Reaction of a Fluorine Atom with Methanol: Potential Energy Surface

Article pubs.acs.org/JPCA

Reaction of a Fluorine Atom with Methanol: Potential Energy Surface Considerations Hao Feng,† Katherine R. Randall,‡,¶ and Henry F. Schaefer, III*,‡ †

School of Physics and Chemistry and Research Center for Advanced Computation, Xihua University, Chengdu 610039, People’s Republic of China ‡ Center for Computational Quantum Chemistry, University of Georgia, Athens, Georgia 30602, United States ¶ Research Focus Area for Chemical Resource Beneficiation, North-West University, Hoffman Street, Potchefstroom, South Africa 2520 ABSTRACT: There are several experimental studies of the F + CH3OH reaction but very little in the way of reliable theory. Here reported are aug-cc-pVQZ CCSD(T) computations for two different reaction pathways. The more exothermic pathway B, which produces methylhydroxy radical (ΔE= −37.7 kcal mol−1), has a van der Waals minimum as the entrance complex and a transition state lying 0.15 kcal mol−1 below F + CH3OH. The exit complex B is bound by 5.0 kcal mol−1 relative to separated HF + CH2OH. Although pathway A, which produces the methoxy radical, is 9.3 kcal mol−1 less exothermic than B, A has a deep entrance complex, 6.6 kcal mol−1 below reactants. Also, transition state A lies 4.1 kcal mol−1 below separated F + CH3OH. The latter two energetic features nicely explain why 40% of the laboratory products follow the less exothermic pathway A.

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reaction rate observed by Perksy and co-workers in 2007,19 which was in disagreement with earlier theoretical predictions.20 Previous findings, particularly the presence or absence of an entrance complex and the height of the reaction barrier, will be discussed in relation to our work on F + CH3OH. The F + CH3OH reaction can proceed via removal of the hydroxyl proton to form the methoxy radical (CH3O)

INTRODUCTION Simple chemical reactions are now accessible to very high levels of theory and with sophisticated experiments. These techniques allow scientists to discover information about potential energy surfaces, electronic states, dynamics, including collision statistics, scattering angles, and the distribution of excess energy among vibrational and rotational excitation of the products. The simplest exchange reaction, H + H2 → H + H2, has been the subject of numerous experimental and computational studies. However, it is still not a closed book.1,2 The latest studies of this reaction raise even more questions about exactly what is happening in the H + H2 reaction. However, such research has already yielded highly valuable insights into chemical reactivity. Proceeding from this simplest “identity exchange” reaction, the next most studied reaction is probably F + H2 → H + HF.3 Highly accurate potential energy surfaces for this reaction have been computed using large basis sets and thousands of CI (configuration interaction) and/or coupled cluster energies.4−6 It has also been the subject of extensive computational dynamics studies.7−9 Very recent experimental advances have allowed scientists to study F + H2 at low temperatures relevant to the interstellar medium.10 With modern techniques in spectroscopy for studying dynamics and recent advances in theoretical capabilities, chemists can now study larger reaction systems. Recently the F + H2O,11,12 Cl + H2O,13 and Br + H2O14,15 reactions have been pursued. Even more complicated systems have also been studied in our group, F + (H2O)2,16 F + NH3,17 and F + CH3NH2.18 In the case of F + NH3, our computational results helped to explain the inverse temperature dependence of the © XXXX American Chemical Society

F + CH3OH → HF + CH3O (A)

(1)

or, with greater exothermicity, removal of a methyl proton to form the hydroxymethyl radical (CH2OH) F + CH3OH → HF + CH 2OH (B)

(2)

Because this reaction is a convenient source of methoxy radicals, past research has focused on the branching ratio of these two pathways21−26 and the dynamics of this reaction based on HF 21,22,24,27 or radical28,29 emission spectra. Interestingly, the hydroxymethyl pathway is nearly 10 kcal mol−1 more exothermic, but the methoxy pathway is favored quite strongly in the experiments. Statistically, the hydroxymethyl pathway is also favored because there are three methyl protons that may be removed but only one hydroxyl proton. However, at 298 K, the product branching ratio is about 40% methoxy radical and 60% hydroxymethyl.26 This is a far larger Special Issue: 25th Austin Symposium on Molecular Structure and Dynamics Received: August 13, 2014 Revised: September 13, 2014

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dx.doi.org/10.1021/jp508189d | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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H distances are essentially unchanged; they are 1.086, 1.089, and 1.092 Å. The two hydrogen atoms are no longer equivalent due to loss of symmetry. The dihedral angle F−H−O−C is 98.2°. The C−O−H angle is slightly smaller than the tetrahedral angle, at 108.8°. The same F···CH3OH stationary point was located by Jodkowski and co-workers in their 1998 paper.31 Their results are based on 6-31G(d) geometry optimizations. Their entrance complex (MC1a) has a much longer H···F contact (2.22 Å) than our results and a much larger F−H−O angle (173.1°). In addition, the F−H−O−C angle is 0°. The earlier (1991) computational paper (Glauser and Koszykowski30) did not locate an entrance complex for this reaction. We now turn to the transition state for pathway A (TSA, Figure 2). This transition state has a slightly shorter (by 0.003

percentage of methoxy radical than expected given that the hydroxymethyl pathway is both statistically and thermodynamically favored. Theoretical studies of this reaction are limited; the most reliable computations optimized the stationary points at the MP2 level30,31 with Pople basis sets and computed energy differences at the G2 level of theory.31 However, there are very small energy differences on this potential energy surface, and the geometries of the stationary points appear to be sensitive to the level of theory.

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METHODS The computations were carried out using coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)].32−35 We used the augmented double, triple, and quadruple-ζ basis sets of Dunning and co-workers (aug-ccpVDZ, aug-cc-pVTZ, and aug-cc-pVQZ).36,37 The CFOUR program package was used for all computations.38

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RESULTS AND DISCUSSION The reaction of a fluorine atom with methanol can proceed via two pathways, removal of the hydroxyl hydrogen to form the methoxy radical (CH3O, pathway A) or removal of a methyl hydrogen to form the hydroxymethyl radical (CH2OH, pathway B). As mentioned above, the hydroxymethyl pathway is favored by thermodynamics and statistics, although around 40% of the product is methoxy radical and only 60% of the methanol is converted to the hydroxymethyl radical. Geometries at the aug-cc-pVQZ CCSD(T) Level. A. Pathway A. Both pathways A and B start with separated methanol and a ground-state fluorine atom and contain four other stationary points. We begin with the separated reactants. In methanol, the computed C−O distance is 1.421 Å, while the experimental bond length is 1.427 ± 0.007 Å.39 The O−H distance is 0.970 Å, compared to the experimental distance of 0.956 ± 0.015 Å, and the C−O−H angle is 109.5°, in good agreement with the experimental angle of 108.9 ± 2°.39 For the methyl hydrogen trans to the hydroxyl proton, the C−H distance is 1.088 Å, while the other two C−H distances are longer, at 1.093 Å. Again, these compare favorably with the measured C−H distance of 1.096 ± 0.010 Å.39 In pathway A, the first stationary point is an entrance complex with a hydrogen bond between the fluorine atom and the hydroxyl group, shown in Figure 1. The H−F distance is 2.110 Å, and the F−H−O angle is 70.6°. The C−O distance is slightly elongated (by 0.010 Å) to 1.431 Å, while the O−H distance is slightly shortened (by 0.007 Å) to 0.963 Å. The C−

Figure 2. Pathway A transition state at the aug-cc-pVQZ CCSD(T) level of theory. This transition state is predicted to lie 4.1 kcal mol−1 below the separated reactants, F + CH3OH.

Å) C−O distance than the entrance complex; it is 1.428 Å. The F−H distance decreases significantly (by 0.644 Å), to 1.466 Å, and the O−H distance increases by 0.032 Å to 0.995 Å. The F− H−O angle increases to 111.4°. The C−O−H angle increases to 109.6°, essentially its original value in methanol. The F−H− O−C dihedral angle decreases to 80.4°. The same transition state was located in previous computational work.31 The H−F distance is slightly shorter in their work (1.355 Å), and the O−H distance is slightly longer (1.023 Å). The other geometric parameters are reasonably similar to our results. Thus, the Jodkowski31 transition-state geometry is far more accurate than their entrance complex geometry. The exit complex for pathway A (exit complex A, Figure 3) is a molecular complex between HF and the methoxy radical.

Figure 3. Pathway A exit complex between the methoxy radical and HF at the aug-cc-pVQZ CCSD(T) level of theory. This complex is predicted to lie 8.1 kcal mol−1 below the separated products, HF + CH3O.

There is a short hydrogen bond between the HF proton and the oxygen atom, with a hydrogen bond distance of 1.728 Å. The C−O distance is 1.378 Å, somewhat shorter than that in the transition state (by 0.050 Å). The H−F distance is 0.934 Å, a significant shortening (0.532 Å) from the transition state

Figure 1. Structure of the pathway A entrance complex between methanol and the fluorine atom at the aug-cc-pVQZ CCSD(T) level of theory. This complex is predicted to lie 6.6 kcal mol−1 below the separated reactants, F + CH3OH. B



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H−O−C dihedral angle is −117.2°. The C−O−H angle is 108.5°. The F−HC angle is 132.0°. The pathway B exit complex involves a strong hydrogen bond between HF and the carbon atom of the hydroxymethyl radical (Figure 6). The HF distance is 0.930 Å, lengthened a bit

value of 1.466 Å. The C−O−H angle is 108.2°, while the F− H−O angle increases greatly to 168.6°. The F−H−O−C dihedral angle decreases to 2.8°. The C−H distances are 1.092, 1.094, and 1.105 Å. The longer C−H distance corresponds to the hydrogen more or less perpendicular to the H−F bond. The pathway A exit complex was also located by the previous computational study of Jodkowski and co-workers.31 Their hydrogen bond distance is 1.786 Å, 0.058 Å longer than our result, while their C−O distance of 1.384 Å is only 0.006 Å longer than our result. Their H−F distance is 0.926 Å, and the C−O−H and F−H−O angles are 100.5 and 157.0°, respectively. The C−H distances are 1.093, 1.102, and 1.095 Å. Like the pathway A transition-state geometry, the Jodkowski MP2 geometry is quite similar to our CCSD(T) results.31 B. Pathway B. The entrance complex for pathway B was not located by either of the two earlier computational papers. Entrance complex B is formed when the fluorine atom interacts with the methyl group, as depicted in Figure 4. The H−F

Figure 6. Pathway B exit complex between the hydroxymethyl radical and HF at the aug-cc-pVQZ CCSD(T) level of theory. This complex is predicted to lie 5.0 kcal mol−1 below the separated products, HF + CH2OH.

from the isolated H−F distance of 0.918 Å. The H−C distance is 2.047 Å. The F−H−C angle is nearly linear, at 169.3°. The C−O distance is 1.360 Å, slightly shorter than that in the free radical (1.367 Å). The O−H distance is 0.961 Å, slightly longer than 0.959 Å in free hydroxymethyl. The C−H distances are nearly identical, at 1.080 and 1.083 Å. In the CH2OH free radical, the C−H distances are 1.081 and 1.078 Å. The shorter C−H bond is trans to the hydroxy proton in both places.The H−C−H angle is almost exactly the sp2 hybridization angle, at 120.5°, slightly smaller than the 121.4° angle in hydroxymethyl. The hydroxymethyl radical is nearly planar; one H−C−O−H dihedral angle is −173.1°, while the other is −23.6°. These dihedral angles are the same (to within 2°) as those in the free radical. Finally, the F−H−C−O dihedral angle is −69.4°. Energetics. A. Pathway A. On both pathways A and B, all of the intermediates and products lie below the separated reactants in energy (see Table 1); the potential energy surface is shown in Figure 7. This indicates that the reaction may proceed spontaneously and does not have to overcome any net barrier at the transition state. On pathway A, which produces methoxy radical, the entrance complex lies 6.59 kcal mol−1 lower in energy than the separated reactants. Transition state A is only 2.50 kcal mol−1 higher in energy than the entrance complex; therefore, it still lies 4.69 kcal mol−1 below the separated reactants. The reaction proceeds to exit complex A, which lies 32.39 kcal mol−1 below the transition state. The interaction between the methoxy radical and HF is energetically favorable, with a dissociation energy of 8.08 kcal mol−1. Comparing separated reactants to separated products, this pathway is exoergic by 28.40 kcal mol−1. Although this is a large ΔE, pathway B is actually more exoergic by almost 10 kcal mol−1. Pathway A was investigated by Jodkowski and co-workers using MP2 6-31G(d) geometries and G2 energies.31 According to their computations, the entrance complex lies 1.0 kcal mol−1 below separated reactants, and the transition state lies 1.3 kcal mol−1 above reactants. The products, methoxy radical and

Figure 4. Pathway B entrance complex between methanol and the fluorine atom at the aug-cc-pVQZ CCSD(T) level of theory. This complex is predicted to lie only 0.5 kcal mol−1 below the separated reactants, F + CH3OH.

distances are 3.023, 3.047, and 3.047 Å, indicating that the fluorine atom is interacting very weakly with the methyl group and not with a particular hydrogen atom. This is a long-range interaction. The C−H distances and the C−O distance are the same (to within 0.001 Å) as those in methanol, while the O−H distance is slightly shorter (0.958 Å) and the C−O−H angle is slightly smaller at 108.2°. The F−H−O−C dihedral angle is −176.0°. The transition state for pathway B involves the removal of a methyl proton by the fluorine atom (Figure 5). The relevant H−F distance is 1.800 Å, much longer than that for transition state A. The other two methyl protons are much farther away from the fluorine atom. The relevant C−H distance is 1.102 Å, while the other C−H distances remain close to 1.09 Å. The C− O distance is 1.414 Å. The O−H distance is 0.958 Å. The F−

Figure 5. Pathway B transition state at the aug-cc-pVQZ CCSD(T) level of theory. This transition state is predicted to lie 0.15 kcal mol−1 below the separated reactants, F + CH3OH. C



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Table 1. Total Energies (hartrees) and Relative Energies (kcal mol−1) for the Different Stationary Points on the F + CH3OH Potential Energy Hypersurface Using the CCSD(T) Method path A

reactants

aug-cc-pVDZ aug-cc-pVTZ aug-cc-pVQZ

−215.005993 −215.190211 −215.246096

aug-cc-pVDZ aug-cc-pVTZ aug-cc-pVQZ path B

0.00 0.00 0.00 reactants


−215.005993 −215.190211 −215.246096


0.00 0.00 0.00

entrance complex A

TS A

Total Energies (hartree) −215.016974 −215.012974 −215.200641 −215.196641 −215.256591 −215.252616 Relative Energies (kcal mol−1) −6.89 −4.38 −6.54 −4.03 −6.59 −4.09 entrance complex B TS B Total Energies (hartree) −215.006886 −215.005638 −215.191000 −215.189999 −215.246806 −215.245853 Relative Energies (kcal mol−1) −0.56 −0.22 −0.50 −0.13 −0.45 −0.15

exit complex A

products A

−215.062064 −215.247765 −215.304224

−215.048730 −215.234694 −215.291353

−35.19 −36.12 −36.48 exit complex B

−26.82 −27.91 −28.40 products B

−215.069339 −215.256855 −215.314238

−215.060812 −215.248477 −215.306240

−39.75 −41.82 −42.76

−34.40 −36.56 −37.74

Figure 7. F + CH3OH potential energy hypersurface at the aug-cc-pVQZ CCSD(T) level of theory. Relative energies with respect to the separated reactants are in kcal mol−1.

hydrogen fluoride, lie 28.4 kcal mol−1 below reactants at this level of theory. The exit complex between the methoxy radical and HF lies 7.5 kcal mol−1 below separated products, or 35.9 kcal mol−1 below reactants. These results differ significantly from ours, indicating that the reported MP2 geometries are not sufficiently accurate, even for a qualitative description of the potential energy surface. An accurate potential energy surface is crucial for kinetic studies because reaction rate constants depend exponentially on energy differences. This means that small errors in the surface propagate to very large errors in the branching ratios. B. Pathway B. Pathway B, which leads to the hydroxymethyl radical, has a much shallower entrance complex, which lies only 0.45 kcal mol−1 below the reactants. However, from the

entrance complex, the energy barrier is only 0.30 kcal mol−1, placing TSB 0.15 kcal mol−1 below reactants. Formation of exit complex B is energetically very favorable, by 42.61 kcal mol−1. The interaction in exit complex B is also favorable, with an interaction energy of 5.02 kcal mol−1, comparable to that for the HF dimer. Overall, pathway B is exoergic by 37.74 kcal mol−1. Pathway B was also investigated by Jodkowski and coworkers.31 They did not locate an entrance complex for this pathway. The transition state for pathway B lies 6.8 kcal mol−1 below separated reactants. The products, hydroxymethyl radical and hydrogen fluoride, lie 40.1 kcal mol−1 below reactants. Again, these results are greatly different from ours. D



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DISCUSSION Early experiments indicated that the F + methanol reaction has an inverse temperature dependence, that is, higher temperatures slow down the reaction. Given the extreme exoergicity of the reaction, on both pathways, this result is clearly in line with our theoretical potential energy surface. Experiments have also indicated that the methoxy pathway makes up about 40% of the product, which is surprising because pathway A is less exoergic and is statistically not as favorable as pathway B. However, examination of the potential energy surface reveals an explanation. The initial steps of pathway A, formation of the entrance complex and the transition state, are much more energetically favorable than the analogous steps on pathway B. For comparison, the F + H2 reaction, has a very shallow (tenths of a kcal mol−1) entrance complex and a classical barrier of approximately 1.5 kcal mol−1.6,9 The reaction is exothermic by some 30 kcal mol−1. The slightly more complex reaction of F + H2O has an entrance complex 3 kcal mol−1 below the reactants and a transition state 2.5 kcal mol−1 above reactants.11 The exit complex stabilizes the products by 6 kcal mol−1. For larger systems, the entrance complexes are significantly more favorable (10 kcal mol−1), and the transition states lie below separated reactants. For instance, in the F + NH3 reaction, the entrance complex lies 14 kcal mol−1 below reactants, and the transition state is 11.5 kcal mol−1 below the reactants. This reaction is exothermic by 25 kcal mol−1.17 The exit complex is stabilized by 10 kcal mol−1. In the F + CH3NH2 reaction, both of the entrance complexes are 20 kcal mol−1 below the reactants, and the barriers are 8 and 18 kcal mol−1 below reactants for removal of amine and methyl protons, respectively.18 Both pathways are exothermic by over 30 kcal mol−1. Again, the exit complexes lie about 10 kcal mol−1 below the separated product molecules. How do our results on the F + CH3OH reaction fit in with previous work? Pathway A, abstraction of the hydroxyl proton, follows the trend set by F + H2O, F + NH3, and F + CH3NH2; the entrance complex features a strong hydrogen-bonding interaction, putting the entrance complex in a potential well around 10 kcal mol−1 deep. The transition state for pathway A is well below the energy of separated reactants. As with all of the hydrogen abstraction reactions that we have discussed, pathway A is exothermic by over 30 kcal mol−1, and the exit complex contains a hydrogen-bonding interaction worth 5 kcal mol−1. However, pathway B seems to have more in common with the F + H2 reaction, a very small entrance well on the order of tenths of a kcal mol−1 and a transition state nearly isoenergetic with the separated reactants. After the transition state, however, pathway B follows the common pattern of 30 kcal mol−1 exothermicity and an exit complex with a 5−10 kcal mol−1 hydrogen-bonding interaction.

barrier are required to explain the kinetics of the OH + CH3OH reaction.40−42 This unexpected behavior at low temperatures is quite relevant to interstellar chemistry.41 Because the methanol plus hydroxyl reaction has a barrier (1.0 kcal mol−1 for the methoxy pathway and 3.6 kcal mol−1 for the hydroxymethyl pathway), interstellar chemists previously excluded it from their models, on the assumption that it could not occur at the temperatures relevant to the ISM. However, recent experiments have shown a large increase in reaction rate as the temperature decreases below 200 K.40,42

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemistry, Computational, and Theoretical Chemistry, the Program for New Century Excellent Talents in University in China (Grant NCET-10- 0949), and the Chinese National Natural Science Foundation (Grant 11174236).

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Reaction of a Fluorine Atom with Methanol: Potential Energy Surface

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