Reactions of Atomic Metal Anions in the Gas phase: Competition

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Reactions of Atomic Metal Anions in the Gas phase: Competition between Electron Transfer, Proton Abstraction and Bond Activation Sharon Curtis, Jason DiMuzio,† Alex Mungham,† Julie Roy,† Dhiya Hassan,† Justin Renaud, and Paul M. Mayer* Chemistry Department, University of Ottawa, Ottawa, Canada K1N 6N5

bS Supporting Information ABSTRACT: Bare metal anions K , Rb , Cs , Fe , Co , Ni , Cu , and Ag , generated by electrospray ionization of the corresponding oxalate or tricarballylate solutions, were allowed to react with methyl and ethyl chloride, methyl bromide, nitromethane, and acetonitrile in the collision hexapole of a triple-quadrupole mass spectrometer. Observed reactions include (a) the formation of halide, nitride, and cyanide anions, which was shown to be likely due to the insertion of the metal into the C X, C N, and C C bonds, (b) transfer of H+ from the organic molecule, which is demonstrated to most likely be due to the simple transfer of a proton to form neutral metal hydride, and (c) in the case of nitromethane, direct electron transfer to form the nitromethane radical anion. Interestingly, Co was the only metal anion to transfer an electron to acetonitrile. Differences in the reactions are related to the differences in electron affinity of the metals and the ΔacidH° of the metals and organic substrates. Density functional theory calculations at the B3-LYP/6-311++G(3df,2p)//B3-LYP/ 6-31+G(d) level of theory shed light on the relative energetics of these processes and the mechanisms by which they take place.

’ INTRODUCTION Substitution reactions are a cornerstone in both natural biological synthetic pathways and modern synthetic chemistry. Oxidative addition/reductive elimination involves the loss of stereochemistry, while bimolecular nucleophilic substitution reactions, notably SN2 reactions, involve the inversion of stereochemistry. These reactions have been explored extensively in solution and the gas phase.1 8 The differences between solution and gas-phase behavior have been used to explore the role of the solvent in these reactions. While organic nucleophiles and substrates have been extensively studied, the use of metallic nucleophilic systems has been limited to a small class of organometallic complexes, both in solution and the gas phase. In their work on metal carbonyl anions, McElvany and Allison proposed that most of the reactions they observed with haloalkanes and nitroalkanes occur by charge transfer in an encounter complex, followed by metal insertion into bonds as is observed for metal cations.9,10 There have been only two previous studies of the reactions of gas phase bare metal anions with neutral substrates. Squires and Freiser generated V , Cr , Fe , Co , and Mo from their corresponding carbonyl compounds and examined their reactions with those parent compounds in an FT-ICR mass spectrometer.11 Stevens and co-workers generated Fe from the parent carbonyl and examined their reactions with methyl halides.12 Except for CH3F, each reaction yielded the halogen anion, presumably by an oxidative addition/reductive elimination mechanism. Recently, our group13 and that of Attygalle and co-workers14 introduced a new way to generate bare metal anions by electrospraying a solution of the metal oxalates. In our preliminary report, the collision-induced dissociation (CID) mass spectra of the singly charged metal oxalate anions was shown to generate the bare metal anion. Dissociation occurs by loss of CO2, which r 2011 American Chemical Society

itself has a negligible (or negative) electron affinity, leaving the electrons in the anionic complex to transfer to the metal. This CID process can also occur in the source, leaving the bare metal anion to be mass selected and undergo a reaction with a neutral reagent. In the present study, we explore the reactions of selected gas-phase metal anions with several neutral substrates, CH3Cl, CH3Br, CH3CH2Cl, CH3NO2, and CH3CN, with a view to examining the competition between dissociative electron transfer, proton transfer, and bond activation in the reactions. These molecules were selected because their respective gas-phase acidities and electron affinities are such that the ΔH of the proton transfer reactions and charge transfer reactions with the metal anions range from highly endothermic with CH3Cl to almost thermoneutral with CH3NO2. Also observed in some cases are substitution and oxidative addition/reductive elimination reactions. A method is also introduced to generate anions from +2 oxidation state metals in solution.

’ EXPERIMENTAL PROCEDURES Electrospray ionization mass spectrometry experiments were carried out on a Micromass Quattro-LC triple-quadrupole mass spectrometer equipped with a Z-Spray source and running the MassLynx 3.5 operating system. Metal oxalate solutions were prepared by combining 0.1 mg/mL solutions of oxalic acid (Sigma Aldrich) with similar concentration of the corresponding metal salt. Solutions, particularly silver oxalate, were shielded from light to prevent degradation. Due to the low solubility of many metal oxalate salts, this procedure was used to generate K , Received: September 8, 2011 Revised: October 21, 2011 Published: October 26, 2011 14006

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The Journal of Physical Chemistry A Rb , Cs , Cu , Ag , and Fe anions. The same procedure was shown to generate Na , however, the flux of this anion was too small to perform conclusive ion molecule reactions in the triplequadrupole mass spectrometer. Oxaloacetic acid (Sigma Aldrich) was also found to produce the desired anion of these metals. The Fe anion was prepared using Fe(III) chloride, and thus, the active species in solution was found to be the Fe(C2O4)2 anion, which dissociates in the skimmer cone region to produce Fe . The anions Co and Ni were made by forming a complex of their +2 state ion with tricarballylic acid (Sigma Aldrich) to form a 1 complex. A total of 20 μL of solution was injected into a 40 μL/min mobile phase of methanol. Neutral gases for ion molecule reactions were introduced directly into the hexapole, while volatile vapors from liquids were introduced via a Granville-Phillips variable leak valve after three freeze pump thaw cycles to remove air. Collisions were performed in the central hexapole. In most cases, the entrance and exit electrode potentials were set at 10 V, and the “collision energy” varied from 0 to ∼100 eV (or less if no changes were observed in the mass spectra). Specific conditions are reported for each mass spectrum reported in the paper. The collision energy was incremented in 5 V steps, except near the onset where 1 V steps were taken. The pressures listed in the figure captions were chosen to avoid multiple collisions of the metal anions. In many cases, evidence for sequential reactions was observed, such as the formation of K(NO2)2 in the reaction of K with CH3NO2, as the pressure of the reagent gas was increased. The reactant pressure was subsequently reduced until these products were unobservable.

’ COMPUTATIONAL PROCEDURES Density functional theory calculations were carried out with the Gaussian 03 suite of programs.15 Geometries were optimized (and vibrational frequencies subsequently obtained) at the B3-LYP/6-31+G(d) level of theory. Single-point energy calculations were carried out at the B3-LYP/6-311++G(3df,2p) level of theory. Transition states were confirmed by the intrinsic reaction coordinate method in Gaussian 03. The level of theory presently employed for the geometry optimization will likely overestimate the stability of some of the more weakly bound anions due to the constraint of the basis set on the added electron. Thus, the results from these calculations should be taken as qualitative, with the view to understanding the general principles surrounding the reactions of these metal anions. For example, the single point calculation does estimate the difference in electron affinity between the metals and organic substrates reasonably well: EA(Na) EA(CH3NO2) = 0.16 eV as compared to the experimentally derived value of 0.06 eV.16 Indeed, Puiatti and co-workers recently demonstrated that the B3-LYP/6-311+G(2df,p) level of theory provided adequate results for EA estimation.17 What are likely less satisfactory are the bond dissociation energies in the organic anions. For the neutral nitromethane molecule, the current approach puts the homolytic C H bond strength at 470 kJ mol 1, while higher level G2 calculations estimate the value to be 431 kJ mol 1,18 leading to the conclusion that the present value for the C H bond strength in the nitromethane anion, 210 kJ mol 1, could be overestimated by as much as 40 kJ mol 1. Similarly, one can examine the thermochemical cycle involving nitromethane, its anion, and the C N dissociation products from each. The relative C N bond strengths in the neutral and anion are a function of the difference in electron affinity of the molecule (0.5 eV) and NO2

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Table 1. Electron Affinity and ΔacidH Values for the Metal and Organic Species in This Study ΔacidH (kJ mol 1)16

EA (eV)16 Na

0.5480 ( 0.0040

NaH

1444 ( 1

K Rb

0.501459 ( 0.000012 0.485940 ( 0.000044

KH RbH

1439.840 ( 0.042 1436.7 ( 2.1 1441.765 ( 0.042

Cs

0.47164 ( 0.00006

CsH

Fe

0.1510 ( 0.0030

FeH

1439 ( 18

Co

0.66330 ( 0.00061

CoH

1411 ( 25

Ni

1.15717 ( 0.00013

NiH

1452 ( 7.9

Cu

1.236 ( 0.033

CuH

1451 ( 11

Ag

1.304 ( 0.028

AgH

1406 ( 7.9

Au CH3Cl

2.3090 ( 0.0010 0.127a

AuH CH3Cl

1378 ( 7.9 1672 ( 10

CH3Br

n/a

CH3Br

1660 ( 10

CH3CN

0.0030 ( 0.0072

CH3CN

1560 ( 8.8

CH3NO2

0.500 ( 0.020

CH3NO2

1498 ( 21

a

Calculated value. The geometry of the anion is essentially an ioninduced dipole complex between Cl and CH3, and the radical anion has never been made by electron attachment to the neutral.19

product (2.3 eV).16 Experimentally, it means that the C H bond in the anion must be 174 kJ mol 1 weaker than it is in the neutral molecule. A previous G2 estimate of the bond strength in neutral nitromethane is 264 kJ mol 1,18 placing the corresponding bond strength in the anion at 90 kJ mol 1, whereas the presently applied level of theory estimates the value at 50 kJ mol 1.

’ RESULTS AND DISCUSSION All mass spectra referred to in this study can be found as Supporting Information. It must be noted that only the K , Rb , Cs , Cu , Ag , and Fe anions were selected for reaction with the methyl halides, while K and Fe were chosen for reaction with ethyl chloride. The electron affinities of all species in this study are listed in Table 1. Alkali Metal Anions. Figure 1 provides examples of the reactions of the alkali metal anions with methyl chloride, acetonitrile, and nitromethane. Reactions involving methyl chloride all produce Cl with the correct isotopic abundance, Figure 1a. Similar reactions are observed with ethyl chloride, while the anions react with methyl bromide to produce Br . In the case of nitromethane, the observed reactions are CH3NO2 (m/z 61), and NO2 (m/z 46), Figure 1b. Reactions with acetonitrile produce m/z 40 (the CH2CN anion) and CN (m/z 26), Figure 1c. Halo, NO2, and CN Anion Formation. In each of the four sets of reactions, the formation of the small anionic leaving group is driven by their large electron affinity (EA(Cl) = 3.6144 eV, EA(Br) = 3.363583 eV, EA(NO2) = 2.273 eV, and EA(CN) = 3.8620 ( 0.0050 eV).16 The mechanism to account for this observation could be any one of the following: (a) general oxidative addition/reductive elimination of the metal anion to the C X, C N, or C C bond; (b) SN2 type nucleophilic substitution involving inversion of the methyl center; or (c) dissociative electron attachment. In their work on metal carbonyl anions, McElvany and Allison proposed that most of the reactions they observed with haloalkanes and nitroalkanes occur by charge transfer in an encounter complex, followed by metal insertion into bonds as is observed by metal cations.9,10 To test this, each of the pathways observed in the present study were 14007

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Figure 1. Examples of the reactions of alkali metal anions with neutral substrates (a) Rb with methyl chloride (hexapole pressure 1.0  10 3 mbar, entrance-cell-exit potentials set to 50:0:50), (b) K with nitromethane (hexapole pressure 3  10 4 mbar, entrance-cell-exit potentials set to 10:10:10 V), and (c) Cs with acetonitrile (hexapole pressure 2  10 4 mbar, entrance-cell-exit potentials set to 10:40:10 V).

computationally explored, using Na reacting with CH3Cl, CH3CN, and CH3NO2 as examples (Figure 2). For CH3Cl and CH3CN, SN2 substitution reactions, resulting in the formation of the leaving anion and a neutral NaCH3 molecule, are overwhelmingly exothermic, the barrier to the process lies significantly higher in energy than the reactants, leading to a kinetic bottleneck in the gas phase. In addition, the initially formed metal anion/ substrate complex has quite different character depending on the

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relative EAs of the metal and substrate. For CH3Cl and CH3CN, the initially formed complex has the metal anion interacting with the partially positive methyl group of the molecular dipole, unlike what was predicted by McElvany and Allison for the metal carbonyl anions.9,10 The subsequent transition state has partial negative charge on both the metal and the leaving group, as expected for this kind of mechanism. In the case of nitromethane, whose EA is slightly larger than that of Na, the calculations predict that charge transfer does largely takes place upon formation of the initial complex leading to a metal atom with slight positive charge interacting with the negatively charged nitro group of the nitromethane anion, Figure 3. There is no formal SN2 process that can occur from such a complex with that charge distribution. While methyl chloride has been calculated to have a slightly positive electron affinity, the geometry of the anion is essentially an ion-induced dipole complex between Cl and CH3, and the radical anion has never been made by electron attachment to the neutral.19 Vertical electron attachment to methyl chloride is endothermic by upward of 114 kJ mol 1 (Figure 2), resulting in a large endothermicity to the dissociative electron transfer mechanism. Acetonitrile will form only a dipole-bound anion upon electron transfer (EA of 3 ( 7 meV),16 and it is unlikely that this species survives electron detachment prior to dissociating to form CN given that the latter reaction is predicted to be endothermic by 80 kJ mol 1 (Figure 2). The electron affinity of nitromethane, on the other hand, is largely positive, 0.49 ( 0.11 eV,16 and so electron transfer to nitromethane can generate a valence-bound anion (and, thus, the observation of CH3NO2 in the mass spectra). There is also the possibility of forming the dipole-bound anion.16 Dissociation of the valence-bound anion would lead to NO2 only, as formation of CH2NO2 is too endothermic to compete, Figure 2. Another argument against dissociative electron transfer is the observation, under certain experimental conditions, of the ions MCH3 from the reactions with methyl chloride and MNO2 from those with nitromethane. Insertion of the metal anion into the C Cl bond produces an intermediate that is estimated to lie 261 kJ mol 1 below reactants and a subsequent loss of Cl that results in the total reaction being exothermic by 97 kJ mol 1, Figure 2. Given the low relative energy of the intermediate, the transition state leading from the reactant should correspondingly be significantly lower than that of the SN2 process, leading to the conclusion that this is likely the kinetically preferred route to the formation of X in the mass spectra. In the case of acetonitrile, the complex formed by insertion of Na into the C C bond is only 114 kJ mol 1 lower in energy than the reactants, and consequently, the reaction products NaCH3 + CN are endothermic by 44 kJ mol 1. Na also forms a thermodynamically stable insertion complex with nitromethane (Figures 2c and 3), and the overall reaction to form MCH3 + NO2 is exothermic by 63 kJ mol 1. Insertion by metalcontaining anions into C Cl and C NO2 bonds has been postulated by McElvany and Allison in the reactions of metal carbonyl anions with a variety of chloroalkanes and nitroalkanes, albeit by a proposed mechanism that first involves transfer of the charge to the organic molecule.9,10 Our calculations, as mentioned earlier, suggest that the metal anion itself can insert into the bond, with electron rearrangement taking place after this has happened. Indeed, the insertion reaction complex involves the sodium atom having significant positive charge in each case (Figure 3), and thus, charge transfer does take place during the course of the insertion reaction for CH3Cl and CH3CN when sodium is the metal. 14008

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Figure 2. B3-LYP/6-311++G(3df,2p)//B3-LYP/6-31+G(d) reaction energetics calculated for (a) Na + CH3Cl, (b) Na + CH3CN, (c) Na + CH3NO2, and (d) Cu + CH3NO2. All values are in kJ mol 1. Metal insertion into C X bonds are denoted with the solid lines ( ), insertion into C H bond are denoted ( •), and the SN2 reaction by the dashed line ( ). Note that no transition states were found for the insertion reactions.

Proton Transfer. Deprotonated organic molecules are also observed from the reactions of metal anions with acetonitrile and nitromethane but not from the haloalkanes. The trend is consistent with the relative acidites of the metal hydrides and each organic molecule (Table 1). The haloalkanes have high acidities placing H+ transfer to the metal anion endothermic by over 200 kJ mol 1. Transfer from acetonitrile is less endothermic and transfer from nitromethane is the least endothermic reaction as the ΔacidH° of the C H bond is most similar to those of the metal hydrides, ∼1498 kJ mol 1. None of these reactions is exothermic and therefore must be driven by the translational energy of the metal anion traveling through the collision hexapole. We explored the mechanism by which this proton transfer takes place. Bond activation by insertion of the metal anion into the C H bond, similar to what was observed previously for C Cl, C C, and C N bonds, can form thermodynamically stable complexes, Figures 2 and 3. In the case of CH3Cl and CH3CN, Na can insert into a C H bond to form a complex of the form H Na CH2Cl and H Na CH2CN, respectively, Figure 3. Both the CH2Cl and CH2CN anions exhibit nonplanar methylene groups as a result of the charge residing largely on the CH2 group (due to the avoidance of an unfavorable 4-electron interaction of the C-lone pair with the lone pair/π-orbital on the substituent).18 Consequently, the charge distribution in the

insertion complexes means it is essentially a negative organic anion interacting with the dipole of a neutral metal hydride (Figure 3). In contrast, the CH2 group in the CH2NO2 anion is planar, resulting from delocalization of the electrons (and thus charge) to the NO2 substituent.18 Thus, when Na insertion into the C H bond was investigated in CH3NO2, the optimized structure obtained was not like that for the other two species, but rather a CH2NO2 anion/NaH complex involving the interaction of the metal hydride dipole with the negative portion of the dipole in the anion, Figure 3, in keeping with the previously discussed formation of an initial collision complex in which charge has transferred to the nitromethane molecule due to its higher EA. There must be a significant barrier to the H transfer in the initially formed CH3NO2/Na complex preventing the formation of the CH2NO2 anion/NaH complex from competing with charge transfer and thus H+ transfer is not observed for the alkali metals. The alternative to an insertion reaction is a harpooningtype mechanism, which should out-compete insertion as it will be associated with more favorable entropic parameters. McElvany and Allison did not observe insertion into C H bonds in haloalkanes and nitroalkanes by transition metal carbonyl anions,9,10 which also suggests a harpoon-type mechanism of H+ transfer. In addition, the intact metal/substrate complex was never observed in our experiments, even though in most cases metal anion insertion 14009

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Figure 3. Optimized B3-LYP/6-31+G(d) geometries of the metal anion/substrate complexes mentioned in this paper (bond lengths in Å, angles in degrees). Partial charges are shown in italics (values may not add to 1 due to rounding).

into C X, C N, C C, and C H bonds produce thermodynamically stable structures. In several cases, this can be explained by the fact that the system is not at equilibrium and the reactants lie well above the products of the dissociation of these complexes. A notable exception is for Cu insertion into the C H bond of nitromethane (see below), the complex of which should be observable based on the large potential energy well it resides in and endothermicity of the overall reaction. The fact it is not observed also points to a harpoon-type mechanism. Transition Metal Anions with a Closed s-Orbital. The noble metal anions Cu and Ag were allowed to react with the three organic substrates. As observed for the alkali metal anions, only X is formed upon reaction with haloalkanes, Figure 4a. Reactions with nitromethane produce CH2NO2 (m/z 60), and NO2 (m/z 46), Figure 4b, while reactions with acetonitrile produce m/z 40 (the CH2CN anion) and CN (m/z 26), Figure 4c. In some cases there was evidence for oxide formation. Due to the high EA of Cu and Ag, electron transfer does not take place to nitromethane, and indeed, the encounter complex involving Cu resembles that observed between Na and CH3Cl and CH3CN, Figure 3. The complex formed by insertion of the Cu atom into the C N bond of nitromethane has relatively less positive charge on the Cu atom, while that for C H bond insertion still has a negatively charged metal atom, both consistent with the greater electron affinity of Cu. So, the degree of charge transfer that takes place between the metal and the organic substrate over the course of a reaction is a function of the relative EA values of the two species. Initial charge transfer in the encounter complex will

Figure 4. Examples of the reactions of Cu and Ag with neutral substrates (a) 107Ag with methyl chloride (hexapole pressure 1.0  10 3 mbar, entrance-cell-exit potentials set to 0:10:0), (b) 107Ag with nitromethane (hexapole pressure 3  10 4 mbar, entrance-cell-exit potentials set to 10:25:10 V), and (c) 63Cu with acetonitrile (hexapole pressure 2  10 4 mbar, entrance-cell-exit potentials set to 10:20:10 V).

only occur if the EA of the metal is less than that of the organic molecule. Transition Metal Anions with an Open Valence Orbital. The transition metal anions explored in this study nominally have an unfilled valence d-orbital, Fe , Co , and Ni . The electron configuration of Fe is 4s23d7 and that of Co is 4s23d8. However, Ni is isoelectronic with Cu, which mean it should take on a 3d104s1 electronic configuration. Examples of the reactions with methyl 14010

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Figure 6. Plot of the center-of-mass collision onset voltage for the observation of ion molecule reactions involving acetonitrile as a function of the metal atom electron affinity. Anions with a closed s-orbital are denoted with the (b) and those with an open valence orbital are denoted with the ().

Figure 5. Examples of the reactions of Fe , Ni , and Co with neutral substrates (a) Fe with methyl chloride (hexapole pressure 1  10 3 mbar, entrance-cell-exit potentials set to 50:0:50), (b) Ni with nitromethane (hexapole pressure 3  10 4 mbar, entrance-cellexit potentials set to 10:10:10 V), and (c) Co with acetonitrile (hexapole pressure 2  10 4 mbar, entrance-cell-exit potentials set to 10:15:10 V).

chloride, acetonitrile, and nitromethane can be found in Figure 5. As expected from the previous examples, Fe reacting with CH3Cl produces Cl , Figure 5a. Reactions with nitromethane produce CH2NO2 (m/z 60), NO2 (m/z 46), and a small amount of metal oxide anion in some cases, Figure 5b. Fe is the only metal anion to exhibit all three reactions with nitromethane, producing m/z 61, 60, and 46.

Reactions of Fe and Ni metal anions with acetonitrile produce m/z 40 (the CH2CN anion) and CN (m/z 26), while Co does not insert into the C C bond to give m/z 26, and was the only metal anion to transfer an electron to acetonitrile itself to produce m/z 41 (Figure 5c). The fact that acetonitrile has an EA of only ∼11.5 meV and that Co has an EA of 0.6633 eV,16 suggests something different is going on in this reaction. The first excited state of neutral Co lies 0.43 0.63 eV above the ground state (depending on the J value).20 If a portion of the Co was formed in an excited state, it is possible this excited state could lie ∼0.63 eV above the ground state, that is, the electron would be barely bound. This would make charge transfer to acetonitrile thermoneutral and allow the observation of the acetonitrile anion. While this is pure speculation at this point in time, the participation of electronic states is a common issue when dealing with the reactions of metal cations and deserves further investigation. Overall Trends in Reactivity. The center-of-mass collision onset energy for the C CN insertion reactions were plotted as a function of metal EA and there is indeed a trend for the metal anions with a closed s-orbital, which supports the fact that charge transfer must occur during the reaction and that collision energy must be used to overcome the endothermicity of this transfer in the complex itself during the insertion process, Figure 6. When the open valence orbital metal anions are plotted on the same graph, clearly this relationship breaks down and insertion appears to be related to the specific electron configuration of the metal anion and, perhaps, the specific electronic states of the metal anions produced in the ion source region of the mass spectrometer.

’ CONCLUSIONS The reactions of the metal anions employed in this study clearly depend both on their thermochemical properties (electron affinity and basicity) and their electron configuration. Metal anions produced by filling an s-orbital tend to react closely according to the EA and aciditiy of the corresponding neutral metal and metal hydride. Metal anions produced by placing the extra electron in an open valence orbital exhibit reactions not accessible based on thermochemistry alone. In all cases, it appears that the metal anion can insert into bonds containing first row elements in an oxidative addition/reductive elimination reaction. 14011

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The Journal of Physical Chemistry A Proton abstraction seems to occur by a harpoon mechanism rather than by insertion into the C H bond itself. Electron transfer from the metal to the organic molecule occurs upon the initial formation of the encounter complex only when the EA of the metal is lower than or equal to that of the organic molecule.

’ ASSOCIATED CONTENT

bS

Supporting Information. All mass spectra referred to in this study. This material is available free of charge via the Internet at http://pubs.acs.org.

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(16) NIST Chemistry Webbook, NIST Standard Reference Database Number 69; National Institute of Standards and Technology: Gaithersburg, MD, 2005. (17) Puiatti, M.; Vera, D. M. A.; Pierini, A. B. Phys. Chem. Chem. Phys. 2009, 11, 9013. (18) Mayer, P. M.; Radom, L. J. Phys. Chem. A 1998, 102, 4918. (19) Miller, T. M.; Friedman, J. F.; Schaffer, L. C.; Viggiano, A. A. J. Chem. Phys. 2009, 131, 084302/1. (20) Ralchenko, Y.; Jou, F.-C.; Kelleher, D. E.; Kramida, A. E.; Musgrove, A.; Reader, J.; Wiese, W. L.; Olsen, K. NIST Chemistry Webbook, NIST Standard Reference Database 78: Atomic Spectra Database, Version 4; National Institute of Standards and Technology: Gaithersburg, MD, 2010; Vol. 2010.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: (613) 567-0241. Fax: (613) 562-5170. Notes †

Undergraduate researcher.

’ ACKNOWLEDGMENT P.M.M. thanks the Natural Sciences and Engineering Research Council of Canada for continuing financial support. Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research. ’ REFERENCES (1) Uggerud, E. Top. Curr. Chem. 2003, 225, 3. (2) Munsch, T. E.; Wenthold, P. G. Annu. Rep. Prog. Chem., Sect. B: Org. Chem. 2003, 99, 420. (3) Schmatz, S. ChemPhysChem 2004, 5, 600. (4) Kato, S. J. Mass Spectrom. Soc. Jpn. 2005, 53, 183. (5) Ren, Y.; Chu, S.-Y. J. Theor. Comput. Chem. 2006, 5, 121. (6) Uggerud, E. J. Phys. Org. Chem. 2006, 19, 461. (7) Uggerud, E. Eur. J. Mass Spectrom. 2007, 13, 97. (8) Laerdahl, J. K.; Uggerud, E. Int. J. Mass Spectrom. 2002, 214, 277. (9) McElvany, S. W.; Allison, J. Organometallics 1986, 5, 1219. (10) McElvany, S. W.; Allison, J. Organometallics 1986, 5, 416. (11) Sallans, L.; Lane, K. R.; Squires, R. R.; Freiser, B. S. J. Am. Chem. Soc. 1985, 107, 4379. (12) Stevens Miller, A. E.; Miller, T. M.; Viggiano, A. A.; Morris, R. A.; Paulson, J. F. Int. J. Mass Spectrom. 2000, 195 196, 341. (13) Curtis, S.; Renaud, J.; Holmes, J. L.; Mayer, P. M. J. Am. Soc. Mass Spectrom. 2010, 21, 1944. (14) Attygalle, A. B.; Axe, F. U.; Weisbecker, C. S. Rapid Commun. Mass Spectrom. 2011, 25, 681. (15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; and Pople, J. A. Gaussian 03, revision c.02; Gaussian, Inc.: Wallingford, CT, 2004. 14012

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