Article pubs.acs.org/JPCA
Combustive, Postcombustive, and Tropospheric Butadiyne Oxidation by O2, Following Initial HO Attack. Theoretical Study Andrea Maranzana, Giovanni Ghigo, and Glauco Tonachini* Dipartimento di Chimica, Università di Torino, via Pietro Giuria, 7, I-10125 Torino, Italy S Supporting Information *
ABSTRACT: Butadiyne (diacetylene, HC4H) is produced during combustions, and has been quantified in different flames as well as a biomass burning emission. Its reaction with the hydroxyl radical, HO(2Π3/2), under combustion conditions, was investigated in a thorough RRKM study by J. P. Senosiain, S. J. Klippenstein, and J. A. Miller (Proc. Combust. Inst. 2007, 31, 185−192). The present density functional theory (DFT) study focuses on the mechanism of further oxidation by O2(3Σ−g ). The DFT(M06-2X)/cc-pVTZ reaction energy hypersurface for the system C4H2/HO•/O2 is studied to define a variety of reaction pathways, and the relevant thermochemistry for temperatures ranging from 200 to 2500 K is assessed, thus encompassing combustive, postcombustive, and tropospheric conditions. Energies are then recomputed at the coupled cluster level [CCSD(T)/cc-pVTZ], and combined with the DFT thermochemistry. Finally, the role of the different reaction channels is assessed by RRKM-ME simulations in the same temperature range for P = 1 atm, to comprise the situation of emission in the troposphere and those pertaining to different flames. This shows that, when considering HO addition to the triple bond, dioxygen takes part in C4H2 oxidation with higher efficiency at lower temperatures, whereas, as T rises, the O2 adducts are inclined to redissociate: for instance, a 50% redissociation is estimated at T = 1800 K. For 200 < T < 1100 K, two polycarbonyl products (CHO.CO.CCH and CHO.CO.CHCO) and two fragmentation products (HCOOH plus OC•−CCH) are the main species predicted as products from the addition channel (fragmentation is entropy-favored by higher T values). However, at higher temperatures, an initial H abstraction by HO can give the butadiynyl radical (HC4•) as the starting point for subsequent dioxygen intervention. Then, new pathways opened by O2 addition become accessible and bring about fragmentations mainly to HC3• + CO2 and also to HC3•O + CO.
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INTRODUCTION Polyynes have been detected under very different physical and chemical conditions, ranging from combustion situations1 to interstellar, protostar cloud, or planetary environment.2 They had already been considered, at an earlier time, as important combustion intermediates by Homann and Wagner,3 and then by Krestinin,4−7 who proposed the ring closure−radical breeding polyyne-based mechanism for PAH and soot growth. Their possible role in the synthesis of an aromatic ring (obenzyne) has been reviewed recently.8 The oxidation of polyynes at combustion temperatures could also bear some relevance to the formation of oxidized PAHs and soot platelets, if it is assumed that oxidation can take place all through their synthesis. Polyyne oxidation in a combustion situation might be likely initiated by small reactive radicals, as hydrogen and oxygen atoms, or hydroxyl, that can be present in significant amounts. Some studies reporting their concentrations are available.9−17 Conversely, once polyynes are emitted in the tropospheric environment, their fate is determined by lower-T oxidation processes modulated also by the very different © XXXX American Chemical Society
concentrations of the reactive species which can start off the oxidation process. Among polyynes, butadiyne (HCC−CCH, also called diacetylene) is an important intermediate18−20 in combustion processes21−23 (related to this, we can also mention two studies on its pyrolysis24 and oxidation25). Two pathways by which it can form are either from the ethynyl radical + ethyne reaction18 or through H loss from the i-C4H3 radical intermediate (H2C C•−CCH). Of the two reactions C2H2 + C2H ⇆ n-C4H3 (1) and C2H2 + C2H ⇆ i-C4H3 (2), the first one seems to be barrierless, hence more important.19 Then, H loss following reaction 1 or 2 can give butadiyne (the reverse process was studied theoretically).20 Otherwise, it can form directly via C2H2 + C2H ⇆ C4H2 + H (3). It is present in the highest concentration among polyyne molecules26 (hence eventually emitted in the troposphere). Its Received: July 8, 2015 Revised: September 17, 2015
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DOI: 10.1021/acs.jpca.5b06548 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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THEORETICAL METHOD All stationary points on the energy hypersurface, i.e., minima and first order saddle points, corresponding to transition structures (TS), were determined by gradient procedures48−52 within the density functional theory (DFT),53 and making use of the M06-2X functional.54,55 Dunning’s correlation-consistent polarized valence triple-ζ basis set cc-pVTZ56 was used throughout in the DFT optimizations. The nature of the critical points (and the thermochemistry) was assessed by vibrational analysis. The M06-2X/cc-pVTZ thermochemical corrections gave estimates of the zero point vibrational energy (ZPE), by which the relative energies were corrected to obtain ΔEZPE (= ΔE + ΔZPE) values and enthalpy (ΔH) and Gibbs free energy (ΔG) differences (energetics in kcal mol−1). The thermochemistry was assessed in all cases at temperatures ranging from troposheric values up to those typical of combustion (T = 200−2500 K). The DFT energetics is reported in the Supporting Information, section 3. The relevant critical points were selected, for comparative purposes, for a series of coupled cluster calculations, carried out at the CCSD(T) level57−65 with the cc-pVTZ basis set. These energy values were combined with the DFT(M06-2X)/cc-pVTZ thermochemistry, to define the ΔEZPE values reported in the schemes, and to ascertain the more viable evolution paths of the reacting system. Geometry optimization and thermochemistry calculations were carried out by using the GAUSSIAN 09 system of programs.66 The Rice Ramsperger Kassel Marcus (RRKM) theory67,68 was then used to obtain the distribution of reaction products. In order to obtain these distributions as functions of time, RRKM and master equation (ME) calculations were carried out by using the Multiwell program suite.69−71 It allows calculation of sum and densities of states, then obtains microcanonical rate constants according to RRKM theory, and finally solves the master equation. Corrections for quantum tunneling were included for all hydrogen transfer reactions (not H dissociations) by incorporating the corrections for onedimensional unsymmetrical Eckart barriers.72 Internal rotations were treated approximately as a symmetric hindered internal rotation.73 MultiWell stores densities and sums of states in double arrays: the lower part of the array consisted of 800 array elements which ranged in energy from 0 to 3995 cm−1. The higher energy part of the double array consisted of 200 elements ranging in energy from 0 to 150000 cm−1 with an energy spacing of 753 cm−1. The Lennard-Jones parameters necessary for the collision frequency calculations, assumed to be the same for all the structures, were σ = 5.1 Å and ε/kB = 535 K. For the N2 collider, σ(A) = 3.75 Å and ε/kB = 80 K. Energy transfer, dependent from the temperature, was treated by assuming the exponential-down model for collision step-size distributions: Edown = 250 cm−1 × (T/298)−0.85, as suggested by SKM.44 Rate constants were calculated in the range 200−2500 K. In the present work, the number of stochastic trials was set to 107, for 300 collisions. When the system is thermalized, it may still undergo thermally activated unimolecular reactions. This situation is common at the high temperatures found in flames or if the reaction time scale is much larger than the thermalization time. In these cases, separating the chemically and thermally activated processes becomes necessary. We used the Post-Processing in Multiwell (PPM) program, developed by Pinches and da Silva,74 to check the consistency of our results: in the worst cases, the errors in the yields after 300 collisions
maximum mole fraction xmax in the oxidation zone of premixed ethyne,27 benzene,28 toluene,29 cyclopentene,1,30 allene,1 propyne,1 or gasoline31 flames has been measured in the range xmax = 2−6 × 10−3, or up to ca. 2.75 × 10−4 in a methane flame.32 Butadiyne has also been quantified as an emission released in the troposphere as a consequence of vegetative fuel combustion.33 We can also mention that butadiyne has been identified in astrochemical investigations, dealing with planetary or satellite atmospheres.34−38 Due also to this astrochemical interest, the photophysical and chemical characteristics of C4H2 have been the subject of several studies. Regarding O2, it has been recently detected in Orion by a Herschel measurement,39 and the presence of oxygen under different forms on Titan has also been discussed.40 Nonetheless, the importance of oxidation processes in those environments appears dubious. The C4H2 + HO· reaction, in particular, has been studied experimentally in a limited number of investigations,41−43 and theoretically by Senosiain, Klippenstein, and Miller (hereafter referred to as SKM) in a thorough RRKM study under conditions relevant to combustion (see also the related papers).44−46 The purpose of the present investigation is to extend the study (1) beyond the reaction with hydroxyl, focusing on the subsequent dioxygen intervention, and (2) to situations ranging from combustion to postcombustion (emission in the environment), and then tropospheric oxidation, to assess under which conditions dioxygen addition can best proceed to oxidation products. In the end, the likelihood of the reaction pathways so defined will be assessed theoretically over a range of temperatures (see Theoretical Method). The initial intermediates which form upon a hydroxyl radical attack onto butadiyne are shown in Scheme 1, and are part of Scheme 1. Initial Steps of the Hydroxyl Radical Attack onto Butadiyne
the study by SKM. Either the addition to the carbon−carbon triple bond (to C1 or C2) or H abstraction (shown by SKM to gain some importance for temperatures beyond 1200 K) takes place. The radical originating from hydroxyl addition to C2 is less stable than A1 (−28.7 vs −43.0 kcal mol−1, at the DFT(M06-2X)/cc-pVTZ computational level). It is also obtained significantly less easily: the “HO addition to C2” transition structure is 5.9 kcal mol−1 above the reactants, to be compared to 0.1 kcal mol−1 for the addition to C1 (at the same computational level). For these reasons, it has not been considered any further. A1 is similar to the radical CH2CHCCH2, whose oxidation by O2 was studied in 2011 by Rutz et al.47 B
DOI: 10.1021/acs.jpca.5b06548 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A Scheme 2. Pathways Stemming from Dioxygen Addition to the Stabler Initial HO−Butadiyne Adducta
a
Labels syn and anti refer to the CC−O−O dihedral angle, E and Z to the positions of the HO and OO substituents with respect to the double bond. ΔEZPE values (see Theoretical Method) are reported as blue figures, referred to the initial hydroxyl adduct.
2), whose presence has been shown to be substantial within the temperature range taken into consideration by SKM (Figure 7 in ref 44). The relevant coupled cluster energetics is reported in Scheme 2. Dioxygen addition to the adduct A1 forms two initial hydroxyl−peroxyl radical adducts A2, for which the Z and E symbols refer to the relative positions of the hydroxyl and peroxyl substituents. Thus, we get syn and anti Z-A2 (two rotamers) or syn and anti E-A2 (two other rotamers), where syn and anti correspond to O−O−CC dihedral angles of 0° and 180°, respectively. From anti E-A2 a 5-ring closure leads to the radical A3, and a subsequent H shift to A4. From A4, a H loss gives the tricarbonyl closed shell product A5. From syn E-A2 two pathways depart. However, they converge in the end to the same outcome. If a 3-ring closure takes place, it gives the peroxirane A6. When the peroxy bond in A6 cleaves, another 3ring closure occurs, to form an epoxy ring, as indicated by the curly arrows, but the cleavage of the epoxy ring C−C bond takes place concertedly (blue carbons). By these three events, the radical A7 forms. Through a final fragmentation, formic acid and an acyl radical are produced, collectively labeled as A8. As an alternative to the step “syn E A2−A6”, a 4-ring closure gives the dioxetane-like radical A9. Peroxy bond cleavage is accompanied concertedly by cleavage of the opposite C−C
are 5%) only below T ≈ 600 K, since it declines almost linearly right from the beginning as T rises (Figure 1). The contribution of the open chain tricarbonyl closed shell product A5 mirrors the behavior of anti E-A2 in the leftmost part of the plot. Its yield attains a maximum in the 500−600 K zone. A11 originates from anti E-A2 through unimolecular steps followed in the end by a hydrogen atom loss. Though fragmentation provides an entropic advantage to this step as T rises, two other fragmentation processes compete with it (compare Scheme 2). One entails the splitting of the molecular system into two almost comparable parts, formic acid, A12, and a radical intermediate, A13. The second one implies hydroxyl loss and formation of a closed shell molecule, A10. The two pathways have very different weight at low T, with A10 dominating over all other individual species. But their behavior is similar to that just described, and presents a crossing
an oxyl/carbonyl substituent of the epoxide ring (see the resonance structures for B4). While B3 is C2v, the ring opening TS slightly departs from it, being actually C1. The symmetry lowering is necessary, in order to have a transition structure instead of a cusp, because B3 and B4 have different wave function symmetry. The former is symmetric with respect to the molecular plane, since the unpaired electron lies on it. The latter, by contrast, has an enol radical-like π system extended on the [CC−O• ↔ •C−CO] atoms. Therefore, its wave function is antisymmetric. The real crossing in C2v symmetry then becomes an avoided crossing if the symmetry is lowered, and consequently this is a case of conical intersection. A bifurcation of the reaction pathways must accordingly be present, with two rather early transition structures, each one made up by two enantiomers, which can however be mentally seen as distinct regarding the involvement of the individual oxygen atoms (an isotopic substitution for one O would make them distinguishable). Their relationship is represented in Chart 1. Chart 1a
a
Up−down relationship: enantiomers (reflection with respect to the plane indicated by a horizontal dashed segment). Left−right: superimposable structures (through a π rotation around the x axis, indicated by an arrow). Diagonal relationship: enantiomers (reflection with respect to sheet plane).
They are however represented in Scheme 4, for clarity, as associated with the unique cleavage, a, and one of two alternative closures, b and b′. The bifurcation to TS B3−B4 and TS B3−B4′ thus entails, past the transition structures, two equivalent all-downhill pathways that lead to the two equivalent epoxy-oxyl/carbonyl intermediates, B4 and B4′. The two superimposable or enantiomeric intermediates can in principle interconvert through TS B4−B4′ (again C2v). From B4 (Scheme 3), CO2 loss is possible. As far as ΔEZPE values are considered, it occurs less easily than formation of B6. We can nevertheless expect that a growing T could greatly favor it. As an alternative, the epoxide ring in B4 can open again, from the side closer to the CO group (cleavage of the bond labeled “c”), resulting in an overall oxygen atom migration from the formerly terminal carbon atom to its neighbor. The open-chain resonant radical B6 so obtained is rather stable. It can give a fragmentation, as B6−B10(−B11), or further rearrange (as B6−B7 or B6−B8−B9). If it undergoes a fragmentation, a delocalized radical plus carbon monoxide form (collectively indicated as B10). This radical might as well further fragment to the ethynyl radical plus CO again (B11). If instead B6 undergoes a rearrangement, this can take place in two different ways. The migrating oxygen atom can in one case proceed to the next carbon, forming the bond labeled “e” in B7, in which step also one C−C bond cleaves concertedly, and therefore an ether functionality forms. This intermediate, rather E
DOI: 10.1021/acs.jpca.5b06548 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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Figure 1. Reaction pathways started by hydroxyl addition to butadiyne. Continuous lines: net product reaction yields as a function of temperature. Labels make reference to Scheme 2. As higher T values are attained, the O2 adducts tend to redissociate more effectively: the dashed black line (which stands alone) indicates the fraction of adduct that survives. The redissociating fraction is hence 1 − (A2 + derivatives).
of the two curves around 1600 K, at which temperature their yield is ca. 40% each. Though intermediate ring products are present in Scheme 2, none of them plays an important role in Figure 1. 3.2. Reaction Pathways Started by Hydroxyl H Abstraction from Butadiyne. RRKM-ME simulations show in this case (Figure 2) that the only products are B5 (HC3• + CO2) and, to a lesser extent, B10 (HC3•O + CO), at all temperatures considered. Higher temperatures further favor B5. These products are formed irreversibly, through fragmentations. In this case, in the range 1100−2500 K, the back-reaction to the reactants (B1 + O2) is negligible.
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CONCLUSIONS
CCSD(T)/cc-pVTZ energetics have been defined for the HO + O2 + C4H2 reacting system, based on the full study of the relevant DFT(M06-2X)/cc-pVTZ energy hypersurface. Then, two RRKM-ME simulations (A, B) have been carried out, in the temperature range 200−2500 K (P = 1 atm), to comprise the situations of emission into the troposphere and those typical of different combustions (Figure 1 and Figure 2). Their results show that, when considering an initial HO addition to the triple bond of C4H2 (A) at rising T values, dioxygen takes part in C4H2 oxidation with decreasing efficiency, because at higher T values the O2 adducts tend to redissociate. At T = 1800 K, for instance, the dioxygen adduct A2 gives back the initial HO adduct A1 and O2 with a yield of
Figure 2. Reaction pathways started by hydroxyl H abstraction from butadiyne. Net product reaction yields as a function of temperature. Labels make reference to Scheme 3.
ca. 50%. Let us begin by considering the gamut of rather low temperatures of environmental interest. These range approximately from 350 K down to ca. 200 K. These extremes correspond to going from a quite high ground-level temperature to that found close to the tropopause.75 If T spans this F
DOI: 10.1021/acs.jpca.5b06548 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A range, two fragmentation species, HCOOH and OC•−CCH, A8, are predicted to be the main products of HO•/O2 oxidation of butadiyne, together with two polycarbonyl molecules. These are CHO.CO.CCH, A5, whose formation is accompanied by H loss; and CHO.CO.CHCO, A11, whose formation entails HO loss (see Figure 1 and Scheme 2). A5, A8, and A11 are produced in irreversible steps. At the coldest temperatures, the initial adduct presence is significant, but its sharp declension upon temperature rise is also apparent. A11 clearly dominates, but also declines as T rises. By contrast, both A5 and A8 gain importance by almost mirroring the trend of the other two species. They gain additional importance, as expected, as temperature further rises toward, and through, temperatures typical of combustion. A8 yield constantly rises, whereas A5 begins to decline beyond ca. 600 K, while A11 decline comes gradually to a halt. Then, in the range 600 K < T < 1200 K, the density of the initial adduct A2 drops to zero, and A11 remains approximately constant. Thus, at combustion temperatures higher than 1200 K, A8 and A11 are described as the chief products of butadiyne oxidation, as far as hydroxyl addition followed by dioxygen addition define the initial steps. However, at higher T, the starting point for dioxygen intervention can be instead the butadiynyl radical, HC4•, formed by H abstraction operated by HO (B). New pathways, pertinent to combustion only, initiated by O2 addition become then accessible, and the RRKM-ME simulations indicate that the fragmentations to HC3• + CO2 (B5) and HC3•O + CO (B10) are in this case the main outcome.
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(4) Krestinin, A. V. Detailed Modeling of Soot Formation in Hydrocarbon Pyrolysis. Combust. Flame 2000, 121 (3), 513−524. (5) Krestinin, A. Formation of Soot Particles as a Process Involving Chemical Condensation of Polyynes. Chem. Phys. Rep. 1998, 17 (8), 1441−1461. (6) Krestinin, A. V. On the Mechanism of Soot Formation from Acetylene. Chem. Phys. Rep. 1994, 13 (1), 191−210. (7) Indarto, A.; Giordana, A.; Ghigo, G.; Maranzana, A.; Tonachini, G. Polycyclic Aromatic Hydrocarbon Formation Mechanism in the Particle Phase. A Theoretical Study. Phys. Chem. Chem. Phys. 2010, 12 (32), 9429−9440. (8) Cahill, K. J.; Ajaz, A.; Johnson, R. P. New Thermal Routes to Ortho-Benzyne. Aust. J. Chem. 2010, 63 (7), 1007−1012. (9) H, OH, CH3: Seepana, S.; Jayanti, S. Flame Structure and NO Generation in Oxy-Fuel Combustion at High Pressures. Energy Convers. Manage. 2009, 50 (4), 1116−1123. (10) H, O, HO, CH: Choudhuri, A. Intermediate Radical Concentrations in Hydrogen Natural Gas Blended Fuel Jet Flames. Int. J. Hydrogen Energy 2004, 29 (12), 1293−1302. (11) H: Butler, C. J.; Hayhurst, A. N. Measurements of the Concentrations of Free Hydrogen Atoms in Flames from Observations of Ions: Correlation of Burning Velocities with Concentrations of Free Hydrogen Atoms. Combust. Flame 1998, 115 (1−2), 241−252. (12) H: Bertagnolli, K. E.; Lucht, R. P.; Bui-Pham, M. N. Atomic Hydrogen Concentration Profile Measurements in Stagnation-Flow Diamond-Forming Flames Using Three-Photon Excitation LaserInduced Fluorescence. J. Appl. Phys. 1998, 83 (4), 2315−2326. (13) HCO, HO, CH2: Lozovsky, V. A.; Derzy, I.; Cheskis, S. Radical Concentration Profiles in a Low-Pressure Methane-Air Flame Measured by Intracavity Laser Absorption and Cavity Ring-down Spectroscopy. Symp. Combust., [Proc.] 1998, 27 (1), 445−452. (14) O: Barlow, R. S.; Fiechtner, G. J.; Chen, J.-Y. Oxygen Atom Concentrations and No Production Rates in a Turbulent H2/N2 Jet Flame. Symp. Combust., [Proc.] 1996, 26 (2), 2199−2205. (15) HO: Cheskis, S.; Derzy, I.; Lozovsky, V. A.; Kachanov, A.; Romanini, D. Cavity Ring-down Spectroscopy of OH Radicals in Low Pressure Flame. Appl. Phys. B: Lasers Opt. 1998, 66 (3), 377−381. (16) HO, NO: Nguyen, Q. V.; Dibble, R. W.; Carter, C. D.; Fiechtner, G. J.; Barlow, R. S. Raman-LIF Measurements of Temperature, Major Species, OH, and NO in a Methane-Air Bunsen Flame. Combust. Flame 1996, 105 (4), 499−510. (17) H, O, HO: Zeegers, P. J. T.; Alkemade, C. T. J. Radical Recombinations in Acetylene-Air Flames. Combust. Flame 1965, 9 (3), 247−257. (18) Eiteneer, B.; Frenklach, M. Experimental and Modeling Study of Shock-Tube Oxidation of Acetylene. Int. J. Chem. Kinet. 2003, 35 (9), 391−414. (19) Ceursters, B.; Nguyen, H. M. T.; Peeters, J.; Nguyen, M. T. Experimental and Theoretical Study of the Reaction of the Ethynyl Radical with Acetylene (HCC + HCCH). Chem. Phys. 2000, 262 (2−3), 243−252. (20) Miller, J. A.; Klippenstein, S. J. From the Multiple-Well Master Equation to Phenomenological Rate Coefficients: Reactions on a C3H4 Potential Energy Surface. J. Phys. Chem. A 2003, 107 (15), 2680−2692. (21) Homann, K. H.; Warnatz, J.; Wellmann, C. The Formation of Higher Hydrocarbons in the Reaction of O-Atoms with Acetylene. Symp. Combust., [Proc.] 1977, 16 (1), 853−860. (22) Homann, K. H.; Schweinfurth, H. Kinetics and Mechanism of Hydrocarbon Formation in the System C2H2/H. Ber. Bunsen. Phys. Chemie 1981, 85 (7), 569−577. (23) Warnatz, J. Hydrocarbon Oxidation at High Temperatures. Ber. Bunsen. Phys. Chemie 1983, 87 (11), 1008−1022. (24) Krestinin, A. V.; Kislov, M. B.; Raevskii, A. V.; Kolesova, O. I.; Stesik, L. N. On the Mechanism of Soot Particle Formation. Kinet. Catal. 2000, 41 (1), 90−98. (25) Hidaka, Y.; Henmi, Y.; Ohonishi, T.; Okuno, T.; Koike, T. Shock-Tube and Modeling Study of Diacetylene Pyrolysis and Oxidation. Combust. Flame 2002, 130 (1−2), 62−82.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b06548. Spin densities of intermediates, geometries and energetics of optimized structures, and energetics at DFT(M06-2X)/cc-pVTZ (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Dipartimento di Chimica, Università di Torino, Corso Massimo D’Azeglio 48, I-10125 Torino, Italy. E-mail: glauco.
[email protected]. Phone: ++39-011-6707648. Fax: ++39-0112367648. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was conducted in the frame of EC FP6 NoE ACCENT and ACCENT−PLUS projects (Atmospheric Composition Change, the European Network of Excellence).
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