Temperature-Dependent Kinetics of Charge Transfer, Hydrogen-Atom

Article pubs.acs.org/JPCA

Temperature-Dependent Kinetics of Charge Transfer, HydrogenAtom Transfer, and Hydrogen-Atom Expulsion in the Reaction of CO+ with CH4 and CD4 Joshua J. Melko,† Shaun G. Ard,† Ryan S. Johnson,‡ Nicholas S. Shuman,† Hua Guo,‡ and Albert A. Viggiano*,† †

Air Force Research Laboratory, Space Vehicles Directorate, Kirtland AFB, New Mexico 87117-5776, United States Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131, United States

‡

ABSTRACT: We have determined the rate constants and branching ratios for the reactions of CO+ with CH4 and CD4 in a variable-temperature selected ion flow tube. We find that the rate constants are collisional for all temperatures measured (193− 700 K for CH4 and 193−500 K for CD4). For the CH4 reaction, three product channels are identified, which include charge transfer (CH4+ + CO), H-atom transfer (HCO+ + CH3), and Hatom expulsion (CH3CO+ + H). H-atom transfer is slightly preferred to charge transfer at low temperature, with the chargetransfer product increasing in contribution as the temperature is increased (H-atom expulsion is a minor product for all temperatures). Analogous products are identified for the CD4 reaction. Density functional calculations on the CO+ + CH4 reaction were also conducted, revealing that the relative temperature dependences of the charge-transfer and H-atom transfer pathways are consistent with an initial charge transfer followed by proton transfer.

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density at the abstracting atom (oxygen).7 Further, it has been shown that this mechanism competes with an indirect pathway in which the hydrocarbon first coordinates to the metal center.7 While this indirect mechanism is not a true HAT mechanism as defined above, it represents the preferred pathway in almost all of the diatomic systems studied.12−15 An exception is the reaction of CO+ with methane,16 an interesting case study for two reasons. First, while heavier members of carbon’s group prefer the indirect pathway where the metal or metalloid facilitates HAT, CO+ opts for the direct HAT mechanism. Second, CO+ represents the only cationic system that can facilitate HAT at two sites, resulting in two isomeric HAT products (HCO+ and COH+).16 The structure and isomerization energy of these formyl isomers have received significant attention, mainly due to their importance in interstellar chemistry.17−21 In this light, the reaction of CO+ with methane has been studied by a handful of groups in the gas phase, as early as the late 1960s.22−27 Rate constant and product branching measurements have been made using selected ion flow tube (SIFT)25 or ion cyclotron resonance (ICR) techniques.16,27 It has been shown that the reaction proceeds

INTRODUCTION Hydrogen-atom transfer (HAT) reactions represent one class of the ubiquitous proton-coupled electron-transfer (PCET) reactions.1−6 HAT is generally described as a concerted reaction in which a proton and an electron are transferred in a single kinetic step with no reaction intermediates. While there is some debate about the distinction of HAT from PCET reactions, recent work suggests that HAT be viewed as a simultaneous transfer of an electron and proton in an electronically adiabatic nature, whereas PCET generally includes nonadiabatic effects (e.g., the electron and proton transfers are at different sites or there is significant charge redistribution).3,4 This distinction is critical in order to appropriately model rate constant expressions and the effects of the local environment. Further, the importance of HAT reactions to a variety of chemical processes, for example, catalysis, combustion, and biological energy conversion, necessitates a mechanistic understanding and accurate theoretical treatment. Recently, gas-phase experiments have been able to isolate and probe the details of HAT reactions in conjunction with computational studies.7−15 Cationic metal oxides have shown an affinity for promoting HAT from hydrocarbons, wherein the oxygen site abstracts the hydrogen directly (i.e., no long-lived coordinated methyl group). In this scenario, the gas-phase approach was able to highlight the importance of unpaired spin © 2014 American Chemical Society

Special Issue: A. W. Castleman, Jr. Festschrift Received: January 17, 2014 Revised: February 19, 2014 Published: March 3, 2014 8141

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organic molecules.35 Ahlrichs basis sets of TZV(2p/s) quality for H and TZV(2df/sp) for other atoms were used. B2PLYPD3 work was performed with the ORCA 3.0 computational suite.36 UHF wave functions on B2PLYP-D3 structures were used for analysis of spin density transfer along the two possible intermediates of the HAT reaction.

at the collision rate, and in addition to the HAT pathway, charge-transfer (CH4+ + CO) and H-atom expulsion (CH3CO+ + H) products are found. The mechanistic details of the HAT pathway in this reaction have only very recently been tied to experimental results, utilizing ICR methods coupled with quantum chemical calculations.16 In the present study, we have examined the CO+ reaction with CH4 and CD4 from 193 to 700 K, establishing the temperature dependencies of the rate constants and product channels. Complementary calculations have also been performed, revealing subtle aspects of the various product pathways.

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RESULTS AND DISCUSSION The rate constants for the reaction of CO+ with CH4 and CD4 are shown in Figure 1. The CH4 reactions were measured at six

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EXPERIMENTAL AND THEORETICAL METHODS The experiments were conducted on a variable-temperature SIFT at the Air Force Research Laboratory, described in detail elsewhere.28 We have recently upgraded the source region from an electron impact source so that it now includes electrospray ionization and a direct insertion probe, details of which are the subject of a future publication. For the present study, the new electron impact source operates essentially identical to our previous setup, only now with a quadrupole bender, ion guide, and additional lenses. The parent ion CO+ was created in the new electron impact source by flowing neat carbon monoxide at levels sufficient to raise the source chamber base pressure to 1 × 10−5 Torr. We confirmed that the ion was CO+ and not N2+ by studying the reactivity of the 28 m/z cation with O2. Initially, a small leak showed a mix of the two ions through curved decay plots. Eliminating the leak produced pure CO+. The CO+ ions are injected into the flow tube via a Venturi inlet, which prevents back-streaming of the helium buffer gas. The buffer gas is maintained at 0.4 Torr within the flow tube, and the CO+ experiences about 104−105 collisions with helium (ensuring that it is thermalized to the flow tube temperature) as it is carried downstream to the reactant gas inlet where CH4 is added. Reaction occurs over the remaining 59 cm of the flow tube before a portion of the ions are sampled through a small orifice in a rounded nose cone; the remaining gas flow is pumped away through an oil-free roots pump equipped with a throttled gate valve. The sampled ions are focused into a quadrupole mass spectrometer and detected with an electron multiplier. Rate constants are obtained by a least-squares fit to the parent ion decay as a function of reactant flow. Product branching ratios are determined from the early part of the decay because secondary chemistry is fast (i.e., CH4+ + CH4 proceeds at the collision rate29). Extrapolating the product branching back to zero flow yields nascent branching ratios.30 We estimate the errors in rate constant measurements at ±25% for absolute values and ±15% for relative changes as a function of temperature, while errors in the product branching ratios are ±0.1 absolute and ±0.05 relative. Mass discrimination was checked by monitoring the total ion signal over a large range of reactant flows and was determined to be negligible, in large part because a high frequency was used for the quadrupole. In the current study, the temperature of the flow tube was varied from 193 to 700 K through the use of pulsed liquid nitrogen or resistance heaters at several zones along the flow tube. The double-hybrid DFT functional B2PLYP31 with the dispersion corrections32 utilizing the Becke and Johnson dampening scheme33 and the Resolution of the Identity (RIJ) and Chain of Spheres (COSX)34 approximations have been used to calculate geometries and zero-point-corrected energies along the CO+ + CH4 pathway. This approach has recently been shown to provide very accurate geometries for small

Figure 1. Experimental rate constants as a function of temperature for the reaction of CO+ with CH4 (red circles) and CD4 (blue diamonds). Absolute error bars are shown at ±25%. CD4 data are offset by 10 K for clarity.

temperatures from 193 to 700 K, while the CD4 measurements were only taken at three temperatures (193, 300, and 500 K) due to minimal gas quantities on hand. Within experimental error, the rate constants for both reactions are collisional at all temperatures measured. The collision rate constants, as calculated using quasi-classical trajectory calculations parametrized by Su and Chesnavich,37 are 1.18 × 10−9 and 1.10 × 10−9 cm3/s for CH4 and CD4 reactions, respectively. Previous rate constant measurements have only been made at 300 K,16,22,23,25−27 and we tabulate these values alongside our data for the methane and deuterated methane reactions in Tables 1 and 2, respectively. We find good agreement with most all of the previous rate constant measurements for the methane Table 1. Comparison of the Current Experimental Rate Constants and Product Branching Fractions (SIFT/2014) with Previous Measurements at 300 K for the Reaction of CO+ with CH4a product branching fractions 3

k (cm /s) 1.20 9.10 1.36 1.30 1.37

× × × × ×

−9

10 10−10 10−9 10−9 10−9

HCO+

CH4+

CH3CO+

method/year

0.53 0.35b 0.22 0.35 1.00

0.43 0.625b 0.72 0.61

0.04 0.025b 0.06 0.04

SIFT/2014 ICR/201316 ICR/198027 SIFT/197725 HPMS 197126

The corresponding calculated collision rate constant is 1.18 × 10−9. The error in our rate constant is ±25%, while error in our branching fractions is ±0.10. bRepresents an average of 13CO and doubleresonance experiments.16 a

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than the other reactant channels. An illustrative example is to think of charge transfer as the first step of the HAT pathway, with subsequent proton transfer creating HCO+; as the temperature is increased, the probability of the additional proton-transfer step is decreased, resulting in more branching to the charge-transfer product. This model of charge transfer followed by proton transfer is supported by our calculations. We find that the positive charge is already located on methane by the time that CO is within 4 Å of methane. This may be rationalized by the fact that the ionization energy of methane is 1.4 eV lower than CO.38 In Table 1, we compare our branching fractions with previous measurements for this reaction, which only exist at 300 K.16,25−27 There are two previous, independent, ICR measurements16,27 and one SIFT measurement25 that are all in fair agreement (at the extremes of the error limits) in terms of branching ratios; an older high-pressure source mass spectrometry (HPMS) measurement of branching appears erroneous, although their rate constant seems accurate.26 Our data show HCO+ to be the dominant channel while the newer ICR study (utilizing 13CO and double resonance) and the SIFT study show that charge transfer is dominant. All reliable studies show that CH3CO+ occurs only in a few percent of the reactions. Because of fair agreement, we carefully checked mass discrimination and other factors that could affect the branching ratio but did not find any problems. The observed trend with temperature has smaller error limits. While the temperature dependence may suggest that the ions in ICR were slightly hot, that would not explain the slight disagreement with the SIFT results. We provide our experimental branching fractions for the case of deuterated methane in Figure 3. One may expect the

Table 2. Comparison of the Current Experimental Rate Constants and Product Branching Fractions (SIFT/2014) with a Previous Measurement at 300 K for the Reaction of CO+ with CD4a product branching fractions 3

k (cm /s) −9

1.11 × 10 9.30 × 10−10

DCO+

CD4+

CD3CO+

method/year

0.41 1.00

0.54

0.05

SIFT/2014 MS/196723

a The corresponding calculated collision rate constant is 1.10 × 10−9. The error in our rate constant is ±25%, while the error in our branching fractions is ±0.10.

reaction; recent ICR experiments16 find a slightly lower value (∼80% of the collision rate constant), which they attribute to neutral CO diffusion from the source and potentially different thermalization conditions. In any case, the results are within error of each other, and it is clear that the reaction is very efficient. To our knowledge, the only previous rate constant measurements for the CD4 reaction were performed by A. G. Harrison et al. ∼45 years ago,22,23 and we agree with their nearcollisional value. Figure 2 shows our experimental branching fractions for the reaction of CO+ and CH4. We observe three product channels,

Figure 2. Experimental branching fractions as a function of temperature for the reaction of CO+ with CH4. Products include HAT (red circles), charge transfer (blue squares), and hydrogen-atom expulsion (green diamonds). Absolute error bars are shown at ±0.1. CH4+ data are offset by 10 K for clarity.

charge transfer (CH4+ + CO), HAT (HCO+ + CH3), and Hatom expulsion (CH3CO+ + H). We note that the ions in the latter two channels have isomers that are exothermic but indistinguishable in our apparatus; for now, we shall not make the isomeric distinction and instead refer to these ions as shown above (until discussing the isomers in the calculation section below). At low temperatures, we find that HAT is slightly preferred to charge transfer (53:43%). As the temperature is increased, the branching preference switches so that by 700, K charge transfer is definitively the majority (∼65%) of the products. We find that at all temperatures, H-atom expulsion is a minor channel (∼5%). The observation that charge transfer begins to dominate at higher temperatures is perhaps not surprising. The ion−molecule interaction time decreases with temperature, and charge transfer occurs on a shorter time scale

Figure 3. Experimental branching fractions as a function of temperature for the reaction of CO+ with CD4. Products include deuterium-atom transfer (red circles), charge transfer (blue squares), and deuterium-atom expulsion (green diamonds). Absolute error bars are shown at ±0.1. CD4+ data are offset by 10 K for clarity.

branching fractions to be quantitatively similar between the CH4 and CD4 reactions, given that the exothermicities of the products are all substantial (discussed below) and the ionization potentials only differ by 0.05 eV.38 There is indeed an analogous minor channel (in this case, CD3CO+ + D) observed at all temperatures; however, the D-atom transfer and charge-transfer products appear to behave differently than those 8143

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in the case of CH4. The charge-transfer product (CD4+ + CO) represents the majority of the products at all temperatures. We note that due to limited gas quantities, the measurements were taken fewer times at each temperature and were conducted over a more limited temperature range than those in the case of CH4. In any case, we can conclude that the identities of the products are analogous to those observed for CH4, and both charge transfer and D-atom transfer are significant products, which is important to note because this was not the case in previous measurements (see Table 2).23 Because both the CH4 and CD4 rate constants are collisional, the total kinetic isotope effect (KIE) is negligible. The difference in product branching discussed above, however, results in subtle KIEs of the product channels. We find that the KIE for HAT (i.e., HCO+/DCO+) at 300 K is 1.3 ± 0.1, and the KIE for hydrogen-atom expulsion (i.e., CH3CO+/CD3CO+) is 0.9 ± 0.1. All KIEs include corrections for the different collision rate constants. In order to probe mechanistic details of the reaction pathways, we have performed calculations on the stationary points of the CO+ + CH4 reaction at the B2PLYP-D3 level. Our results are shown in Figure 4 for the two isomeric forms of

due to the nature of the bonding orbitals in CO, which are polarized toward the oxygen atom. When a proton is placed at the carbon site, the electron density becomes less polarized, leading to a stronger and more covalent bond (like in N2). When the proton is added to the oxygen site to create the isoformyl cation COH+, the C−O bond is weakened. Our calculations indicate a different structure for the intermediates from that reported by Schwarz et al.16 Under both B3LYP and B2PLYP-D3, we observed an intermediate that is closer to the reactants than Schwarz et al., in that the CH bond in methane is 1.24 Å and the H−O bond is 1.14 Å compared to 1.36 Å for both distances in the previous study. Also, their structure displays a nearly planar CH3 moiety perpendicular to C−O−H, compared to our structure in which methane is still nearly tetrahedral with a compressed HCH bond angle of 69.9° for the H being abstracted. The geometry of the methane moiety in our structure is close to that of CH4+, consistent with the localized charge having already transferred to CH4 before a subsequent proton transfer. Starting from the Schwarz geometry, minimization leads to this current CH4+ like structure. Calculations using constrained DFT,41 which localizes the charge on either the CO or CH4 moiety, confirm that our intermediate structure is the one in which the majority of the charge is located in CH4, and that of Schwarz et al. corresponds to a cationic CO. These calculations also indicate that our structure is more stable. The H-atom expulsion pathway has been less well studied, and Figure 4 depicts the results of our calculations for this pathway. An intermediate complex and a transition state have been located for the reaction resulting in OC−CH3+ + H, and they are much lower (∼−2 eV) than the reactant asymptote. For CO−CH3+, neither an intermediate complex nor transition state could be located. Our overall calculated exothermicity reproduces the experimental value very well in the case of the acetylium ion CH3CO+; experimental data for the isomeric CH3OC+ does not appear to be available.

Figure 4. Energies of the stationary points relative to reactants. For clarity reasons, the charge-transfer pathway is not shown. The dashed line represents a pathway for which we were unable to locate an intermediate complex or transition state.

Table 3. Calculated Exothermicities of the Various Product Channels Compared to Experimentally Determined Values from the Literature

CO+ CO+ CO+ CO+ CO+

+ + + + +

CH4 CH4 CH4 CH4 CH4

→ → → → →

CO + CH4+ HCO+ + CH3 COH+ + CH3 CH3CO+ + H CH3OC+ + H

calculated ΔrH° (eV)

experimental ΔrH° (eV)

−1.51 −2.18 −0.40 −3.18 −0.69

−1.40 ± 0.0142 −2.00 ± 0.0142 −0.41 ± 0.2619,42 −3.00 ± 0.0142 not available

CONCLUSIONS

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AUTHOR INFORMATION

SIFT studies are carried out as a function of temperature on the reaction of CO+ with CH4 and CD4. All measured rate constants are collisional, agreeing with previous findings that are only available at 300 K. Our experiments identify three product masses, two of which have isomers that are exothermic but indistinguishable in our experiments; charge transfer creates CH4+, HAT produces the isomers HCO+ and COH+, and Hatom expulsion produces the isomers CH3CO+ and CH3OC+. Analogous products are identified for the reaction with deuterated methane. Our calculations show that all five pathways are indeed exothermic, agreeing with literature values where available. The product branching as a function of temperature for the reaction of CO+ with CH4 reveals that charge transfer begins to dominate over HAT as the temperature is increased. We rationalize this with a two-step model where HAT consists of charge transfer followed by proton transfer, and our calculation results are consistent with this picture. Lastly, for the H-atom expulsion pathway, we are able to identify the intermediate complex and transition state for one of the isomers.

HAT and the two isomeric forms of H-atom expulsion (the charge-transfer pathway is not shown for clarity reasons). In Table 3, we provide the experimentally determined exothermicities from literature alongside of our calculated exothermicities for these reactions. We find, similar to other work,16 that the HAT pathway occurs without a transition state at either the carbon or oxygen site of the CO+. The carbon site that leads to the formyl cation HCO+ represents the more exothermic channel. As previously discussed in the literature,16,39,40 this is

reaction

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Corresponding Author

*E-mail: [email protected]. 8144

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Notes

(15) Chen, K.; Wang, Z.-C.; Schlangen, M.; Wu, Y.-D.; Zhang, X.; Schwarz, H. Thermal Activation of Methane and Ethene by Bare MO•+ (M=Ge, Sn, and Pb): A Combined Theoretical/Experimental Study. Chem.Eur. J. 2011, 17, 9619−9625. (16) Dietl, N.; Troiani, A.; Schlangen, M.; Ursini, O.; Angelini, G.; Apeloig, Y.; de Petris, G.; Schwarz, H. Mechanistic Aspects of GasPhase Hydrogen-Atom Transfer from Methane to [CO]•+ and [SiO]•+: Why Do They Differ? Chem.Eur. J. 2013, 19, 6662−6669. (17) George D.X., F.; Kumar, S. Ab Initio Ground and the First Excited Adiabatic and Quasidiabatic Potential Energy Surfaces of H+ + CO System. Chem. Phys. 2010, 373, 211−218. (18) Burgers, P. C.; Holmes, J. L.; Mommers, A. A. Laboratory Experiments on the Interstellar Species Oxomethylium ([HCO]+) and Hydroxymethyliumylidene ([COH]+). J. Am. Chem. Soc. 1985, 107, 1099−1101. (19) McMahon, T. B.; Kebarle, P. The Formyl and Isoformyl Cations. A Pulsed Electron Beam High Pressure Mass Spectrometric Study of the Energetics of HCO+ and HOC+. J. Chem. Phys. 1985, 83, 3919−3923. (20) Holmes, J. L.; Fridgen, T. D. A Study of the Methane Catalyzed Isomerization of HCO+ to HOC+ and the Elimination of Methane from Metastable Methoxymethyl Cation. Eur. J. Mass Spectrom. 2004, 10, 747−754. (21) Yamaguchi, Y.; Richards, C. A.; Schaefer, H. F. High Level Ab Initio Study on the Ground State Potential Energy Hypersurface of the HCO+−COH+ System. J. Chem. Phys. 1994, 101, 8945−8954. (22) Harrison, A. G.; Ivko, A.; Shannon, T. W. Reactions of Thermal Energy Ions. II. Rates of Some Hydrogen Transfer Ion−Molecule Reactions. Can. J. Chem. 1966, 44, 1351−1359. (23) Harrison, A. G.; Myher, J. J. Ion−Molecule Reactions in Mixtures with D2 or CD4. J. Chem. Phys. 1967, 46, 3276−3277. (24) McAllister, T. High-Resolution Ion Cyclotron Resonance Spectroscopy of Mixtures of CH4 with N2O and CO2. Int. J. Mass Spectrom. Ion Processes 1977, 25, 55−60. (25) Adams, N. G.; Smith, D.; Grief, D. Reactions of HnCO+ Ions with Molecules at 300 K. Int. J. Mass Spectrom. Ion Phys. 1978, 26, 405−415. (26) Chong, S.-L.; Franklin, J. L. High-Pressure Ion−Molecule Reactions in Carbon Monoxide and Carbon Monoxide−Methane Mixtures. J. Chem. Phys. 1971, 54, 1487−1495. (27) Huntress, W. T., Jr.; McEwan, M. J.; Karpas, Z.; Anicich, V. G. Laboratory Studies of Some of the Major Ion−Molecule Reactions Occurring in Cometary Comae. Astrophys. J. Suppl. Ser. 1980, 44, 481− 488. (28) Viggiano, A. A.; Morris, R. A.; Dale, F.; Paulson, J. F.; Giles, K.; Smith, D.; Su, T. Kinetic Energy, Temperature, and Derived Rotational Temperature Dependences for the Reaction of Kr+(2P3/2) and Ar+ with HCl. J. Chem. Phys. 1990, 93, 1149−1157. (29) Anicich, V. An Index of the Literature for Bimolecular Gas Phase Cation−Molecule Reaction Kinetics; Jet Propulsion Laboratory: La Cañada Flintridge, CA, 2003. (30) Anderson, D. R.; Bierbaum, V. M.; DePuy, C. H.; Grabowski, J. Flowing Afterglow Studies of Organic Positive Ions Generated by Penning Ionization Using Metastable Argon Atoms. Int. J. Mass Spectrom. Ion Processes 1983, 52, 65−94. (31) Grimme, S. Semiempirical Hybrid Density Functional with Perturbative Second-Order Correlation. J. Chem. Phys. 2006, 124, 034108. (32) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H−Pu. J. Chem. Phys. 2010, 132, 154104. (33) Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the Damping Function in Dispersion Corrected Density Functional Theory. J. Comput. Chem. 2011, 32, 1456−1465. (34) Neese, F.; Wennmohs, F.; Hansen, A.; Becker, U. Efficient, Approximate and Parallel Hartree−Fock and Hybrid DFT Calculations. A ‘Chain-of-Spheres’ Algorithm for the Hartree−Fock Exchange. Chem. Phys. 2009, 356, 98−109.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.A.V. and J.J.M. wish to dedicate this article to Prof. Will Castleman, Jr. in recognition of his many important contributions to physical chemistry and his positive influence as a teacher, colleague, mentor, and friend. We are grateful for the support of the Air Force Office of Scientific Research for this work under Project AFOSR-2303EP. J.J.M. and S.G.A. acknowledge the support of the National Research Council. R.S.J. and H.G. are supported by the Department of Energy (DE-FG02-05ER15694).

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