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Potential Energy Surfaces for the Reactions of HO2 Radical with CH2O and HO2 in CO2 Environment Artem E. Masunov, Arseniy Alekseyevich Atlanov, and Subith S Vasu J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b07257 • Publication Date (Web): 23 Aug 2016 Downloaded from http://pubs.acs.org on August 24, 2016

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The Journal of Physical Chemistry

Potential Energy Surfaces for the Reactions of HO2 Radical with CH2O and HO2 in CO2 Environment Artëm E. Masunov,*1,2,3,4 Arseniy A. Atlanov,1,5 Subith S. Vasu6

1

NanoScienece Technology Center, 2Department of Chemistry, 3Department of Physics, University of Central Florida, 12424 Research Parkway, Ste 400, Orlando, Florida 32826, USA 4 National Research Nuclear University MEPhI, Kashirskoye shosse 31, Moscow, 115409, Russia 5 Department of Chemistry and Biochemistry, 95 Chieftan Way Rm. 118 DLC, Florida State University, Tallahassee, Florida, 32806, USA 6 Center for Advanced Turbomachinery and Energy Research (CATER), Mechanical and Aerospace Engineering University of Central Florida, Orlando, Florida, 32816, USA

Abstract. We report on potential energies for the transition states, reactant and product complexes along the reaction pathways for hydrogen transfer reactions to hydroperoxyl radical from

formaldehyde

H2CO+HO2→HCO+H2O2

and

another

hydroperoxyl

radical

2HO2→H2O2+O2 in the presence of one carbon dioxide molecule. Both covalently bonded intermediates and weak intermolecular complexes are identified and characterized. We found that reactions that involve covalent intermediates have substantially higher activation barriers and are not likely to play role in hydrogen transfer kinetics. The van der Waals complexation with carbon dioxide does not affect hydrogen transfer from formaldehyde, but it lowers the barrier for hydroperoxyl self-reaction by nearly 3 kcal/mol. This indicates that CO2 environment is likely to have catalytic effect on HO2 self-reaction, which needs to be included in kinetic combustion mechanisms in supercritical CO2.

* To whom correspondence should be addressed, Phone: 1-407-374-3783; E-mail: [email protected]

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1. Introduction Carbon capture and sequestration is a technology that can capture up to the carbon dioxide (CO2) emissions produced from the fossil fuels combustion.1 One of the challenges for this technology is the separation of flue gases, which can be simplified when nitrogen is removed from the oxidant. This is known as oxycombustion. In order to control the combustion temperature, the CO2 can be used as diluent.2 The ability of CO2 to reach a supercritical state and hence increase in efficiency of the turbine is another advantage of oxycombustion.3, 4 However, the published studies on the diluent CO2 affecting combustion kinetics are rather limited,5 and computational chemistry methods6,7,8,9 can greatly assist in this understanding. In the first paper of this series10 we investigated effect of additional CO2 molecule on the mechanism of CO2 formation from CO and OH. We found a new, lower activation energy pathway that involves intermediates where CO2 is covalently bonded. Here we report the study of two other reactions, critically important in combustion, HO2• + CH2O → H2O2 + CHO•, and HO2• + HO2• → H2O2 + O2. Potential energy surfaces (PES) of several reaction pathways derived from the system HO2+CH2O were previously reported by Anglada and Domingo.11 They found two transition state structures for hydrogen abstraction from formaldehyde (cyclic 2TS11, and linear 2TS12), connecting two reactant complexes (CR) to one product complex (CP). Their relative energies, along with reactant and product complexes are shown in Table 1 and Figure 1af.


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Table 1. Relative energies (kcal/mol) for the critical points on HO2•+CH2O→HCO•+H2O2 reaction (origin is set at isolated reactants, H2CO+HO2). Stationary points on Figure 1a-c ΔE+ZPE

a

2

CR11 -7.4

2

TS11 12.4

2

CP11

Ea

-1.2

19.8

PES

for

M11D3+ZPE -9.6 10.2 -1.6 19.8 CCSD(T)+ZPE -6.9 14.0 -1.9 20.9 2 2 2 Stationary points on Figure 1d-f CR12 TS12 CP12 Ea a ΔE+ZPE -1.6 12.8 14.4 M11D3+ZPE -2.2 10.9 -3.3 13.1 CCSD(T)+ZPE -1.0 13.9 -0.9 14.9 Isolated reactants and products H2CO+HO2 HCO+H2O2 a ΔE+ZPE 0.0 1.2 M11D3+ZPE 0.0 1.9 CCSD(T)+ZPE 0 1.0 a) CCSD(T)/aug-cc-pVTZ//QCISD/6-311+G(d,p), ZPE: B3LYP/6-311+G(2df,2p), Ref.11

The mechanisms of self-reaction for two hydroperoxyl radicals were reported by several authors (Table 2, Figure 2a-r).12,13,14 Zhang et al.12 studied both triplet and singlet potential surfaces and found two transition states for hydrogen abstraction by one hydroperoxyl from another one: linear (3TS21 and 1TS23) and cyclic (3TS22 and 1TS24). The singlet potential surfaces had activation energies (8.4 and 13.1 kcal/mol) comparable to those for triplet surfaces (8.4 and 8.7 kcal/mol). Hydrogen tetroxide, H2O4, was proposed as a possible intermediate 1IN25 in a hydroperoxyl self-reaction by Anglada et al.13,14 This intermediate was found to be 5 kcal/mol more stable than H-bonded dimers of hydroperoxyl radicals 1CR25,14 and had a low (7.3 kcal/mol) barrier for its formation via 1TS25.13 However, its rearrangement into the products proceeds through a strained four-membered ring transition state structure 1TS27 with activation barrier of 45.2 kcal/mol.13 This is unfavorable when compared with the hydrogen atom transfer mechanism 1TS24. Finally, the role of additional water molecule(s) on the creation of new reaction pathways was investigated14,15 and their catalytic effect was reported. Here will focus on 3 ACS Paragon Plus Environment


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the role of additional carbon dioxide molecules. We study three types of such effects: 1) formation of van der Waals (or H-bonded) complexes; 2) hydrogen transfer mediated by CO2; and 3) covalent bonding between the reactive radical and CO2.

Table 2. Relative energies (kcal/mol) for the critical points on PES for 2HO2•→H2O2+O2 reaction (origin is set at isolated reactants, 2HO2). Stationary points on Figure 2a-c

3

CR21

3

TS21

3

CP21

Ea

ΔE+ZPEa -4.3 4.1 -38.0 8.4 M11D3+ZPE -5.3 1.7 -35.1 7.0 CCSD(T) +ZPE -4.1 4.5 -37.9 8.6 3 3 3 Stationary points on Figure 2d-f CR22 TS22 CP22 Ea ΔE+ZPEa -9.3 -0.6 -38.0 8.7 M11D3+ZPE -11.8 -3.2 -35.3 8.6 CCSD(T) +ZPE -9.0 0.1 -38.1 9.2 1 1 1 Stationary points on Figure 2g-i CR23 TS23 CP23 Ea a ΔE+ZPE 0.4 13.5 -8.8 13.1 M11D3+ZPE -6.0 4.4 -4.3 10.4 CCSD(T)+ZPE -4.0 8.0 -4.0 12.1 1 1 1 Stationary points on Figure 2j-l CR24 TS24 CP24 Ea a ΔE+ZPE 0.4 8.8 -8.8 8.4 M11D3+ZPE -11.9 -0.5 -4.1 11.4 CCSD(T)+ZPE -9.1 3.7 -26.9 12.8 1 1 1 Stationary points on Figure 2m-o CR25 TS25 CP25 Ea ΔE+ZPEb -9.1 -1.8 -6.7 7.3 M11D3+ZPE -11.9 3.0 -9.4 14.9 CCSD(T) +ZPE -9.1 -4.7 -10.7 4.4 1 1 1 Stationary points on Figure 2p-r CR26 TS26 CP26 Ea M11D3+ZPE -10.3 5.5 -6.0 15.8 CCSD(T)+ZPE -4.1 4.5 -11.3 8.5 1 1 1 Stationary points on Figure 2s-u CR27 TS27 CP27 Ea ΔE+ZPEb -7.6 37.6 -16.2 45.2 M11D3+ZPE -10.3 37.2 -3.7 47.5 CCSD(T) +ZPE -11.5 18.6 -26.6 30.1 3 1 Isolated reactants and products HO2+HO2 H2O2+ O2 H2O2+ O2 a ΔE+ZPE 0.0 -38.0 -8.8 M11D3+ZPE 0.0 -33.6 -1.3 CCSD(T)+ZPE 0 -37.5 -6.7 a) CCSD(T)/6-311++G(3d,2p)//CCSD/6-31+G(d,p), ZPE: CCSD/6-31+G(d,p), Ref.12 b) CASPT2/6-311G(3df,2p), ZPE: CASSCF/6-311G(3df,2p), Ref.13 4 ACS Paragon Plus Environment

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2. Computational Details All calculations were performed with the Gaussian 2009 suite of programs.16 MOLDEN was used to analyze the results.17 Density Functional Theory (DFT) was shown to be very useful in

a several areas of chemistry,18, 19, 20, 21, 22 providing an excellent accuracy to computational cost ratio.23,

24, 25, 26

In this work all geometry optimizations and normal mode analysis were

performed using DFT with M11 exchange-correlation functional.27 This functional belongs to the class of meta-GGA hybrids, where larger fraction of Hartree-Fock exchange ensures the accurate description of the transition states, while dependence on the kinetic energy density recovers accurate description of the stable molecular configurations. Unlike older meta-GGA hybrids, M11 includes range separation, so that fraction of Hartree-Fock exchange varies from 42.8% at short interelectron distances to 100% at the asymptotic limit. M11 was optimized for across-theboard superior performance (including main-group atomization energies, barrier heights, and noncovalent interaction energies). However, long-range dispersion is still missing in M11 functional.28 For this reason, we used Grimme’s 3-body dispersion correction (GD3)29 with S8 and SR6 parameters chosen to be S8=0.0 and SR6=1.619 (values, optimized for a similar M062X functional). The 6-311G(d,p) basis set30 was selected for compatibility with CBS-QB3 model chemistry.31 Thereafter, we will call this theory level M11D3. In addition, the relative energies of stationary points were refined with single point calculation at CCSD(T)/6-311++G(3d,2p) theory level. Zero-point vibrational energy (ZPE) correction obtained in vibrational analysis at M11D3 level was added to M11D3 and CCSD(T) energies as well. In order to describe the open shell electronic structure of diradicals correctly, DFT selfconsistent field procedure needs a guess with broken spin symmetry. In order to generate such a guess, we tried mixing HOMO and LUMO (Guess=Mix) and found it to be insufficient in some 5 ACS Paragon Plus Environment


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cases. As a more robust alternative, we prepared open shell guess using stability checking option in Gaussian (Stable=Opt) for geometry with elongated covalent bond(s). The reaction pathway search was using the following protocol: 1) optimization of the product/reactant van der Waals complex; 2) relaxed potential energy scan along selected bond formation/breaking; 3) transition state (TS) optimization starting form the highest point on that scan; 4) frequency analysis for identification of that TS as first-order critical point; 5) Intrinsic Reaction Coordinate32 (IRC) search for two (forward and backward) minimum energy pathways (MEP) starting from the transition state; 6) optimization into the local minima to continue both MEPs found in step 5), then repeat step 2) above. 3. Results and Discussion 3.1 Method validation In order to validate the exchange-correlation functional M11D3+ZPE model chemistry, we compared our predictions of the relative energies for the gas-phase complexes and transition states to the literature values in Tables 1 and 2. Geometries of the stationary points optimized with M11D3 method are reported in Table S1 in the Supporting Information and illustrated in Figures 1 and 2. Both relative energies and activation barriers predicted for hydrogen abstraction form formaldehyde by hydroperoxyl all agree with the coupled cluster values to within 2 kcal/mol. In addition, we found reaction pathway leading from 2TS12 to the new complex of products (2CP12), while the original study11 reported 2CP11 in its place. Critical points on triplet potential surface are also described well (within 2.5 kcal/mol). The relative energies on singlet potential surface are predicted less accurately (with deviations 5-9 kcal/mol compared to couple cluster values). However, the systematic errors for reactants and transition state largely cancel each other, and the activation barriers agree to the higher theory level within 3 kcal/mol. The only exception is the central covalent bond formation in hydrogen tetroxide. For this system, we 6 ACS Paragon Plus Environment

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also found a new transition state 1TS26, which was not reported previously. The described inaccuracies can be attributed to multiconfiguration character of the wavefunction in singlet oxygen diatomic (last two rows in Table 2), and transition states involved in its formation. The energies of these systems are not easy to capture with DFT method. Therefore, our predictions for the singlet potential surface should be considered as preliminary, to be verified by higher theory level in the future. 3.2 Interactions of HO2 radical with CO2 Before we describe the effect of CO2 on the binary reactions, it is instructive to investigate formation of adducts between CO2 and one of the reactants. Our previous study10 reported the easy formation of adducts between OH radical and CO2 molecule. It would not be unreasonable to expect similar behavior from hydroperoxyl radical. After an extensive search we located three mechanisms, reported on Table 3 and Figure 3a-l. Formation of the new CO bond results in adduct 2CP31 with activation barrier of 45 kcal/mol (Fig.3a-c). Hydrogen transfer from hydroperoxyl to the oxygen atom of CO2 molecule also results in formation the new CO bond and another adduct 2IN32 (Fig.3e-f), which has a lower activation energy 21.7 kcal/mol. After a conformational transition into 2IN33 this adduct can dissociate, forming triplet oxygen diatomic weakly bound to HOCO (2CI33). The dissociation processes have the activation barrier of 35.2 kcal/mol. In turn, hydrogen transfer to the carbon atom of CO2 molecule resulting in formation of HCO2 and oxygen molecule (Fig.3j-l) has even higher barrier of 68 kcal/mol. The activation barriers for all these processes are rather high, and only the formation of the 2CP32 intermediate may be comparable with the main channels of hydroperoxyl dissipation. For this reason, we will only consider 2TS32 in the following.



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Table 3. Relative energies (kcal/mol) for the critical points on PES for HO2+CO2 reactive system (origin is set at isolated reactants, HO2+CO2). 2 2 2 Stationary points on Figure 3a-c CR31 TS31 CP31 Ea M11D3+ZPE -4.2 40.8 37.3 45.0 CCSD(T)+ZPE -2.4 39.3 35.5 41.7 2 2 2 Stationary points on Figure 3d-f CR32 TS32 IN32 Ea M11D3+ZPE -4.1 17.6 12.3 21.7 CCSD(T)+ZPE -2.4 19.8 13.6 22.2 2 2 2 Stationary points on Figure 3g-i IN33 TS33 CI33 Ea M11D3+ZPE 15.1 50.2 48.6 35.2 17.6 76.4 45.8 CCSD(T)+ZPE 58.7 2 2 2 Stationary points on Figure 3j-l CR34 TS34 CP34 Ea 68.5 -4.2 64.3 64.1 M11D3+ZPE 62.6 -2.4 60.1 61.1 CCSD(T)+ZPE

3.3 Effect of CO2 on reaction pathways of hydrogen abstraction from formaldehyde We started by adding a spectator CO2 molecule to 2TS12, that corresponds to the lowest activation barrier in the gas phase (Table 4, Figure 4a-o). The result (2TS41) is shown on Fig.4b. One can see that the added CO2 molecule forms a hydrogen bond with the hydrogen atom of the hydroperoxyl, and another close C..O contact with the terminal oxygen atom, stabilizing the transition state by 4 kcal/mol compared to 2TS12. However, the reactant complex is equally stabilized by the same interactions so that the activation barrier remains nearly the same (13 kcal/mol). In addition to 2TS41, we found two other transition states (2TS42 and 2TS43) that can be classified as 2TS12 with a spectator CO2 molecule. The relative energies of all three complexes are nearly the same. However, IRC search found corresponding reactant complexes (2CR42 and 2CR43) where an H-bond is formed between hydroperoxyl and formaldehyde, and a CO2 molecule forms two additional close C..O contacts with formaldehyde. These reactant complexes are considerably more stable than 2CR41, which increases the activation barrier from 13.2 kcal/mol to 19.6 kcal/mol. It is worth noting that stabilization of reactant complexes may take place in a vacuum, increasing the activation barrier corresponding to 2TS11 to 19.8 kcal/mol 8 ACS Paragon Plus Environment

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(compared to 13.1 kcal/mol for 2TS12). The analogs of 2TS11 with a spectator CO2 molecule added were also located (2TS44 and 2TS45). The corresponding reactant complexes are stabilized by the same interaction, and have similar relative energies compared to 2CR42 and 2

CR43. Transition states themselves are slightly more stable than 2TS41, 2TS42, and 2TS43,

which results in somewhat lower activation barriers (17.7 and 18.1 kcal/mol). From these data one can conclude that spectator CO2 molecules either do not change activation barriers, or increase them by extra stabilization of the reaction complexes. Table 4. Relative energies (kcal/mol) for the critical points CH2O+HO2→CHO+H2O2 reaction with spectator CO2 molecule. 2 2 2 Stationary points on Figure 4a-c CR41 TS41 CP41 M11D3+ZPE -6.3 6.9 -5.3 CCSD(T) +ZPE -3.7 11.1 -3.4 2 2 2 Stationary points on Figure 4d-f CR42 TS42 CP42 M11D3+ZPE -12.7 6.9 -6.3 CCSD(T) +ZPE -8.6 11.5 -3.4 2 2 2 Stationary points on Figure 4g-i CR43 TS43 CP43 M11D3+ZPE -12.8 6.9 -5.5 CCSD(T) +ZPE -8.4 11.1 -3.2 2 2 2 Stationary points on Figure 4j-l CR44 TS44 CP44 M11D3+ZPE -12.6 5.7 -6.1 CCSD(T) +ZPE -8.3 11.2 -3.3 2 2 2 Stationary points on Figure 4m-o CR45 TS45 CP45 M11D3+ZPE -12.0 5.7 -5.5 CCSD(T) +ZPE -8.1 11.2 -3.2

on

PES

for

Ea 13.2 14.8 Ea 19.6 20.2 Ea 19.7 19.5 Ea 18.1 19.5 Ea 17.7 19.3

Next, let us consider the possibility of covalent intermediate for this hydrogen abstraction reaction. The reaction pathways that involve the intermediates, are reported in Table 5 and Figure 5a-u. As it was discussed in Section 3.2, there is only one kinetically accessible intermediate formed by CO2 addition to hydroperoxyl (2CP32). In the presence of spectator formaldehyde, the activation barrier of its formation drops slightly lower to 21.4 kcal/mol (2TS51) as compared to a barrier of 21.7 kcal/mol in the gas phase (2TS32). The resulting 9 ACS Paragon Plus Environment


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intermediate 2CP33 abstracts the hydrogen atom from formaldehyde via 2TS52, or 2TS53 (activation barriers of 13.3 kcal/mol and 13.4 kcal/mol respectively), then undergoes internal rotation of the peroxide group (2TS54, with 8.7 kcal/mol barrier) and 1-3 hydrogen shift (2TS55 with 35.9 kcal/mol barrier). Alternatively, the latter two steps may proceed synchronously (2TS56 with 36.5 kcal/mol barrier). These relatively high values for activation barriers are expected from the strained four-membered ring transition state structures. Therefore, we investigated the possibility for the product (formyl radical) to extend this ring by two atoms and stabilize the transition state. The resulting 2TS57 indeed was found to have lower activation barrier of 29.0 kcal/mol. In either case, the mechanisms that involve covalently bound CO2 molecule demonstrate considerably higher barriers compared to direct hydrogen abstraction, and therefore may not be kinetically competitive. Table 5. Relative energies (kcal/mol) for the critical points CH2O+HO2→CHO+H2O2 reaction with covalently bound CO2 molecule. 2 2 2 Stationary points on Figure 5a-c CR51 TS51 CP51 M11D3+ZPE -6.0 15.4 10.2 CCSD(T) +ZPE -3.5 18.7 12.2 2 2 2 Stationary points on Figure 5d-f CR52 TS52 CP52 M11D3+ZPE 3.5 16.8 7.2 CCSD(T) +ZPE 7.3 22.6 7.9 2 2 2 Stationary points on Figure 5g-i CR53 TS53 CP53 M11D3+ZPE 5.1 18.5 5.7 CCSD(T) +ZPE 8.5 23.1 10.3 2 2 2 Stationary points on Figure 5j-l CR54 TS54 CP54 M11D3+ZPE 2.2 10.9 2.4 CCSD(T) +ZPE 6.3 14.4 7.1 2 2 2 Stationary points on Figure 5m-o CR55 TS55 CP55 M11D3+ZPE 4.4 40.4 -6.3 CCSD(T) +ZPE 8.3 45.6 -3.4 2 2 2 Stationary points on Figure 5p-r CR56 TS56 CP56 M11D3+ZPE 2.2 38.7 -8.1 CCSD(T) +ZPE 6.3 44.4 -4.6 2 2 2 Stationary points on Figure 5s-u CR57 TS57 CP57

on

PES

for

Ea 21.4 22.2 Ea 13.3 15.2 Ea 13.4 14.6 Ea 8.7 8.0 Ea 35.9 37.3 Ea 36.5 38.1 Ea 10

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M11D3+ZPE CCSD(T) +ZPE

3.2 6.5

32.2 41.4

-4.8 -2.9

29.0 34.9

3.4 Effect of the third molecule on reaction pathways of hydroperoxyl self-reaction It is conceivable that the addition of a third molecule, mediating hydrogen transfer may relieve the steric strain that is present in a transition state for hydrogen tetroxide rearrangement (1TS27). In order to validate this assumption, we inserted hydroperoxyl and water molecule into the four-member ring of 1TS27 and obtained 2TS61 and 1TS62 (Table 6, Figure 6a-i). The respective barriers were found to be 33.1 kcal/mol and 29.4 kcal/mol, which is considerably lower than 47.5 kcal/mol in the gas phase isolated tetroxide. Inserting the second water molecule (1TS63) further lowers the barrier to 24.5 kcal/mol, but even this mechanism is still not competitive with direct hydrogen exchange between two hydroperoxyl molecules (7.0 kcal/mol and 8.4 kcal/mol for the triplet and singlet channels respectively). Table 6. Relative energies (kcal/mol) for the critical points on PES for H2O4→O2+H2O2 rearrangement reaction, catalyzed by HO2, H2O, and 2 H2O molecules. 2 2 2 Stationary points on Figure 6a-c CR61 TS61 CP61 Ea M11D3+ZPE -16.1 17.0 -11.2 33.1 CCSD(T) +ZPE -14.9 19.8 -25.6 34.7 1 1 1 Stationary points on Figure 6d-f CR62 TS62 CP62 Ea M11D3+ZPE -20.1 9.3 -11.8 29.4 CCSD(T) +ZPE -16.3 16.4 -5.7 32.6 1 1 1 Stationary points on Figure 6g-i CR63 TS63 CP63 Ea M11D3+ZPE -34.4 -9.9 -22.3 24.5 -21.5 4.5 -15.9 26.0 CCSD(T) +ZPE

Last we consider effect of spectator CO2 molecule on the direct hydrogen exchange mechanisms (Table 7, Figure 7a-l). Out of several configuration of CO2 molecule approaching 3

TS21 and 3TS22 structures, the lower activation barriers were obtained for 3TS71 and 3TS72

(6.5 kcal/mol and 5.4 kcal/mol for linear and cyclic transition states respectively). These barriers 11 ACS Paragon Plus Environment


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compare favorably to the isolated 3TS21 and 3TS22 transition states (7.0 kcal/mol and 8.6 kcal/mol for linear and cyclic transition states respectively). Similar structures 1TS73 and 1TS74 on singlet potential energy surface, but their activation barriers (11.9 kcal/mol and 8.8 kcal/mol) are found to be close to the barriers in isolated 1TS21 and 1TS22 transition states (10.4 kcal/mol and 11.4 kcal/mol). Table 7. Relative energies (kcal/mol) for the critical reaction with spectator CO2 molecule. 3 Stationary points on Figure 7a-c CR71 M11D3+ZPE -6.5 CCSD(T) +ZPE -4.4 3 Stationary points on Figure 7d-f CR72 M11D3+ZPE -13.2 CCSD(T) +ZPE -8.0 1 Stationary points on Figure 7g-i CR73 M11D3+ZPE -9.2 -4.6 CCSD(T) +ZPE 1 Stationary points on Figure 7j-l CR74 M11D3+ZPE -13.7 -9.4 CCSD(T) +ZPE

points on PES for HO2+HO2→O2+H2O2 3

TS71 0.0 4.1 3 TS72 -7.8 -2.5 1 TS73 2.7

3

CP71 -39.7 -40.3 3 CP72 -39.1 -40.0 1 CP73 -8.3

Ea 6.5 8.4 Ea 5.4 5.5 Ea 11.9

-9.2

12.7

CP74 -9.7

Ea 8.8

-7.7

10.8

8.1 1

TS74 -4.9 1.4

1

4. Conclusions We investigated the potential energies for the transition states, reactant and product complexes along the reaction pathways for hydrogen transfer reactions of hydroperoxyl radical with carbon dioxide, formaldehyde, and with another hydroperoxyl in the presence of another carbon dioxide molecule. Both covalently bonded intermediates and weak intermolecular complexes were identified and characterized. We found that reactions that involve covalent intermediates have substantially higher activation barriers and are not likely to play role in hydrogen transfer kinetics. In particular, covalent adducts of hydroperoxyl and carbon dioxide have 45.0 kcal/mol formation barrier (2TS31), 21.7 and 68.5 kcal/mol hydrogen transfer barriers (2TS32 and 2TS34), and 35.2 kcal/mol rearrangement barrier (2TS33). Even if subsequent 12 ACS Paragon Plus Environment

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hydrogen transfer from resulting HCO2 intermediates to HO2 were associated with lower activation barriers, these mechanisms would not be competitive with direct hydrogen abstraction from HO2 by another HO2 that has only 7 kcal/mol barrier. Presence of spectator CO2 molecule does not change the barrier to direct hydrogen abstraction from H2CO by HO2, either. The hydrogen tetroxide intermediate rearrangement into H2O2 and O2 is catalyzed by HO2, H2O and 2H2O molecules, relieving the strain of 4-member cycle in transition state. But even two water molecules are able to bring the barrier down to 24.5 kcal/mol, much higher than direct HO2 self-reaction barrier of 7.0 kcal/mol. However, the spectator CO2 molecule is able to stabilize the cyclic transition state and lower the barrier to 5.4 kcal/mol. This effect is even more pronounced (3 kcal/mol stabilization) at CCSD(T) theory level. Thus, CO2 environment is likely to have a catalytic effect on HO2 self-reaction. This can be incorporated into future combustion kinetic mechanism33 currently under development for supercritical CO2.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b03242. Cartesian coordinates for transition states, reactant and product complexes.

Acknowledgments This work was supported in part by the Department of Energy (grant number: DEFE0025260). The authors acknowledge the National Energy Research Scientific Computing Center (NERSC), and the University of Central Florida Advanced Research Computing Center (https://arcc.ist.ucf.edu) for providing computational resources and support. A.E.M. is grateful to



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the Russian Science Foundation, contract no. 14-43-00052, operated by Center of Photochemistry, Russian Academy of Science. •

•

Figure 1. Geometries for the critical points on PES for HO2 +CH2O→HCO +H2O2 reaction.

a.

2

b.

2

c.

2

d.

2

e.

2

f.

2

CR11

CR12

TS11

TS12

CP11

CP12


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•

Figure 2. Geometries for the critical points on PES for 2HO2 →H2O2+O2 reaction.

a.

3

b.

3

c.

3

d.

3

e.

3

f.

3

CR21

CR22

TS21

TS22

CP21

CP22



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g.

1

h.

1

i.

1

j.

1

CR24

k.

1

l.

1

m. 1CR25

n.

1

o.

1

IN25

q.

1

r.

1

IN26

p.

CR23

1

CR26

TS23

TS24

TS25

TS26

CP23

CP24


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s.

1

IN27

t.

1

TS27

u.

1

CP27

Figure 3. Geometries for the critical points on PES for HO2+CO2 reactive system.

a.

2

CR31

b.

2

c.

2

IN31

d.

2

CR32

e.

2

f.

2

IN32

TS31

TS32



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g.

2

j.

2

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IN33

h.

2

i.

2

CR34

k.

2

l.

2

TS33

TS34

CI33

CI34


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Figure 4. Geometries for the critical points on PES for HO2•+H2CO reactive system with spectator

CO2 molecule.

a.

2

CR41

b.

2

c.

2

d.

2

CR42

e.

2

f.

2

TS41

TS42

CP41

CP42



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g.

2

CR43

h.

2

TS43

i.

2

j.

2

CR44

k.

2

TS44

l.

2

m. 2CR45

n.

2

o.

2

TS45

CP43

CP44

CP45


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Figure 5. Geometries for the critical points on PES for HO2•+H2CO reactive system with

covalently bonded CO2 molecule.

a.

2

CR51

b.

2

c.

2

d.

2

CR52

e.

2

f.

2

g.

2

CR53

h.

2

i.

2

TS51

TS52

TS53

CP51

CP52

CP53



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CR54

k.

2

l.

2

m. 2CR55

n.

2

o.

2

q.

2

r.

2

j.

p.

2

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2

CR56

TS54

TS55

TS56

CP54

CP55

CP56


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s.

2

CR57

t.

2

TS57

u.

2

CP57



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Figure 6. Geometries for the critical points on PES for H2O4 rearrangement, catalyzed by HO2 (a-

c), H2O (d-f) and two H2O molecules (g-i).

a.

2

b.

2

c.

2

d.

1

e.

1

f.

1

g.

1

h.

1

i.

1

CR61

CR62

CR63

TS61

TS62

TS63

CP61

CP62

CP63


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Figure 7. Geometries for the critical points on PES for HO2 self-reaction in the presence of

spectator CO2 molecule.

a.

3

b.

3

c.

3

d.

3

e.

3

f.

3

g.

1

h.

1

i.

1

CR71

CR72

CR73

TS71

TS72

TS73

CP71

CP72

CP73



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j.

1

CR74

k.

1

TS74

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l.

1

CP74


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