Gas-Phase Ion Chemistry of Small Gold Cluster Anions

The reactivity of small anionic gold clusters (Aun−, n = 1−4) toward small .... On the other hand, the studied gold clusters show a strongly size ...
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Organometallics 2010, 29, 3001–3006 DOI: 10.1021/om100228y

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Gas-Phase Ion Chemistry of Small Gold Cluster Anions Robert F. H€ ockendorf,†,‡ Yali Cao,†,‡ and Martin K. Beyer*,†,‡ †

Institut f€ ur Chemie, Sekretariat C4, Technische Universit€ at Berlin, Strasse des 17. Juni 135, 10623 Berlin, Germany, and ‡Institut f€ ur Physikalische Chemie, Christian-Albrechts-Universit€ at zu Kiel, Olshausenstrasse 40, 24098 Kiel, Germany Received March 25, 2010

The reactivity of small anionic gold clusters (Aun-, n = 1-4) toward small organic molecules was investigated by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). With trifluoroacetic acid (TFA) proton transfer, gold loss, and radiative association were observed. Sulfide-sulfide bond activation, sulfide-carbon bond activation, gold loss, and radiative association were observed in collisions with dimethyl disulfide (DMDS). The small gold cluster anions show a strongly size dependent reactivity, with the gold trimer being completely unreactive. Regardless of cluster size, no reaction was observed for methanol, acetonitrile, acetaldehyde, acetone, dimethyl sulfide, methyl mercaptan, benzene, ethynylbenzene, and difluoroacetic acid. Introduction The gas-phase chemistry of metal clusters serves as a bridge between the atomic scale and the bulk. The possibility of using gas-phase reactions to gain insight into catalytic mechanisms is especially attractive.1-4 Mass spectrometry is a suitable tool to observe the size-dependent reactivity of metal clusters.5-9 Reports that ultrafine supported gold particles exhibit high catalytic activity sparked interest in the chemistry of this seemingly inert metal in the 1990s. One of the first catalyses, the low-temperature oxidation of CO on gold supported by a metal oxide, was reported by Haruta et al.10-13 Subsequently, nanoscale gold catalysis has become one of the fastest growing topics in chemical science.14,15 Research shows that the catalytic activity is in direct correlation with the dispersion.16 This makes gold an interesting material for cluster reactions in which elementary steps are

elucidated. Gold was the subject of numerous experiments in the gas phase, where mass-selected clusters have been studied.17-23 Gold, as one of the heaviest elements, is of special interest in the investigation of relativistic effects. Therefore, the structures and properties of small gold clusters have been the focus of research.24-28 Small gold cluster anions as well as cations preferentially form planar structures.29-31 This is due to strong relativistic effects, which reduce the size of the 6s orbital and expand the s-d hybridization.32 The reactivity of gold cluster cations has been studied extensively by W€ oste, Bernhardt, and co-workers.33-41 Theoretical works

*To whom correspondence should be addressed at the Institut f€ ur Physikalische Chemie. E-mail: [email protected]. (1) Kappes, M. M.; Staley, R. H. J. Am. Chem. Soc. 1981, 103, 1286. (2) Jackson, G. S.; White, F. M.; Hammill, C. L.; Clark, R. J.; Marshall, A. G. J. Am. Chem. Soc. 1997, 119, 7567. (3) Balaj, O. P.; Balteanu, I.; Rossteuscher, T. T. J.; Beyer, M. K.; Bondybey, V. E. Angew. Chem., Int. Ed. 2004, 43, 6519. (4) Armentrout, P. B. Annu. Rev. Phys. Chem. 2001, 52, 423. (5) Koszinowski, K.; Schr€ oder, D.; Schwarz, H. J. Phys. Chem. A 2003, 107, 4999. (6) Bohme, D. K.; Schwarz, H. Angew. Chem., Int. Ed. 2005, 44, 2336. (7) Bohme, D. K.; Schwarz, H. Angew. Chem. 2005, 117, 2388. (8) Armentrout, P. B. Annu. Rev. Phys. Chem. 1990, 41, 313. (9) Achatz, U.; Berg, C.; Joos, S.; Fox, B. S.; Beyer, M. K.; Niedner-Schatteburg, G.; Bondybey, V. E. Chem. Phys. Lett. 2000, 320, 53. (10) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 405. (11) Iizuka, Y.; Fujiki, H.; Yamauchi, N.; Chijiiwa, T.; Arai, S.; Tsubota, S.; Haruta, M. Catal. Today 1997, 36, 115. (12) Date, M.; Haruta, M. J. Catal. 2001, 201, 221. (13) Haruta, M.; Date, M. Appl. Catal. A: Gen. 2001, 222, 427. (14) Bond, G. C.; Thompson, D. T. Catal. Rev. Sci. Eng. 1999, 41, 319. (15) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (16) Naito, S.; Tanimoto, M. J. Chem. Soc., Chem. Commun. 1988, 832.

(17) Balteanu, I.; Balaj, O. P.; Fox, B. S.; Beyer, M. K.; Bastl, Z.; Bondybey, V. E. Phys. Chem. Chem. Phys. 2003, 5, 1213. (18) Wallace, W. T.; Whetten, R. L. J. Am. Chem. Soc. 2002, 124, 7499. (19) Sanchez, A.; Abbet, S.; Heiz, U.; Schneider, W. D.; Hakkinen, H.; Barnett, R. N.; Landman, U. J. Phys. Chem. A 1999, 103, 9573. (20) Heiz, U.; Sanchez, A.; Abbet, S.; Schneider, W. D. Eur. Phys. J. D 1999, 9, 35. (21) Li, F. X.; Armentrout, P. B. J. Chem. Phys. 2006, 125. (22) Heiz, U.; Schneider, W. D. J. Phys. D: Appl. Phys. 2000, 33, R85. (23) Chowdhury, A. K.; Wilkins, C. L. J. Am. Chem. Soc. 1987, 109, 5336. (24) Pyykk€ o, P. Chem. Rev. 1988, 88, 563. (25) Pyykk€ o, P. Angew. Chem., Int. Ed. 2004, 43, 4412. (26) Pyykk€ o, P. Angew. Chem. 2004, 116, 4512. (27) Taylor, K. J.; Pettiettehall, C. L.; Cheshnovsky, O.; Smalley, R. E. J. Chem. Phys. 1992, 96, 3319. (28) Schr€ oder, D.; Schwarz, H.; Hrusak, J.; Pyykk€ o, P. Inorg. Chem. 1998, 37, 624. (29) Furche, F.; Ahlrichs, R.; Weis, P.; Jacob, C.; Gilb, S.; Bierweiler, T.; Kappes, M. M. J. Chem. Phys. 2002, 117, 6982. (30) Gilb, S.; Weis, P.; Furche, F.; Ahlrichs, R.; Kappes, M. M. J. Chem. Phys. 2002, 116, 4094. (31) H€akkinen, H.; Landman, U. Phys. Rev. B 2000, 62, R2287. (32) H€akkinen, H.; Moseler, M.; Landman, U. Phys. Rev. Lett. 2002, 89. (33) Lang, S. M.; Bernhardt, T. M.; Barnett, R. N.; Landman, U. Angew. Chem., Int. Ed. 2010, 49, 980. (34) Lang, S. M.; Bernhardt, T. M. Int. J. Mass Spectrom. 2009, 286, 39. (35) Lang, S. M.; Bernhardt, T. M. J. Chem. Phys. 2009, 131. (36) Lang, S. M.; Bernhardt, T. M.; Barnett, R. N.; Yoon, B.; Landman, U. J. Am. Chem. Soc. 2009, 131, 8939. (37) Lang, S. M.; Bernhardt, T. M. Eur. Phys. J. D 2009, 52, 139.

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by Gr€ onbeck, H€akkinen, and co-workers have addressed the chemistry of neutral gold clusters with thiolates and other small compounds.42-49 Combined experimental and theoretical efforts by several groups have addressed various aspects of CO oxidation on gold clusters.18,50-58 The reactivity of the gold monomer anion has been experimentally studied by El-Nakat et al. and Cox et al.59,60 It was found not to react with benzene, alkyl-substituted benzenes, H2S, H2, and CH4. Proton transfer was observed for benzenethiol, propanethiols, and butanethiols.60 The exclusive reactivity of cluster anions with an even number n of gold atom cluster anions was observed for the reaction with O2.59 In general, the small gold anions seem to be less reactive toward the reactants used than the corresponding cations. Here we report a reactivity study of small gold cluster anions, Aun- (n = 1-4), with standard reactants and potentially interesting molecules for self-assembled monolayers (SAMs)61-63 in the gas phase studied with FT-ICR mass spectrometry. The small gold cluster anions remain unreactive toward the majority of the reactants used. A rich chemistry was observed for dimethyl disulfide (DMDS) and the very strong Brønsted acid trifluoroacetic acid (TFA), with a strong (38) Bernhardt, T. M.; Hagen, J.; Lang, S. M.; Popolan, D. M.; Socaciu-Siebert, L. D.; W€ oste, L. J. Phys. Chem. A 2009, 113, 2724. (39) Bernhardt, T. M.; Socaciu-Siebert, L. D.; Hagen, J.; W€ oste, L. Appl. Catal. A: Gen. 2005, 291, 170. (40) Bernhardt, T. M. Int. J. Mass Spectrom. 2005, 243, 1. (41) Popolan, D. M.; Bernhardt, T. M. Chem. Phys. Lett. 2009, 470, 44. (42) Kacprzak, K. A.; Akola, J.; H€akkinen, H. Phys. Chem. Chem. Phys. 2009, 11, 6359. (43) Gr€ onbeck, H.; H€akkinen, H.; Whetten, R. L. J. Phys. Chem. C 2008, 112, 15940. (44) H€ akkinen, H. Chem. Soc. Rev. 2008, 37, 1847. (45) Walter, M.; Akola, J.; Lopez-Acevedo, O.; Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Whetten, R. L.; Gr€ onbeck, H.; H€akkinen, H. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 9157. (46) Yoon, B.; Koskinen, P.; Huber, B.; Kostko, O.; von Issendorff, B.; H€ akkinen, H.; Moseler, M.; Landman, U. ChemPhysChem 2007, 8, 157. (47) Koskinen, P.; H€akkinen, H.; Huber, B.; von Issendorff, B.; Moseler, M. Phys. Rev. Lett. 2007, 98. (48) Gr€ onbeck, H.; Walter, M.; H€akkinen, H. J. Am. Chem. Soc. 2006, 128, 10268. (49) H€ akkinen, H.; Walter, M.; Gr€ onbeck, H. J. Phys. Chem. B 2006, 110, 9927. (50) Zhai, H. J.; B€ urgel, C.; Bonacic-Koutecky, V.; Wang, L. S. J. Am. Chem. Soc. 2008, 130, 9156. (51) B€ urgel, C.; Reilly, N. M.; Johnson, G. E.; Mitric, R.; Kimble, M. L.; Castleman, A. W.; Bonacic-Koutecky, V. J. Am. Chem. Soc. 2008, 130, 1694. (52) Mitric, R.; Werner, U.; B€ urgel, C.; Bonacic-Koutecky, V. Eur. Phys. J. D 2007, 43, 201. (53) Kimble, M. L.; Moore, N. A.; Castleman, A. W.; B€ urgel, C.; Mitric, R.; Bonacic-Koutecky, V. Eur. Phys. J. D 2007, 43, 205. (54) Kimble, M. L.; Moore, N. A.; Johnson, G. E.; Castleman, A. W.; B€ urgel, C.; Mitric, R.; Bonacic-Koutecky, V. J. Chem. Phys. 2006, 125. (55) Kimble, M. L.; Castleman, A. W.; B€ urgel, C.; Bonacic-Koutecky, V. Int. J. Mass Spectrom. 2006, 254, 163. (56) Kimble, M. L.; Castleman, A. W.; Mitric, R.; B€ urgel, C.; Bonacic-Koutecky, V. J. Am. Chem. Soc. 2004, 126, 2526. (57) H€ akkinen, H.; Landman, U. J. Am. Chem. Soc. 2001, 123, 9704. (58) Socaciu, L. D.; Hagen, J.; Bernhardt, T. M.; Woeste, L.; Heiz, U.; H€ akkinen, H.; Landman, U. J. Am. Chem. Soc. 2003, 125, 10437. (59) Cox, D. M.; Brickman, R.; Creegan, K.; Kaldor, A. Z. Phys. D: At., Mol. Clusters 1991, 19, 353. (60) El-Nakat, J. H.; Dance, I. G.; Fisher, K. J.; Willett, G. D. Polyhedron 1993, 12, 2477. (61) McDonagh, A. M.; Zareie, H. M.; Ford, M. J.; Barton, C. S.; Ginic-Markovic, M.; Matisons, J. G. J. Am. Chem. Soc. 2007, 129, 3533. (62) Tour, J. M.; Jones, L.; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529. (63) Heimel, G.; Romaner, L.; Bredas, J. L.; Zojer, E. Langmuir 2008, 24, 474.

H€ ockendorf et al.

size dependence. DMDS was chosen because it showed high reactivity with the mixed gold-silicon clusters AunSi-.64

Experimental Section The experiments were performed on a modified Bruker/ Spectroscopin CMS47X mass spectrometer, equipped with an Apex III data station and a 4.5 T superconducting magnet. The small gold cluster anions were produced in a home-built laser vaporization source65-69 via laser vaporization of a rotating gold target pressed from gold nuggets (99,99%, Chempur) with a 5 ns pulse of a frequency-doubled Nd:YAG laser (Continuum Surelite II, 10 Hz, 5 mJ pulse energy), followed by supersonic expansion of the hot plasma in a triggered helium pulse. The transport of the produced gold cluster anions was accomplished by an electrostatic lens system through differential pumping stages into the ultrahigh vacuum (UHV) region, where the ions have been mass-selected and stored in the ICR cell. The reactants benzene (99.8%, Aldrich), ethynylbenzene (98%, Aldrich), acetonitrile (99.8%, Aldrich), acetaldehyde (g99.5%, Aldrich), acetone (g99.9%, Aldrich), methanol (99.8%, Aldrich), methyl mercaptan (purum, Aldrich), DMS (Janssen Chimica, 99%), DMDS (Janssen Chimica, 99%), DFA (98%, Aldrich), and TFA (99,99%, Aldrich) were admitted at a constant pressure into the UHV region of the FT-ICR by a needle valve. To avoid the introduction of impurities in the UHV, liquid reactants were degassed by several freeze-pump-thaw cycles. Relative rate constants were obtained by fitting the experimental data to pseudo-first-order reaction kinetics using a genetic algorithm70 and converted into absolute rate constants.71 Theoretical Calculations. To support our experimental results, we performed quantum chemical calculations using the density functional theory (DFT) method B3LYP72 as implemented in Gaussian03. 73 For gold, the Stuttgart/Dresden effective core potential ECP60MWB basis set74 with two f-type polarization functions (exponents R = 0.498 and R = 1.461)75 was used. The 6-311þþG(3df,3pd) basis sets are used for the other atoms (C, O, F, H, S) in our work. Harmonic frequencies were calculated by using analytical second derivatives to examine the nature of the stationary points. Transition states were identified with the QST2 method.76 All reported energies are zero-point corrected.

Results and Discussion No reaction was observed in the collisions of mass-selected gold cluster anions with acetonitrile, acetaldehyde, acetone, dimethyl sulfide, methyl mercaptan, benzene, methanol, ethynylbenzene, and difluoroacetic acid, regardless of cluster size. On the other hand, the studied gold clusters show a strongly size dependent reactivity toward TFA and DMDS. (64) Cao, Y.; H€ ockendorf, R. F.; Beyer, M. K. ChemPhysChem 2008, 9, 1383. (65) Bondybey, V. E.; English, J. H. J. Chem. Phys. 1981, 74, 6978. (66) Berg, C.; Schindler, T.; Niedner-Schatteburg, G.; Bondybey, V. E. J. Chem. Phys. 1995, 102, 4870. (67) Dietz, T. G.; Duncan, M. A.; Powers, D. E.; Smalley, R. E. J. Chem. Phys. 1981, 74, 6511. (68) Bondybey, V. E. Science 1985, 227, 125. (69) Maruyama, S.; Anderson, L. R.; Smalley, R. E. Rev. Sci. Instrum. 1990, 61, 3686. (70) Goldberg, D. E. Gentic Algorithms in Search, Optimization & Machine Learning; Addison-Wesley: Boston, MA, 1989. (71) Bartmess, J. E.; Georgiadis, R. M. Vacuum 1983, 33, 149. (72) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (73) Frisch, M. J.; et al. Gaussian 03, Revision C.02; Gaussian, Inc., Wallingford, CT, 2004. (74) Andrae, D.; Haussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123. (75) Martin, J. M. L.; Sundermann, A. J. Chem. Phys. 2001, 114, 3408. (76) Peng, C. Y.; Schlegel, H. B. Isr. J. Chem. 1993, 33, 449.

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Figure 1. Kinetic fit for the reactions of the gold monomer anion Au- toward TFA at a reactant pressure of 6.2  10-8 mbar: (2) Au-; (b) CF3COO-; (9) (CF3COO)2H-; (;) noise. Table 1. Absolute Rate Constants kabs in 10-10 cm3 s-1 and Calculated Zero-Point Corrected Energies ΔEzp in kJ mol-1 for the Reactions of Aun- (n = 1-4) with TFAa kabs -

Aun þ CF3COOH f

eq

n

fast

slow

ΔEzp

AunH þ CF3COOAu þ Au n-1CF3COOH-

1 2

AunCF3COOH-

3

1 2 4 4

0.55 0.14 0.06 0.01

4.04 0.66 0.13 0.31

-41 -33 -15 -109

a The two columns fast and slow for kabs represent the different energy fractions of the initial ions.

Reactions with TFA. For the reaction of the gold monomer anion toward TFA a strong peak corresponding to the deprotonated TFA molecule was detected. The neutral product of the reaction has to be gold hydride. The kinetics for Au- toward TFA is shown in Figure 1. In the semilogarithmic plot it is obvious that the decay of the reactant anion does not perfectly follow pseudo-first-order kinetics. In general such a discrepancy can be ascribed to the presence of at least two ion populations, of different isomers, electronic excitations, or translational energies which exhibit different reactivity.64,77,78 Since we see this effect even with the gold monomer anion and since the excited states are far from the ground state, the effect is most likely due to some kinetic excitation. We fitted the kinetics by considering one fast and one slow fraction of the reactant ions, illustrated as dotted lines in Figure 1. The fast ions can be slowed down by collisions, converting them to the slow fraction. In reality, of course, it is a distribution of kinetic energies without sharp transitions. We do not expect problems by this circumstance, because all observed reactions are exothermic, and the fits indicate negative temperature dependence. After a short reaction delay the clusters are cooled down and the kinetics change to the linear characteristics of pseudo-first-order kinetics. The proton transfer, reaction 1, was about 7 times (77) Berg, C.; Beyer, M.; Schindler, T.; Niedner-Schatteburg, G.; Bondybey, V. E. J. Chem. Phys. 1996, 104, 7940. (78) Harding, D.; Ford, M. S.; Walsh, T. R.; Mackenzie, S. R. Phys. Chem. Chem. Phys. 2007, 9, 2130.

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Figure 2. Kinetic fit for the reactions of the gold dimer anion Au2- toward DMDS at a pressure of 3.0  10-8 mbar: (2) Au2-; (O) Au2SMe-; (9) AuMeSSMe-; ([) Au-; (0) Au2SSMe-; (;) noise. Table 2. Absolute Rate Constants kabs in 10-10 cm3 s-1 and Calculated Zero-Point Corrected Energies ΔEzp in kJ mol-1 for the Reactions of Aun- (n = 1-4) with DMDS Aun- þ H3CSSCH3 f

eq

n

kabs

ΔEzp

AunCH3SSCH3 AunCH3SS- þ CH3

4 5

AunCH3S- þ SCH3

6

1 2 4 2 4 2 2

0.8 0.09 1.96 0.83 0.70 0.13 0.08

-220 -16 -35 -34 -37 -64 -3

-

Aun-1CH3SSCH3 þ Au [Aun-1,CH3SSCH3] þ Au-

7 8

more effective for the slow ions. Since no proton transfer is observed with difluoroacetic acid, the proton affinity of the gold monomer anion can be bracketed between -1385 and -1355 kJ mol-1.79

Au- þ CF3 COOH f CF3 COO- þ AuH

ð1Þ

The proton transfer to the monomer can be easily understood, because this closed-shell system with its lone pair is an ideal proton acceptor. As a secondary reaction product deprotonated acid dimer ((CF3COO)2H-) is formed via radiative association,80,81 with an absolute rate constant of kabs= 1.0  10-11 cm3 s-1. The loss of a neutral gold atom combined with the formation of a charged gold-TFA complex was observed for both open-shell systems (n = 2, 4, reaction 2). The reaction is for n = 2 about 5 times and for n = 4 only 2 times more effective for the slow fraction.

Aun- þ CF3 COOH f Au þ Aun - 1 CF3 COOH- for n ¼ 2, 4

ð2Þ

(79) Caldwell, G.; Renneboog, R.; Kebarle, P. Can. J. Chem.-Rev. Can. Chim 1989, 67, 611. (80) Dunbar, R. C. Mass Spectrom. Rev. 1992, 11, 309. (81) Klippenstein, S. J.; Yang, Y. C.; Ryzhov, V.; Dunbar, R. C. J. Chem. Phys. 1996, 104, 4502.

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Figure 3. (a) Potential energy surface of Au4- with doublet spin multiplicity. (b) Geometries of the stationary points.

radiative association (reaction 4). Radiative association is very sensitive to the available energy in the collision complex. As expected, it was exclusively observed for the slow fraction.

Au- þ H3 CSSCH3 f AuCH3 SSCH3-

Figure 4. Geometries for the reaction products of small gold cluster anions with TFA. Bond lengths are given in angstroms.

Radiative association80,81 was only observed for the slow fraction of the gold tetramer anion Au4- (reaction 3).

Au4- þ CF3 COOH f Au4 CF3 COOH-

ð3Þ

For the gold trimer anion no reactions were observed. This can be rationalized because all four valence electrons of this linear molecule are used for the two σ bonds. Absolute rate constants and zero-point corrected relative energies for all observed reactions of the gold anions with TFA are shown in Table 1. Reactions with DMDS. In the case of DMDS we observe the aforementioned sensitivity to kinetic excitation only for the gold monomer anion, which reacts toward DMDS via

ð4Þ

Again, both open-shell systems show similar reactivities. The gold dimer and tetramer anion are activating the S-C bond of DMDS (reaction 5). A second common reaction is the activation of the S-S bond (reaction 6). While the absolute rate constants for reaction 6 are nearly the same for n = 2, 4, reaction 5 is 20 times faster for the gold tetramer anion.

Aun- þ H3 CSSCH3 f Aun CH3 SS- þ CH3 for n ¼ 2, 4

ð5Þ

Aun- þ H3 CSSCH3 f Aun CH3 S- þ SCH3 for n ¼ 2, 4

ð6Þ

Exclusively for the dimer, ligand exchange of a neutral gold atom against DMDS was observed (reaction 7) and with reversed charge distribution in the products (reaction 8). The gold monomer anion resulting from the gold exchange (reaction 8) undergoes radiative association (reaction 4), as the mass-selected Au- coming from the ion source does. Figure 2 displays the relative intensity of Au2- and of the

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Figure 5. Geometries for the reaction products of small gold cluster anions with DMDS. Bond lengths are given in angstroms.

products for the reaction with DMDS as a function of time at a constant pressure.

Au2- þ H3 CSSCH3 f AuCH3 SSCH3- þ Au

ð7Þ

Au2- þ H3 CSSCH3 f AuCH3 SSCH3 þ Au-

ð8Þ

Absolute rate constants and calculated relative energies for the reactions of Aun- with DMDS are shown in Table 2. At Δm = þ1 we observed a peak relative to the gold dimer anion which corresponds to Au2H- from the ion source. Again, no reaction is observed for the trimer anion with DMDS. Calculations of Aun- (n = 1-4). An electron affinity of 214 kJ mol-1 is calculated for the singlet state of the gold monomer anion. The doublet state of the gold dimer anion with a bond length of 2.69 A˚ is energetically preferred to the quartet state with a bond length of 2.83 A˚, which lies 373 kJ mol-1 higher in energy. For the singlet state of Au3the linear D¥h structure with a bond length of 2.63 A˚ is stable. The triplet state of the gold trimer anion is a regular D3h triangle with a bond length of 2.78 A˚, 123 kJ mol-1 higher in energy than the singlet state. The calculated structures for the small anionic gold clusters correspond to previous works of Ahlrichs, Kappes, and co-workers.29 The tetramer is the only cluster in our size range with structural isomers in the ground state which potentially contribute to the observed deviation from pseudo-first-order kinetics. In our examination we recalculated the already known isomers of the tetramer29 and newly identified two transition states. For cluster ions produced in a laser vaporization source, higher lying isomers can be rapidly frozen out in the supersonic expansion of the hot plasma in helium.82 We therefore calculated the density of states83-87 of the isomers as a function of energy to get an idea whether higher lying isomers are expected in the experimental cluster population. Local minima and connecting transition states for the doublet of the gold tetramer anion are shown in Figure 3b, (82) Cao, Y.; H€ ockendorf, R. F.; Beyer, M. K. J. Chem. Phys. 2010, 132, 224307. (83) Wales, D. J. Mol. Phys. 1993, 78, 151. (84) Wales, D. J.; Berry, R. S. Phys. Rev. Lett. 1994, 73, 2875. (85) Wales, D. J.; Doye, J. P. K. J. Chem. Phys. 1995, 103, 3061. (86) Miller, M. A.; Wales, D. J. J. Chem. Phys. 1997, 107, 8568. (87) Bogdan, T. V.; Wales, D. J.; Calvo, F. J. Chem. Phys. 2006, 124, 044102.

with the corresponding PES in Figure 3a. The most stable geometry is the zigzag C2h structure, denoted 2IVa. The second isomer with the C2v Y-structure 2IVb is 10 kJ mol-1 and the third isomer with the C2v rhombic structure 2IVc is 32 kJ mol-1 above structure 2IVa. The quartet of the tetramer is a regular Td tetrahedron with a bond length of 2.85 A˚, lying 134 kJ mol-1 above 2IVa. The calculated isomerization of the gold tetramer anion from 2IVa to 2IVb with a barrier of 23 kJ mol-1 (2IV-TSab) and the isomerization of 2IVb to 2IVc with a barrier of 38 kJ mol-1 (2IV-TSbc) relative to 2IVa is shown in Figure 4. The calculation of the density of states as a function of total energy indicates that the lowest lying isomer 2IVa dominates the population with more than 96% at energies above 100 kJ mol-1. This contribution increases to 99% at the barrier of the lowest lying transition state 2IV-TSab. Calculation of Possible Products for the Reaction of Aunwith TFA. For the calculation of the thermochemistry of the reactions, we always used the minimum energy geometries of the bare clusters. Optimized structures for the products of the reaction of Aun- with TFA are presented in Figure 4. A conceivable pathway for the deprotonation of the acid, reaction 1, is first a barrierless and exothermic (-102 kJ mol-1) adduct formation of the gold monomer anion and TFA, forming the negatively charged AuHO2CCF3- complex 1S2. During this formation the O-H bond of TFA at 0.97 A˚, 1S1, is extended to 1.06 A˚ in 1S2. The new formed Au-H bond has a length of 2.04 A˚ and an Au-H-O angle of 169. The adduct formation is followed by a barrierless dissociation into the deprotonated acid, CF3COO-, denoted 1S4, and the neutral gold hydride with a bond length of 1.54 A˚. For the gold loss (reaction 2) the most stable structure is the insertion of one gold atom into the OH bond, 1S3 for n = 2. The favorable structure of the radiative association (reaction 3) is again one terminal gold inserted into the O-H bond with the rest of the gold chain sticking out, structure 2S5. As shown in Table 1, reaction 3 was found to be at -109 kJ mol-1 the most exothermic reaction of Aun- with TFA. Calculation of Possible Products for the Reaction of Aunwith DMDS. Minimum geometries of possible reaction products of DMDS with Aun- are shown in Figure 5. Radiative association (reaction 4) and ligand exchange (reaction 7) form the same charged product. The structure 1S6 with the gold atom inserted into the S-S bond, forming a nearly linear S-Au-S group with an Au-S-C angle of 105, is the most stable.

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In reactions 5 and 6 of the dimer anion, the cleaved part of DMDS is simply added to the gold cluster, 1S7 and 1S9. S-S activation by the gold tetramer results in the H3CS moiety being symmetrically inserted into the gold cluster, 1S10. If CH3 is released as a neutral product, the second sulfur atom inserts into a Au-Au bond, 1S8. The neutral product of reaction 8 is again the gold inserted into the sulfide-sulfide bond, with the S-Au-S bond weakly bent, an Au-S-C angle of 107, and the methyl groups on the same side, 2S11. Table 2 shows that radiative association is at -220 kJ mol-1 again the most exothermic reaction. The suggested structures 1S6 and 2S11 in Figure 5 correspond nicely to recently calculated gold thiolate complexes on the Au(111) surface.43

Conclusion Overall the small gold cluster anions are very unreactive. They need a strong acid or a disulfide bond to interact. A sizedependent reactivity is observed for small anionic gold clusters Aun- (n = 1-4) toward TFA and DMDS. The gold dimer and tetramer anions show largely similar reactivities and exhibit the richest chemistry in this cluster size range, which is rationalized by their radical character. The most efficient reaction

H€ ockendorf et al.

was observed for the gold monomer undergoing proton transfer with TFA, which is a barrierless reaction with a straightforward reaction path. The absence of reactivity for the gold trimer anion can be ascribed to the linear structure of this closed-shell system with its valence electrons localized in the σ bonds. In DFT calculations, products were found for which all observed reactions are exothermic. The calculations of cluster isomers show unambiguously that a mild kinetic excitation of the mass-selected ions is responsible for the deviation from pseudo-first-order kinetics. All reactions which show the deviation also exhibit negative temperature dependence.

Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft and by the Fonds der Chemischen Industrie is gratefully acknowledged. M.K.B. acknowledges a Heisenberg fellowship from the Deutsche Forschungsgemeinschaft. We thank Milan Onc ak for helpful discussions. Supporting Information Available: Text giving the full ref 73, figures giving additional kinetics data and relative densities of states, and tables giving Cartesian coordinates of the calculated structures. This material is available free of charge via the Internet at http://pubs.acs.org.