H-Atom Abstraction Reactions by Ground-State Ozone from Saturated

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H-Atom Abstraction Reactions by Ground State Ozone from Saturated Oxygenates Judith Wurmel, and John M Simmie J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b07760 • Publication Date (Web): 04 Oct 2017 Downloaded from http://pubs.acs.org on October 4, 2017

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The Journal of Physical Chemistry

H-atom Abstraction Reactions by Ground State Ozone from Saturated Oxygenates J. Würmel∗,† and J. M. Simmie‡ †Galway Mayo Institute of Technology, Galway H91 T8NW, Ireland ‡School of Chemistry, National University of Ireland, Galway H91 TK33, Ireland E-mail: [email protected] Phone: +353-91-742391 Abstract Theoretical insights into H-abstraction by ozone from saturated species have been virtually nonexistent which is in sharp contrast to reactions with unsaturated species. Our computed rate constants at various levels of theory for reaction with tetrahydrofuran and its methyl derivatives shows that abstraction occurs primarily at the carbons situated beside the heterocyclic oxygen with least likely reaction from the methyl groups. All the methods tested are in broad relative agreement with this conclusion and with recent experiments although they do differ widely as regards their absolute values. Abstraction leads variously to the formation of hydrotrioxides, ROOOH, or more directly to R – OH + O2 (1 ∆g ). To understand some of the observed behaviour dimethyl and diethyl acyclic ethers, methyl and ethyl hydroperoxides and the alcohols methanol and ethanol were also included in this study. For the simplest system, CH3 OH + O3 −−→ H2 C – O + H2 O2 , an all-electron scalar relativistic CCSD(T)/CBS approach yields a barrier height which differs by > 5 kJ mol−1 when an additional multi-reference treatment up to CCSDT(Q) is considered. Draft: October 2, 2017

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Introduction There are ongoing efforts to ‘green’ manufacturing processes and one strand is the utilisation of green solvents, that is, a solvent manufactured from renewable resources with a low life cycle footprint. 1,2 In particular 2-methyltetrahydrofuran has found promise as an alternative solvent suitable for environmentally benign syntheses because of its remarkable stability, low miscibility with water, etc. 3 Concurrently there is a pressing need to produce high-energy density liquid fuels from lignocellulosic biomass, the so-called ‘next generation biofuels’, to reduce our dependency on fossil fuels and to lessen their negative environmental impact. 4–6 Whilst the disposal, accidental or otherwise, of such materials may impact on the water table — here we are concerned with emissions to the atmosphere and their fate thereof. In particular we focus on four oxolanes, more commonly known as tetrahydrofurans, oxolane (THF), both 2-methyl and 3-methyl oxolanes (2MTHF, 3MTHF) and 2,5-dimethyloxolane (25DMTHF). These are classified as heterocyclic compounds and more specifically as members of the cyclic ether family. The rates of reactions with the hydroxyl radical, for example, have been extensively studied. 7 We are concerned here with their reactivity towards ozone O3 (X1 A1 ) and although ozone is present in the troposphere the highest concentrations are to be found in heavily polluted urban areas where solvent spills might reasonably be expected to occur. Recent experimental determinations of the total rates of reaction of THF, 2MTHF and 25DMTHF with ozone in a photochemical reactor supply the essential verification that is needed. 8 Andersen and colleagues used both relative and absolute rate methods at 298 ± 3 K and report that the relative rates are as 1:2.9:7.1 for THF:2MTHF:25DMTHF. These are slow hydrogen-atom abstraction reactions in comparison say to reaction even with an unsaturated cyclic ether such as furan. Mechanistically, reaction with an alkene moiety C – C proceeds via a fast 1,3-dipolar addition reaction — an option not available to saturated compounds. In fact, in the over 1,800 records in a comprehensive kinetics database 9 there are very few entries featuring neutral saturated molecules, M + O3 −−→ products. 2 ACS Paragon Plus Environment

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Cerkovnik et al. have shown that ozone inserts into the aldehydic C–H bond in benzaldehyde, C6 H5 CHO, leading to the formation of an organic hydrotrioxide which can then undergo a series of conformational changes ending up with the elimination of singlet oxygen, O2 (1 ∆g ), and benzoic acid. 10 Their experiments were backed up by B3LYP/6-31G(d,p) calculations which determined an activation energy of 46 kJ mol−1 and ∆r H of −240 kJ mol−1 , for the highly exothermic formation of the benzoyl hydrotrioxide, C6 H5 C(O)OOOH. More generally there are a number of studies of the mechanisms and kinetics of ozone reacting with unsaturated oxygenates, 11–13 acetaldehyde, 14 fluorocarbons, 15 methane, 16 formaldehyde, 17,18 unsaturated ketones, 19 alkyl hydroperoxides 20,21 and summaries in a recent review of atmospheric reaction mechanisms in the troposphere. 22 However, although there have been these experimental studies, the intrinsic multireference character of ozone 23 has probably accounted for the dearth of theoretical calculations for the reactions of systems larger than H2 CO + O3 . 17 On the application side the enhancement of combustion by ozone is of interest as, for example, seen in studies of the increase in burning velocities of methane/air flames and similar. 24–28 The aims and objectives of this study are therefore to determine the reactivity of the various sites in the cyclic ethers and thereby validate the experimental measurements, to contrast cyclic and acyclic ethers, and to understand how the reactivity of cognate hydroperoxides and alcohols compares.

Computational methodology The bulk of our computations used a recent Minnesota density functional, RMN12SX, 29 a screened-exchange hybrid functional coupled with a split-valence triple-ζ basis set with added diffuse and polarization functions, 6-311++G(d,p). Thus, zero-point corrected electronic energies, vibrational frequencies (scaled by 0.9755), moments of inertia and relaxed potential



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energy scans were carried out with this model chemistry on a singlet surface with Gaussian16. 30 Wang and co-workers 17 explored both the singlet and triplet surfaces in H2 CO + O3 −−→ HCO• + HOOO• and found that the singlet barrier is considerably lower than that of the triplet. The H-atoms occupy non-equivalent positions on the THF ring and they can be labelled as axial and equatorial as shown in Fig 1 for just those on the vicinal carbon atoms to the heterocyclic oxygen.

Figure 1: Axial and equatorial sites on THF. Intrinsic reaction coordinate calculations were used to verify the connection between reactants, transition state and products. 31,32 The keyword Recorrect=Never was used to check that the end points of the IRC corresponded to the actual structures because of the variability of the products for apparently similar reactions. Very weakly-bonded prereaction complexes were found for which basis set superposition error corrections 33 were applied; given their very weak nature they are kinetically insignificant. The simplest transition states were checked in CCSD(T)-F12A/VDZ-f12//M06-2X/6311++G(d,p) molpro calculations 34–36 which showed T1 diagnostics well above the recommended limit, 37 for example, T1=0.045 for the C3 axial H-abstraction transition state from 3MTHF at CCSD(T)-F12a/VDZ-F12 — thereby necessitating multireference methods which are however unaffordable with the resources available. The sole exception to this limitation was for the small system CH3 OH + O3 −−→ CH2 O + HOOOH for which T1 > 0.06 and for which it was possible to employ much higher level composite methods, W2X and W3X-L. The former provides an accurate approximation to the all-electron scalarrelativistic CCSD(T)/CBS energy whilst the latter provides additionally a multi-reference 4 ACS Paragon Plus Environment

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post-CCSD(T) treatment up to CCSDT(Q). 38 These model chemistries have been used previously, employing the applications molpro 34 and mrcc, 39 to compute formation enthalpies via an atomisation procedure, validated against the most credible database of formation enthalpies known, the Active Thermochemical Tables, 40 and shown to give excellent results. 41 The results of these calculations were then used to determine thermochemical parameters such as entropy, S(298.15K) and the specific heat at constant pressure, Cp (298.15K), and hence the kinetics of reaction via canonical transition state theory as framed in the thermo module of the application MultiWell. 42 Thermo includes a correction for unsymmetrical Eckart tunnelling and treats 1-dimensional hindered rotors by fitting Fourier series to both the potential energy, and the rotational constant, as a function of dihedral angle. 43 In the case of highly symmetric methyl rotors, determining the barrier height is sufficient to compute the contribution that these modes make with the more elaborate treatment being unnecessary.

Pseudo-rotational considerations Tetrahydrofuran, in common with most n-ring systems with n > 3, exhibits the phenomenon of pseudo-rotation with a recent study on the robustness of fitting molecular mechanics parameters labelling it as a ‘complex and subtle’ energy surface. 44 Thus, these authors have gone to great lengths to determine reliable parameters because of the ubiquity of the THF ring in furanose carbohydrates including ribose components in RNA and DNA. Experiment 45 has shown that the CS structure with a C–C–C–C dihedral angle of 0◦ is the global minimum; it is only at quite high levels of theory, MP2/aug-cc-pVTZ, that this is reflected by calculations which suggest that the C2 structure is the more stable. 46 However the structures are nearly isoenergetic with a difference of ≤ 0.2 kJ mol−1 . 47 And while there has been much work on aspects of the rotational spectroscopy not much has been done on the thermochemical implications, specifically on the nature of the ring puckering vibrations which in essence drive pseudo-rotation. Current codes do not readily lend themselves to a simple treatment of out-of-plane skeletal 5 ACS Paragon Plus Environment


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modes. 48,49 In addition relaxed potential energy scans of the CCCC dihedral over a narrow range of values from −15 → +15◦ exhibit different behaviour at B3LYP, MN12SX, M06-2X and MP2 all at a common basis set of 6-311++G(d,p). Also Ghahremanpour et al. have argued that the pseudorotation inherent in five-membered rings is described relatively well by the harmonic approximation. 50 Consequently we have assumed that it is sufficient to consider ring puckering as pure vibrational modes although these low frequencies will have the greatest impact on the computed vibrational entropy.

Results The calculation of a reaction rate constant depends essentially on two quantities, (1) the difference in zero-point corrected electronic energies between the transition state, TS, and the reactant(s), A and B, more commonly known as the barrier height, E ‡ and (2) the ratio of partition functions, (QTS /QA QB ). The partition functions can be separated into individual contributions due to translational (T), rotational (R), vibrational (V) and electronic components. Of these the most critical are the vibrational terms which are very species dependent as opposed to translational or whole-molecule rotational partition functions. Unfortunately partition functions are not directly verifiable but their contribution to experimentally accessible quantities such as entropies and heat capacities enables validation.

Thermochemistry In the calculation of rate constants computed values for S and CP are tabulated and compared with the literature, Table 1. The calculated values include symmetries C2 for THF and DMTHF and C1 for 2MTHF and 3MTHF with the latter two being optically active. Only the methyl groups are treated as hindered rotors; the ring puckering modes are assumed to be pure vibrations except for THF itself for which a pseudo-rotational constant is known 51 and for which well characterised calculations are available. 52 The agreement is very good in


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this particular case; a purely vibrational treatment gives S = 297.3 and CP = 80.79 J K−1 mol−1 which differ from the most accurate values. However since ring-opening, which would compromise the puckering modes, is not involved in these abstraction reactions we have assumed that neglecting the proper treatment will not materially affect the kinetic results which are based on the difference in the entropies of reactants and transition state. Table 1: Thermochemical data at 298.15 K S/J K−1 mol−1 calc. lit. THF 303.1 302.3 52 2MTHF 335.3 305.6 53 3MTHF 336.0 311.2 53 25DMTHF 357.5

Species

Cp /J K−1 mol−1 calc. lit. 76.7 77.2 52 105.9 103.2 53 106.4 95.1 53 132.4

For the transition states a hindered rotor analysis in Gaussian-16 shows that only rotation about the dihedral H···O – O – O needs to be considered and to a very good approximation a vibrational mode treatment is perfectly adequate. So for both reactants and transition states only methyl hindered rotors must be taken into account.

Energetics Although absolute enthalpies of formation are not required in determining the kinetics of reaction xTHF + O3 −−→ TS the following ∆f H(298.15 K) values were found for 2MTHF, 3MTHF and 25DMTHF of −225.1, −213.3 and −265.6 kJ mol−1 respectively, anchored by a value of −184.6 kJ mol−1 for THF. 54 A summary of the barrier heights at MN12SX/6-311++G(d,p), and the corresponding rate constants is given in Table 2 from which it can be seen that: 1. Abstraction from carbon atoms adjacent to the heterocyclic oxygen is the dominant site of attack 2. Followed by abstraction from the more remote carbon atoms and



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Table 2: Barriers / kJ mol−1 and rate constants / cm3 s−1 molecule−1 . E† k (298.15 K) tetrahydrofuran C2 ax 43.9 5.38 × 10−22 C2 eq 41.1 1.77 × 10−21 C3 ax 86.9 7.88 × 10−30 C3 eq 83.4 2.35 × 10−29 2-methyltetrahydrofuran C2 ax 27.4 1.49 × 10−19 CH3 103.0 1.95 × 10−33 C3 ax 82.8 9.06 × 10−30 C3 eq 88.4 5.53 × 10−31 C4 ax 77.4 6.06 × 10−29 C4 eq 88.0 1.61 × 10−30 C5 ax 34.1 5.78 × 10−21 C5 eq 43.4 1.85 × 10−22 3-methyltetrahydrofuran C2 ax 49.3 6.04 × 10−23 C2 eq 37.1 1.24 × 10−21 CH3 100.1 2.64 × 10−32 C3 ax 63.1 1.15 × 10−25 C4 ax 75.1 2.43 × 10−28 C4 eq 90.9 6.87 × 10−31 C5 ax 25.0 1.26 × 10−19 C5 eq 46.9 3.90 × 10−23 2,5-dimethyletrahydrofuran C2 ax 25.1 5.71 × 10−19 CH3 101.5 6.34 × 10−33 C3 ax 81.0 4.48 × 10−29 C3 eq 80.3 2.15 × 10−29 Site

3. Finally, least likely from the methyl groups (these always occupy an equatorial site). These observations are entirely consistent with known C – H bond dissociation energies which have C2 – H at 388–392, C3 – H at 412–416 and CH2 – H substantially higher at 431 kJ mol−1 and indeed they also parallel the barriers found for H-abstraction by H-atoms and CH3 radical. 54


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Kinetics There are very few quantitative measurements of the room temperature rates of reaction between saturated oxygenates and ozone. This is in contrast to the wealth of data for unsaturated oxygenates which typically react approximately 103 times faster. In this case by a different and well-known mechanism of addition across the double bond, ozonolysis, as elucidated by Criegee, which has synthetic significance. 55,56 As regards the total reactivity of each molecule with ozone comparisons can be made against very recent experimental measurements. The values recommended by Andersen and colleagues 8 are an average of determinations obtained from absolute measurements of the concentration of reactant as a function of time via FTIR in the presence of an excess of O3 and from relative experiments using ethyne as a standard. The rather large uncertainties in their final values are quite understandable given the difficulties in measuring the rates of these very slow reactions. Perhaps, somewhat fortuitously, the computed rate rate constant for THF lies within a factor of 3 of the experimental value. However in general the agreement is poor although the computed rate constants capture the relative reactivities of THF, 2MTHF and 25DMTHF in good agreement with experiment, albeit the absolute values are off by a large factor of up to ×10; note however that modest changes in barrier heights by ≈ 6 kJ mol−1 do bring these into very satisfactory agreement. These barriers do show large variabilities depending on the level of theory. Thus, for example, at M06-2X/6-311++G(d,p) the C2 axial and equatorial barriers for THF are 67.2 and 62.3 kJ mol−1 respectively, some 50% greater than the MN12SX barriers whereas the C3 sites only increase by about 20%. Even higher barriers emerge from ωB97X-D/aug-cc-pVTZ calculations. Computations of reactions of these oxolanes with ground state oxygen, 3O2 , show comparable patterns of reactivity at the same sites on the ring structures but at much slower rates as a result of barrier heights up to 5 fold higher than those calculated for reactions with ozone. 9 ACS Paragon Plus Environment


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Table 3: Experimental 8 and computed total rate constants at 298 K / cm3 s−1 molecule−1 Total rate constant, 10+21 × k reaction measured calc. THF 6.41 ± 2.90 2.3 2MTHF 18.7 ± 8.2 149 3MTHF — 126 25DMTHF 45.8 ± 21.8 571 Acyclic ethers In order to gain some insight into the cyclic ether, or THF results, abstraction from acyclic dimethyl and diethyl ethers was investigated. Judged purely on the barriers to reaction the cyclic ethers should be considerably more reactive than the acyclic ethers, Table 4. In the case of the highly reactive hydroxyl radical this distinction is blurred and the cyclic and acyclic ethers have comparable reactivities. 57,58 For diethyl ether abstraction of a secondary H-atom leads to the formation of 1-ethoxy ethanol, C2 H5 OCH(OH)CH3 , and O2 (1 ∆g ). The former was probably observed during a study on the atmospheric oxidation of diethyl ether initiated by chlorine atom but the author could not be certain. 59 However the unidentified IR bands match quite closely our calculated spectrum. Abstraction of a primary H-atom in dimethyl ether leads this time to the formation of methoxymethylhydrotrioxide, CH3 OCH2 OOOH. In neither case are pre-reaction complexes observed. Alkyl hydroperoxides Since total reaction rates for CH3 OOH + O3 and C2 H5 OOH + O3 of 7.3×10−21 and 8.4×10−21 cm3 molecule−1 s−1 respectively, were available in the literature 20,21 we determined barrier heights and IRCs for these reactions. The computed rate constants are not in good agreement with these literature values for reasons previously discussed. It is however somewhat surprising that ethyl hydroperoxide is not more reactive than experiment indicates since the barrier to abstraction faced by a secondary H-atom in ethyl hydroperoxide at 65 kJ mol−1 is


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Table 4: Barriers / kJ mol−1 . Species THF C2 CH3 CH2 OC2 H5 CH3 OCH3 CH3 CH2 OOH CH3 CH2 OOH CH3 OOH CH3 CH2 OH CH3 CH2 OH CH3 OH

E† 41.1 55.5 58.3 65.3 98.8 76.2 56.0 80.5 60.9

quite lower than that by a primary H-atom in methyl hydroperoxide at 76 kJ mol−1 , Table 4. For methyl hydroperoxide abstraction of a primary H-atom leads to the formation of a αperoxymethylhydrotrioxide, HOOOCH2 OOH. The simpler α-hydroxymethylhydroperoxide, HOCH2 OOH, has been the focus of recent studies on its catalysed and un-catalysed decomposition in the atmosphere. 60 For ethylhydroperoxide C2 H5 OOH abstraction of a secondary H-atom forms 1-hydroperoxy ethanol and singlet oxygen, CH3 CH(OH)OOH + O2 . Alcohols In an attempt to select a system which might be computationally affordable with higher level methods we have found that H-abstraction from methanol, CH3 OH, exhibits a comparable barrier of 61 kJ mol−1 to that for dimethyl ether of 58 kJ mol−1 . W2X and W3X-L calculations at an MN12SX/6-311++G(d,p) geometry give barrier heights of 76.3 and 71.1 kJ mol−1 — a clear indication of the necessity of including a post-CCSD(T) treatment. However the nature of the transition state (cyclic) and the IRC indicates the direct formation of methanal and trioxidane H2 CO + HOOOH, effectively abstracting two H-atoms from CH3 OH in a single step, Fig. 2. By contrast for ethanol, abstraction of a secondary Hatom is more straightforward with reaction leading to the hydrotrioxide CH3 CH(OOOH)OH.



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Figure 2: CH3 OH + O3 transition state. Formaldehyde CCSD(T)/6-311+G(d,p)//M05-2X/6-311+G(d,p) calculations by Voukides et al. 18 and BMCCCSD//BHandHLYP/6-311+G(d,p) by Wang and colleagues 17 showed that both addition and abstraction occurs in CH2 – O + O3 , Fig. 3. They both agree that abstraction faces lower barriers of 68 and 72 kJ mol−1 , respectively, but disagree on the magnitude of this difference. In this study a value of 54.3 kJ ml−1 was obtained for abstraction at MN12SX/6311++G(d,p) and of 76.3 kJ mol−1 for W2X — yet another illustration of the sensitivity of these reactions.

Figure 3: CH2 O + O3 transition states.

Products Although the products of reaction are of very little kinetic significance as regards these calculations, the nature of the products of xTHFs + O3 is very interesting. Intrinsic reaction 12 ACS Paragon Plus Environment

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coordinate computations show that as the initially formed hydrogen trioxide radical, HOOO• , departs it can re-attach to the newly formed radical carbon C• in one of two ways. If the terminal O-atom attaches then a hydrotrioxide is formed, ROOOH, whereas if the other ‘terminal’ O-atom is involved then a furanol will result concomitantly with the elimination of singlet dioxygen, R – OH + O2 (1 ∆g ). For 25DMTHF abstraction at a C3 equatorial site neatly illustrates the fine balance between these two possibilities, Fig 4, C···O(H) = 2.608 ≈ C···O(OOH) = 2.612 Å; as the IRC is traversed the hydrotrioxide is formed. For the case of abstraction at the C3 axial site the IRC goes on to eliminate O2 and form the furanol.

Figure 4: A snapshot along an IRC path This contrasting behaviour can be simply shown in plots of the C···H, C···O(H), C···O(OOH) and O···O(OH) bondlengths, Fig 5, as a function of energy along the IRCs. In the only other known case Cerkovnik and colleagues 10 found that a trioxide is formed, benzoyl hydrotrioxide, during the low temperature ozonation of benzaldehyde in preference to the formation of a cyclic tetraoxolane. However, the trioxide rapidly decomposes in an 13 ACS Paragon Plus Environment


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Figure 5: Bondlengths along IRC paths; ⊙ C3 axial,


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C3 equatorial

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intramolecular H-atom transfer to benzoic acid and singlet dioxygen and they concluded that ROOOH should not be observable by 1H,

13

C and

17

O NMR above −80 C in a number

of different organic solvents, contrary to previous reports.

This latter mechanism is not possible for our systems but the equivalent process, formation of an alcohol and elimination of dioxygen is. The prediction of hydrotrioxides as products from our theoretical calculations is somewhat surprising. The chemistry of organic hydrotrioxides is but sparsely known 61,62 and there are very few examples of their formation particularly via an H-atom abstraction reaction. Hydrotrioxides The only member of this family sufficiently well known to be included in the Active Thermochemical Tables 40 or to feature in a recent review 63 is trioxidane or dihydrogen trioxide, HOOOH, the trans conformer of which has a formation enthalpy, ∆f H(0 K), of −81.40±0.78 kJ mol−1 ; this is in good agreement with a W3X-L value of −82.1 kJ mol−1 . A recent communication considered the reaction between methyl peroxy and hydroxyl radicals from which an enthalpy of formation for methyl hydrotrioxide of −65.3±3.5 kJ mol−1 can be derived. 64 Earlier Jungkamp and Seinfeld 65 had derived values for ∆f H(298.15K) of −94 kJ mol−1 from G2M(RCC) calculations. None of these are in satisfactory accord with W2X and W3X-L calculations of −69.5 and −71.1 kJ mol−1 at 0 K and −84.7 and −86.9 kJ mol−1 at 298.15 K respectively, for the same conformer. These key values can now be used in isodesmic reactions with lower, more affordable, levels of theory to compute the formation enthalpies of larger hydrotrioxides. The bond dissociation energies of 3-substituted tetrahydrofuran hydrotrioxides, hydroperoxides and alcohols, calculated with the G4 model chemistry, 66 are shown in Table 5. Although most of these values are novel where comparisons are possible, for example, in the 15 ACS Paragon Plus Environment


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case of tetrahydro-3-furanol, R–OH typically range from 390–400 kJ mol−1 and for RO–H 435-445 kJ mol−1 are not untypical. 67 The O–OH bond in tetrahydro-3-hydroperoxyfuran, RO–OH, at 177 kJ mol−1 is comparable to values of 180–190 kJ mol−1 to be found in a comprehensive tabulation of chemical bond energies. The only direct comparison that can be made is with methyl hydrotrioxide for which Lay and Bozzelli 68 calculated CH3 OO – OH of 143 kJ mol−1 and 124 kJ mol−1 for CH3 O – OOH using a variety of methods up to G2 model chemistry. Table 5: Bond dissociation energy, BDE(298.15 K) / kJ mol−1 ROOO–H ROO–OH RO–OOH R–OOOH

319 121 104 251

ROO–H RO–OH R–OOH

353 177 286

RO–H R–OH

440 390

Conclusions The reactivities of the various sites in the oxolanes THF, 2-MTHF, 3-MTHF and 25DMTHF is mapped out and it is shown that these are concentrated at those carbon atoms situated beside the heterocyclic oxygen with abstraction from the methyl groups being least likely. All the functionals and basis set combinations tested are in broad relative agreement with this conclusion although they do differ widely as regards their absolute values. Hence, absolute computed rate constants are not in good agreement with experiment almost certainly due to the limited theoretical treatment; however modest but consistent changes to the barrier heights does improve the situation. The need for the highest levels of theory to adequately determine the potential energy surface was a difficulty which could only be overcome by reducing the number of ‘heavy’ atoms to two in the reactant, methanol. The fact that the reaction mechanism then changes from the usual abstraction of a single H-atom renders a general finding somewhat less certain. However a 10 kJ mol−1 change in barrier height leads to a 50-fold change in rate constant 16 ACS Paragon Plus Environment

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at room temperature — an illustration of the extreme sensitivity encountered with these reactions. Moreover, the difference in barrier heights from W2X, from all-electron scalarrelativistic CCSD(T)/CBS energies, to W3X-L, which includes in addition a multi-reference treatment up to CCSDT(Q)/VDZ, leads to an 8-fold change in rate constant. In comparison to the cyclic ethers abstraction of a secondary hydrogen from an acyclic ether such as C2 H5 OC2 H5 exhibits a much higher barrier as does abstraction of secondary hydrogens from peroxides CH3 CH2 OOH and alcohols, CH3 CH2 OH, Table 4. Abstraction of primary hydrogens from acyclics, peroxides and alcohols are yet higher again reflecting the stronger C–H bonds. The nature of the products initially formed also varies with remote H-sites favouring the formation of a furanic hydrotrioxide and the vicinal sites favouring the formation of furanols. The properties of these novel hydrotrioxides are shown to correspond with some of the limited data available for the simplest alkyl hydrotrioxide CH3 OOOH. In this latter case we are able to provide an accurate value for its formation enthalpy using a methodology which is itself validated against the best collection of such data. 40

Acknowledgement The Irish Centre for High-End Computing, ICHEC, is thanked for the provision of computational resources to projects gmche001c and ngche041c. We also acknowledge advice from J.L Bao and D. G. Truhlar.

Supporting Information Available Cartesian coordinates, vibrational frequencies, symmetries and rotational constants for all reactive species are archived.



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H-Atom Abstraction Reactions by Ground-State Ozone from Saturated

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