by Criegee intermediate. Considering that a complete understanding of the factors that influence the H- X activation by transition metal complexes continues to

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remain a significant challenge,28 the distortion-interaction model may also provide a different perspective to understand those missing mechanistic links.

Computational kinetics of the CH2OO-HX reactions. To gain deeper insight into the H-X activation by Criegee intermediate, we next calculated the rate constants for the bimolecular Criegee reactions employing the canonical transition state theory in combination with the Eckart tunneling correction. The calculated rate constants are given in Table 2. For detailed information on the calculated kinetic results, see Tables S1-S5. The calculated rate constants suggest that the CH2OO-H2 and CH2OO-SiH4 reactions are significantly faster than the CH2OO-CH4 or other reactions. Specifically, the calculated rate constants for the CH2OO-H2 and CH2OO-SiH4 reactions at 200 K and 300 K are 812 and 9-11 orders of magnitude larger than the other reactions, respectively. This is consistent with the fact that the calculated barriers for the CH2OO-H2 and CH2OO-SiH4 reactions are nearly half of those for the other reactions. Overall, the CH2OO-H2 reaction is the fastest reaction whereas the CH2OO-CH2F2 and CH2OO-CHF3 are the two slowest reactions. At 300 K, the rate constant for the CH2OO-H2 reaction is 4.4 x 10-22 s-1, which is 2.5 times larger than that for the CH2OO-SiH4 reaction (1.6 x 10-22 s-1) and nearly 9 orders of magnitude larger than that for the CH2OO-CH4 reaction (1.3 x 10-31 s-1). The rate constants for the CH2OO-CH2F2 and CH2OO-CHF3 reactions at 300 K are 1.7 x 10-33 s-1 and 7.7 x 10-33 s-1, respectively, which are nearly 2 orders of magnitude smaller than that for the CH2OO-CH4 (1.3 x 10-31 s-1) or CH2OO-CH3F reaction (5.3 x 10-31 s-1). The rate constants of all reactions increase with the increase in temperature, exhibiting a positive temperature dependence. As the temperature is increased from 200 to 300 K, the

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rate constants of these reactions are increased by 4-6 orders of magnitude. The rate constants for the CH2OO-CH4 reaction are increased the most (by 6 orders of magnitude) with temperature whereas those of the CH2OO-CH3F are increased the least (by 3 orders of magnitude) with temperature. The rate constants of CH2OO-H2 and CH2OO-SiH4 reactions are increased by nearly 4 orders of magnitude in going from 200 to 300 K temperature.

Table 2. CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ calculated rate constants for the reaction of CH OO with CH , CH F, CH F , CHF , H , and SiH . 2

4

Temperature (K)

CH4

200 210 220 230 240 250 260 270 280 290 300

8.2 x 10 2.4 x 10-36 7.3 x 10-36 2.3 x 10-35 7.7 x 10-35 2.7 x 10-34 9.3 x 10-34 3.3 x 10-33 1.1 x 10-32 3.9 x 10-32 1.3 x 10-31

-37

3

2

CH3F

-34

8.0 x 10 1.2 x 10-33 1.9 x 10-33 3.3 x 10-33 5.9 x 10-33 1.1 x 10-32 2.3 x 10-32 4.7 x 10-32 1.0 x 10-31 2.3 x 10-31 5.3 x 10-31

2

3

2

4

kTS (s-1) CH2F2 CHF3 -37

4.1 x 10 6.9 x 10-37 1.3 x 10-36 2.6 x 10-36 5.5 x 10-36 1.3 x 10-35 3.1 x 10-35 8.0 x 10-35 2.1 x 10-34 5.9 x 10-34 1.7 x 10-33

14

-38

2.5 x 10 6.9 x 10-38 2.1 x 10-37 7.3 x 10-37 2.7 x 10-36 1.0 x 10-35 4.0 x 10-35 1.6 x 10-34 6. 0 x 10-34 2.2 x 10-33 7.7 x 10-33


H2

-26

6.2 x 10 1.9 x 10-25 5.3 x 10-25 1.5 x 10-24 3.8 x 10-24 9.4 x 10-24 2.2 x 10-23 5.0 x 10-23 1.1 x 10-22 2.2 x 10-22 4.4 x 10-22

SiH4

3.2 x 10-26 8.8 x 10-26 2.4 x 10-25 6.2 x 10-25 1.5 x 10-24 3.7 x 10-24 8.5 x 10-24 1.9 x 10-23 3.9 x 10-23 8.0 x 10-23 1.6 x 10-22

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Figure 4. M06-2X/aug-cc-pVTZ calculated electrostatic map, HOMO, and LUMO of the transition state for the CH2OO-H2 reaction.

Energetics of the (R1)(R2)COO-H2 Reaction. We next examined the gas-phase Criegee-H2 reaction in greater detail. The reaction involves the heterolytic cleavage of HH bond, which results in the proton transfer to the terminal oxygen and hydride transfer to the central carbon of Criegee intermediate. The calculated electrostatic map and the frontier orbitals for the transtion state of the CH2OO-H2 reaction clearly supports the heterolytic activation mechanism (Figure 4). The effect of anti and syn substituents on the reaction was evaluated by considering Cl, F, CF3, CH3, and -CH=CH2 substituents. The H2 reactions of anti substiuented Criegee intermediates have 12.3-12.9 kcal/mol barriers, that are atleast ~6.0 kcal/mol lower than their syn substituted analogues (17.818.6 kcal/mol, Figure 5). This is consistent with prior reports30-32 suggesting that antisubstituted Criegee intermediates are generally more reactive towards bimolecular addition reactions than the syn-substituted ones. The electron withdrawing substiuents (F, Cl, CF3) at anti or syn tends to cause a noticeable lowering in the barrier of the reaction compared to electron releasing substituents (CH3). The lowest barrier of 12.3 kcal/mol is calculated for the anti-CHFOO-H2 reaction. The exothermicity of the Criegee-H2 reaction is also impacted by the nature of substitution, i.e., the electron withdrawing substituents

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Figure 5. CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ calculated reaction profiles (kcal/mol, 298.15 K, 1 atm) for the reaction of various Criegee intermediates with H2 in the gas-phase. results in relatively more stable adducts, at least by 3.5 kcal/mol compared to electron releasing ones. The adduct for the anti-CF3CHOO-H2 reaction was the most stable (∆E = -61.2 kcal/mol), 5.8 kcal/mol less energetic than that for the CH2OO-H2 reaction. We also examined the effect of water mediation on the Criegee-H2 reaction. Though the watermediated reaction has ~5.0-9.0 kcal/mol larger barrier than that without water (Figure 6), the reaction is now mediated by prereaction and postreaction complexes that are more stable than separated reactants and products, respectively. This may provide an efficient

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channel for achieving the Criegee-H2 reaction. The water-mediated H2 reaction is less likely to occur in the troposphere, but this reaction could be utilized in the laboratory conditions to access carbonyl compounds.

Figure 6. CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ calculated reaction profiles (kcal/mol, 298.15 K, 1 atm) for the gas-phase reaction of various Criegee intermediates with H2 in the presence of one water molecule. The (R1)(R2)CHOOH adduct formed from the Criegee-H2 reaction could decompose further into carbonyl compounds that have broad utility as value-added chemicals. The reaction involves dehydration of adduct with a large barrier of ~44.0-50.0

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kcal/mol (Figure 5). This barrier slightly varies with the nature of substitution on the Criegee carbon. Since the adducts from the reactions of anti Criegee intermediates with H2 are formed with an excess energy of atleast 52.1 kcal/mol, this barrier should be surmountable atleast for the reactions of anti substituted Criegee intermediates. Overall, the (R1)(R2)COO + H2 → (R1)(R2)CO + H2O reaction is

at least 110.7 kcal/mol

exothermic. As far as we know, this is the first report suggesting the usefullness of Criegee chemistry beyond troposphere. Overall, the results presented here show that Criegee intermediates are an interesting class of metal-free systems that could be explored for the activation of small molecules, which is an active area of research in solar energy conversion. Computational kinetics of the (R1)(R2)COO-H2 reactions. We next calculated the rate constants for the (R1)(R2)COO-H2 reactions to gain deeper insights into the reactivity of various Criegee intermediates towards the H2 activation. The calculated rate constants are collected in Table 3. For more detailed information into the calculated kinetic data, see Tables S5-S14. Just like the CH2OO-HX reactions, the (R1)(R2)COO-H2 reactions also exhibit positive temperature dependence, i.e., the rate constants for these reactions are enhanced by 3-6 orders of magnitude in going from 200 to 300 K temperature. Compared to the CH2OO-H2 reaction, the rate constants for the reactions of anti-substituted Criegee intermediates are increased by 1-2 orders of magnitude whereas those for the reactions of syn-substituted Criegee intermediates are decreased by 3-5 orders of magnitude. The rate constants for the (CH3)2COO-H2 reaction are 4-5 orders of magnitude smaller than those for the CH2OO-H2 reaction whereas those for the CCl2OOH2 reaction are quite comparable to those for the CH2OO-H2 reaction. Among all the

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reactions, the anti-CH3CHOO-H2 reaction is predicted to be the fastest whereas the (CH3)2COO-H2 reaction is predicted to be the slowest reaction at all temperatures. At 300 K, the rate constant for the anti-CH3CHOO-H2 reaction is 7.9 x 10-21 s-1, which is nearly an order of magnitude larger than that for the CH2OO-H2 reaction (4.4 x 10-22 s-1) and nearly 3 orders of magnitude larger for the syn-CH3CHOO-H2 reaction (1.5 x 10-24 s-1). The rate constant for the (CH3)2COO-H2 reaction at 300 K is 2.3 x 10-26 s-1, which is 4 and 5 orders of magnitude smaller than that for the CH2OO-H2 and anti-CH3CHOO-H2 reactions, respectively.

Table 3. CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ calculated rate constants for the reaction of various Criegee intermediates with H . 2

Temperature (K) 200 210 220 230 240 250 260 270 280 290 300

CH2OO

-26

6.2 x 10 -25 1.9 x 10 -25 5.3 x 10 -24 1.5 x 10 -24 3.8 x 10 -24 9.4 x 10 -23 2.2 x 10 -23 5.0 x 10 -22 1.1 x 10 -22 2.2 x 10 -22 4.4 x 10

anti-CH3CHOO -24

6.8 x 10 -23 1.5 x 10 -23 3.5 x 10 -23 7.6 x 10 -22 1.6 x 10 -22 3.4 x 10 -22 6.8 x 10 -21 1.3 x 10 -21 2.5 x 10 -21 4.5 x 10 -21 7.9 x 10

anti-CHClOO

anti-CHFOO

7.5 x 10 -25 2.2 x 10 -25 6.1 x 10 -24 1.6 x 10 -24 4.1 x 10 -24 9.9 x 10 -23 2.3 x 10 -23 5.0 x 10 -22 1.0 x 10 -22 2.1 x 10 -22 4.0 x 10

4.7 x 10 -24 1.3 x 10 -24 3.7 x 10 -24 9.6 x 10 -23 2.4 x 10 -23 5.6 x 10 -22 1.2 x 10 -22 2.6 x 10 -22 5.4 x 10 -21 1.0 x 10 -21 2.0 x 10

-26

-25

19

kTS (s-1) anti-CF3CHOO -25

1.7 x 10 -25 5.3 x 10 -24 1.6 x 10 -24 4.4 x 10 -23 1.2 x 10 -23 2.9 x 10 -23 6.8 x 10 -22 1.5 x 10 -22 3.2 x 10 -22 6.4 x 10 -21 1.2 x 10


syn-CH3CHOO -30

5.9 x 10 -29 2.6 x 10 -28 1.1 x 10 -28 4.5 x 10 -27 1.7 x 10 -27 6.3 x 10 -26 2.2 x 10 -26 6.9 x 10 -25 2.0 x 10 -25 5.7 x 10 -24 1.5 x 10

syn-CHClOO -31

4.1 x 10 -30 2.1 x 10 -30 9.7 x 10 -29 4.2 x 10 -28 1.7 x 10 -28 6.3 x 10 -27 2.2 x 10 -27 6.8 x 10 -26 2.0 x 10 -26 5.6 x 10 -25 1.5 x 10

(CH3)2COO -31

1.3 x 10 -31 5.1 x 10 -30 2.0 x 10 -30 7.5 x 10 -29 2.8 x 10 -29 9.8 x 10 -28 3.3 x 10 -27 1.0 x 10 -27 3.1 x 10 -27 8.7 x 10 -26 2.3 x 10

CCl 2OO

-26

1.1 x 10 -26 4.2 x 10 -25 1.5 x 10 -25 5.5 x 10 -24 1.9 x 10 -24 5.9 x 10 -23 1.8 x 10 -23 4.9 x 10 -22 1.3 x 10 -22 3.2 x 10 -22 7.6 x 10


Figure 7. Snapshots of representative gas-phase BOMD trajectories starting from the transition state for the CH2OO-H2 reaction to CH3OOH adduct in the absence (Figure 7a) and presence (Figure 7b) of a water molecule.

The BOMD Simulations. We also performed gas-phase BOMD simulations to gain deeper insight into the dynamics of the CH2OO-H2 reaction in the absence and presence of a water molecule. The BOMD simulations were initiated from the transition states and representative trajectories were analyzed. Snapshots of typical trajectories for both reactions are illustrated in Figures 7a and 7b. From inspection of the sample trajectory starting at the transition state, one can see that the addition of heterolytically cleaving H2 across the Criegee COO moeity is quickly achieved at 90 fs, resulting in the formation of CH3OOH (Figure 7a). The large internal energy of CH3OOH is randomly distributed in different vibrational modes, and the hydroperoxy proton of CH3OOH also undergoes internal rotation, as indicated at 0.20-0.48 ps. Over the simulated time scale of 15 ps, CH3OOH remains dynamically stable. For the water-mediated reaction, the H2 addition across the Criegee COO moeity is complete at 65 fs (Figure 7b). However, the

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internal rotation of hydroperoxy proton across the O-O axis occurs relatively slowly at 0.48-1.33 ps. The water molecule remains hydrogen-bonded to CH3OOH during the entire simulation time. The time scale of these Criegee events is quite similar to the recently reported other uni- and bimolecular Criegee reactions.26,33-35 IMPLICATIONS There is strong interest in catalyst systems that can activate small molecules such as hydrogen, methane, silane and so on. Our results suggest that Criegee intermediate could be an interesting metal-free system that can activate small molecules in an environmentally benign manner. Though the Criegee intermediates are usually very reactive, the judicious substitution at the central carbon atom could be exploited to control their reactivity. For example, the canonical transition state theory calculations suggest that the rate constants for the H2 reaction of syn-CH3CHOO and (CH3)2COO are 2-4 and 4-5 orders of magnitude smaller than those for the CH2OO-H2 reaction in the 200-300 K temperature range. The aqueous medium provides another handle for performing the Criegee intermediate-induced small molecule activation in a controlled fashion as the barriers for the water-mediated Criegee-H2 reactions are larger than those without them and the reaction is mediated by prereaction and postreaction complexes that will ensure the efficient release of products. It is important to mention here that the reactions of C1 and C2-based Criegee intermediates with small molecules in aqueous media may not be possible due to strong competition from the Criegee-water reactions. However, larger Criegee intermediates such as (CH3)2COO may react with H2 or any other small molecule in an aqueous media because their water reactions are relatively slower36. Finally, our calculations suggest that the distortion interaction model can be

21



successfully applied to understand the preferentially activation of a particular small molecule by a Criegee intermediate. The insights from our calculations could be used to rationalize the selective activation of a given small molecule by any other catalyst system. CONCLUSIONS Quantum chemical calculations and BOMD simulations show that Criegee intermediates constitute an interesting class of metal-free systems that activate H2 and thus, are of interest for their potential in H2 storage applications and in small molecules activation. The canonical transition state theory calculations suggest that the rate constants for the CH2OO-H2 reaction are 9-11 orders of magnitude larger than those for the CH2OO-CH4 reaction. Among various Criegee intermediates considered, the H2 reaction of antiCH3CHOO is found to be the fastest as their rate constants are 1-2 orders of magnitude larger than those for the CH2OO-H2 reaction in the 200-300 K temperature range. The use of water as a medium may not only be advantageous, but make the reaction environmentally benign. These findings may open up new vistas in the small-molecule activation via metal-free reactions and catalysis.

ASSOCIATED CONTENT Supporting Information Available: Tables containing detailed kinetic information on the (R1)(R2)COO-HX reactions. The Supporting Information is available free of charge on the ACS Publications website.

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