Article pubs.acs.org/JPCA
Reactivity of Silver Clusters Anions with Ethanethiol Zhixun Luo,*,†,§ Gabriel U. Gamboa,‡ Meiye Jia,† Arthur C. Reber,‡ Shiv N. Khanna,*,‡ and A. W. Castleman, Jr.*,§ †
State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Department of Physics, Virginia Commonwealth University, Richmond, Virginia 23284, United States § Departments of Chemistry and Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *
ABSTRACT: We have investigated the gas-phase reactivity of silver clusters with ethanethiol in a fast-flow tube reactor. The primary cluster products observed in this reaction are AgnSH− and AgnSH2−, indicating C−S bond activation, together with interesting byproducts H3S− and (H3S)2−. Agn− clusters with an odd number of valence electrons (n = even) were observed to be more reactive than those with an even number of electronsa feature previously only observed in the reactivity of Agn− with triplet oxygen, indicating that radical active sites play a role in their reactivity. Furthermore, the reactivity dramatically increases with large flow rate of ethanethiol being introduced in the flow tube. Theoretical investigations on the reactivity of Ag13− and Ag8− with ethanethiol indicate that both Ag13− and Ag8− face significant barriers to reactivity with a single ethanethiol molecule. However, Ag8− reacts readily in a cooperative reaction with two ethanethiol molecules, consistent with the dramatic increase in reactivity with a large flow rate. Further hydrogen-transfer reactions may then release an ethylene molecule or an ethyl radical resulting in the observed AgnSH− species.
1. INTRODUCTION One of the central questions concerning reactions which cleave a RH bond is whether a radical active site is critical for accepting the H atom.1,2 Solution-phase studies have found that the radical character or spin state of an oxidant is not a primary determinant of hydrogen-atom-transfer (HAT) abstracting ability,1 whereas in gas-phase experiments of metal oxide clusters, radical O sites are critical for bond activation.2−5 In our previous work, we have investigated the cleavage of the OH bond, the CO bond, and the SC and SH bonds by complementary active sites on aluminum cluster anions.6−11 In these complementary active sites, both a Lewis acid site that preferentially binds the oxygen lone pair and a Lewis base site that binds the H were found to be necessary to lower the barrier for the cleavage of the OH or CO bonds. It was found that the charge density alone was sufficient for a Lewis base active site; Al17− was highly reactive with water/methanol and had no unpaired spin density, whereas Al20− that has an unpaired electron was resistant to reaction with water and methanol.8 Hence, a radical Lewis base site was not necessary for reactivity between O−H bonds and aluminum cluster anions. RH bond breaking reactions are ubiquitous in many chemical, environmental, and biological processes, from the action of antioxidants to industrial and metalloenzyme catalysis.2,12−14 To better understand the role of radical sites in the cleavage of R−H bonds, we have investigated the gasphase reactivity of ethanethiol (ETSH) and size-selected silver © XXXX American Chemical Society
cluster anions. In this study, the cleavage of the S−H bond of ETSH when reacting with silver clusters is expected to be one of the major reaction pathways, and the final products include AgnSH2− species which require a HAT from the ethyl group to the cluster resulting in an ethylene molecule release. As the n = even Agn− clusters have an odd number of electrons and a radical active site, but those n = odd have an even number of electrons and no such radical active sites, this study of “Agn− + ETSH” allows a comparison in reactivity between clusters with and without radical active sites. Recent investigations on silver cluster anions have determined that Ag13− exhibits unique resistance to reactivity with oxygen.15 The large spin excitation energy of Ag13− was unexpected because this cluster has 14 valence electrons, and its stability is due to a crystal-field like splitting of the 1D orbitals caused by the oblate deformation of the cluster’s atomic structure.15,16 The role of the spin excitation energy on the reactivity of clusters with O2 is well documented,17−19 but it is less clear how important spin effects are in other reactions. On the other hand, joint experimental and theoretical studies revealed that Cu8− and Ag8− are very reactive with chlorine gas through a novel harpoon mechanism.20 Ag13− has a closed Special Issue: A. W. Castleman, Jr. Festschrift Received: February 1, 2014 Revised: May 25, 2014
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electronic shell, so it does not contain unpaired charge density, whereas Ag8− has an unpaired electron so will have a radical site. Further interest in the silver-ethanethiol system is due to the synthesis of numerous ligand-protected silver and other noble metal thiol clusters/nanoparticles.21−25 As is well-known, ligand-protected nanoparticles/clusters have potential applications in areas ranging from catalysis to medicine. The size, shape, and composition can dramatically affect the stability and their physical and chemical properties. In Brust−Schiffrin twophase synthesis the average size of the particles/clusters and their stability can be inferred from the thiol/metal ratio;26,27 in the gas phase, however, there have been limited studies of the chemical and physical behavior of metal−thiol clusters. Here we have performed an investigation of the gas-phase reaction of silver clusters with ethanethiol in the fast-flow tube reactor. Experimental and theoretical results illustrate interesting reactivity of Agn− with ETSH, enabling an insight into the HAT-related reaction process. Considering the ligand-protected metal clusters provide a pathway for constructing clusterassembled materials,28,29 this work may also offer insight into the mechanism by which ligand-protected clusters could be synthesized.30−32 Figure 1. Typical distributions of Ag cluster anions tuned by different parameters of the MagS source.
2. EXPERIMENT AND CALCULATION METHODS The reactions leading to the current findings were carried out in an apparatus that has been previously described by a magnetron sputtering (MagS) cluster source,33 where a dc power supply (Power Supply Make/Model, 1.6 kW) was used to provide the high voltages needed for the MagS source. High purity helium (Praxair, Inc., purity >99.995%) was introduced from the inlet at the rear of the magnetron chamber, to carry the clusters through an adjustable iris, then into the flow tube where they encountered and reacted with ethanethiol (SigmaAldrich, 99% purity) in a room-temperature laminar flow vessel (maintained at 0.7 Torr by a high-volume Roots pump). The ETSH reactant gas was introduced to the cluster beam approximately ∼30 cm downstream from the source and allowed us to react with the Ag clusters over a 60 cm distance and a time of ∼8 ms, respectively, and then the products were extracted into a differentially pumped ion guide vacuum system and analyzed by a quadrupole mass spectrometer (Extrel CMS). Ultra high purity argon (Praxair, Inc., purity >99.99%) was used as sputtering matter, and the silver disk (99.99% pure, Φ50 mm, and 6 mm thickness) was obtained from Kurt J. Lesker Company. To compare the reactivities of Ag8− and Ag13−, extensive investigations corresponding to differing relative-intensity distributions of nascent Ag8− and Ag13− clusters were undertaken by adjusting the various parameters of the MagS source (e.g., different target-cathode distance, He and Ar partial pressures, cathode voltages),33 as well as parameters of the mass analyzer (quadrupole bias and lens setting). Some of the typical mass spectra under different conditions are displayed in Figure 1. When running reactions for chosen conditions, we ascertained that all these parameters were kept at the same value. We carried out theoretical investigations on the ground state geometries and reactivity of anionic Agn− clusters with ETSH by using deMon2k34 set of computer codes. A first-principles molecular orbital approach was used wherein the cluster wave function is expressed as a linear combination of atomic orbitals centered at the atomic sites, and the exchange−correlation effects were included within the PBE generalized gradient
density functional theory formalism.35 The silver atom is described using a 19 electron quasi-relativistic effective core potential (QECP) with a corresponding valence basis set as proposed by Andrae et al.,36,37 and the S, C, and H atoms used the DZVP basis.38 Reaction barriers were calculated using the hierarchical transition state search.39
3. RESULTS AND DISCUSSION Figure 2a presents a typical mass spectrum of silver cluster anions produced via a MagS source. Nearly all the Agn− cluster anions of n = 1−16 are identified although there are a few contaminant peaks, and the size distribution is centered at Ag10−11−, allowing a comparison in reactivity between Ag8− and Ag13−. Parts b and c of Figure 2 display the spectra after exposure to different quantities of ethanethiol. In Figure 2b, a small flow (∼2.0 sccm) ETSH results in no significant change in the Agn− clusters size distributions, except for Ag1− and Ag3−. Several reaction products are observed that appear in four series: (i) products with direct binding of ETSH molecules such as Ag7(ETSH)3−, which are observed at very small flow rate of ETSH (Supporting Information, Figure S1) but mostly disappear in flow rates as seen in Figure 2b; (ii) species of Ag2n+1(H2S)1,2− seen as Ag(H2S)1,2−, Ag7(H2S)1,2−, Ag9(H2S)−, and Ag11H2S−; (iii) products of Ag2n(SH)− such as Ag8SH−, Ag10SH−, Ag12SH−, and Ag14SH−; (iv) molecular anion products of H3S− and (H3S)2−. These classes of products indicate a variety of size selective reactivity in Agn− clusters with ETSH. The mass spectrum of Agn− clusters when exposed to a large flow rate of ETSH at 9.7 sccm is shown in Figure 2c. In this case, all the Agn− clusters and related products become quite weak in intensity and an odd−even alternation is still observed. The even-electron Ag clusters (odd number of silver atoms, i.e., Ag2n+1−) show greater intensity than the odd-electron clusters. Further, even-electron species (odd-number silver atoms) have an even number of H atoms as Ag2n+1(H2S)1,2−, but the clusters B
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Figure 2. Mass spectrum of silver cluster anions produced via a MagS source (a) and the spectra after exposure to different quantities of ethanethiol (b/c). Insets display the enhanced area from 600 to 1800 amu.
It is interesting to note that the reaction products assigned to H3S− and (H3S)2− dominate the observed products. H3S+ is a well-known ion; however, H3S is known to be metastable with a lifetime of less than 3 μs.40 Furthermore, H2S has a negative electron affinity, so H2S− cannot be the observed species. Considering that H3O− is a weakly bound species that forms two structures, a H2−OH− and H2O(H−) species,41 it is inferred that H3S− also follow this principle. The ground state structure of H3S− is found to be H2−SH− in which the binding energy of the H2 molecule to the SH− anion is 0.18 eV. This seems to imply that there is a significant density of H2 in the reaction chamber, as the binding energy of H2 to SH− is weak.40 To gain a better understanding of the reactivity of Ag cluster anions with ETSH, we have performed first-principles calculations to identify the reaction coordinates. We choose two representative species, Ag8− and Ag13−. Figure 3 presents the energy profile for Ag13− in reacting with ETSH, where it binds by 0.23 eV. Reaction barrier for cleaving the C−S bond on Ag13− is found to be 1.45 eV, whereas the reaction barrier for the cleavage of the S−H bond is found to be 1.01 eV. The large reaction barriers are consistent with the experimental observation of reduced reactivity of “Ag13− + ETSH” at the room-temperature gas-phase condition. The dissociated chemisorption intermediates and final products exhibit reasonably low energy relative to the initial reactants; however, the high
with an even number of silver atoms have a SH species bound to the cluster. These reactivity patterns can be simply summarized into two reaction channels, Ag n− + mC2H5SH → Ag n(H 2S)m− + mC2H4
(1)
Ag n− + mC2H5SH → Ag n(HS)m− + m ·C2H5
(2)
A striking result in Figure 2c is that the species Ag8(SH)−, Ag10(SH)−, and Ag12(SH)− seem to have been formed from the loss of the ethyl radical as in eq 2; however, these clusters all have an odd number of electrons. This implies that the HAT between the leaving ethyl group and the silver sulfur cluster does not significantly occur on the cluster with unpaired electrons. On the other hand, Ag9(H2S)2− and Ag11(H2S)2− are observed, which seem to imply that the Ag clusters with an even number of electrons do undergo HAT as the ethyl radical leaves the cluster. It is possible that this unusual pattern is due to secondary fragmentation of the cluster; however, this should produce a variety of products, rather than one major product for each cluster size. One of the large peaks in Figure 2c is Ag(H2S)2−, which suggests that the clusters could be fragmented with the loss of ethylene as seen in eq 3. Ag n− + 2C2H5SH → Ag(H 2S)2− + 2C2H4 + Ag n − 1
(3)
This reaction produces small silver clusters, so the Agn(SH)− and Agn(H2S)2− peaks are due to reactions in eqs 1 and 2. C
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Figure 5. Energy profile for Ag8− reacting with two C2H5SH molecules (fast): (a) Ag8− + 2 C2H5SH, (b) chemisorption, (c) transition states, (d) a dissociated chemisorbed intermediate, (e1) another transition state, and (f1) the cleavage of a C−S bond.
Ag13−
Figure 3. Energy profile for reacting with C2H5SH (slow): (a) Ag13− + ETSH, (b) chemisorption, (c1/c2) transition states, (d1/d2) the dissociated chemisorbed products, (e1) another transition state, and (f1) the proposed product of Ag13(HSH)− with C2H4 released.
barrier prevents the reaction. The inactive reactivity of Ag13− with ETSH is similar to the previously finding of inertness of Ag13− toward oxygen.15 In comparison, Figure 4 shows the energy profile for Ag8− in reacting with an ETSH molecule, where two reaction channels
binding energy of 0.48 eV (Figure 5b), which then allows one of the adsorbed −SH groups to come into contact with another adjacent Ag atom (transition state, Figure 5c) leading to a dissociated chemisorptive intermediate (Figure 5d). It is worthwhile noting that this transition state (Figure 5c) is fairly low in energy (−0.05 eV). Subsequently, the cleavage of a C−S bond can occur (Figure 5f) after another feasible transition state as shown in Figure 5e. This results in the weakening of the C−C bond so the formation of ethylene, or the loss of the ethyl radical becomes more feasible. Furthermore, the −C2H5 and −SC2H5 species on the silver cluster may dimerize and leave the cluster as of C2H5SC2H5 (or H2 plus two C2H4 molecules).
4. CONCLUSION We have investigated the gas-phase reactivity of silver clusters with ethanethiol in a fast-flow tube reactor and found that the primary cluster products are Agn(SH)− and Agn(H2S)− species. It is evidenced that the Agn− clusters with an unpaired electron react more readily than those with even number of electrons, suggesting that unpaired electrons increase the cluster reactivity. Our theoretical investigations focused on Ag13− and Ag8−, where we examined the reactivity of the two species with single and two ETSH molecules. It was found that cooperative reactivity was necessary because the transition state barrier is too large for the reaction of a single ethanethiol molecule; however, the reaction of Ag8− with two ethanethiol molecules is energetically favored. This result is consistent with the experimental observation that a drastic increase in reactivity of Agn− with ETSH occurs at the large flow rate of ethanethiol, and hint why the thiol/metal ratio is very sensitive to the sizes in preparing thiol-protected metal clusters.
Figure 4. Energy profile for Ag8− reacting with C2H5SH (slow): (a) Ag8− + C2H5SH, (b) chemisorption, (c1/c2) transition states, (d1/d2) the dissociated chemisorbed products, (e1) another transition state, and (f1) the proposed product of Ag8(HSH)− with C2H4 released.
are displayed. One reaction pathway refers to the cleavage of the S−H bond (c2), whereas the other pathway follows the cleavage of the C−S bond (c1). Subsequently, the dissociated chemisorption product d1 produces an intermediate with a −SH and −C2H5, respectively, bonded to a different site of the cluster and then undergoes another transition state with the second H atom transferred onto the cluster, leading to a product of Ag8(HSH)− with C2H4 released. In contrast, d2 forms a dissociatively chemisorbed product that is 1.14 eV more stable than the original reactants. Although Ag8− has a lower barrier for S−H and C−S bond activation than Ag13−, neither are low enough to explain the observed products as showed in Figure 2. In view of the fact that the reactivity mostly occurs when a large flow rate of ETSH is introduced, we then investigated the cooperative reactivity of Agn− with two ETSH molecules.42,43 Figure 5 shows the reaction coordinates for Ag8− encountering two C2H5SH molecules. As displayed there, two ETSH molecules chemisorb on the active sites of a Ag8− cluster and display a
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ASSOCIATED CONTENT
S Supporting Information *
More experimental and calculation details, including mass spectra and energy profiles. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*Z. Luo: e-mail, [email protected]. *S. N. Khanna: e-mail, [email protected]. *A. W. Castleman, Jr.: e-mail, [email protected]. D
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This material is based upon work supported by the Air Force Office of Science Research under AFOSR Award No. FA955010-1-0071. Z.L. acknowledges the support by the 100-Talent Program of Chinese Academy of Sciences (ICCASY3297B1261). G.U.G., A.C.R., and S.N.K. are grateful to Air Force Office of Scientific Research under Basic Research Initiative grant AFOSR FA9550-12-1-0481 for financial support.
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Reactivity of Silver Clusters Anions with Ethanethiol Zhixun Luo,*,†,§ Gabriel U. Gamboa,‡ Meiye Jia,† Arthur C. Reber,‡ Shiv N. Khanna,*,‡ and A. W. Castleman, Jr.*,§ †
State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Department of Physics, Virginia Commonwealth University, Richmond, Virginia 23284, United States § Departments of Chemistry and Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *
ABSTRACT: We have investigated the gas-phase reactivity of silver clusters with ethanethiol in a fast-flow tube reactor. The primary cluster products observed in this reaction are AgnSH− and AgnSH2−, indicating C−S bond activation, together with interesting byproducts H3S− and (H3S)2−. Agn− clusters with an odd number of valence electrons (n = even) were observed to be more reactive than those with an even number of electronsa feature previously only observed in the reactivity of Agn− with triplet oxygen, indicating that radical active sites play a role in their reactivity. Furthermore, the reactivity dramatically increases with large flow rate of ethanethiol being introduced in the flow tube. Theoretical investigations on the reactivity of Ag13− and Ag8− with ethanethiol indicate that both Ag13− and Ag8− face significant barriers to reactivity with a single ethanethiol molecule. However, Ag8− reacts readily in a cooperative reaction with two ethanethiol molecules, consistent with the dramatic increase in reactivity with a large flow rate. Further hydrogen-transfer reactions may then release an ethylene molecule or an ethyl radical resulting in the observed AgnSH− species.
1. INTRODUCTION One of the central questions concerning reactions which cleave a RH bond is whether a radical active site is critical for accepting the H atom.1,2 Solution-phase studies have found that the radical character or spin state of an oxidant is not a primary determinant of hydrogen-atom-transfer (HAT) abstracting ability,1 whereas in gas-phase experiments of metal oxide clusters, radical O sites are critical for bond activation.2−5 In our previous work, we have investigated the cleavage of the OH bond, the CO bond, and the SC and SH bonds by complementary active sites on aluminum cluster anions.6−11 In these complementary active sites, both a Lewis acid site that preferentially binds the oxygen lone pair and a Lewis base site that binds the H were found to be necessary to lower the barrier for the cleavage of the OH or CO bonds. It was found that the charge density alone was sufficient for a Lewis base active site; Al17− was highly reactive with water/methanol and had no unpaired spin density, whereas Al20− that has an unpaired electron was resistant to reaction with water and methanol.8 Hence, a radical Lewis base site was not necessary for reactivity between O−H bonds and aluminum cluster anions. RH bond breaking reactions are ubiquitous in many chemical, environmental, and biological processes, from the action of antioxidants to industrial and metalloenzyme catalysis.2,12−14 To better understand the role of radical sites in the cleavage of R−H bonds, we have investigated the gasphase reactivity of ethanethiol (ETSH) and size-selected silver © XXXX American Chemical Society
cluster anions. In this study, the cleavage of the S−H bond of ETSH when reacting with silver clusters is expected to be one of the major reaction pathways, and the final products include AgnSH2− species which require a HAT from the ethyl group to the cluster resulting in an ethylene molecule release. As the n = even Agn− clusters have an odd number of electrons and a radical active site, but those n = odd have an even number of electrons and no such radical active sites, this study of “Agn− + ETSH” allows a comparison in reactivity between clusters with and without radical active sites. Recent investigations on silver cluster anions have determined that Ag13− exhibits unique resistance to reactivity with oxygen.15 The large spin excitation energy of Ag13− was unexpected because this cluster has 14 valence electrons, and its stability is due to a crystal-field like splitting of the 1D orbitals caused by the oblate deformation of the cluster’s atomic structure.15,16 The role of the spin excitation energy on the reactivity of clusters with O2 is well documented,17−19 but it is less clear how important spin effects are in other reactions. On the other hand, joint experimental and theoretical studies revealed that Cu8− and Ag8− are very reactive with chlorine gas through a novel harpoon mechanism.20 Ag13− has a closed Special Issue: A. W. Castleman, Jr. Festschrift Received: February 1, 2014 Revised: May 25, 2014
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electronic shell, so it does not contain unpaired charge density, whereas Ag8− has an unpaired electron so will have a radical site. Further interest in the silver-ethanethiol system is due to the synthesis of numerous ligand-protected silver and other noble metal thiol clusters/nanoparticles.21−25 As is well-known, ligand-protected nanoparticles/clusters have potential applications in areas ranging from catalysis to medicine. The size, shape, and composition can dramatically affect the stability and their physical and chemical properties. In Brust−Schiffrin twophase synthesis the average size of the particles/clusters and their stability can be inferred from the thiol/metal ratio;26,27 in the gas phase, however, there have been limited studies of the chemical and physical behavior of metal−thiol clusters. Here we have performed an investigation of the gas-phase reaction of silver clusters with ethanethiol in the fast-flow tube reactor. Experimental and theoretical results illustrate interesting reactivity of Agn− with ETSH, enabling an insight into the HAT-related reaction process. Considering the ligand-protected metal clusters provide a pathway for constructing clusterassembled materials,28,29 this work may also offer insight into the mechanism by which ligand-protected clusters could be synthesized.30−32 Figure 1. Typical distributions of Ag cluster anions tuned by different parameters of the MagS source.
2. EXPERIMENT AND CALCULATION METHODS The reactions leading to the current findings were carried out in an apparatus that has been previously described by a magnetron sputtering (MagS) cluster source,33 where a dc power supply (Power Supply Make/Model, 1.6 kW) was used to provide the high voltages needed for the MagS source. High purity helium (Praxair, Inc., purity >99.995%) was introduced from the inlet at the rear of the magnetron chamber, to carry the clusters through an adjustable iris, then into the flow tube where they encountered and reacted with ethanethiol (SigmaAldrich, 99% purity) in a room-temperature laminar flow vessel (maintained at 0.7 Torr by a high-volume Roots pump). The ETSH reactant gas was introduced to the cluster beam approximately ∼30 cm downstream from the source and allowed us to react with the Ag clusters over a 60 cm distance and a time of ∼8 ms, respectively, and then the products were extracted into a differentially pumped ion guide vacuum system and analyzed by a quadrupole mass spectrometer (Extrel CMS). Ultra high purity argon (Praxair, Inc., purity >99.99%) was used as sputtering matter, and the silver disk (99.99% pure, Φ50 mm, and 6 mm thickness) was obtained from Kurt J. Lesker Company. To compare the reactivities of Ag8− and Ag13−, extensive investigations corresponding to differing relative-intensity distributions of nascent Ag8− and Ag13− clusters were undertaken by adjusting the various parameters of the MagS source (e.g., different target-cathode distance, He and Ar partial pressures, cathode voltages),33 as well as parameters of the mass analyzer (quadrupole bias and lens setting). Some of the typical mass spectra under different conditions are displayed in Figure 1. When running reactions for chosen conditions, we ascertained that all these parameters were kept at the same value. We carried out theoretical investigations on the ground state geometries and reactivity of anionic Agn− clusters with ETSH by using deMon2k34 set of computer codes. A first-principles molecular orbital approach was used wherein the cluster wave function is expressed as a linear combination of atomic orbitals centered at the atomic sites, and the exchange−correlation effects were included within the PBE generalized gradient
density functional theory formalism.35 The silver atom is described using a 19 electron quasi-relativistic effective core potential (QECP) with a corresponding valence basis set as proposed by Andrae et al.,36,37 and the S, C, and H atoms used the DZVP basis.38 Reaction barriers were calculated using the hierarchical transition state search.39
3. RESULTS AND DISCUSSION Figure 2a presents a typical mass spectrum of silver cluster anions produced via a MagS source. Nearly all the Agn− cluster anions of n = 1−16 are identified although there are a few contaminant peaks, and the size distribution is centered at Ag10−11−, allowing a comparison in reactivity between Ag8− and Ag13−. Parts b and c of Figure 2 display the spectra after exposure to different quantities of ethanethiol. In Figure 2b, a small flow (∼2.0 sccm) ETSH results in no significant change in the Agn− clusters size distributions, except for Ag1− and Ag3−. Several reaction products are observed that appear in four series: (i) products with direct binding of ETSH molecules such as Ag7(ETSH)3−, which are observed at very small flow rate of ETSH (Supporting Information, Figure S1) but mostly disappear in flow rates as seen in Figure 2b; (ii) species of Ag2n+1(H2S)1,2− seen as Ag(H2S)1,2−, Ag7(H2S)1,2−, Ag9(H2S)−, and Ag11H2S−; (iii) products of Ag2n(SH)− such as Ag8SH−, Ag10SH−, Ag12SH−, and Ag14SH−; (iv) molecular anion products of H3S− and (H3S)2−. These classes of products indicate a variety of size selective reactivity in Agn− clusters with ETSH. The mass spectrum of Agn− clusters when exposed to a large flow rate of ETSH at 9.7 sccm is shown in Figure 2c. In this case, all the Agn− clusters and related products become quite weak in intensity and an odd−even alternation is still observed. The even-electron Ag clusters (odd number of silver atoms, i.e., Ag2n+1−) show greater intensity than the odd-electron clusters. Further, even-electron species (odd-number silver atoms) have an even number of H atoms as Ag2n+1(H2S)1,2−, but the clusters B
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Figure 2. Mass spectrum of silver cluster anions produced via a MagS source (a) and the spectra after exposure to different quantities of ethanethiol (b/c). Insets display the enhanced area from 600 to 1800 amu.
It is interesting to note that the reaction products assigned to H3S− and (H3S)2− dominate the observed products. H3S+ is a well-known ion; however, H3S is known to be metastable with a lifetime of less than 3 μs.40 Furthermore, H2S has a negative electron affinity, so H2S− cannot be the observed species. Considering that H3O− is a weakly bound species that forms two structures, a H2−OH− and H2O(H−) species,41 it is inferred that H3S− also follow this principle. The ground state structure of H3S− is found to be H2−SH− in which the binding energy of the H2 molecule to the SH− anion is 0.18 eV. This seems to imply that there is a significant density of H2 in the reaction chamber, as the binding energy of H2 to SH− is weak.40 To gain a better understanding of the reactivity of Ag cluster anions with ETSH, we have performed first-principles calculations to identify the reaction coordinates. We choose two representative species, Ag8− and Ag13−. Figure 3 presents the energy profile for Ag13− in reacting with ETSH, where it binds by 0.23 eV. Reaction barrier for cleaving the C−S bond on Ag13− is found to be 1.45 eV, whereas the reaction barrier for the cleavage of the S−H bond is found to be 1.01 eV. The large reaction barriers are consistent with the experimental observation of reduced reactivity of “Ag13− + ETSH” at the room-temperature gas-phase condition. The dissociated chemisorption intermediates and final products exhibit reasonably low energy relative to the initial reactants; however, the high
with an even number of silver atoms have a SH species bound to the cluster. These reactivity patterns can be simply summarized into two reaction channels, Ag n− + mC2H5SH → Ag n(H 2S)m− + mC2H4
(1)
Ag n− + mC2H5SH → Ag n(HS)m− + m ·C2H5
(2)
A striking result in Figure 2c is that the species Ag8(SH)−, Ag10(SH)−, and Ag12(SH)− seem to have been formed from the loss of the ethyl radical as in eq 2; however, these clusters all have an odd number of electrons. This implies that the HAT between the leaving ethyl group and the silver sulfur cluster does not significantly occur on the cluster with unpaired electrons. On the other hand, Ag9(H2S)2− and Ag11(H2S)2− are observed, which seem to imply that the Ag clusters with an even number of electrons do undergo HAT as the ethyl radical leaves the cluster. It is possible that this unusual pattern is due to secondary fragmentation of the cluster; however, this should produce a variety of products, rather than one major product for each cluster size. One of the large peaks in Figure 2c is Ag(H2S)2−, which suggests that the clusters could be fragmented with the loss of ethylene as seen in eq 3. Ag n− + 2C2H5SH → Ag(H 2S)2− + 2C2H4 + Ag n − 1
(3)
This reaction produces small silver clusters, so the Agn(SH)− and Agn(H2S)2− peaks are due to reactions in eqs 1 and 2. C
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Figure 5. Energy profile for Ag8− reacting with two C2H5SH molecules (fast): (a) Ag8− + 2 C2H5SH, (b) chemisorption, (c) transition states, (d) a dissociated chemisorbed intermediate, (e1) another transition state, and (f1) the cleavage of a C−S bond.
Ag13−
Figure 3. Energy profile for reacting with C2H5SH (slow): (a) Ag13− + ETSH, (b) chemisorption, (c1/c2) transition states, (d1/d2) the dissociated chemisorbed products, (e1) another transition state, and (f1) the proposed product of Ag13(HSH)− with C2H4 released.
barrier prevents the reaction. The inactive reactivity of Ag13− with ETSH is similar to the previously finding of inertness of Ag13− toward oxygen.15 In comparison, Figure 4 shows the energy profile for Ag8− in reacting with an ETSH molecule, where two reaction channels
binding energy of 0.48 eV (Figure 5b), which then allows one of the adsorbed −SH groups to come into contact with another adjacent Ag atom (transition state, Figure 5c) leading to a dissociated chemisorptive intermediate (Figure 5d). It is worthwhile noting that this transition state (Figure 5c) is fairly low in energy (−0.05 eV). Subsequently, the cleavage of a C−S bond can occur (Figure 5f) after another feasible transition state as shown in Figure 5e. This results in the weakening of the C−C bond so the formation of ethylene, or the loss of the ethyl radical becomes more feasible. Furthermore, the −C2H5 and −SC2H5 species on the silver cluster may dimerize and leave the cluster as of C2H5SC2H5 (or H2 plus two C2H4 molecules).
4. CONCLUSION We have investigated the gas-phase reactivity of silver clusters with ethanethiol in a fast-flow tube reactor and found that the primary cluster products are Agn(SH)− and Agn(H2S)− species. It is evidenced that the Agn− clusters with an unpaired electron react more readily than those with even number of electrons, suggesting that unpaired electrons increase the cluster reactivity. Our theoretical investigations focused on Ag13− and Ag8−, where we examined the reactivity of the two species with single and two ETSH molecules. It was found that cooperative reactivity was necessary because the transition state barrier is too large for the reaction of a single ethanethiol molecule; however, the reaction of Ag8− with two ethanethiol molecules is energetically favored. This result is consistent with the experimental observation that a drastic increase in reactivity of Agn− with ETSH occurs at the large flow rate of ethanethiol, and hint why the thiol/metal ratio is very sensitive to the sizes in preparing thiol-protected metal clusters.
Figure 4. Energy profile for Ag8− reacting with C2H5SH (slow): (a) Ag8− + C2H5SH, (b) chemisorption, (c1/c2) transition states, (d1/d2) the dissociated chemisorbed products, (e1) another transition state, and (f1) the proposed product of Ag8(HSH)− with C2H4 released.
are displayed. One reaction pathway refers to the cleavage of the S−H bond (c2), whereas the other pathway follows the cleavage of the C−S bond (c1). Subsequently, the dissociated chemisorption product d1 produces an intermediate with a −SH and −C2H5, respectively, bonded to a different site of the cluster and then undergoes another transition state with the second H atom transferred onto the cluster, leading to a product of Ag8(HSH)− with C2H4 released. In contrast, d2 forms a dissociatively chemisorbed product that is 1.14 eV more stable than the original reactants. Although Ag8− has a lower barrier for S−H and C−S bond activation than Ag13−, neither are low enough to explain the observed products as showed in Figure 2. In view of the fact that the reactivity mostly occurs when a large flow rate of ETSH is introduced, we then investigated the cooperative reactivity of Agn− with two ETSH molecules.42,43 Figure 5 shows the reaction coordinates for Ag8− encountering two C2H5SH molecules. As displayed there, two ETSH molecules chemisorb on the active sites of a Ag8− cluster and display a
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ASSOCIATED CONTENT
S Supporting Information *
More experimental and calculation details, including mass spectra and energy profiles. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*Z. Luo: e-mail, [email protected]. *S. N. Khanna: e-mail, [email protected]. *A. W. Castleman, Jr.: e-mail, [email protected]. D
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This material is based upon work supported by the Air Force Office of Science Research under AFOSR Award No. FA955010-1-0071. Z.L. acknowledges the support by the 100-Talent Program of Chinese Academy of Sciences (ICCASY3297B1261). G.U.G., A.C.R., and S.N.K. are grateful to Air Force Office of Scientific Research under Basic Research Initiative grant AFOSR FA9550-12-1-0481 for financial support.
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