Theoretical Prediction of SâH Bond Rupture in Methanethiol upon

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Theoretical Prediction of S-H Bond Rupture in Methylthiol upon Interaction with Gold Mikhail Askerka, Daria Pichugina, Nikolay Kuz'menko, and Alexander Shestakov J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp303001x • Publication Date (Web): 17 May 2012 Downloaded from http://pubs.acs.org on May 31, 2012

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The Journal of Physical Chemistry

Theoretical Prediction of S-H bond Rupture in Methylthiol upon Interaction with Gold Authors: Mikhail Askerka, Daria Puchugina, Nikolay Kuz’menko and Alexander Shestakov Affilation: Department of chemistry, Lomonosov Moscow State University, Leninskie gory, 1 str. 3, 119991, Moscow, Russian Federation Corresponding author: Daria Pichugina Telephone: 0079394765 Fax: 0079328846 e-mail: [email protected]

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Abstract Organic thiols are known to react with gold surface to form self-assembled monolayers (SAMs), which can be used to produce materials with highly attractive properties. Although the structure of various SAMs is widely investigated, some aspects of their formation still represent a matter of debate. One of these aspects is the mechanism of S-H bond dissociation in thiols upon interaction with gold. This work presents a new suggestion for this mechanism on the basis of DFT study of methylthiol interaction with single gold atom and Au20 cluster. The reaction path of dissociation is found to be qualitatively independent on the model employed. However, the highest activation barrier of S-H bond dissociation on the single gold atom (12.9 kcal/mol) is considerably lower than that on the Au20 cluster (28.9 kcal/mol), which can be attributed to the higher extent of gold unsaturation. The energy barrier of S-H cleavage decreases by 4.6 kcal/mol in the presence of the second methylthiol molecule at the same adsorption site on the model gold atom. In the case of Au20 cluster we have observed the phenomenon of hydrogen transfer from one methylthiol molecule to another, which allows reducing the energy barrier of dissociation by 9.1 kcal/mol. This indicates the possibility of the “relay” hydrogen transfer to be the key step of the thiol adsorption observed for the SAMs systems. Keywords: DFT, Hydrogen Transfer, Gold Cluster, Transition State, Relay Mechanism, Self-Assembled Monolayer


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I.

Introduction Thiol molecules form dense, stable self-assembled monolayers (SAMs) on Au surface.

Such SAMs offer a unique opportunity to functionalize surfaces with controlled morphology and reactivity as well as to stabilize gold clusters.1-3 SAM-functionalized materials are of potential interest in molecular electronics, surface science, chemistry, biology and molecular engineering.4-9 Thiol molecules (R-SH) are known to be adsorbed on a reconstructed10 gold surface in a form of thiolates (R-S·) without the hydrogen atom originally attached to the sulfur atom.11,12 Structure and electronic properties of thiolate monolayers as well as gold thiolate-binding sites have been widely investigated both theoretically and experimentally.13 In theoretical studies methylthiol becomes the most frequent model due to the difficulties of the computational treatment of long-chain thiols. Early theoretical studies, based on DFT calculations, are devoted mostly to prediction of the possible adsorption sites of the sulfur head-group binding and orientation of the alkyl radical on the clean Au(111) surface. The first theoretical assumption about methilthiolate being located at the Hollow-Fcc site forming three covalent bonds with gold atoms,14,15 confirmed experimentally with STM,16 was argued by more recent calculations, which indicate the most favorable position of the head-group at the Bridge site.17,18 This later hypothesis found its experimental support in the experiments of Maksymovich et al. who detected the position of twofold coordinated single thiolate molecules on Au(111) surface through STM.19 The presence of surface defects is also of prime importance in the process of thiol adsorption. Experimental and theoretical studies indicate the key role of gold adatoms on Au(111) surface in formation of monolayers. In the work20 thiolate molecules are found to be adsorbed through the RS-Au-SR, so-called “staple” motif. Such complexes consist of an Au adatom at a bridge site of the Au(111) surface and two thiolate radicals each of which forms two bonds with the Au adatom and a surface atom. This work by Maksymovych et al. has led to many new structural models involving Au adatoms and vacancies in the first atomic layer of the substrate surface.21-24 This paper also confirms a “pulling mechanism”, first suggested by R. Mazzarello et al.25 under which sulfur atoms pull gold atoms out of the first layer forming a nearby vacancy and a RS-Au-SR “staple”. The most recent experimental breakthrough in the structure on thilolate monolayers was made by Cossaro et al.; the work26 evidences in favor of formation of …-(R)S-Au-S(R)-…zigzag chain structure aligned along the cell short axis as well as the single RS-Au molecules adsorbed at the Bridge sites of Au(111) surface. This work drew the attention to cyclic thiolated gold clusters2,27 representing a simple model of such zigzag motif. ACS Paragon Plus Environment


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However, both the way thiolate is formed and the fate of the thiol hydrogen atom still represent a matter of debate. Since the purpose of the present paper is to bring some light to the problem of the S-H bond cleavage, the next part of introduction is devoted to the key aspects of non-dissociative adsorption of thiols on gold surface. Molecular adsorption of thiols is also investigated theoretically and experimentally. The position of the sulfur headgroup, however, is not as contradictory as in the case of thiolates. The major conclusions are: (i) thiol molecules are adsorbed at the Top sites of Au(111) surface, (ii) the adsorption energy is much lower than that of thiolates and (iii) the molecular adsorption doesn’t cause the reconstruction of the surface.18,28-31 The atop position of the sulfur head-group, proposed in DFT studies,

is confirmed by STM.30 The calculated energy of molecular

adsorption (about 9 kcal/mol) is also in good agreement with a measured one19 (about 10 kcal/mol). In order to further investigate the pathways of S-H bond cleavage, we should identify the initial reagents and final products of the reaction of dissociation. On the basis of theoretical and experimental facts noted above we refer to thiols molecularly adsorbed on the clean Au(111) surface as reagents; thiolates chemisorbed at adatoms through a chain …-(R)S-Au-S(R)-… along with single S-R fragments chemisorbed at the Bridge sites are referred to as products after S-H bond cleavage. In the work32 several pathways of methylthiol dissociation on the clean Au(111) surface are found. However, since we consider Au adatoms the key elements of S-H dissociation, we assume that the search for the possible ways of this reaction still remains an unsolved problem.33 Our report presents the results of theoretical investigation of thiol adsorption mechanism on gold clusters at DFT level. The study of interaction between thiol and model gold cluster implied the formation of a gold-thiolate complex without electron transfer.11,12 In order to avoid huge computation time from one hand and omit the effects of dispersion interaction between hydrocarbon chains from the other hand we have chosen methylthiol to investigate the mechanism. Both isolated single gold atom and Au20 cluster are used as models to investigate the reaction pathway upon interaction with methylthiol. These cluster models provide less complicated computations than those of slab models. A single gold atom represents the simplest model of a gold adatom34. In order to study how the cluster size influences the reaction pathway, the interaction with Au20 cluster has been thoroughly investigated. Among all known gold clusters with magic numbers, Au20 has been chosen because of its extraordinary stability due to closed shell electronic structure. Its facet atoms are arranged like in the first layer of Au(111) surface.35 Moreover, using tetrahedral Au20 cluster, we get an opportunity to simulate unevenness of gold surface in the form of defects which are expected to be active in adsorption of thiols. ACS Paragon Plus Environment

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II.

Computational details All calculations have been performed using Gaussian 0336 and Priroda37 program

packages. In order to choose the proper functional and basis set to be employed in further largescale DFT computations, the structures and binding energies of small gold methylthiolate AuSCH3 and gold methylthiol AuSHCH3 complexes have been determined by different quantum-chemical approaches. Since AuSCH3 and AuSHCH3 molecules represent closed- and open-shell systems containing different types of S-Au bonding, it was particularly important to estimate the accuracy of DFT methods. The lack of available experimental data on the characteristics of these molecules made it urgent to use calibrations through the highly correlated CCSD and CCSD(T) approaches. Generalized Gradient Approximation with functionals of Perdew, Wang (PW91)38 and Perdew, Bücke (PBE)39 along with B3LYP hybrid approach40-42 have been employed in conjunction with SBK,43 LANL2DZ44 and combined 631G**/LANL2DZ (6-31G** for H, C, S and LANL2DZ for Au atoms) basis sets. In these approaches significant relativistic effects of gold are taken into consideration using 19 electron core potentials. The binding energies, including ZPVE correction, of AuSCH3 and AuSHCH3, defined by ∆E0

complex=

(E

atom

– E0

molecule)

with gold atom and methylthiol (methylthiolate) being in

appropriate spin states are compiled in Tables 1,2 in addition to the most relevant structural parameters. In order to avoid considerable computation costs ZPVE correction in CCSD(T) method has been calculated using Hess matrix obtained through CCSD approach. Table 1 Table 2 Structural and electronic parameters of molecules, fully optimized using the CCSD(T) method, are employed as a criterion to evaluate the validity of DFT approaches using the particular basis set. In both complexes SBK basis set is found to provide the most reliable values for S-Au distances, which are of prime importance for further computations, while the other two basis sets lead to huge errors of minimum 0.046 Å in PW91/6-31G** and 0.024 Å in B3LYP/LANL2DZ compared to 0.005 Å in PBE/SBK for Au-S distance in CH3SAu molecule. B3LYP functional is found to highly overestimate Au-S distances, especially in open-shell CH3HS-Au system (maximum 0.127 Å in B3LYP/SBK), and thus considered unreliable. Comparison of dissociation energies using SBK basis set has revealed PBE and B3LYP functionals to yield the best results; taking into consideration the previous comparison, we have chosen PBE functional with SBK basis set for the further calculations. Moreover, PBE functional ACS Paragon Plus Environment


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is proved to be a fair compromise between the description of the metallic and ligand part of the gold-thiolate system.2 In order to confirm the applicability of the PBE functional we have compared the properties of CH3SHAu and CH3SAu molecules, calculated in double-ζ basis set (631G**/LANL2DZ) with those, obtained in CCSD(T) approach but in triple-ζ basis set (631G**/LANL2TZ). It can be inferred from the last blocks of Tables 1 and 2 that PBE functional yields best results for both energetic characteristics and interatomic distances. The equilibrium geometry of methylthiol obtained with PBE/SBK approach is in line with experimental results45 dS-H = 1.362 Å, dS-C = 1.843 Å, dC-H = 1.096 Å, ∠H-S-C = 96.5˚ (compared with experimental values 1.340 Å, 1.819 Å, 1.090 Å and 97.1˚ respectively). The types of stationary points on the potential energy surfaces (PES) have been determined from the analysis of Hess matrix; the second derivatives have been calculated analytically. We have determined the structures of transition states (TS) according to the procedure of eigenvector following with the Berny algorithm.46 The initial approximation for the geometry of TS has been proposed by different ways: on the basis of structures of adsorbed methylthiol-gold complexes, intermediates, or methylthiolate-gold complexes. For each TS, which contained only one imaginary frequency, we have performed the calculation of the intrinsic reaction coordinate47 through this mode.

Zero-point vibration energies have been

computed in harmonic approximation. As basis set superposition errors BSSE48 are a frequent source of uncertainty, especially in the systems containing gold and thiols,17 the calculated PBE/SBK adsorption energies have been checked for BSSE. The largest binding energy change of less than 0.4 kcal/mol has been obtained for the Au20CH3SH system. Due to this counterpoise corrections are judged to be negligible on the relevant accuracy scale so that all presented data are not BSSE corrected. III.

Results and Discussion

Interaction of methylthiol and single gold atom. In this section we consider the energetics of non-dissociative adsorption of methylthiol on a single gold atom and explore possible ways of SH bond cleavage upon dissociative interaction. Single gold atom serves as a rough model of an adatom on Au(111) surface. The energetics of such interaction can be by no means extended to the surface model; however, this system may represent a kind of a ‘reference point’ to understand the stages of dissociative interaction. It can be inferred from the Table I, that binding energy of thiol in CH3SHAu complex (I1) is 14.0 kcal/mol. Formation of this complex is a result of non-dissociative adsorption of methylthiol on gold atom. Subsequent hydrogen transfer from the sulfur to gold atom according to the scheme (Fig. 1, dash line) ACS Paragon Plus Environment

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CH3-SH + Au → H-Au-S-CH3

(1)

has a favoring energy change (20.7 kcal/mol). However, the reaction (1) is unlikely due to high energy barrier (TS1, 17.5 kcal/mol). Therefore, we have suggested that two methylthiol molecules take part in the S-H bond dissociation with formation of molecular hydrogen: 2 CH3-SH + Au → CH3-S-Au-S-CH3 + H2

(2)

The adjunction of the second methylthiol molecule to I1 can occur through the formation of S-Au and H-Au bonds. We have found the complex formed via H-Au bond to be more stable with an energy difference of 2.8 kcal/mol. Its structure (I2) is shown in the Fig. 1. H-Au bond is formed through the conjunction of LUMO of methylthiol and the highest double-occupied molecular orbital of CH3SHAu complex. Since the complex I2 represents a more stable electronic structure, it has been chosen as an initial molecule in the pathway of hydrogen elimination. Figure 1 We have found two alternative ways to realize the reaction 2 (Fig. 1, solid line). The way a (see Fig.1 caption) involves three transition states and represents a step-by-step migration of two hydrogen atoms from sulfur to gold atoms (TS2, TS3) and then association of the hydrogen atoms (TS4) into a H2 molecule. An alternative way b implies a direct formation of H2 without migration of the second hydrogen atom from sulfur to gold atom (TS5). The reaction pathway b is more probable for two reasons. First, the highest energy barrier in this case (I3→TS5→(CH3S)2-Au+H2) is lower by 2.7 kcal/mol. Second, all the elementary stages of this pathway are exothermic, which makes the direct reaction more probable than the inverse one. The activation energy of the reaction 2, which corresponds to association of two hydrogen atoms (TS5) is lower than that for the reaction 1 by 7.3 kcal/mol. Therefore, the addition of the second methylthiol molecule at the same gold atom can facilitate hydrogen elimination. Interaction of methylthiol and Au20 cluster. In this section we apply the patterns obtained for single gold atom to investigate molecular and dissociative adsorption of methylthiol on Au20 cluster. The Au20 cluster is an attractive model to investigate thiol adsorption for several reasons (some of them are noted in introduction): (i) this cluster represents an extremely stable closed shell electronic structure;35

(ii) the atoms at the faces of the cluster are arranged

according to Au(111) motif and the average Au-Au distance in the cluster (2.61 Å)35 approaches that of the bulk gold (2.88 Å); (iii) at the same time this cluster has low coordinated atoms which are supposed to be potential sites of thiol adsorption and can represent surface defects.



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Thus, using this cluster, we can at least qualitatively compare the patterns observed with those obtained for Au(111) surface. Au20 cluster is a complicated object to investigate adsorption process due to existence of several nonequivalent adsorption sites. From the examples in the literature we know that thiols are adsorbed at the “on-top” site of Au(111) surface, demonstrating monodentate nature. Possessing a high tetrahedral symmetry, Au20 cluster has three potential adsorption sites which correspond to its nonequivalent apex, edge and facet atoms. In order to choose the most appropriate adsorption site for first methylthiol molecule, molecular orbitals of both cluster and thiol have been visualized (Fig. 2). Figure 2 Since the Au-S bonds in gold-thiol complexes are formed by the transfer of electron density from thiol HOMO to LUMO of gold,49 we have visualized LUMO of Au20 cluster and HOMO of methylthiol. As far as Au20 cluster has high tetrahedral symmetry, it has three degenerate unoccupied orbitals (LUMO – LUMO-2) one of which is presented in the Fig. 2. The shape of molecular orbitals is in good qualitative agreement with the results obtained in traditional PW91/LANL2DZ approach.50 Fig. 2 shows that the electron density is less likely to be located (LUMO) at the apex atom of the cluster. Thus, the apex atom is the most active adsorption site in the electron-accepting process. This suggestion has been confirmed by the calculation of methylthiol binding energy at different adsorption sites of Au20 cluster. The binding energy at the apex atom is 16.0 kcal/mol, at the edge atom – 8.9 kcal/mol and at the facet atom – 7.0 kcal/mol, the latter being in good agreement with theoretical value obtained for Au(111) surface (8.6 kcal/mol)30 through solid-state comtutations. Similarly to single gold atom, the direct dissociation of methylthiol S-H bond on Au20 CH3-SH + Au20 → H-Au20-S-CH3

(3)

is improbable because of high energy barrier (23.7 kcal/mol, TS6), which made us search the preferable reaction pathway with two methylthiol molecules. Figure 3 The most favorable complexes of Au20 cluster and two methylthiol molecules are shown in the Fig. 3. Like in the case of a single Au atom, adsorption of the second methylthiol molecule can occur through the formation of S-Au (I6) or H-Au (I7) bonds. Unusual behavior of methylthiol in the I7 complex can be explained in the terms of frontier molecular orbitals. H-Au bond is formed through the transport of electron density from HOMO of the I5 complex to the


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LUMO of methylthiol (Fig.2). The I7 and I8 complexes (isomers of I6) have an additional stabilization (2.1 kcal/mol) due to the formation of intermolecular hydrogen bond. Figure 4 We have found the reaction pathway of methylthiol dissociation on the Au20 cluster, starting with the complex I6, to be completely similar to that on single Au atom. 2 CH3-SH + Au20 → CH3-S-Au20-S-CH3 + H2

(4)

The reaction 4 is accompanied by a large energy gain (43 kcal/mol) because of formation of four exclusively stable Au-S bonds and a strong H-H bond. Structure and bonding orbitals of P2 complex are presented in the Fig. 3. Notable is the structure of the reaction 4 product (P2). The principle arrangement of thiolate fragments and gold atoms is in line with the “staple” motif observed both experimentally and theoretically. Au-S distances in this “staple”, obtained in the present work (2.37-2.43 Å), are also in good agreement with previous theoretical studies.21,23,51 However, in the case of the Au20 cluster the values of energy barriers are much higher. The high energy barrier (almost 50 kcal/mol, pathway с) of the last stage, corresponding to association of two hydrogen atoms into a molecule (TS9), is due to the high stability of the complex I10, where four extremely stable Au-S bonds have been already formed. However, there is an opportunity to avoid this barrier using an alternative reaction pathway d, where the elimination of hydrogen takes place without migration of the second hydrogen from sulfur to the Au atom (TS10). Similarly to the case of single Au atom, this alternative way d, is more preferable. Nevertheless, despite the reaction 4 leads to an attractive “staple”, its activation energy, corresponding to the highest barrier (I6→TS7→I9), is still considerable (28.9 kcal/mol); this fact, along with experimentally observed effortless formation of thiolates, points at the existence of another reaction pathway, different from the one, found on a single Au atom. On the basis of the transition state found (TS11), we have shown the feasibility of the “relay” (or “push-pull”) mechanism of thiol dissociation on Au20 cluster. This “relay” mechanism implies the migration of a hydrogen atom not to a gold atom of the cluster, but to the sulfur atom of nearby thiol (Fig. 5). Figure 5 This migration is possible because of the high electronegativity of sulfur and represents a phenomenon, common to the elements of the 6-th group in the Periodic Table. The key step of the “relay” mechanism is represented by the transition state TS11 (ωi = 445 cm-1). The comparatively low (19.8 kcal/mol) energy barrier corresponds to the migration of hydrogen in ACS Paragon Plus Environment


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the structure I6. However, this barrier can be even lower (12 kcal/mol), if the hydrogen shift occurs in the complex I8, where the hydrogen bond, which promotes the hydrogen migration, is already formed. The mechanism itself and the energy barriers of its stages can be compared with those obtained by R. Nadler et al.52 Through MD simulations the authors found a reaction pathway of S-H dissociation including hydrogen transfer; in this pathway the hydrogen atom of a thiol is captured by a molecule of water from the solution. The calculated activation energy of the hydrogen transfer is 18.9 kcal/mol. The “relay” mechanism as it’s formulated above, could lead to the formation of series of “staples” resulting in the appearance of …-(R)S-Au-S(R)-… chains observed in experiment.26 IV.

Conclusions The mechanism of gold-thiol adsorption have been investigated using first principle

methods, with single gold atom and Au20 cluster being the models of the fragments of Au(111) surface, and methylthiol being a model of an organic thiol. The model of the adsorption site doesn’t have a significant influence on the principal mechanism of thiol adsorption but it has a noticeable impact on its activation energy. The presence of an additional methylthiol leads to a more complicated mechanism of thiol dissociation but reduces the effective activation energy of the reaction. In the presence of two thiol molecules the adsorption may occur through a “relay” mechanism, when the intermolecular hydrogen atom exchange results in the transformation of one of thiols into a thiolate. This process is accompanied by comparatively low energy barriers and may represent a key step of thiol adsorption, observed in the experiment. The mechanism, found for an isolated gold cluster, can take place in more complicated supramolecular systems, where it represents one of the stages of an intricate chemistry of gold-thiolate SAM formation. V.

Acknowledgment This research was supported by the Russian Foundation for Fundamental Research

through the Projects 10-03-00999, 11-01-00280, 11-03-01011 and by the Council for Grants of the Russian Federation President (State Program of Support of Young Candidates of Science) through the Project MK-158.2010.3. The calculations were performed using the supercomputer SKIF MSU “Chebyshev”. Supporting information. Cartesian coordinates of the reactants, products and intermediates of the reactions, described in the article. This material is available free of charge via the Internet at http://pubs.acs.org. References 1. Jin R. Nanoscale. 2010, 2, 343-362.


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25. Mazzarello, R.; Cossaro, A.; Verdini, A.; Rousseau, R.; Casalis, L.; Danisman, M.F.; Floreano, L.; Scandolo, S.; Morgante, A.; Scoles, G. Phys. Rev. Lett. 2007, 98, 016102– 016105. 26. Cossaro, A.; Mazzarello, R.; Rousseau, R.; Casalis, L.; Verdini, A.; Kohlmeyer, A.; Floreano, L.; Scandolo, S.; Morgante, A.; Klein, M.L.; Scoles, G. Science. 2008, 321, 943–946. 27. Barngrover, B.M.; Aikens, C.M. J. Phys. Chem. Lett. 2011, 2, 990–994. 28. Yourdshahyan, Y.; Rappe, A. J. Chem. Phys. 2002, 117, 825–835. 29. Gronbeck, H.; Curioni, A.; Andreoni, W. J. Am. Chem. Soc. 2000, 122, 3839–3842. 30. Maksymovych, P.; Sorescu, D.; Dougherty, D.; Yates, J.T. J. Phys. Chem. B 2005, 109, 15992–15996. 31. Lavrich, D.; Wetterer, S.; Bernasek, S.; Scoles, G. J. Phys. Chem. B 1998, 102, 3456– 3465. 32. Lustemberg, P.G.; Martiarena, M.L.; Martínez, A.E.; Busnengo, H.F. Langmuir, 2008, 24, 3274–3279. 33. Vericat, C.; Vela, M.E.; Benitez, G.; Carro, P.; Salvarezza, R.C. Chem. Soc. Rev. 2010, 39, 1805–1834. 34. Maksymovych, P.; Voznyy, O.; Dougherty, D. B.; Sorescu, D. C.; Yates J. T. Prog. Surf. Sci. 2010, 85, 206–240. 35. Li, J.; Li, X.; Zhai, H.-J.; Wang, L.-Sh. Science. 2003, 299, 864–867. 36. Frisch, M. J., Gaussian 03, Revision B.02; Gaussian, Inc., Pittsburgh, PA, 2003. 37. Laikov, D.N. Chem. Phys. Lett. 1997, 281, 151–156. 38. Perdew, J.P.; Chevary, J.A.; Vosko, S.H.; Jackson, K.A.; Pederson, M.R.; Singh, D.J.; Fiolhais, C. Phys. Rev. B 1992, 46, 6671–6687. 39. Perdew, J.P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865–3868. 40. Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. 41. Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. 42. Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989, 157, 200–206. 43. Stevens, W.J.; Krauss, M.; Basch, H.; Jasien, P.G.; Can. J. Chem. 1992, 70, 612–630. 44. Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299-310. 45. CRC Handbook of Chemistry and Physics, 81st ed. Lide, D.R., Ed.; CRC Press: Boca Raton, FL, 2001. 46. Schlegel, H.B. J. Comp. Chem. 1982, 3, 214–218. 47. Gonzalez, C.; Schlegel, H.B. J. Chem. Phys. 1989, 90, 2154–2161. 48. Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553–566. ACS Paragon Plus Environment

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Tables Table 1. Comparison of Bond Lengths (Å), Angles (˚) and Binding Energies (in kcal/mol) of CH3SHAu system for various electronic structure methods Method

∆E0

d Au-S

d S-H

d S-C

d C-H

∠ Au-S-H

∠ Au-S-C

SBK PBE

-13.7

2.516

1.369

1.850

1.104

95.3

103.9

B3LYP

-7.3

2.631

1.355

1.850

1.355

95.2

103.4

PW91

-14.2

2.519

1.368

1.851

1.102

94.9

104.2

CCSDa

-8.5

2.528

1.350

1.844

1.098

98.4

100.4

-10.2

2.504

1.353

1.849

1.100

96.9

101.3

CCSD(T)b

LANL2DZ PBE

-11.7

2.743

1.392

1.904

1.099

93.6

99.7

B3LYP

-6.7

2.829

1.378

1.900

1.090

95.4

100.5

PW91

-12.3

2.738

1.391

1.904

1.097

93.8

99.5

CCSD

-6.4

2.781

1.381

1.914

1.101

98.5

100.0

CCSD(T)

-7.2

2.762

1.382

1.917

1.103

96.1

99.3

6-31G**/LANL2DZ PBE

-11.2

2.615

1.359

1.837

1.098

92.3

101.0

B3LYP

-5.7

2.717

1.348

1.837

1.090

93.5

101.1

PW91

-11.8

2.613

1.358

1.838

1.096

92.2

100.9

CCSD

-3.7

2.711

1.335

1.823

1.086

97.5

100.7

CCSD(T)

-4.7

2.682

1.337

1.830

1.088

96.0

100.7

1.088

98.3

100.4

6-31G**/LANL2TZ CCSD(T)

-8.8

2.584

1.338

1.832

a – coupled cluster method with single and double excitations, b – coupled cluster method with single, double and perturbatively estimated triple excitations


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Comparison of Bond Lengths (Å), Angles (˚) and Binding Energies (in kcal/mol) of CH3SAu system for various electronic structure methods Method

∆E0

d Au-S

d S-C

d C-H

∠ Au-S-C

SBK PBE

-53.8

2.276

1.845

1.111

104.1

B3LYP

-44.9

2.298

1.849

1.097

104.3

PW91

-54.0

2.277

1.847

1.103

104.3

CCSD

-46.8

2.283

1.848

1.099

103.0

CCSD(T)

-49.4

2.281

1.860

1.101

102.7

LANL2DZ PBE

-47.3

2.366

1.902

1.104

102.5

B3LYP

-39.1

2.381

1.901

1.096

103.0

PW91

-47.7

2.365

1.902

1.097

102.4

CCSD

-37.7

2.407

1.925

1.102

101.9

CCSD(T)

-39.1

2.407

1.935

1.104

101.6

6-31G**/LANL2DZ PBE

-49.7

2.288

1.835

1.098

103.2

B3LYP

-41.3

2.308

1.838

1.090

103.4

PW91

-50.2

2.288

1.836

1.096

103.1

CCSD

-38.4

2.326

1.829

1.086

102.4

CCSD(T)

-40.1

2.334

1.839

1.089

101.9

1.089

103.3

6-31G**/LANL2TZ CCSD(T)

-46.9

2.275

1.836



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Figure Captions Figure 1. Optimized structures of reagents, products, intermediates (In) and transition states (TSn) for the mechanism of dissociative adsorption of methylthiol on a single Au atom along with most relevant bond distances (Å) and the relative energies on the section of PES (dash line – reaction 1, sold line – reaction 2). The imaginary frequencies of TSi are: ωi(TS1) =544 cm-1, ωi(TS2)=496 cm-1, ωi(TS3)=666 cm-1, ωi(TS4)=714 cm-1, ωi(TS5)=432 cm-1. The reaction pathway a described in the section: I3→TS3→I4→TS4→(CH3S)2-Au+H2; pathway b: I3→TS5→(CH3S)2-Au+H2. In order to make the comparison of reactions 1 and 2 more convenient, the energy of the CH3SH is added to the energies of TS1 and CH3S-Au-H. Figure 2. Molecular orbitals of Au20 cluster, CH3SH and CH3SHAu20 complex (I5).

Figure 3. The structures of I6, I7, I8, P2 complex with the most relevant bond lengths (Å) and molecular orbitals of P2 complex. Figure 4. Optimized structures of reagents, products, intermediates (In) and transition states (TSn) for the mechanism of dissociative adsorption of methylthiol on Au20 cluster along with most relevant bond distances (Å) and the relative energies on the section of PES (dash line – reaction 3, sold line – reaction 4). The imaginary frequencies of TSi are: ωi(TS6) = 104 cm-1, ωi(TS7)= 660 cm-1, ωi(TS8)=250 cm-1, ωi(TS9)= 951 cm-1, ωi(TS10)= 1046 cm-1. The reaction pathway c described in the section: I9→TS8→I10→TS9→P2+H2; pathway d: I9→TS10→ P2+H2. In order to make the comparison of reactions 3 and 4 more convenient, the energy of the CH3SH is added to the energies of TS6 and P1.

Figure 5. Optimized structure of the transition state (TS11) for the “relay” (“push-pull”) mechanism of dissociative adsorption of methylthiol on Au20 cluster and relative energies of the corresponding intermediates and transition state on the PES section. A web-enhanced object is available in AVI format.


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Figures

Figure 1



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Figure 2


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Figure 3



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Figure 4


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Figure 5



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Table of Contents Image


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Figure 1. Optimized structures of reagents, products, intermediates (In) and transition states (TSn) for the mechanism of dissociative adsorption of methylthiol on a single Au atom along with the most relevant bond distances (Å) and relative energies on the section of PES (dash line – reaction 1, sold line – reaction 2). The imaginary frequencies of TSi are: ωi(TS1) =544 cm-1, ωi(TS2)=496 cm-1, ωi(TS3)=666 cm-1, ωi(TS4)=714 cm1 , ωi(TS5)=432 cm-1. The reaction pathway a described in the section: I3→TS3→I4→TS4→(CH3S)2-Au+H2; pathway b: I3→TS5→(CH3S)2-Au+H2. In order to make the comparison of reactions 1 and 2 more convenient, the energy of the CH3SH is added to the energies of TS1 and CH3S-Au-H. 177x145mm (300 x 300 DPI)



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Figure 2. Molecular orbitals of Au20 cluster, CH3SH and CH3SHAu20 complex (I5). 177x81mm (300 x 300 DPI)


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Figure 3. The structures of I6, I7, I8, P2 complex with the most relevant bond lengths (Å) and molecular orbitals of P2 complex. 177x132mm (300 x 300 DPI)



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Figure 4. Optimized structures of reagents, products, intermediates (In) and transition states (TSn) for the mechanism of dissociative adsorption of methylthiol on Au20 cluster along with the most relevant bond distances (Å) and relative energies on the section of PES (dash line – reaction 3, sold line – reaction 4). The imaginary frequencies of TSi are: ωi(TS6) = 104 cm-1, ωi(TS7)= 660 cm-1, ωi(TS8)=250 cm-1, ωi(TS9)= 951 cm-1, ωi(TS10)= 1046 cm-1. The reaction pathway c described in the section: I9→TS8→I10→TS9→P2+H2; pathway d: I9→TS10→ P2+H2. In order to make the comparison of reactions 3 and 4 more convenient, the energy of the CH3SH is added to the energies of TS6 and P1. 177x204mm (300 x 300 DPI)


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TOC image 78x35mm (300 x 300 DPI)


Theoretical Prediction of SâH Bond Rupture in Methanethiol upon

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