Hydrogen Abstraction from n-Butanol by the Hydroxyl Radical: High

Apr 9, 2010 - Chemistry Centre, National UniVersity of Ireland, Galway, Ireland ... January 30, 2010; ReVised Manuscript ReceiVed: March 17, 2010...
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Hydrogen Abstraction from n-Butanol by the Hydroxyl Radical: High Level Ab Initio Study of the Relative Significance of Various Abstraction Channels and the Role of Weakly Bound Intermediates Jerzy Moc*,† and John M. Simmie‡ Faculty of Chemistry, Wroclaw UniVersity, F. Joliot-Curie 14, 50-383 Wroclaw, Poland and Combustion Chemistry Centre, National UniVersity of Ireland, Galway, Ireland ReceiVed: January 30, 2010; ReVised Manuscript ReceiVed: March 17, 2010

We have investigated the mechanism of the reaction of H abstraction from n-butanol by the hydroxyl radical (HO•) using high level ab initio methods in conjunction with the correlation consistent basis sets up to quadruple-ζ quality (cc-pVQZ). This reaction is of significance in the atmosphere and combustion. The focus of the study has been on the relative importance of the abstractions from the specific n-butanol sites and on the role of reaction intermediates involved. Our results show that abstractions from the CR and Cγ positions are kinetically most favored and nearly barrierless, with barrier height estimates of 0.10 and 0.47 kcal/mol, respectively, at the CCSD(T)/cc-pVQZ level. We have determined that the indicated barrier height order, CR < Cγ < Cβ < Cδ < OH, parallels that for the n-butanol bond dissociation energies established recently. The kinetically and thermodynamically most favored CR abstraction occurs via a mechanism including the formation of the n-butanol · · · HO• prereaction complex. The weakly bound postreaction complexes between the product radicals and H2O have been identified for all the specific site abstraction reactions, with their calculated CCSD(T) binding energies of up to about 3 kcal/mol after correcting for the basis set superposition error. G3 method has been found to yield consistent results with those obtained from the CCSD(T) calculations for the predicted orders of both the H abstraction barrier heights and their exothermicities. 1. Introduction

SCHEME 1

One of the most pressing problems facing mankind today is the search for new transport liquid fuels that will not impact negatively on human health by degrading local air quality1 or water supplies2 and which will not accelerate global warming.3–6 The new generation of fuels (biofuels) under active consideration include n-butanol that can be produced by classical anaerobic fermentation7 but which can also be made in a new and potentially more significant way through the manipulation of biological systems or metabolic engineering.8 More recently, the production of n-butanol (biobutanol) from macroalgae has attracted considerable attention because seaweed is a potentially sustainable and scalable new source of biomass that crucially does not require arable land or potable water.9 n-Butanol is currently employed as an industrial solvent and is also emitted by plants.10 Consequently, its reaction with the hydroxyl radical (HO•) has been so far of interest primarily to atmospheric chemists. Yujing and Mellouki11 reported that H abstraction from the terminal or δ carbon atom and from the OH group of n-butanol is much less important compared to the other sites considered and concluded that abstraction rates of R, β, and γ hydrogens are 58:20:17 at room temperature. A more recent estimate by Cavalli et al.12 based on structure-activity relationships suggests that the probabilities of attack by HO• at the O, R, β, γ, and δ hydrogens are 2.2:42.7:38.3:14.7:2.2. Because of the importance of the reaction of n-butanol with HO• in the atmosphere and combustion environments, we set out to study the mechanism of this process by using high-level ab initio methods. To this end, reaction pathways have been

CH3CH2CH2CH2OH + HO• f CH3CH2CH2CH2O• + H2O (R1)

* To whom correspondence should be addressed. E-mail: [email protected]. † Wroclaw University. ‡ National University of Ireland.

CH3CH2CH2CH2OH + HO• f CH3CH2CH2CH•OH + H2O (R2) CH3CH2CH2CH2OH + HO• f CH3CH2CH•CH2OH + H2O (R3) CH3CH2CH2CH2OH + HO• f CH3CH•CH2CH2OH + H2O (R4) CH3CH2CH2CH2OH + HO• f CH•2CH2CH2CH2OH + H2O (R5) calculated for H abstraction by HO• from the OH group (R1) and from the R (R2), β (R3), γ (R4), and δ (R5) carbon positions in n-butanol (Scheme 1). The focus of the study is on the relative significance of hydrogen abstraction from the specific n-butanol sites and on the role of the weakly bound reaction intermediates involved. In our recent studies, the results of thermochemical calculations of the bond dissociation energies of n-butanol have been reported,13 preceded by accounting for the alcohol conformational isomerism and molecular mechanism of its dehydration.14 Previously, Galano and co-workers15 as part of a computational study of the reactions of C1-C4 alcohols with HO• determined the geometries, energetics, and rate coefficients for H abstraction from the R, β, and γ C atoms as well as from the

10.1021/jp1009065  2010 American Chemical Society Published on Web 04/09/2010

Hydrogen Abstraction from n-Butanol by HO•

Figure 1. Structures of the reactants: TGt conformer of n-butanol and HO•(2∏) radical, optimized at the MP2/6-311G(d,p) level (bond lengths in Ångstroms, bond and dihedral angles in degrees).

OH group of n-butanol. It should be noted that H abstraction from the Cδ position was not examined by these authors. The Galano et al. study, at the CCSD(T)/6-311G(d,p)//BHandHLYP/ 6-311G(d,p) level, also reported some reaction intermediates. As we specify below, there are differences between the current work and the previous one15 dealing with the mechanism of the n-butanol + HO• reaction. 2. Computational Methods All structures were optimized and subjected to vibrational analysis by using second-order Møller-Plesset perturbation theory (MP2)16 and the 6-311G(d,p) basis set.17 To verify that each transition state was connected to the appropriate adjacent minima on the potential energy surface (PES), the intrinsic reaction coordinate (IRC) was determined at the MP2/6311G(d,p) level. All the minimum energy paths were generated by using the Gonzalez-Schlegel second-order method.18 In addition, the single-point energies were calculated at the MP2/ 6-311G(d,p) structures using the coupled cluster method with single, double and noniterative triple excitations (CCSD(T))19 and the correlation-consistent cc-pVnZ, n ) D, T, Q, basis sets.20 Spin-restricted and spin-unrestricted calculations were carried out for the closed- and open-shell systems, respectively. The structures and energies of the species involved in the n-butanol + HO• reaction were also computed using the multilevel G3 scheme21 (the respective G3 optimized Cartesian coordinates are included in the Supporting Information). To compare the CCSD(T) energies with the G3 thermochemical values, the electronic energy differences ∆E have been converted into 0 K enthalpy differences ∆H(0 K) by adding the zeropoint energy contributions (∆ZPE). All calculations were performed using the Gaussian 03 program.22 3. Results and Discussion A. Reactants. Our recent14 theoretical study of n-butanol addressed its complex conformational isomerism. Fourteen energetically distinct conformers of the n-butanol molecule were found, lying within a narrow range of 1.86 kcal/mol at the ZPE corrected CCSD(T)/cc-pVTZ//MP2/6-311G(d,p) level, with the TGt conformer (Figure 1) indicated to be the lowest-energy minimum structure.14,23 Additionally, a number of transition states for their interconversion has been located.14 In the notation used23 to label the n-butanol structures, trans (T or t) and gauche (G or g) is with respect to the CC-CC, CC-CO, and CC-OH dihedral angles, respectively. Consistent with the CCSD(T) results,14 the TGt conformer is also predicted to be the most stable structure of n-butanol at the G3 level (the G3 relative energies of the 14 n-butanol conformers are included in the Supporting Information). On the basis of the CCSD(T) and G3

J. Phys. Chem. A, Vol. 114, No. 17, 2010 5559 energetics, the lowest-energy TGt conformer has been used here as the actual reactant in the study of the n-butanol + HO• reaction. For the hydroxyl radical HO•(2∏), the equilibrium bond length of 0.967 Å was calculated (Figure 1), in good accord with the experimental24 distance of 0.970 Å. B. Hydrogen Abstraction Transition States. The TSOH, TSr, TSβ, TSγ, and TSδ transition states (Figure 2) involve hydrogen transfer from the OH group and from the R, β, γ, and δ carbon positions in n-butanol, respectively, to the hydroxyl oxygen radical site. These transition states have the cleaving C-H (O-H) bond stretched to 1.17-1.20 (1.07) Å and the forming H · · · O bond length of 1.23-1.41 Å. As seen in Figure 2, the four transition structuressTSOH, TSr, TSβ, and TSγsare similar in that they preserve the starting TGt conformation of the n-butanol reactant. By contrast, TSδ is cyclic and H-bonded. Within the latter TS, the CCCC backbone of the n-butanol molecule is “twisted” (with the CC-CC dihedral angle of -128.9°) to make it possible to form an eight-membered transition state structure with the attacking HO• radical. Transition states for H atom abstraction by the hydroxyl radical from the OH group and from the Ci-H (i ) R, β, γ) carbon sites of n-butanol were computed previously by Galano et al.15 using density functional theory (DFT) with the BHandHLYP functional and 6-311G(d,p) basis set. As mentioned above, these authors did not examine the Cδ-H abstraction channel. There is a modest agreement between the TS geometries in Figure 2 and those reported by Galano et al. One reason might be purely computational (MP2 vs DFT). Secondly, as these authors did not specify the actual n-butanol reactant conformer(s), it is somewhat difficult to make a direct comparison of the corresponding H abstraction TSs. C. Product Radicals. CH3CH2CH2CH2O• (1-butoxy), CH3CH2CH2CH•OH (r-rad), CH3CH2CH•CH2OH (β-rad), CH3CH•CH2CH2OH (γ-rad), and CH2•CH2CH2CH2OH (δ-rad) radicals are the products of the reaction of H abstraction by HO• occurring at the OH position and at the Ci-H (i ) R, β, γ, and δ) carbon sites of n-butanol, respectively (for the radical structures, see Figure S1 of Supporting Information). Since the TGt conformer of n-butanol has no symmetry, the two hydrogen atoms on each of the R through γ carbon atoms and the three hydrogens on the terminal carbon are not actually identical, but the differences in barrier heights are small and have been neglected here based on the results of the related reaction system involving n-butanol.25 Short names, given in bold (in parentheses), are used throughout for convenience to denote the radicals. Note that the actual conformations of the radicals with respect to the CC-CC, CC-CO, and CC-OH dihedrals were reached by tracing the respective minimum energy paths, followed by their full geometry optimization. Similar to the n-butanol reactant, the product radicals’ conformations are TGt (TG for 1-butoxy, Figure S1). To the best of our knowledge, there are no experimental structures available for any of these radicals. D. Reaction Intermediates. The formation of entrance channel H-bonded complex involving the hydroxyl radical has been noted for its reactions with C1-C4 alcohols15 and other oxygenates.26,27 Consistent with these prior results, we have located the reactant complex COM0, n-butanol · · · HO•, in which the hydroxyl radical acts as the hydrogen bond donor (Figure 3). This initial complex shows the intermolecular O · · · H distance and O · · · H-O angle of 1.88 Å and 171.3°, respectively. Importantly, COM0 serves a prereaction complex for the three reaction paths, those of R1, R2, and R5 abstraction channels,

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Figure 2. Structures of the transition states for the reaction of H abstraction from the OH group (TSOH) and from the CR (TSr), Cβ (TSβ), Cγ (TSγ), and Cδ (TSδ) carbon sites in n-butanol by HO• optimized at the MP2/6-311G(d,p) level (bond lengths in Ångstroms, bond and dihedral angles in degrees). The associated imaginary vibrational frequencies (cm-1) are shown in brackets. Note that for the cyclic transition state TSδ, two views are shown.

as confirmed by the IRC following. By contrast, walking down the minimum energy paths from the TSβ (R3) and TSγ (R4) transition states toward reactants resulted in the prereaction complexes involving a relatively weaker C-H · · · O interaction. The two complexes, COMβ and COMγ, respectively, have been studied here only at the MP2 level (see Figure S2 of Supporting Information for their structures and binding energies). The postreaction H-bonded complexes between the product radicals and water, in which the water acts either as the H-bond donor or H-bond acceptor, have been identified for all the specific site abstraction reactions (Figure 3). The first group of the postreaction complexes involve an O-H · · · O interaction. For the R1 channel, the complex COM1 has the H atom of water pointing toward the 1-butoxy radical oxygen at a distance of 2.07 Å and making the O · · · H-O angle of 150.1°. The complex COM2 of the R2 channel shows a structure in which the hydrogen of the r-rad radical O-H group interacts with the water’s oxygen at a relatively short distance of 1.84 Å, with the O · · · H-O angle of 167.4°. For the complex COM5 of the R5 channel, the principal H-bonding interaction occurs

between the H atom of H2O and the oxygen of the δ-rad radical at a distance of 1.94 Å and with the O · · · H-O angle of 167.7°. The different kind of postreaction complexes, denoted COM3 and COM4, have been identified for the R3 and R4 abstraction paths, respectively. The two complexes involve interaction between the unpaired electron density on the Cβ and Cγ centers of the β-rad and γ-rad radicals and the water’s hydrogen (see Figure 3). These complexes show the intermolecular C · · · H distances of 2.37 and 2.29 Å and the corresponding C · · · H-O angles of 146.5° and 152.3°, respectively. An interaction of this type has been recently recognized28 as being somewhat weaker than a traditional hydrogen bond (we discuss the related CCSD(T) energetics in section F). E. Method Dependence of the H Abstraction Barrier Height. The results presented in this section illustrate a dependence of the calculated H abstraction barrier height on the computational method. Table 1 shows the performance of various ab initio methods for predictions of the barrier height (with respect to the free reactants) for the case of the R1 channel,

Hydrogen Abstraction from n-Butanol by HO•

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Figure 3. Structures of the intermediates formed during the reaction of H abstraction from n-butanol by HO•: reactant complex (COM0) and product complexes (COM1, COM2, COM3, COM4, and COM5) optimized at the MP2/6-311G(d,p) level (bond lengths in Ångstroms, bond and dihedral angles in degrees).

TABLE 1: Method Dependence of Classical Barrier Heighta for H Abstraction from the OH Group in n-Butanol by HO• (R1 Channel)

a In kcal/mol. Relative to the n-butanol (TGt) + HO• reactants. At the MP2/6-311G(d,p) geometries. c Values in parentheses were corrected by ∆ZPE contributions.

It is seen from Table 1 that the non-ZPE-corrected barrier height varies vastly between 0.12 and 7.25 kcal/mol. Clearly, the cc-pVDZ basis set is not flexible enough to ensure an acceptable accuracy of both the CCSD(T) and DFT calculations, whereas the MP2 barrier converges to the wrong basis set limit. On the other hand, the MPWB1K/cc-pVQZ estimate of 3.89 kcal/mol is within 0.28 kcal/mol of the reference. Similarly, comparing the G3 and CCSD(T)/cc-pVQZ results (ZPE including values, in parentheses) reveals even better agreement. From this comparison, at least a reasonable agreement between the CCSD(T)/cc-pVQZ and G3 barrier heights compared below is expected. F. PES Profile of the n-Butanol + HO• Reaction. On the basis of the results presented in sections A-D, the emerging mechanism of the n-butanol + HO• reaction including all the specific site H abstraction channels can be described as follows:

using CCSD(T)/cc-pVQZ as reference. The MPWB1K density functional has been elected for comparison as the one specifically designed for kinetics.29

n-butanol + HO• f COM0 f TSOH f COM1 f 1-butoxy+H2O (R6)

methodb

barrier height

MP2/6-311G(d,p) MP2/cc-pVDZ MP2/cc-pVTZ MP2/cc-pVQZ MPWB1K/6-311G(d,p) MPWB1K/cc-pVDZ MPWB1K/cc-pVTZ MPWB1K/cc-pVQZ CCSD(T)/6-311G(d,p) CCSD(T)/cc-pVDZ CCSD(T)/cc-pVTZ CCSD(T)/cc-pVQZ G3

7.25 6.28 6.92 7.52 1.84 0.12 2.99 3.89 4.37 3.81 3.82 4.17 (3.19)c (3.21)c

b

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n-butanol + HO• f COM0 f TSr f COM2 f r-rad + H2O (R7) n-butanol + HO• f COMβ f TSβ f COM3 f β-rad + H2O (R8) n-butanol + HO• f COMγ f TSγ f COM4 f γ-rad + H2O (R9)

where the value after the slash includes the BSSE correction. It is consequently expected that, at the CCSD(T)/cc-pVQZ level, the BSSE involved in the interaction energy of all the weakly bound intermediates is comparatively less, with the restored binding effect of the complex COM3. In the remainder of our discussion, from now on, both the CCSD(T)/cc-pVQZ and G3 values of ∆H(0 K) will be used in the CCSD(T)/cc-pVQZ (G3) manner. As seen from Table 2 and Figure 4, the CCSD(T) and G3 methods predict consistently that the barriers to H abstraction from the specific n-butanol sites increase in the order:

n-butanol + HO• f COM0 f TSδ f COM5 f δ-rad + H2O (R10) Let us consider next the associated energetics. ∆E and ∆H(0 K) energies of the intermediates, transition states, and products (relative to the separated reactants) calculated using the CCSD(T) and G3 methods are summarized in Table 2. The corresponding ∆H(0 K) profile is depicted in Figure 4 with both the CCSD(T)/cc-pVQZ and G3 results indicated. As seen from Table 2 and Figure 4, the five pathways available for H atom abstraction from n-butanol by the hydroxyl radical are significantly exothermic and three of them begin with the formation of the prereaction complex COM0. The basis set superposition error (BSSE) estimated for the complex COM0 by using a counterpoise method30 at the highest affordable (for this purpose) CCSD(T)/cc-pVTZ level is 2.26 kcal/mol. This leads to the BSSE-corrected complexation energy of COM0 of -3.36 kcal/mol at this computational level. The BSSE of similar magnitude of 2.10-2.87 kcal/mol is found at CCSD(T)/cc-pVTZ for the postreaction complexes COM1, COM2, and COM5. The following counterpoisecorrected CCSD(T)/cc-pVTZ stabilization energies result (relative to the respective radical plus H2O products, ∆H(0 K), kcal/ mol): COM1 (-1.53), COM2 (-3.47), and COM5 (-2.10). For the two unique radical complexes COM3 and COM4, the corresponding CCSD(T)/cc-pVTZ stabilization energies with respect to the β-rad + H2O and γ-rad + H2O products are -2.38/0.08 kcal/mol and -2.87/-0.55 kcal/mol, respectively,

CR < Cγ < Cβ < Cδ < OH At the CCSD(T)/cc-pVQZ level, all the energy barriers are small and positive, whereas the two lowest G3 barriers, those for abstraction from the CR and Cγ positions, are found to be actually negative:

CR0.10(-1.67) < Cγ0.47(-0.90) < Cβ1.40(0.77) < Cδ1.83(1.52) < OH 3.19(3.21) Clearly, an accurate estimation (with the error of 1 kJ/mol or less) of the lowest barrier height for H abstraction from n-butanol by HO• is computationally challenging. We have recognized that the predicted here barrier height order parallels that for the bond dissociation energies of n-butanol established recently.13 Therefore, one interpretation of the barrier order is in terms of Evans-Polanyi relationship,31 correlating the barrier height to the strength of the breaking bond: (Do(CR-H) ) 397 ( 3) < (Do(Cγ-H) ) 413 ( 2) < (Do(Cβ-H) ) 419 ( 3) < (Cδ-H ) 424 ( 2) < (Do(O-H) ) 439 ( 2) (from ref 13, in kJ/mol). The other interpretation of the trend relies on the structures of the H abstraction transition states involved. If one takes the forming H · · · O bond distance as a measure of its early character (Figure 2) one finds that TSr having the longest distance (1.41 Å) is the earliest, followed by TSγ (1.39 Å), TSβ (1.37 Å), TSδ (1.31 Å), and TSOH (1.24 Å), with the latter being the latest. Thus, the increasing order

TABLE 2: Relative Energiesa,b of the Intermediates, Transition States and Products for the Reaction of H Abstraction from n-Butanol by HO• Calculated at the CCSD(T) and G3 Levels CCSD(T)/cc-pVDZ species n-butanol(TGt) + HO COM0 TSOH COM1 1-butoxy + H2O TSr COM2 r-rad + H2O TSβ COM3 β-rad + H2O TSγ COM4 γ-rad + H2O TSδ COM5 δ-rad + H2O

•

CCSD(T)/cc-pVTZc

CCSD(T)/cc- pVQZ

G3d

∆E

∆H(0 K)

∆E

∆H(0 K)

∆E

∆H(0 K)

∆H(0 K)

0.00 -9.19 3.81 -17.18 -10.42 3.15 -26.64 -17.03 4.36 -15.44 -10.23 3.37 -17.46 -12.00 3.59 -18.38 -9.38

0.00 -6.93 2.82 -15.95 -11.02 2.25 -24.96 -17.18 3.14 -14.73 -11.05 2.30 -16.80 -12.91 3.14 -17.29 -10.55

0.00 -7.88/-5.62 3.82 -17.14/-15.04 -11.68 1.44 -28.41/-26.28 -20.97 2.82 -18.83/-16.36 -14.92 1.79 -20.88/-18.56 -16.44 2.39 -21.15/-18.28 -13.92

0.00 -5.62/-3.36 2.84 -15.91/-13.81 -12.28 0.55 -26.73/-24.59 -21.12 1.60 -18.12/-15.66 -15.74 0.72 -20.22/-17.90 -17.35 1.95 -20.06/-17.19 -15.09

0.00 -7.29 4.17 -17.06 -12.22 1.00 -29.05 -22.46 2.62 -19.67 -16.64 1.54 -21.72 -18.06 2.28 -21.79 -15.51

0.00 -5.03 3.19 -15.83 -12.83 0.10 -27.37 -22.61 1.40 -18.96 -17.46 0.47 -21.07 -18.97 1.83 -20.70 -16.69

0.00 -5.28 3.21 -16.62 -12.88 -1.67 -27.92 -22.97 0.77 -19.64 -17.78 -0.90 -21.84 -19.30 1.52 -21.52 -17.08

At the MP2/6-311G(d,p) geometries. b Relative to the energy of the n-butanol (TGt) + HO• reactants (in kcal/mol). c The values after slash include the BSSE correction. d Not BSSE corrected. a

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Figure 4. Schematic potential energy profile for the five channels of the reaction of H abstraction from n-butanol (TGt) by HO•. ∆H(0 K) in kcal/mol from the CCSD(T)/cc-pVQZ calculations; the corresponding G3(0 K) values are shown in parentheses. For definitions of the intermediates, transition states, and products involved, see Sections B-D in the text.

of the barrier heights shown above can be correlated with the decreasing of the early nature of the H abstraction TS in terms of the forming H · · · O bond distance. Consistent with the Hammond postulate,32 the latter trend correlates with the decreasing order of the abstraction channels exothermicity (Table 2, kcal/mol): (CR ) 22.61 (22.97)) > (Cγ ) 18.97 (19.30)) > (Cβ ) 17.46 (17.78)) > (Cδ ) 16.69 (17.08)) > (OH ) 12.83 (12.88)). 4. Summary (1) We have studied the mechanism of the reaction of H abstraction from n-butanol by HO• using high level ab initio methods. The abstractions from the CR and Cγ positions are found to be kinetically most favored and nearly barrierless at the CCSD(T)/cc-pVQZ level, with the former abstraction reaction being most exothermic (by about 23 kcal/mol) and occurring via a mechanism including the formation of the n-butanol · · · HO• prereaction complex. (2) The weakly bound postreaction complexes between the product radicals and water have been identified for all the distinct site abstraction reactions with their calculated CCSD(T) binding energies of up to about 3 kcal/mol after correcting for the BSSE. (3) Our indicated barrier height order, CR < Cγ < Cβ < Cδ < OH, parallels that for the n-butanol bond dissociation energies established recently.13 It also corresponds to the order of the H abstraction barrier heights obtained for the related n-butanol + HO2• reaction.25,33 In contrast to the present case, all the H abstractions for the latter reaction system are endothermic and are of significant energy barrier of 13-21 kcal/mol.25,33

(4) G3 method has yielded consistent results with those obtained from the CCSD(T) calculations for the orders of both the H abstraction barrier heights and their exothermicities. (5) The following significant differences have been noticed between the current work and the previous one15 dealing with the mechanism of the n-butanol + HO• reaction: (i) the lowestenergy TGt conformer14,23 was used herein as the reactant, whereas the actual n-butanol conformer(s) involved was not specified before; (ii) abstractions from all the distinct sites of n-butanol were examined in this work, while that from the Cδ position was not studied earlier; (iii) our determined minimum energy paths were based on the full IRC calculations; (iv) the different orders of the H abstraction barrier heights were predicted in the two studies (at 0 K): the previous15 order is as follows (the barriers are relative to the separated reactants, in kcal/mol): (Cγ ) -0.61) < (Cβ ) -0.11) < (CR ) -0.05) < (OH ) 3.21). Acknowledgment. Funding from an EU Marie Curie Transfer of Knowledge grant (MKTD-CT-2004-517248) is acknowledged. Computational resources were provided by the Wroclaw Centre for Networking and Supercomputing, WCSS and Irish Centre for High-End Computing, ICHEC. Supporting Information Available: The G3 relative energies of the 14 conformers of n-butanol molecule. The MP2/6311G(d,p) and G3 optimized Cartesian coordinates of the intermediates, transition states, and products discussed in the paper. Figure S1 illustrating the MP2/6-311G(d,p) optimized structures of the product radicals. Figure S2 depicting the structures of the prereaction complexes COMβ and COMγ

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