Positively Charged Active Sites for the Adsorption of Five-Membered

Article pubs.acs.org/JPCC

Positively Charged Active Sites for the Adsorption of Five-Membered Heterocycles on Silver Colloids Maurizio Muniz-Miranda*,†,‡ and Marco Pagliai† †

Dipartimento di Chimica “Ugo Schiff”, Università degli Studi di Firenze, via della Lastruccia 3, 50019 Sesto Fiorentino (FI), Italy European Laboratory for Nonlinear Spectroscopy (LENS), via Nello Carrara 1, 50019 Sesto Fiorentino (FI), Italy

‡

S Supporting Information *

ABSTRACT: The adsorption of isothiazole on silver colloids was studied by surface-enhanced Raman scattering (SERS) spectroscopy in comparison with other five-membered heterocycles. The experimental results coupled with those obtained by density functional theory calculations for molecule/metal complexes evidenced the presence of positively charged active sites on the surface of the silver nanoparticles, along with a close relation between ligand basicity and strong SERS effect.

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INTRODUCTION The adsorption of molecules on metal surfaces plays a fundamental role in phenomena of heterogeneous catalysis as well as for electroactivated reactions in electrochemical cells. These processes can be monitored and studied by means of surface-enhanced Raman scattering (SERS) spectroscopy.1,2 By using this technique, the Raman signal of molecules adsorbed on metal substrates can be intensified by several orders of magnitude, usually 106−107 times, when the surface of high reflectivity metals such as silver, gold, or copper exhibits a nanostructured roughness. This enhancement mainly derives from the intensification of the local electromagnetic field undergone by the molecules adhering the metal surface, when the Raman exciting radiation is in resonance with the excitation waves of the conduction electrons localized at the metal surface. These latter, called surface plasmon resonance (SPR) bands, can be observed in UV−visible absorption spectra, with peak positions, intensities, and widths closely depending on the type of metal, along with the size and shape of the metal nanostructures. Indeed, a strong SERS signal is generally observed when the molecules are chemically adsorbed on the active sites of the surface, which could be represented as defects of the metal structure at the subnanometer scale. Chemisorption is usually responsible for a Raman enhancement factor up to 102, which is small in comparison with the electromagnetic one but quite important for the SERS observation, because it strongly affects the positions and the relative intensities of the bands, which appear different from those of the normal Raman spectrum. Different metal platforms were used as SERS substrates: colloidal dispersions of nanoparticles, roughened surfaces obtained by electrochemical treatment or chemical etching, thin layers formed in high-vacuum chamber, and island crystalline films. However, the metal colloids present peculiar © 2013 American Chemical Society

advantages owing to the large specific area and the easy monitoring of the adsorption process by observing the modifications of the SPR bands in the visible spectral region. However, the adsorption of ligand molecules in aqueous dispersions of silver nanoparticles is a quite complex process, involving the solvation in the water environment, the ligand affinity to the metal, and the reactivity of the active sites located at the surface of the metal nanoparticles.3,4 The most common ligands employed in the SERS experiments are organic molecules containing heteroatoms, which can provide electronic lone pairs for interaction with the metal surface. Actually, until the discovery of the SERS effect of pyridine adsorbed on a silver electrode,5 the nitrogen-containing heterocycles showed the highest enhancements of the Raman signal. The SERS responses of different five-membered rings with two or three heteroatoms were previously investigated by some of us,6−9 in order to understand the conditions of adsorption on colloidal silver. However, the presence of heteroatoms is not sufficient for reliable adsorption on metal and consequently for a strong SERS effect, because the number and type of heteroatoms, as well as their position in the ring, play a very important role. For example, a strong Raman enhancement was observed for adsorbed thiazole,7 whereas similar molecules such as oxazole and isoxazole exhibited low enhancement factors, also after SERS activation by addition of chloride ions. The strong SERS enhancement of thiazole was attributed to chemisorption and related to the occurrence of a plasmon band in the 500−700 nm spectral region, due to silver particles aggregated by the ligand adsorption, in addition to the usual SPR band around 400 nm, due to monodisperse nanoparticles. Received: September 27, 2012 Revised: December 19, 2012 Published: January 15, 2013 2328

dx.doi.org/10.1021/jp309600p | J. Phys. Chem. C 2013, 117, 2328−2333

The Journal of Physical Chemistry C

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RAMAN AND SERS SPECTRA The Raman spectrum of liquid isothiazole is compared in Figure 1 with the DFT-simulated spectrum, while the observed

In the present paper, we want to study the adsorption of isothiazole on silver colloids by SERS spectroscopy, in relation to its aromaticity and basicity, in order to extend the conclusions to other five-membered heterocycles. Isothiazole is a heterocyclic compound structurally quite similar to thiazole, but its Raman and SERS spectra were not previously analyzed. Moreover, we want also to interpret the mechanism of the chemical SERS enhancement on the basis of positively charged active sites formed on the Ag colloidal surface. For this aim, we have performed density functional theory (DFT) calculations of ligand/metal complexes, which were found able not only to accurately reproduce the SERS profiles10−13 but also to provide useful indications about the molecular sites involved in the adsorption.8,14−20

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Article

EXPERIMENTAL SECTION

Sample Preparation. Isothiazole (ABCR, purity 97%) and isoxazole (Aldrich, purity 99%) were repeatedly purified by distillation. Silver colloids were obtained according to the Creighton’s procedure21 by reduction of AgNO3 (Aldrich, purity 99.998%) with excess NaBH4 (Aldrich, purity 99%). Isothiazole or isoxazole was added to the Ag colloid in 10−3 M concentration, after several hours from the sol preparation, in order to avoid the presence of reduction products.22 The adsorption of isothiazole on silver sol was performed by long stirring after ligand addition. A SERS signal was observed for both ligands only by adding NaCl (10−3 M concentration). Raman Spectra. Raman spectra were recorded using the 514.5 nm line of a Coherent argon ion laser and a Jobin−Yvon HG2S monochromator equipped with a cooled RCA-C31034A photomultiplier. The SERS spectra of isothiazole and isoxazole were only obtained by using silver colloids activated by the presence of chloride anions.

Figure 1. (upper) Raman spectrum of liquid isothiazole; (lower) DFT-simulated spectrum.

and simulated spectra of isoxazole are reported in Figure 2. The comparison between Figures 1 and 2 allows understanding of the effect of the sulfur or the oxygen atom on the vibrational behaviors of the two heterocycles.

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COMPUTATIONAL DETAILS DFT calculations were performed with the Gaussian suite of programs,23 using the B3LYP exchange and correlation functional along with the 6-311++G(d,p) basis set for all atoms but silver, which was described with the LanL2DZ basis set. The molecular structures and the vibrational frequencies were computed adopting a very tight criterion and an improved grid in the numerical evaluation of the integrals, INTEGRAL(GRID = 199974). The Raman and SERS intensities of the vibrational modes, Ii, were obtained from the Raman activities, Ai, using the following relationship (eq 1), as reported in the literature:24,25 Ii =

Figure 2. (upper) Raman spectrum of liquid isoxazole; (lower) DFTsimulated spectrum.

The agreement between experimental and calculated results of both molecules is quite satisfactory, concerning band positions and relative intensities. This result represents a necessary warranty for obtaining reliable results when the same computational approach is adopted for the corresponding ligand/metal complexes. The observed Raman frequencies are compared in Tables 1 and 2 with the calculated ones, and the vibrational assignments are reported in terms of potential energy distributions.26 According to the conventional numbering, S (or O) is the first atom in the ring, and N is the second one. Almost all observed Raman bands of the two compounds are attributable to in-plane normal modes (A′ species). The presence of sulfur in isothiazole, instead of oxygen in isoxazole, namely, a more polarizable and heavy atom, gives rise to a downshift of the observed Raman bands, along with a marked intensity increase of those below 1000 cm−1, which correspond to S−C and S−N stretching modes. The bands occurring at

f (ν0 − νi)4 Ai νi(1 − e−hcνi / kT )

(1)

where ν0 is the exciting frequency (in cm−1), νi is the vibrational frequency of the i-th normal mode (in cm−1), h, c, and k are fundamental constants, and f is a suitably chosen common normalization factor for all peak intensities. The calculated spectra were reported by assigning to each normal mode a Lorentzian shape with a 5 cm−1 full width at halfmaximum. The vibrational frequencies were scaled by a 0.98 factor. Potential energy distributions were obtained by the help of the GAR2PED program.26 2329



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Table 1. Observed and Calculated Raman Frequencies (in cm−1) of Isothiazole species

Raman

calcd

SERS

calcd

A″ A′

468 634

464 630

464 640

460 638

A′

752

716

762

737

A′

812

798

838

829

A′ A′ A′ A′

1041 1055 1234 1293

1034 1067 1244 1315

1050 1063 1238 1294

1062 1079 1247 1311

A′

1389

1396

1400

1411

potential energy distribution (%) ring tors (88), H wag (6) ring bend (58), H bend (16), νSN (15), νC5S (7) νSN (54), νC5S (28), ring bend (10) νC5S (34), ring bend (31), H bend (25), νSN (7) νC4C3 (68), H bend (24) H bend (80), νC4C5 (14) H bend (77), νC3N (11) H bend (42), νC3N (27), νC4C5 (21) H bend (37), νC4C3 (25), νC4C5(16), νC3N (10), ring bend (7)

Table 2. Observed and Calculated Raman Frequencies (in cm−1) of Isoxazole species

Raman

calcd

SERS

calcd

A″ A′ A′

591 844 1029

589 852 1021

597 860 1034

585 889 1049

A′

1088

1096

1087

1095

A′

1131

1125

1134

1136

A′

1220

1222

1224

1233

A′

1372

1368

1370

1379

A′

1432

1430

1432

1428

A′

1560

1565

1564

1560

potential energy distribution (%) ring tors (88), H wag (11) νON (81), H bend (14) H bend (42), νC4C3 (40), νC5O (6) νC5O (52), νC4C3 (18), H bend (10), ring bend (7) νC5O (44), νC4C5 (23), H bend (15), νNO (7) H bend (68), νC3N (12), νC5O (8), ring bend (6) H bend (64), νC4C3 (18), νC5O (9), νC3N (6) νC3N (26), νC4C5 (24), H bend (16), ring bend (14), νC3C4 (10) νC4C5 (36), H bend (31), νC3N (21)

Figure 3. Upper panel: SERS spectrum of isothiazole. DFT-simulated SERS spectra of isothiazole bound to Ag3+, Ag3(0), Ag+, and Ag(0).

higher wavenumbers, instead, mainly correspond to stretching modes of the C−C and N−C bonds. This vibrational analysis is essential for a correct interpretation of the SERS spectra, which are reported in Figures 3 and 4. It is important to remark that the SERS spectra of both molecules can be observed only in silver colloids activated by coadsorption of chloride anions, as discussed in the next section. The SERS bands of isothiazole and isoxazole show positions and intensities that differ from those observed in the normal Raman spectra. Actually, the most evident feature is the strong enhancement of the higher frequency bands with respect to those occurring in the lower frequency region. The goal of the DFT calculations performed on ligand/metal complexes is the capability to reproduce not only the observed SERS frequencies but also the changes in the relative intensities. For this aim, different surface complexes are considered, with one or more silver atoms, as neutral or positively charged metal clusters. As shown in Table 3, the presence of a positive charge in these complexes induces more relevant charge transfers from molecule to metal, with larger values of the stabilization energy. From the inspection of Figures 3 and 4, it is evident that the SERS profiles of both isothiazole and isoxazole can be correctly reproduced only by considering the binding of the two

Figure 4. Upper panel: SERS spectrum of isoxazole. DFT-simulated SERS spectra of isothiazole bound to Ag3+, Ag3(0), Ag+, and Ag(0).

molecules with Ag3+ clusters. Thus, the relative intensities of the strongest SERS bands at 1294 and 1400 cm−1 of isothiazole 2330



Article

to Ag42+ clusters, which offer a worse agreement with the experiment. These results, along with those obtained with Ag4+ and Ag4(0) clusters, are reported in the Supporting Information (Figures S1 and S2): the best agreement with the experimental features is obtained with the Ag4+ clusters, which provide results quite similar to those of ligand/Ag3+ but with less stabilization energy (Table S1 of the Supporting Information). The presence of positively charged clusters such as Ag42+ and Ag3+ were experimentally ascertained in silver colloids by UV absorption during the early stages of colloidal formation by pulse radiolytic reduction of silver ions28 and by mass spectrometry in the nucleation stage of a solution-phase synthesis employing AgNO3 as a precursor to silver.29 In our chemically prepared Ag colloids, we cannot detect by UV absorption these clusters, whose presence could be related to the sizable amount of oxidized silver detected by X-ray photoelectron spectroscopy measurements on Ag colloidal particles with adsorbed chloride ions.27 Actually, in our colloids, the addition of Cl− anions to silver hydrosols could promote the formation of Ag3+ clusters, as shown by reaction 2:

Table 3

a

complex

Ag···N (Å)

Ag···X (Å)

Δqa (e)

ΔEb (kJ/mol)

Ag(0)/isothiazole Ag(0)/isoxazole Ag+/isothiazole Ag+/isoxazole Ag3(0)/isothiazole Ag3(0)/isoxazole Ag3+/isothiazole Ag3+/isoxazole

2.61 2.71 2.19 2.20 2.33 2.34 2.24 2.25

3.80 3.66 3.39 3.03 3.54 3.28 3.44 3.10

0.013 0.016 −0.096 −0.083 0.047 0.047 −0.088 −0.085

−6.999 −4.119 −194.138 −185.712 −52.761 −48.663 −124.898 −120.348

Molecule → metal charge transfer. bStabilization energy.

and at 1224 and 1432 cm−1 of isoxazole are well reproduced, as well as the largest frequency shifts with respect to the normal Raman bands. These results are analogous to those obtained by the DFT simulations of the SERS spectra of thiazole and oxazole adsorbed on Ag colloids activated by chloride anions.27 The structures of the ligand/Ag3+ complexes are reported in Figure 5, along with the negative parts of the electrostatic potentials of the two molecules.

Ag4 2 + + Cl− → Ag 3+ + AgCl

(2)

where the Ag3+ clusters represent the surface active sites of the silver nanoparticles, whereas the adsorbed AgCl species is responsible for the occurrence of the strong band around 240 cm−1, generally observed in the SERS spectra of Ag colloids activated by chloride anions11 (see Figure S3 of the Supporting Information). By adopting instead the ligand/Ag42+ model system, we previously obtained a satisfactory agreement with the SERS spectra of pyridine and pyrazole in free-chloride Ag colloids, regarding both band positions and relative intensities.10,11

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CONCLUSIONS As reported in the previous section, we cannot observe the SERS signal of both isothiazole and isoxazole without coadsorption of chloride anions on the silver colloidal nanoparticles. This is evidence of the scarce affinity of these heterocycles to silver, which could be confirmed by the lack of modifications of the SPR band of the Ag nanoparticles around 400 nm (data not shown) after ligand addition. This result, analogous to that observed for 2,1-benzisoxazole in Ag hydrosol,30 emphasizes the important role played by the chloride anions in the SERS experiments. Actually, for most molecules,7,31−39 the SERS signal resulted in enhancement by 2 or 3 orders of magnitude upon addition of halide anions such as Cl−, Br−, or I− to the silver colloids. Two possible explanations for the halide-induced SERS activation were previously proposed:40 (i) the increase of the local electromagnetic field by aggregation of the silver colloidal nanoparticles; (ii) the formation of complexes with a large charge transfer between ligand and metal. This second hypothesis is consistent with our DFT calculations, which point to the formation of surface complexes with positively charged active sites of the silver surface, stabilized by ligand/metal charge transfer. Instead, the first explanation cannot be proposed in the present case, because our Ag colloids, activated by chloride anions in small concentrations (10−3 M), do not undergo aggregation, as demonstrated by the unaltered SPR spectrum. Moreover, our DFT-assisted SERS investigation allows interpretation in a detailed way of the SERS spectra of heterocycles, including band positions, relative intensities, and frequency shifts with

Figure 5. Negative parts of the electrostatic potentials of (a) isothiazole and (b) isoxazole; optimized structures of the silver complexes of (c) isothiazole and (d) isoxazole.

The adsorption of isothiazole is expected to occur only through the nitrogen atom, whereas for isoxazole, also oxygen can be weakly involved in the interaction with the metal surface. Actually, the chemisorption process could resemble an acid− base reaction between a ligand molecule as the nucleophilic agent and a positively charged silver cluster as the Lewis acid. In this picture, the negative electrostatic potential of the ligand molecule becomes very useful to identify the molecular sites responsible for the chemical interaction with the metal. On the other hand, the good agreement between the observed SERS profiles and those calculated for the ligand/Ag3+ complexes represents a reliable indication to understand the nature of the active sites on the silver colloidal surface. In addition, we have corroborated our conclusions obtaining the simulated SERS spectra of isothiazole and isoxazole bound 2331



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respect to the normal Raman spectra, on the basis of simple model systems with small positively charged silver clusters. This approach is substantially based on the chemical enhancement mechanism proposed by Otto et al.,41 by interaction of ligand molecules with adatom/adclusters, which can be considered almost isolated on the metal surface. Finally, we want to extend our conclusions to the SERS response of different heterocycles (listed in Table 4), in relation to their properties of aromaticity and basicity.

aromaticitya (IA)

basicityb (pKa)

strong SERS effectc

oxazole isoxazole thiazole isothiazole imidazole pyrazole

47 52 79 91 79 90

0.8 −2.98 2.53 −0.51 6.95 2.52

no no yes no yes yes

*E-mail: maurizio.muniz@unifi.it. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully thank the Italian Ministero dell’Istruzione, Università e Ricerca (MIUR) for the financial support.

Unified aromaticity index.42 bRelated to the N-protonated cation.43 SERS effect.6,7,9

All molecules of Table 4 are five-membered heterocyclic compounds with one nitrogen atom and a second one like oxygen or sulfur or N(H). The presence of the second heteroatom is responsible for sizable changes in the ring aromaticity, as well as in the basicity. The oxygen atom, present in oxazole and isoxazole, instead of sulfur or N(H), decreases the aromaticity, due to its larger electronegativity,44 along with the basicity. Also, the relative position of the heteroatoms in the ring is important, because a larger aromaticity occurs when they are vicinal: isoxazole, isothiazole, and pyrazole are more aromatic than oxazole, thiazole, and imidazole, respectively. This finds a justification by considering that the vicinal heteroatom increases the inductive effect for the lone-pair electrons of the sp2 nitrogen atom, which are attracted in the anular current. On the contrary, the vicinal presence decreases the basicity, because the lone-pair electrons are less available to interact with electrophilic reagents. By considering the ligand chemisorption on silver as a reaction between a nucleophile (the heterocyclic compound) and a Lewis acid (the Ag3+ cluster) to form a surface complex, a large value of basicity ensures the possibility of chemisorption and consequently a strong SERS effect, as experimentally observed for imidazole, thiazole, and pyrazole (see Table 4). For ligands like isoxazole, oxazole, and isothiazole with low basicity values, instead, a SERS signal is detectable only by chloride activation. Instead, no relevant correlation is found between a strong SERS effect and aromaticity, because aromatic heterocycles such as thiazole and isothiazole exhibit quite different SERS responses. These considerations regarding SERS enhancement and basicity could be extended to other N-containing ligands and represent a useful guideline to foresee chemisorption on silver and strong SERS effect.

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REFERENCES

(1) Le Ru, E. C.; Etchegoin, P. G. Principles of Surface-Enhanced Raman Spectroscopy and Related Plasmonic Effects; Elsevier: Oxford, U.K., 2009. (2) Surface Enhanced Raman Spectroscopy; Schlücker, S., Ed.; WileyVCH: Weinheim, Germany, 2011. (3) Pagliai, M.; Muniz-Miranda, M.; Cardini, G.; Schettino, V. J. Phys. Chem. A 2009, 113, 15198−15205. (4) Muniz-Miranda, M.; Pagliai, M.; Muniz-Miranda, F.; Schettino, V. Chem. Commun. 2011, 47, 2951−2955. (5) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 26, 163−166. (6) Muniz-Miranda, M.; Neto, N.; Sbrana, G. J. Mol. Struct. 1992, 267, 281−286. (7) Muniz-Miranda, M. Vib. Spectrosc. 1999, 19, 227−232. (8) Pergolese, B.; Muniz-Miranda, M.; Bigotto, A. J. Phys. Chem. B 2005, 109, 9665−9671. (9) Cardini, G.; Muniz-Miranda, M. J. Phys. Chem. B 2002, 106, 6875−6880. (10) Muniz-Miranda, M.; Pagliai, M.; Cardini, G.; Schettino, V. J. Phys. Chem. C 2008, 112, 762−767. (11) Cardini, G.; Muniz-Miranda, M.; Pagliai, M.; Schettino, V. Theor. Chem. Acc. 2007, 117, 451−458. (12) Zhao, L.-B.; Huang, R.; Huang, Y.-F.; Wu, D.-Y.; Ren, B.; Tian, Z.-Q. J. Chem. Phys. 2011, 135, 134707. (13) Zhao, L.; Jensen, L.; Schatz, G. C. J. Am. Chem. Soc. 2006, 128, 2911−2919. (14) Tanaka, T.; Nakajima, A.; Watanabe, A.; Ohno, T.; Ozaki, Y. Vib. Spectrosc. 2004, 34, 157−167. (15) Kundu, J.; Neumann, O.; Janesko, B. G.; Zhang, D.; Lal, S.; Scuseria, A. B. G. E.; Halas, N. J. J. Phys. Chem. C 2009, 113, 14390− 14397. (16) Liu, S.; Zheng, G.; Li, J. Spectrochim. Acta, Part A 2011, 79, 1739−1746. (17) Muniz-Miranda, M.; Gellini, C.; Pagliai, M.; Innocenti, M.; Salvi, P. R.; Schettino, V. J. Phys. Chem. C 2010, 114, 13730−13735. (18) Li, X.; Liu, S.; Li, W.; Mo, Y.; Hu, J. Z. Phys. Chem. 2011, 225, 661−672. (19) Costa, J. C. S.; Ando, R. A.; Camargo, P. H. C.; Corio, P. J. Phys. Chem. C 2011, 115, 4184−4190. (20) Pagliai, M.; Caporali, S.; Muniz-Miranda, M.; Pratesi, G.; Schettino, V. J. Phys. Chem. Lett. 2012, 3, 242−245. (21) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 2 1979, 75, 790−798. (22) Muniz-Miranda, M.; Neto, N.; Sbrana, G. J. Mol. Struct. 1986, 143, 275−278. (23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (24) Keresztury, G. In Handbook of Vibrational Spectroscopy; Chalmers, J. M., Griffiths, P. R., Eds.; John Wiley & Sons: Hoboken, NJ, 2002; Vol. 1, pp 71−87. (25) Krishnakumar, V.; Keresztury, G.; Sundius, T.; Seshadri, S. Spectrochim. Acta, Part A 2007, 68, 845−850. (26) Martin, J. M. L.; Van Alsenoy, C. GAR2PED; University of Antwerp: Antwerp, Belgium, 1995.

a c

AUTHOR INFORMATION

Corresponding Author

Table 4 heterocycle

Article

ASSOCIATED CONTENT

S Supporting Information *

SERS spectrum of isothiazole in Ag/chloride colloid; Ag···N distances, molecule to metal charge transfers, stabilization energies, and DFT-simulated Raman spectra of the Ag4x/ isothiazole and Ag4x/isoxazole complexes, with x = 0, 1+, or 2+. This material is available free of charge via the Internet at http://pubs.acs.org. 2332



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(27) Pagliai, M.; Muniz-Miranda, F.; Schettino, V.; Muniz-Miranda, M. Prog. Colloid Polym. Sci. 2012, 139, 39−44. (28) Lawless, D.; Kapoor, S.; Kennepohl, P.; Meisel, D.; Serpone, N. J. Phys. Chem. 1994, 98, 9619−9625. (29) Xiong, Y.; Washio, I.; Chen, J.; Sadilek, M.; Xia, Y. Angew. Chem., Int. Ed. 2007, 46, 4917−4921. (30) Baia, M.; Baia, L.; Kiefer, W.; Popp, J. J. Phys. Chem. B 2004, 108, 17491−17496. (31) Hildebrandt, P.; Stockburger, M. J. Phys. Chem. 1984, 88, 5935− 5944. (32) Heard, S. M.; Grieser, F.; Barraclough, C. G.; Sanders, J. V. J. Phys. Chem. 1985, 89, 389−392. (33) Kim, M.; Itoh, K. J. Phys. Chem. 1987, 91, 126−131. (34) Sbrana, G.; Neto, N.; Muniz-Miranda, M.; Nocentini, M. J. Phys. Chem. 1990, 94, 3706−3710. (35) Hildebrandt, P.; Keller, S.; Hoffmann, A.; Vanhecke, F.; Schrader, B. J. Raman Spectrosc. 1993, 24, 791−796. (36) Schneider, S.; Grau, H.; Halbig, P.; Freunscht, P.; Nickel, U. J. Raman Spectrosc. 1996, 27, 57−68. (37) Wehling, B.; Hill, W.; Klockow, D. J. Mol. Struct. 1995, 349, 117−120. (38) García-Ramos, J.; Sánchez-Cortés, S. J. Mol. Struct. 1997, 405, 13−28. (39) Liang, E.; Ye, X.; Kiefer, W. Vib. Spectrosc. 1997, 15, 69−78. (40) Grochala, W.; Kudelski, A.; Bukowska, J. J. Raman Spectrosc. 1998, 29, 681−685. (41) Otto, A.; Billmann, J.; J. Eickmans, U. E.; Pettenkofer, C. Surf. Sci. 1984, 138, 319−338. (42) Bird, C. W. Tetrahedron 1992, 48, 335−340. (43) Kurita, Y.; Takayama, C. J. Phys. Chem. A 1997, 101, 5593− 5595. (44) Minkin, V. I.; Glukhovtsev, M. N. Aromaticity and Antiaromaticity: Electronic and Structural Aspects; Wiley: New York, 1994.

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Positively Charged Active Sites for the Adsorption of Five-Membered

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