ARTICLE pubs.acs.org/JPCA
Experimental and Theoretical Study of Hydrogen Atom Abstraction from n-Butane by Lanthanum Oxide Cluster Anions Bo Xu,†,‡ Yan-Xia Zhao,†,‡ Xiao-Na Li,†,‡ Xun-Lei Ding,† and Sheng-Gui He*,† †
Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People's Republic of China ‡ Graduate School of Chinese Academy of Sciences, Beijing 100039, People's Republic of China
bS Supporting Information ABSTRACT: Lanthanum oxide cluster anions are prepared by laser ablation and reacted with n-C4H10 in a fast flow reactor. A time-of-flight mass spectrometer is used to detect the cluster distribution before and after the reactions. (La2O3)m=13OH and La3O7H are observed as products, which suggests the occurrence of hydrogen atom abstraction reactions: (La2O3)m=13O + n-C4H10 f (La2O3)m=13OH + C4H9 and La3O7 + n-C4H10 f La3O7H + C4H9. Density functional theory (DFT) calculations are performed to study the structures and bonding properties of La2O4, La3O7, and La4O7 clusters. The calculated results show that each of La2O4 and La4O7 contains one oxygen-centered radical (O•) which is responsible for the high reactivity toward n-C4H10. La3O7 contains one oxygen-centered radical (O•) and one superoxide unit (O2•), and the O• is responsible for its high reactivity toward n-C4H10. The O• and O2• can be considered to be generated by the adsorption of an O2 molecule onto the singlet La3O5 with electron transfer from a terminally bonded oxygen ion (O2) to the O2. This may help us understand the mechanism of the formation of O• and O2• radicals in lanthanum oxide systems. The reaction mechanisms of La2O4 + n-C4H10 and La3O7 + n-C4H10 are also studied by the DFT calculations, and the calculated results are in good agreement with the experimental observations.
1. INTRODUCTION Transition metal oxides (TMOs) including lanthanide oxides are widely used as both catalysts and catalytic support materials.16 In oxidation reactions involving molecular oxygen, the O2 is considered to dissociate in the following scheme: O2 (molecular oxygen) f O2• (superoxide) f O22 (peroxide) f 2O• (mononuclear oxygen-centered radical) f 2O2 (lattice oxygen).7 Among these oxygen species, the O2•, O22, and O• are reactive oxygen species.710 To characterize the role of the reactive oxygen species in surface reactions, numerous condensedphase studies have been conducted and the achievements have been frequently reviewed.79,1113 In some cases, the reactive oxygen species may not be generated with sufficient concentrations or their lifetimes may be too short for condensed-phase studies.7,14 As a result, alternative ways are needed to investigate the chemistry of the reactive oxygen species involved in surface reactions. One such alternative way is to study the TMO clusters (MxOyq, in which M is the metal atom and q is the charge number) under isolated, controlled, and reproducible conditions. Several reviews on the studies of TMO clusters are available in the literature.1522 In a previous study,23 for MxOyq cluster, we defined Δ 2y nx þ q
predicted that for early transition metals (M = groups 37 and 3d5d metals, except for M = Cr and Mn), all of the oxide clusters MxOyq, with Δ = 1 and x = 13 (x = 16 for M = Sc) have the character of oxygen-centered radicals (O•). This conclusion supports2436 and is supported22,3747 by many studies on the reactivity of TMO clusters. It should be noted that the studies on the reactivity of TMO clusters have been mostly concentrated on cationic clusters while anionic clusters has been much less reported.33,3941,47 This is because the anionic TMO clusters were identified to be much less reactive than the corresponding cationic ones.33 Therefore, it is worthy to identify TMO cluster anions with high oxidative reactivity to enrich the chemistry of gas phase clusters. Lanthanum oxides are used as catalysts in various reactions, for example, oxidative coupling of methane48,49 and reduction of NO.50,51 A number of condensed-phase experimental5259 and theoretical60,61 investigations have centered around the reactive oxygen species on lanthanum oxide surface. Electron parametric resonance (EPR)52,5557,59 and X-ray photoelectron spectroscopy (XPS)58 studies identified the presence of O2• and O22 over lanthanum oxide surface. Theoretical calculated results showed that O• and O22 may be the active species in the activation of methane on lanthanum oxide surface.60,61 Many
ð1Þ
in which n is the number of metal valence electrons. According to the results calculated by the density functional theory (DFT), we r 2011 American Chemical Society
Received: April 29, 2011 Revised: August 2, 2011 Published: August 11, 2011 10245
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The Journal of Physical Chemistry A gas-phase experimental studies and theoretical calculations have been conducted to investigate the preparations, structure, stabilities, and spectra of lanthanum oxide clusters (LaxOyq),6270 while the studies concerned with their reactivity are very limited.71,72 In this study, we focus on the reactivity of LaxOy clusters toward n-C4H10, and we hope this study can improve our understanding of reactive oxygen species such as O• over lanthanum oxide systems at a molecular level.
2. METHODS 2.1. Experimental Section. The reactivity of anionic lanthanum oxide clusters with n-C4H10 is studied by a time-of-flight mass spectrometer (TOF-MS) coupled with a laser ablation/ supersonic expansion cluster source and a fast flow reactor. The experimental setup was described in detail in previous studies,43,7375 only a brief outline of the experiments is given below. The LaxOy clusters are pulsed generated by laser ablation of a rotating and translating lanthanum metal disk in the presence of about 0.3% O2 seeded in a He carrier gas with backing pressure of 3 atm. A 532 nm (second harmonic of Nd3+:yttrium aluminum garnet-YAG) laser with an energy of 58 mJ/pulse and repetition rate of 10 Hz is used. The gas is controlled by a pulsed valve (General Valve, Series 9). Possibly due to residual water in gas handling system, undesirable hydroxo species, such as LaxOy(HO)zq (z > 0) often occur in the distribution of transition metal oxide clusters although high purity gases (He and O2, 99.995%) are used. To eliminate or to significantly decrease contributions of these undesirable species in the cluster distributions, the prepared gas mixture (O2/He, denoted as cluster generation gas) is passed through a 10 m long copper tube coil at low temperature (T = 77 K) before entering into the pulsed valve. Similar treatment (T = 273 K) is also applied in the use of the reactant gases (see below). The clusters formed in a gas channel (2 mm diameter 25 mm length) are expanded and reacted with 5% n-C4H10 (n-C4D10)/He in a fast flow reactor (6 mm diameter 60 mm length). The reactant gases with backing pressures of 520 kPa (depending on the reaction rate of particular reaction) are pulsed into the reactor 20 mm downstream from the exit of the narrow cluster formation channel by a second pulsed valve (General Valve, Series 9). By using the method in ref 73, the instantaneous total gas pressure in the fast flow reactor is estimated to be around 260 Pa at T = 350 K. The number of collisions that a cluster (radius = 0.5 nm) experiences with the bath gas (radius = 0.05 nm, T = 350 K, P = 260 Pa) in the fast flow reactor is about 50 per 1 mm of forward motion. This corresponds to a collision rate (kcollision) of 5.0 107 s1 for an approaching velocity of 1 km/s. Because the length (60 mm) of the reactor is much longer than 1 mm, the intracluster vibrations are likely equilibrated (cooled or heated, depending on the vibrational temperature after exiting cluster formation channel with a supersonic expansion) to close to the bath gas temperature before reacting with the diluted n-C4H10 (n-C4D10) molecules. The bath gas temperature is around 300400 K considering that the carrier gas can be heated during the process of laser ablation. After reacting in the fast flow reactor, the reactant and product ions are skimmed (3 mm diameter) into a vacuum system of the TOF-MS for mass (to charge ratio) measurement. Ion signals are generated by a dual microchannel plate detector and recorded with a digital oscilloscope (LeCroy WaveSurfer 62Xs) by averaging 1000 traces of independent mass spectra (each corresponds to one laser shot). The uncertainties of the reported
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relative ion signals are about 10%. The mass resolution is about 400500 (M/ΔM) with the current experimental setup. 2.2. Computational. The DFT calculations using the Gaussian 03 program76 are employed to study the structures of reactive LaxOy clusters (La2O4, La3O7, and La4O7) and the reaction mechanisms of La2O4 + n-C4H10 and La3O7 + n-C4H10. The hybrid B3LYP exchange-correlation functional77,78 is used throughout this work. The effective core potentials (ECPs)79 and the polarized triple-ζ valence basis sets (Def2-TZVP)80 are used to take into account scalar relativistic effects for La, while the allelectron TZVP basis sets81 are used for H, C, and O elements. Geometry optimizations are performed starting from a high number of possible candidate structures with full relaxation of all atoms and all possible spin multiplicities. The reaction mechanism calculations involve geometry optimizations of reaction intermediates (IMs) and transition states (TSs) through which the IMs transfer to each other. The TSs are optimized using the Berny algorithm method.82 The initial guess structure of the TS species is obtained generally through relaxed potential energy surface (PES) scans using an appropriate internal coordinate. Vibrational frequency calculations are performed to check that reaction IMs and TSs have zero and only one imaginary frequency, respectively. Natural bond orbital (NBO) analysis is also conducted for specific clusters using NBO 3.183 implemented in Gaussian 03. Test calculations indicate that basis set superposition error (BSSE)84,85 is negligible, so the BSSE is not taken into consideration in this study. The zero-point vibration corrected energies (ΔH0K) and the relative Gibbs free energies at 298 K (ΔG298K) are reported in this work. Test calculations indicate that the ΔG values at 350 K (cluster vibrational temperature in the experiment is in between 300 and 400 K and may be very close to 300 K86) differ from ΔG298K values by very small values (0.030.07 eV). It is thus reasonable to use the DFT calculated energies under the standard conditions to interpret the experimental data.
3. RESULTS AND DISCUSSION Figure 1a displays the mass spectra of selected regions that cover (La2O3)m=13O, La3O7, and La4O8 clusters generated under the condition of 0.3% O2 in 3 atm He. Note that a weak peak at 531 amu next to La3O7 can be assigned as a H2O containing cluster La3O6H2O, which may be generated from the reaction of La3O6 with residual H2O in the gas handling system. When 5% n-C4H10/He are pulsed into the reactor, the intensities of La2O4, La3O7, and La4O7 decrease, while the signals at the position of Δmass = +1 amu increase simultaneously (Figure 1b). This suggests that these clusters can abstract one hydrogen atom from n-C4H10 to produce the hydroxide cluster anions LaxOyH, as shown in reaction 2. Lax Oy þ n-C4 H10 f Lax Oy H þ C4 H9 ðx, yÞ ¼ ð2, 4Þ, ð3, 7Þ, ð4, 7Þ
ð2Þ
This reaction channel is confirmed by isotopic labeling experiments using n-C4D10 (Figure 1c). For La6O10, there is no sufficient resolution to resolve La6O10 and La6O10H, however, the generation of La6O10D can be observed clearly, which suggests that La6O10 can abstract a deuterium atom from n-C4D10, as shown in reaction 3. La6 O10 þ n-C4 D10 f La6 O10 D þ C4 D9
ð3Þ
Figure 1b and c also demonstrate that the presence of n-C4H10 and n-C4D10 in the reactor do not cause apparent signal depletion 10246
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Figure 1. TOF mass spectra for reactions of (La2O3)m=13O, La3O7, and La4O8 clusters with (a) He, (b) 5% n-C4H10/He, and (c) 5% n-C4D10/He. Numbers x,y denote LaxOy and x,yX denote LaxOyX (X = H, D, and H2O).
Table 1. Estimated Absolute Rate Constants (k1) for Reactions of LaxOy with n-C4H10 and n-C4D10 in the Fast Flow Reactor (k1 is in Unit of cm3 Molecule1 s1 and KIE is the Kinetic Isotope Effect) LaxOy La2O4 La3O7 La4O7 La6O10
k1 (LaxOy + n-C4H10) 11
3.6 10
k1 (LaxOy + n-C4D10) 1.0 10
11
KIE 3.6
8.4 1011 9.1 1011
3.7 1011
2.5
5.1 1011
for La4O8 (and other clusters including LaO25, La2O5,6, La3O5,6,8,9, La4O6,8,9, and La5O8,9 not shown in Figure 1), indicating that these clusters are inert toward n-C4H10 and n-C4D10 under the same experimental conditions. The pseudo-first-order rate constant (k1) of the hydrogen abstraction reaction can be estimated by using the following equation: k1 ¼ lnðI0 =IÞ=ðF ΔtÞ
ð4Þ
in which I and I0 are signal intensity in the presence and absence of reactant gas, respectively; F is the concentration of reactant gas; and Δt is the reaction time in the fast flow reactor. The estimated rate constants for some clusters with n-C4H10 and n-C4D10 are provided in Table 1. The uncertainties for the relative rate constants are within 20%. Because it is hard to determine accurately the F and Δt values in the pulse experiment, the absolute k1 value can be systematically under or overestimated. The systematic deviation may be within a factor of 5 by comparing the rate constants from our fast flow reaction experiments with those from other independent experiments for known reactions such as CeO2+ + C2H4 f CeO+ + C2H4O43,87 and V4O10+ + CH4 f V4O10H+ + CH3.30,45 The values of kinetic isotope effect [KIE: defined as k1(LaxOy + n-C4H10)/k1(LaxOy + n-C4D10)] are determined to be 3.6 and 2.5 for La2O4 and La4O7 reaction systems, respectively. Assuming that La6O10 and La4O7 have the same KIE value (2.5), the intensity ratio of product La6O10H to unreacted La6O10 in Figure 1b is about 2:1. To verify the reactivity of (La2O3)m=13O and La3O7 toward n-C4H10, theoretical calculations are performed to find the stable isomers and the locations of the spin densities (SDs) in these clusters. The structures of the lowest and some low-lying energy isomers of La2O4, La3O7, and La4O7 are plotted in
Figure 2. The energies and Cartesian coordinates of these cluster isomers are listed in Supporting Information. In the lowest energy structure of La2O4, the SDs of the cluster are highly localized in the 2p orbital of the terminal oxygen atom (Ot). Such an Ot atom with SDs close to one unit μB is a mononuclear oxygen-centered radical (denoted as O•) because it has similar electronic structure as the free O ion does. In the lowest energy structure of triplet La3O7, SDs of one unit μB are localized on one terminal oxygen atom (O•), while SDs of another unit μB are equally distributed around two oxygen atoms in a η2-O2 unit, and the resulting η2-O2 unit can be denoted as O2•. Similar to La2O4 and La3O7, the lowest energy structure of La4O7 also contains one unit of O•. To gain insight into the mechanistic details of hydrogen atom abstraction from n-C4H10 by La2O4 and La3O7, the reaction pathway is studied by the DFT calculations. The potential energy profile of the most stable isomer of La2O4 with n-C4H10 is plotted in Figure 3. The structures and energies of the reaction IMs and TSs are also given. This reaction is thermodynamically favorable (ΔH0K = 0.74 eV). The n-C4H10 molecule approaches La2O4 to form IM1 with a binding energy of 0.14 eV. In IM1, two hydrogen atoms interact with the terminal O• in La2O4 moiety through two weak hydrogen bonds, and the O1H1 distance is 226 pm. Then, the reaction proceeds by TS1 to overcome 0.01 eV barrier (ΔH0K), and the distance of O1H1 shortens from 226 to 129 pm together with the elongation of C1H1 bond from 110 to 123 pm, which suggests that the C1H1 in n-C4H10 moiety is greatly activated. Next, the O1H1 distance continues to shorten from 129 pm in TS1 to 96 pm in IM2. Significant amount of energy (ΔH0K = 0.85 eV) can be released after formation of IM2 from the separated reactants (La2O4 and n-C4H10). Note that, in terms of the zero-point-vibration corrected energy (ΔH0K), the energy difference between P1 and IM2 (0.11 eV) is larger than that between TS1 and IM1 (0.01 eV) in Figure 3. However, in terms of the Gibbs free energy (ΔG298K), P1 is lower in energy than IM2 by 0.26 eV, while TS1 is still higher than IM1 (0.04 eV). It means that formation of the separated products (La2O4H and 2-C4H9) from IM2 is facile (not the rate-limiting step) at room temperature. In addition, for the gas phase reaction under our experimental conditions, the energy released (ΔH0K = 0.85 eV) from formation of IM2 may not be quickly dissipated into bath gas, which further facilitates the dissociation of reaction intermediate (IM2) into separated products (P1). As a result, the step from IM1 to TS1 (hydrogen atom transfer from carbon to oxygen atoms) is expected to be the rate-limiting step of the whole reaction. This is also consistent with the experimental KIE value of 3.6 for the La2O4 reaction system (Table 1). The hydrogen atom transfer process (IM1 f TS1 f IM2) is subject to one small overall positive free energy barriers (ΔG298K = 0.25 eV). In the experiments, the center-ofmass collisional energy Ek is 0.26 eV (Ek = 1/2μυ2, μ is the reduced mass of La2O4 with C4H10 and υ ∼ 1 km/s), the cluster vibrational energy Evib is 0.30 eV (calculated by DFT from the vibrational temperature T = 350 K). The sum of Ek and Evib is larger than 0.25 eV. Given that the reaction intermediates are not fully localized at thermal equilibrium due to the low pressure in the fast flow reactor, the overall free energy barrier is easily surmountable, which agrees with the experimental observation of the La2O4H product. Figure 4 plots the reaction pathway of the most stable isomer of La3O7 with n-C4H10, which is similar to that of La2O4 + n-C4H10. 10247
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Figure 2. DFT optimized structures of La2O4, La3O7, and La4O7. I01, I02, and so on denote different structure isomers. The symmetry, electronic state, and energy (in eV) with respect to the most stable isomer are given below each isomer. The profiles of spin density distributions for the most stable isomers are also given. Some bond lengths are given in pm. Mulliken spin density values (μB) over some O atoms are given in parentheses.
Figure 3. DFT calculated reaction pathway for [La2O4 (2A1) + nC4H10 (1Ag)] (R1) f [La2O4H (1A) + C4H9 (2A)] (P1). The reaction intermediates and transition states are denoted as IMm and TSn, respectively. The zero-point vibration corrected energy (ΔH0K in eV) and Gibbs free energy (ΔG298K in eV) are given in the brackets as (ΔH0K/ΔG298K). Some of the bond lengths are given in pm.
This reaction is exothermic by 0.81 eV (ΔH0K). There is one small overall barrier (ΔG298K = 0.27 eV) for CH activation (IM3 f TS2 f IM4). The overall free energy barrier can be overcome by the collisional and vibrational energies (Ek = 0.27 eV and Evib = 0.51 eV at T = 350 K). During the whole reaction process, the bond length of OO unit is kept at 134 pm. Studies on reaction pathways of La2O4 + n-C4H10 and La3O7 + n-C4H10 suggest that the O• in La2O4 and La3O7 are responsible for their high reactivity toward n-C4H10. As a result, we can believe that the O• exists in La6O10 as well
Figure 4. DFT calculated reaction pathway for [La3O7 (3A) + nC4H10 (1Ag)] (R2) f [La3O7H (2A) + C4H9 (2A)] (P2). See caption of Figure 3 for explanations.
according to its high reactivity toward n-C4H10 (Figure 1). Therefore, each of the (La2O3)m=13O and La3O7 reactive clusters investigated in this paper contains one unit of O•. The experimental studies and the DFT calculations suggest that each of (La2O3)m=13O clusters with Δ = 1 contains one unit of O•, which is responsible for the high reactivity toward n-C4H10. These results not only support our previous prediction that La2O4 with Δ = 1 contains one unit of O•,23 but also suggest that larger La4O7 and La6O10 clusters with Δ = 1 contain the O• as well. In addition, La3O7 with Δ = 4 also contain an O•, which is beyond our expectation. This interesting observation is discussed below. The oxygen-rich clusters with Δ > 1 tend to be superoxo or peroxo complexes8890 that are usually less oxidative than the 10248
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O2. This study not only supports and extends our previous prediction, but also improves our understanding of the mechanism of the formation of O• and O2• radicals by the adsorption of the O2 on lanthanum oxide surface.
’ ASSOCIATED CONTENT Figure 5. Adsorption of the O2 on La3O5 generates the O• and O2• in La3O7. The profiles of spin density distributions for La3O7 are given. The lengths of LaOt bond are given in pm.
corresponding Δ = 1 clusters with the O•.91 TOF-MS experiments suggest that La3O7 is highly reactive toward n-C4H10 although it is oxygen-very-rich (Δ = 4). The DFT calculations indicate that La3O7 contains one unit of O• and one unit of O2•, and the O• is responsible for its high reactivity. The triplet La3O7 can be considered to be generated by the adsorption of a triplet O2 molecule on the singlet La3O5 (Figure 5). This reaction is exothermic by 2.18 eV (ΔH0K). NBO analysis indicates that a charge of 0.75 |e| transfers from the terminally bonded oxygen ion (O2) to the O2. This is accompanied by the elongation of the LaOt bond from 205 to 240 pm and the OO bond from 120 pm (free O2) to 134 pm. These changes result in the generation of the O• and O2• through the following channel: O2 þ O2 f O• þ O2 •
ð5Þ
The similar mechanisms were also proposed in other cases. MS experiments and DFT calculations indicated that the adsorption of an O2 molecule on Zr2O6 can generate one unit of O• and one unit of O2• in Zr2O8 (Δ = 7), and the O• is responsible for the high reactivity toward n-C4H10.39 Photoelectron spectroscopy (PES) experiments and DFT calculations also suggested that M2O7 (M = Nb, Ta) with Δ = 4 contains one unit of O• and one unit of O2• generated by the adsorption of an O2 molecule on M2O5 (M = Nb, Ta).92 In condensed phase, EPR studies on TMO surface reactions suggested that an electron can transfer from low coordinated surface oxygen ion (O2 LC ) to an absorbed molecule with sufficient electron affinity to form an adsorbed anion radical at high temperature. However, there has been no EPR signal of the corresponding hole or O center formed when the O2 LC gives up an electron.7 The studies on gas-phase clusters suggest that the transfer of an electron from a terminal oxygen ion (O2) to O2 will result in the generation of the O• and O2• radicals. These studies shed light on the mechanism of the formation of O• and O2• radicals on TMO oxide surface.7
4. CONCLUSIONS Reactions of lanthanum oxide cluster anions with n-C4H10 have been studied by time-of-flight mass spectrometry. The (La2O3)m=13O and La3O7 clusters can abstract one hydrogen atom from n-C4H10, while a lot of other clusters are inert toward n-C4H10. The density functional theory calculations on reactive clusters and reaction systems indicated that each of (La2O3)m=13O clusters with Δ = 1 contains one unit of O•, which is responsible for the high reactivity toward n-C4H10. La3O7 cluster with Δ = 4 contains one unit of O• and one unit of O2• generated by the adsorption an O2 molecule on singlet La3O5 together with the electron transfer from the O2 to the
bS
Supporting Information. DFT determined energies and Cartesian coordinates for cluster isomers in Figure 2. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Phone: +86-10-62536990. Fax: +86-10-62559373.
’ ACKNOWLEDGMENT This work was supported by Chinese Academy of Sciences (Knowledge Innovation Program KJCX2-EW-H01, Hundred Talents Fund), the National Natural Science Foundation of China (Nos. 20803083 and 20933008), CMS Foundation of the ICCAS (No. CX-201002), and Major Research Plan of China (No. 2011CB932302). ’ REFERENCES (1) Ertl, G.; Knozinger, H.; Weitkamp, J. Handbook of Heterogeneous Catalysis; Wiley-VCH: Weinheim, Germany, 1997. (2) van Santen, R. A.; Neurock, M.; Corporation, E. Molecular Heterogeneous Catalysis; Wiley-VCH: Weinheim, Germany, 2006. (3) Fierro, J. L. G. Metal Oxides; Taylor & Francis Group: Boca Raton, FL, 2006. (4) Rosynek, M. P. Catal. Rev. Sci. Eng. 1977, 16, 111. (5) Barteau, M. A. Chem. Rev. 1996, 96, 1413. (6) Trovarelli, A. Catal. Rev. Sci. Eng. 1996, 38, 439. (7) Che, M.; Tench, A. J. Adv. Catal. 1982, 31, 77. (8) Che, M.; Tench, A. J. Adv. Catal. 1983, 32, 1. (9) Panov, G. I.; Dubkov, K. A.; Starokon, E. V. Catal. Today 2006, 117, 148. (10) Li, C.; Domen, K.; Maruya, K.; Onishi, T. J. Am. Chem. Soc. 1989, 111, 7683. (11) Dyrek, K.; Che, M. Chem. Rev. 1997, 97, 305. (12) Chiesa, M.; Giamello, E.; Che, M. Chem. Rev. 2010, 110, 1320. (13) Lunsford, J. H. Catal. Rev. Sci. Eng. 1974, 8, 135. (14) Zhao, C. L.; Wachs, I. E. Catal. Today 2006, 118, 332. (15) Zemski, K. A.; Justes, D. R.; Castleman, A. W., Jr. J. Phys. Chem. B 2002, 106, 6136. (16) O’Hair, R. A. J.; Khairallah, G. N. J. Clust. Sci. 2004, 15, 331. (17) B€ ohme, D. K.; Schwarz, H. Angew. Chem., Int. Ed. 2005, 44, 2336. (18) Asmis, K. R.; Sauer, J. Mass. Spectrom. Rev. 2007, 26, 542. (19) Gong, Y.; Zhou, M. F.; Andrews, L. Chem. Rev. 2009, 109, 6765. (20) Zhai, H. J.; Wang, L. S. Chem. Phys. Lett. 2010, 500, 185. (21) Roithova, J.; Schr€oder, D. Chem. Rev. 2010, 110, 1170. (22) Zhao, Y. X.; Wu, X. N.; Ma, J. B.; He, S. G.; Ding, X. L. Phys. Chem. Chem. Phys. 2011, 13, 1925. (23) Zhao, Y. X.; Ding, X. L.; Ma, Y. P.; Wang, Z. C.; He, S. G. Theor. Chem. Acc. 2010, 127, 449. (24) Irikura, K. K.; Beauchamp, J. L. J. Am. Chem. Soc. 1989, 111, 75. (25) Schr€oder, D.; Fiedler, A.; Hrusak, J.; Schwarz, H. J. Am. Chem. Soc. 1992, 114, 1215. (26) Kretzschmar, I.; Fiedler, A.; Harvey, J. N.; Schr€oder, D.; Schwarz, H. J. Phys. Chem. A 1997, 101, 6252. 10249
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