with the Super Greenhouse Gas SF5CF3 - American Chemical Society

Oct 21, 2014 - William S. Taylor,* Jerald M. Manion, Christopher M. Church, Xavier S. .... which differ by either the presence or absence of an s elec...
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State-Specific Reactions of Cu+(1S,3D,1D) with the Super Greenhouse Gas SF5CF3 William S. Taylor,* Jerald M. Manion, Christopher M. Church, Xavier S. Redmon, and Benjamin A. Scheuter Department of Chemistry, University of Central Arkansas, Conway, Arkansas 72035, United States ABSTRACT: State-specific reactions of the potent greenhouse gas SF5CF3 with Cu+ were carried out in a selected ion drift cell apparatus. Copper ions were prepared in a glow discharge utilizing Ne as the working gas. Analysis of these ions using ion mobility mass spectrometry (IMS) indicated the presence of both Cu+(3d10) and Cu+(3d94s1) configurations. Subsequent analysis indicates that the 3d10 configuration consists of Cu+(1S) exclusively whereas the 3d94s1configuration is composed primarily of Cu+(3D) with small contributions from Cu+(1D). State-specific product formation in reactions of these ions with SF5CF3 was determined using IMS along with the known energetic requirements for product formation. These experiments reveal that Cu+ excited states initiate fragmentation of SF5CF3 to yield SF2+, SF3+, SF5+, and CF3+, where SF3+ represents the largest branching fraction at 90% of the total bimolecular product formation. The energetics associated with the formation of these ions suggest that molecular Cu-containing products must also be formed in all cases, indicating that the governing reaction mechanisms are more complicated than simple dissociative charge transfer. Production of SF2+ and SF3+ are shown to proceed via Cu+(3D) and can be rationalized with a two-step mechanism proceeding through the common intermediate SF3CF3+. Production of CF3+ can be explained using this same mechanism but is also energetically possible from Cu+(1D) in a more direct process. Energetic requirements indicate that Cu+(1D) is the sole source of SF5+ with concomitant formation of CuCF3. Cu+(1S) exhibits adduct formation exclusively, but IMS spectra of the resulting Cu+·SF5CF3 suggest that as many as three association structures are formed.

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INTRODUCTION A continuing line of inquiry in our laboratory has been the exploration of factors affecting σ-bond activation by bare metal ions. In particular, Cu+ has provided an interesting and accessible model system in that it is readily prepared in a sputtering glow discharge in both the 1S (ground) and 3D (first excited) states. Provided that a means of distinguishing between these two states is possible, comparisons between their respective chemical behaviors can be made. Our previous studies of the reactions of Cu+(1S,3D) with a variety of halogenated methane analogs have shown that under nearthermal conditions, Cu+(3D) exhibits halogen atom abstraction to yield CuX+, provided that a labile C−X bond is available.1−3 Indeed, with the neutral reactants we have studied, this process appears to occur in all cases in which abstraction is thermochemically favorable and conserves spin. In the work described here, we extend our examination of the chemistry of Cu+(1S,3D) to include trifluoromethyl sulfur pentafluoride (SF5CF3). Unlike the methane analogs discussed above, Cu+induced halogen abstraction from the carbon is not possible with this neutral; however, it is readily consumed by both Cu+ states to yield a number of different products arising via other pathways. In addition to providing a somewhat more complex molecular substrate with which to examine the state-specific behavior of Cu+, SF5CF3 has generated considerable interest in its own right. First discovered in the atmosphere in 2000, © XXXX American Chemical Society

SF5CF3 has a global warming potential approximately 18 000 times that of CO2, thus classifying it as a “super greenhouse gas”.4 Initial studies indicated that atmospheric concentrations of SF5CF3 rose steadily from zero in the early 1960s, to approximately 0.15 ppt in 2002, where it has essentially stabilized.5 Interestingly, the halt in further increases in the atmospheric concentrations of this gas appears to be linked to the discontinuation of certain industrial processes related to the production of perfluorooctanyl sulfonate;5,6 however, some question remains as to whether or not these processes can account for the entirety of the SF5CF3 concentration currently in the atmosphere.7,8 Given that the lifetime of SF5CF3 has been estimated at approximately 1000 years,9 a number of reactions of this molecule with ions of atmospheric significance have been examined in an effort to better understand its environmental fate.10−12 Although certain metal ions (e.g., Mg+, Ca+, and Fe+) have long been known to occur in the mesosphere as a result of meteoric ablation, Cu+ is not known to be among them.13−15 Moreover, SF5CF3 is thought to be degraded photolytically at altitudes where atmospheric metal ions are known to exist.9 Thus, it is unlikely that reactions of this neutral with metal ions contribute significantly to depletion of this molecule within the atmosphere. This being Received: September 11, 2014 Revised: October 21, 2014

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Determination of Cu+ State Distribution. For the work described here, specific configurations of Cu+ ions produced in the glow discharge were differentiated within the drift cell using ion mobility spectrometry (IMS), which characterizes them on the basis of their mobilities in He.19,20 (Note that this particular application of IMS was originally introduced as “electronic state chromatography” by Kemper and Bowers.) Thus, applied, IMS is effective in distinguishing between electronic configurations which differ by either the presence or absence of an s electron. The larger size of the s orbital results in a less attractive interaction between the ion and the He bath gas, which reduces the number of capture-collisions. For Cu+, this means that the 3d94s1 configuration has a higher reduced zero-field mobility (22 cm2/(V·s)) in He than the 3d10 configuration (15.8 cm2/ (V·s)).18 As a consequence, a pulse of ions containing both configurations will be separated within the drift cell such that they appear as different peaks in an arrival time distribution (ATD). Under all discharge conditions employed here, both Cu+ configurations were formed. A recent analysis of the energetics of excited state production within the discharge provides evidence that it is capable of producing excited metal ion states with energies up to approximately 11.5 eV above the atom ground state.21 This upper limit includes the 1S(3d10) ground state for Cu+, as well as the 3D(3d94s1) and 1D(3d94s1) excited states that lie 2.808 eV (averaged over J-levels) and 3.257 eV above the 1S state. Without question, the low-mobility feature in our Cu+ ATDs is the 1S ground state, but we cannot rule out the presence of both Cu+(3D) and Cu+(1D) within the 3d94s1 feature on the basis of IMS alone because both are indistinguishable on the basis of their mobilities. We have argued previously that, though some amount of Cu+(1D) may be present in the 3d94s1 ATD feature, it is likely that the major contributors to this configuration are the energetically more accessible 3D3,2,1 states.2 Charge-transfer bracketing tests utilizing C3F6 do in fact result in the production of small amounts of C3F6+, consistent with the presence of Cu+(1D) within the 3d94s1 population extracted from the discharge. On the basis of this diagnostic, we estimate that Cu+(1D) represents less than 2% of the 3d94s1 population. Reagents. Copper cathodes used as sputter targets were fashioned from used oxygen-free copper gaskets into 5.0 mm diameter rods. Research grade neon (the discharge gas) was obtained from Matheson Tri-Gas Inc. Helium used as the buffer gas in the drift cell was obtained from Air Products Inc. with a purity of 99.9999%. SF5CF3 was obtained from Synquest Gases Inc. with a purity of 99%. Computational Methods. Singlet association product structures and transitions states were determined with the Gaussian 09 suite of programs using density functional methods.22 Transition states were located using the synchronous transit-guided quasi-Newton (STQN) method developed by Peng et al.23 All calculated energies were corrected for zeropoint energy contributions. These calculations were carried out using the B3LYP functional in conjunction with the aug-ccpVTZ basis set.24−26 This is a correlation-consistent basis set that includes polarization functions with triple-ζ split-valence and diffuse functions in the augmented versions.

noted, however, the chemistry we report below bears some similarities to that observed with several of these other ions, and indicates that energetic requirements governing the outcomes of these reactions necessitate bond formation in the neutral products.

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EXPERIMENTAL METHODS Instrumental Description. Experiments were carried out using a selected ion drift cell apparatus that has been described previously.1 Cu+ ions used in this work were generated with a dc glow discharge ion source (discussed below) and directed through a quadrupole deflector (turning lens) and then to a quadrupole mass filter for mass selection. This selection quadrupole was operated in resolving mode, with a sufficiently low resolution such that both Cu+ isotopes were transmitted. The reactant ion beam was then focused onto the entrance aperture of a 4.0 cm drift cell which has been described in detail elsewhere.16 The pressure in the drift cell was maintained at a constant pressure of 3.5 Torr with a mixture of SF5CF3 in He. SF5CF3 mole fractions (denoted in the results by XSF5CF3) were on the order of 10−4. Though small relative to the He buffer gas, this concentration was sufficiently greater than the ion number density that pseudo-first-order conditions existed with respect to depletion of Cu+. The extent of reaction was controlled by variation in the SF5CF3 mole fraction. Reactant ions were drawn through the drift cell by means of a small electric field maintained by a set of seven guard rings during which time reaction occurred. Experiments described here were carried out at an E/N of 8.5 Td (1 Td = 1 × 10−17 cm2·V), and residence times for reactant ions were on the order of 100 μs. These reaction conditions are such that little translational heating occurs and only exothermic or thermoneutral reactions are typically observed. Temperature control of the drift cell is accomplished via a copper shroud through which heated or cooled gases can be circulated. The reactions described in this work were carried out at room temperature and 173 K, where liquid nitrogen was used as the cryogen. Temperatures within the drift cell were monitored using a Pt-RTD (resistance temperature device). Ions exiting the drift cell were massanalyzed with the use of a second quadrupole, and detected using an electron multiplier operated in pulse-counting mode. Metal Ion Source. Cu+ ions for use in this work were produced by a sputtering glow discharge source that has been described previously.17 When operated as in the experiments described here, this ion source produces metal ions via a sputter bombardment process in which ions of a working gas (Ne in this case) are accelerated to a cathode made from the desired metal and sputter atoms from its surface. These sputtered atoms diffuse into the discharge plasma and are subsequently ionized either by Penning ionization via metastables of the working gas or by electron impact ionization via fast electrons being accelerated from the cathode. Metal ions are sampled directly from the discharge plasma. We have previously demonstrated that this ion source is capable of producing metal ions in excited states as well as in their ground states. Further, excited state populations can be controlled to some extent by manipulation of both the working gas pressure and the distance between the cathode and the sampling aperture.18 Both methods were utilized in this work to alter the relative amounts of excited and ground states of Cu+ extracted from the discharge.

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RESULTS AND DISCUSSION Under the experimental conditions described above, Cu+ induces fragmentation pathways with SF5CF3 resulting in the formation of SF3+ as the major product along with SF2+, SF5+, and CF3+ in lesser amounts. These ions are listed in Table 1, B

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Table 1. Possible Reaction Pathways and Related Thermochemistry thermochemistry (kJ/mol) observed product iona

branching fraction (%)b

possible neutral productsa

SF2+[D] SF3+[S]

6±1 90 ± 2

SF5+[S] CF3+[S]

2±1 2±1

CuF2[D] + CF4[S] CuF2[D] + CF3[D] CuF[S] + CF4[S]c CuCF3[S] CuF2[D]+SF3[D] CuF[S] + SF4[S] CuSF5[S]

1

S

113 ± 21 183 ± 8 −22 ± 28 +301 ± 32d 266 ± 5 260 ± 5 264 ± 24e

3

D

−158 ± 21 −88 ± 8 ΔΣ≠0 ΔΣ≠0 −5 ± 5 ΔΣ≠0 ΔΣ≠0

1

D

−201 ± 21 −131 ± 8 −336 ± 28 −13 ± 32d −48 ± 5 −54 ± 5 −50 ± 24e

a Molecular singlet and doublet states indicated by S and D, respectively. bIncludes possible contribution from both Cu+(3D) and Cu+(1D) excited states where thermochemically possible. cProduct channel not believed to occur (see text). dIncorporates calculated Cu−CF3 binding energy (see text). eIncorporates calculated Cu−SF5 binding energy (see text).

different neutral byproducts. However, the energetic environment within the drift cell limits the number of possible reaction pathways to only those that are exothermic. Thus, the processes listed in Table 1 include only those that satisfy this criterion with respect to at least one of the Cu+ states known to be present within the drift cell. In particular, this energetic requirement cannot be satisfied by dissociative charge transfer alone−indicating a more complicated process in which the neutral byproducts in these reactions involve some degree of additional bond formation. Enthalpy of formation data (at 298 K) and excitation energies used in calculating the thermochemistry for the reactions in Table 1 were obtained from the work of Afeefy et al.27 and Kramida et al.,28 along with ionization energies obtained from tabulations by Lias29 (SF2 and SF5) and Lias et al.30 (SF3). A number of enthalpies of formation can be found for SF5CF3. The thermochemistry in Table 1 uses the value of −1717.05 kJ/mol as listed in ref 27. Likewise, a wide range of ionization energies has been reported for CF3 (necessary in calculating the reaction energetics for CF3+ formation). In this work we have used the value of 9.04 ± 0.04 eV recommended by Tuckett31 as measured by Garcia et al.32 In all cases, reaction pathways exist which satisfy the requirement of exothermicity and also conserve overall spin from one or more Cu+ states. In light of this, and in the absence of compelling evidence to suggest that spin-forbidden processes are occurring, we have not invoked them in any of the reactions studied here. Where not otherwise excluded on the basis of energetics, additional information regarding state-specificity for individual product channels was obtained by correlating reactant and product ATDs obtained via the IMS experiment. In this technique, the Cu+ ATD was first collected in the absence of SF5CF3. A small amount of the neutral reactant was then admitted into the drift cell and the mass filter was tuned to the desired product ion mass while the Cu+ beam was pulsed. The ATDs for both species were then corrected to account for differences in flight times through the analysis quadrupole, and then overlaid. The reactant ion can be converted into the product at any point within the drift cell, but a product ion formed near the exit of the drift cell will exhibit a flight time characteristic of the reactant ion producing it. Thus, the product ion ATD will correlate to its reactant ion precursor configuration. SF2+ and SF3+. On the basis of thermochemistry, the most likely means of producing SF2+ originates from Cu+(3D) with concomitant formation of CuF2 and CF4 as the neutral byproducts. Minor contributions from Cu+(1D) are also possible but are less likely to be significant given that

along with possible neutral byproducts and related reaction thermochemistry. We note that one or more of these same product ions has been reported in the reactions of SF5CF3 with a number of other ions, including N2+, N+, CO+, N2O+, H2O+, O2+,10,11 Ne+, Ar+, F+, CO2+, SF4+, CF2+, SF+, SF5+, CF+, CF3+,11 and O+.10−12 Growth curves are shown in Figure 1 and

Figure 1. Growth curves for bimolecular product ion formation at 306 K under pseudo-first-order conditions with respect to Cu+. Solid lines represent exponential fits.

indicate that these product ions appear at the earliest extents of reaction. This suggests that they are all formed in primary steps from Cu+. Further, the branching fractions for all four remain constant within experimental error over a wide range of extents of reaction, indicating that little or no secondary depletion (or formation) occurs. For SF3+ and SF2+, this is consistent with the results by Atterbury et al., who observed no reaction of either of these ions with SF5CF3.11 In addition, both CF3+ and SF5+ have themselves been previously shown to react with SF5CF3 to form SF5+ (from CF3+) and CF3+ (from SF5+) as minor products, with SF3+ being by far the most abundant product ion in both reactions.11 Thus, we cannot rule out the possibility that some amount of secondary formation of these two ions occurs in the Cu+ reaction at higher extents of reaction. Having said this, we believe that any secondary contribution to either is insignificant because it would constitute the minor product formed from a precursor that is itself present in small amounts. State-Specificity. For each product ion, it is possible to envision several product channels initiated by Cu+ that result in C

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Cu+(3D) is the dominant excited state present. Thermochemistry alone is insufficient to determine the precursor Cu+ state(s) for SF3+ as formation of this ion is possible from all three Cu+ states; however, the large branching fraction exhibited by this product indicates that here also, little or no contribution from Cu+(1D) occurs. This being said, there are still two exothermic processes that can be envisioned which are initiated by either Cu+(1S) or Cu+(3D) and differ in the neutral byproducts that are formed (CuF and CF4 from Cu+(1S); CuF2 and CF3 from Cu+(3D)). However, the product correlations shown in Figure 2 indicate that the ground state process can be

corresponding enhancement in the SF3+ intensity that occurs in constant proportion to the Cu+(3D) ATD feature. Further, manipulation of the Cu+(3D)/Cu+(1S) state distribution results in no apparent change in the shape of the SF3+ ATD (when normalized to each other, they are coincident), suggesting that there are no features of this ATD originating from the Cu+(1S) state which are simply unresolved. Thus, we conclude that SF3+ production occurs exclusively via Cu+(3D) and is accompanied by formation of CuF2 and CF3. Given that there is no apparent thermochemical prohibition for production of SF3+ via Cu+(1S), this suggests an activation barrier. We note that the only exothermic, spin-allowed process by which Cu+(1S) can yield SF3+ is accompanied by formation of CF4, which necessarily requires the migration of a fluorine from sulfur to carbon. It seems reasonable therefore to conclude that this fluorine migration is associated with an activation barrier sufficient to preclude this product channel on kinetic grounds. The production of CuF2 in the parallel formation of both SF2+ and SF3+ suggests the possibility of mechanisms that share common features, as proposed in Figure 3. In these two-step

Figure 2. Correlation of SF3+ (dotted lines) and Cu+ ATDs (solid lines) at different relative populations of Cu+(3D). All ATDs are acquired at 173 K, XSF5CF3 = 6.5 × 10−5, and normalized to the intensity of the Cu+(1S) feature. SF3+ ATDs are representative of identical acquisition times relative to one another.

ruled out despite favorable thermochemistry. This is specifically indicated by the leading edges of the SF3+ ATDs (dotted lines), which show a clear correlation with the Cu+(3D) feature, consistent with production from this state. These ATDs also indicate that SF3+ has a lower mobility in He than that of Cu+(3D). As such, SF3+ ions detected at the earliest times are those formed near the exit of the drift cell and exhibit flight times representative of Cu+(3D). Conversely, the peak of the SF3+ ATD corresponds to ions produced early in the drift cell. These “promptly-formed” ions exhibit flight times indicative of the mobility of SF3+. The position of this maximum in the SF3+ ATDs in Figure 2 suggests that the mobility of SF3+ falls somewhere between that of Cu+(3D) (22 cm2/(V·s)) and Cu+(1S) (15.8 cm2/(V·s)).18 A more precise measurement of this value was obtained independently by producing SF3+ prior to the drift cell via electron ionization of SF6 and measuring its mobility in He directly. These determinations yielded a mobility for this species of 18.4 ± 0.4 cm2/(V·s), which is consistent with the position of the SF3+ ATD relative to the ATDs for the two Cu+ states. Further confirmation that SF3+ arises exclusively from Cu+(3D) is provided by an examination of the dependence of the intensity of the SF3+ ATD relative to the contribution of Cu+(3D) to the total Cu+ population. When the ion source conditions are adjusted to enhance formation of Cu+(3D) relative to Cu+(1S) (indicated by the solid ATDs) at a constant SF5CF3 mole fraction of XSF5CF3 = 6.5 × 10−5, there is a

Figure 3. Proposed mechanisms for parallel formation of SF2+, SF3+, and CF3+ via Cu+(3D).

processes, Cu+ reacts with SF5CF3 initially to form the common intermediate SF3CF3+, which subsequently undergoes unimolecular decay to yield either SF2+ (reaction 2a) or SF3+ (reaction 2b). Mass spectra taken at various extents of reaction revealed no evidence of the proposed SF3CF3+ species. We therefore conclude that if formed, the lifetime of this intermediate is fleeting. DFT calculations carried out as described above indicate that ground state SF3CF3+ is a stable species. Thus, if occurring, its absence in our product spectra suggests the possibility that it is formed in an energized state that facilitates its subsequent decomposition. In this mechanistic scenario, SF2+ production requires that one of the fluorines bonded to sulfur migrate to carbon, whereas SF3+ formation results from simple homolytic cleavage of the S−C bond. As discussed above for SF3+, it is not unreasonable to D

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expect that the fluorine migration in the SF2+ channel likewise represents an inhibiting mechanistic feature. In this case, the resulting kinetic inefficiency is reflected in a reduced branching fraction for SF2+ relative to that of SF3+. CF3+. A previous TPEPICO study of SF5CF3 has shown that this molecule exhibits two dissociative ionization thresholds at 12.9 eV (to yield CF3+) and 13.6 eV (to yield SF5+).33 Thus, the most direct pathway to form CF3+ is by dissociative charge transfer to Cu+. However, the sum of the 7.7268 eV recombination energy (RE) of Cu+(1S)29 and the additional excitation energies of 2.808 and 3.256 eV in Cu+(3D) and Cu+(1D) is only 10.348 and 10.983 eV for the two excited states respectively28both well below the required 12.9 eV. Thus, formation of CF3+ via direct charge transfer is thermochemically unfavorable from all Cu+ states produced in the discharge. Therefore, as in the production of SF3+ and SF2+, the mechanism to form CF3+ must likewise incorporate the formation of one or more bonds in the neutral products thereby supplying the necessary additional exothermicity. The three product channels listed in Table 1 present possible candidates that satisfy this energetic requirement while also conserving spin. In the absence of additional information regarding the structures of the neutral byproducts, these processes are indistinguishable from one another; however, some general observations are possible. Though none are thermochemically accessible from Cu+(1S), one of these processes is essentially thermoneutral via Cu+(3D) and all three are exothermic from Cu+(1D). Because it is impossible to determine unequivocally whether the small branching fraction for CF3+ is due to inefficient production via the more abundant Cu+(3D) state, or the result of it being formed from the less populous Cu+(1D) state, we are compelled to consider both possibilities. We point out, however, that CF3+ formation via both Cu+(1D) and Cu+(3D) can be easily explained within the context of the mechanism shown in Figure 3 via heterolytic cleavage of the S−C bond in the SF3CF3+ intermediate to yield CF3+ and SF3 as shown in step 2c. If occurring in this manner, inefficient participation of Cu+(3D) (and the resulting low branching fraction for CF3+) is consistent with the near thermoneutrality of this product channel. Alternatively, CF3+ can be formed exclusively from Cu+(1D) via fluoride abstraction followed (or accompanied) by heterolytic S−C bond cleavage as shown in Figure 4:

ΔHf(SF5) + ΔHf(CF3) + IE(CF3) − (ΔHf(SF5CF3) + IE(Cu)) − E1D − D298(Cu − SF5) < 0

where E1D and D298(Cu−SF5) represent the 3.256 eV excitation energy of the Cu+(1D) state relative to the ion ground state28 and the energy of the Cu-SF5 bond, respectively. We are unaware of any experimentally measured values for the Cu−SF5 binding energy. Therefore, the overall thermochemistry for the formation of CuSF5 given in Table 1 utilizes a value for this bond strength of 201 kJ/mol calculated using the density functional methods described above and includes a thermal correction to 298 K. An indication of the accuracy of this value was obtained by a comparison of the calculated binding energies in other neutral Cu complexes where experimental values are known. We have previously shown that calculated bond energies for Cu−Cl and Cu−Br obtained using this level of theory underestimate the experimental values by 10.6% and 7.5% respectively.2 These same methods also predict a binding energy for CuF of 390 kJ/mol, which is likewise lower than the experimental value of 413 kJ/mol by 5.6%.34,35 These simple species are similar to CuSF5 in that Cu is also formally +1. It is therefore prudent to consider the possibility that our calculated value for the Cu−SF5 binding also underestimates the actual bond strength, and that the reaction yielding CuSF5 via Cu+(1D) is even more exothermic than the −50 kJ/mol listed in Table 1. Finally, an additional exothermic pathway in the CF3+ channel can be envisioned, which results in the formation of FCuSF4 as the neutral byproduct. Located utilizing the DFT methods described above, this structure represents an additional stationary point that differs from the CuSF5 isomer in that Cu is inserted into one of the S−F bonds. Further, it is predicted to be more stable than the CuSF5 isomer by 104 kJ/ mol. Thus, its formation represents a more thermochemically favorable reaction pathway than the CuSF5 structure, resulting in an overall exothermicity of −154 kJ/mol relative to Cu+(1D). It is impossible to determine from our results which of these structures occurs, or if both do. Having said this, we have located a transition state separating the two structures which is calculated to occur only 4 kJ/mol above the CuSF5 isomer, suggesting that it undergoes facile conversion to the FCuSF4 structure. We conclude from this that, if occurring, production of CF3+ via S−C bond activation results in concomitant formation of the latter, whether it occurs directly or via rapid rearrangement of CuSF5. SF5+. As shown in Table 1, SF5+ was also observed as a minor product. In this case, reaction energetics limit primary production of this ion solely to Cu+(1D). Here also, the requisite exothermicity dictates that observation of SF5+ in our reaction must be accompanied by bond formation in the neutral products. The most likely process is one in which CuCF3 occurs as the neutral byproduct. This implies a Cu−CF3 binding energy >205 kJ/mol. DFT calculations suggest that this is possible in that this binding is predicted to be 218 kJ/mol. As in the case of the Cu−SF5 binding, we feel it likely that this calculated value represents a lower limit, and that the actual thermochemistry for SF5+ production via Cu+(1D) is more favorable that that shown in Table 1. Cu+·SF5CF3. In addition to the bimolecular products listed in Table 1, association was also observed as a major product channel representing approximately half of the total product ion signal from all Cu+ states combined. Secondary association

Figure 4. Alternative mechanism for production of CF3+ via Cu+(1D).

Even if this reaction proceeds efficiently, it can only yield limited amounts of CF3+ due to the small population of Cu+(1D) present in the drift cell and is therefore also consistent with the minor branching fraction for this product. The simplest process for CF3+ formation that can be envisioned would involve activation of the S−C bond to yield CF3+ and CuSF5. To conserve spin, this process must occur on a singlet surface, thus making its formation possible only from Cu+(1D). Energetically, this reaction can only occur if the CuSF5 bond energy is of sufficient magnitude such that the following condition is met: E

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the relative amount of the Cu+(3D) state, thus pointing to it as the source of these high-mobility association species. Conversely, the larger, lower mobility feature is unaffected in both shape and intensity by variation in the proportion of Cu+(3D), indicating Cu+(1S) as the source of the portion of the association product ions exhibiting longer residence times. Although too poorly resolved in the IMS spectra to measure, the position of the Cu+·SF5CF3 moiety arising from Cu+(3D) suggests that it has a mobility somewhere between that of Cu+(3d94s1) (22 cm2/(V·s)) and Cu+(3d10) (15.8 cm2/(V·s)). The occurrence of Cu+·SF5CF3 species with different mobilities is significant in that it suggests physical differences in the nature of the different association products giving rise to their respective ATD features, including structural differences and/ or spin states. Thus, we conclude that the high-mobility Cu+· SF5CF3 population originating from Cu+(3D) is either a distinct structural isomer or a triplet of a structurally similar, lowmobility singlet isomer, or both. The larger, low-mobility Cu+·SF5CF3 singlet feature in Figure 5 exhibits its own interesting characteristics. Clearly, the ions contributing to this feature exhibit a range of residence times within the drift cell. This behavior either can be the result of the presence of more than one product isomer with slightly different mobilities whose ATDs are unresolved or might be a manifestation of the kinetics of association as has been described by Mason and McDaniel.36 We believe, however, that this second possibility can be ruled out. If correct, this latter case would be similar to that of SF3+ as shown in Figure 2, where the product ion also has a lower mobility than its reactant precursor. Because the low-mobility Cu+·SF5CF3 feature has been shown to arise exclusively from Cu+(1S), and it clearly does not correlate at its trailing edge, it must therefore correlate on the left to the Cu+ ground state (obscured by the presence of the triplet feature discussed above), indicating products formed just prior to leaving the drift cell. As in the case of the SF3+ ATDs, ions at the trailing edge of the Cu+·SF5CF3 ATD would then represent promptly formed ions and should exhibit the highest intensity in the ATD. Because this is obviously not the case, we believe that it is more likely that this portion of the Cu+·SF5CF3 ATD is composed of two different singlet isomers of Cu+·SF5CF3 that were not resolved in the IMS experiment. On the basis of the position of the maximum intensity in the ATD, the isomer with the shortest residence time has a mobility slightly lower than Cu+(1S). However, because Cu+·SF5CF3 ions cannot be formed prior to the drift cell in our apparatus, a direct independent measure of the mobility of this species was not possible. It is unclear on the basis of our experiments whether these two isomers are formed independently of one another or if one is produced by the other within the drift cell. Notably, ATDs collected at room temperature and 170 K display no significant differences in resolution, consistent with the idea that isomerization occurs within the drift cell. However, experiments to test this in which the residence time in the drift cell was varied to provide more or less time for such an isomerization to occur were inconclusive.

was also observed at higher extents of reaction. Product correlation ATDs such as that shown in Figure 5 for Cu+·

Figure 5. Product correlation ATD for Cu+·SF5CF3. T = 170 K, XSF5CF3 = 2.3 × 10−5.

SF5CF3 exhibit features indicating association from both ground and excited Cu+ states. This is in contrast to results in our previous studies of state-specific Cu+ reactions where association was observed to arise almost exclusively from Cu+(1S).2 As we discuss below, the form of this ATD suggests the presence of at least two, and likely three, association species. More specifically, the Cu+·SF5CF3 ATD in Figure 5 suggests that the product ion population incorporates ions with both higher and lower mobilities than that of Cu+(1S). The high-mobility shoulder occurring at the shortest arrival times can be shown without question to arise exclusively from the high-mobility Cu+(3D) state. This is illustrated in Figure 6, where normalized product ATDs collected at the different 3 D/1S Cu+ distributions (inset graph) show that the intensity of the high-mobility Cu+·SF5CF3 feature is directly dependent on

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SUMMARY AND CONCLUSIONS Despite the fact that there is little likelihood of Cu+ having a significant impact on the environmental fate of atmospheric SF5CF3, this molecule provides an excellent substrate to extend our understanding of the state-specific reactions of this metal ion. In addition, the processes described in this work provide

Figure 6. Cu ·SF5CF3 ATDs as a function of the proportion of Cu+(3D) in the reactant ion state distribution. T = 170 K, XSF5CF3 = 2.3 × 10−5. Line colors indicate the corresponding Cu+ state distribution as indicated in inset graph. Reactant and product ATDs are normalized independently. +

F

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useful comparisons to our previous studies of Cu+(1S,3D) with simpler halogenated methane congeners. Whereas the bimolecular chemistry exhibited in those simpler systems is limited primarily to halogen abstraction and, in some cases, charge transfer, excited states of Cu+ induce a variety of decomposition processes in SF5CF3. These reaction pathways can be rationalized on the basis of the available energetics only by invoking the formation of neutral molecular byproducts whose nascent bonds pay the energetic cost associated with ionizing and fragmenting SF5CF3. The results reported here for the reactions of Cu+(3D) are consistent with a two-step mechanism in which CuF2 is initially formed along with the transient radical cation intermediate, SF3CF3+. This intermediate subsequently decomposes to yield SF2+, SF3+, and CF3+. It is also possible that CF3+ results from C−S bond activation by Cu+(1D) and accompanied by formation of CuSF5. The minor product, SF5+, is likely formed via C−S bond activation with concomitant production of CuCF3. For this product ion, however, energetic requirements indicate that it is formed exclusively from Cu+(1D). Like the bimolecular chemistry described above, the association of Cu+ with SF5CF3 presents a much more nuanced picture than the systems we have examined previously. In contrast to those simpler systems, where a single association species arises almost exclusively from Cu+(1S), adduct formation with SF5CF3 occurs from both excited and ground states. In addition, the data presented here provide good evidence that multiple isomers are formed that can be observed via IMS by virtue of their different mobilities. In summary, the environmental significance of SF5CF3 in the atmosphere notwithstanding, the reactions of this species described in this work provide an intriguing contribution to the somewhat limited catalog of its reactions examined to date.

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AUTHOR INFORMATION

Corresponding Author

*W. S. Taylor. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support for this research was provided by the National Science Foundation under Grant No. CHE-0956393.

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