J. Phys. Chem. C 2008, 112, 13851–13855
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Theoretical and Experimental Vibrational Characterizations of Amine-Coated Silver Nanoparticles Motoi Tobita*,† and Yusuke Yasuda‡ Hitachi, Ltd., AdVanced Research Laboratory, 1-280 Higashi-Koigakubo, Kokubunji-shi, Tokyo 185-8601, Japan, and Hitachi, Ltd., Materials Research Laboratory, 1-1 Omika-cho 7-chome, Hitachi-shi, Ibaraki 319-1292, Japan ReceiVed: April 13, 2008; ReVised Manuscript ReceiVed: July 2, 2008
Vibrational spectra of neutral and cationic amines on a silver surface were studied by using experimental and theoretical methods in combination. This work provides a prescription for identifying the charged state of amine on silver nanoparticles. In regards to the experiment, the infrared (IR) spectrum of silver nanoparticles (Ag-NPs) coated by neutral octylamine was measured by Fourier transformed IR. In regards to the calculations, vibrational spectra of neutral and cationic octylamine, adsorbed on a silver surface, were obtained by density-functional BPW91 calculations. The calculations show that amines of different charged states, i.e., neutral or cationic, can clearly be distinguished by the characteristic frequencies of N-H stretching and wagging modes. A comparison between the calculated and experimentally measured spectra of the Ag-NPs revealed that the amine in the synthesized Ag-NPs actually takes the neutral form. In addition, the synthesized Ag-NPs are shown to contain a non-negligible amount of residual ascorbic acid as well as some oxidized amines. In one of the IR spectra of amine-coated Ag-NPs measured prior to this study, the amine was found to be likely to take the cationic form. The current study demonstrates that a combination of theoretical calculations and experiments will help in characterizing the structure and constitution of organic species adsorbed on a metal surface. 1. Introduction Metal nanoparticles (NPs) exhibit unique characteristics that are not observed in bulk metal. For instance, the melting point of NPs decreases as particle size decreases.1 This property leads to many industrial applications of metal NPs, including fine wiring on a substrate and formation of interconnects in electronic packages.2-4 Among the many kinds of metal NPs, such as gold, silver, and copper, silver NPs (Ag-NPs) are important because they are less costly than gold NPs and are more tolerant to oxidation than copper NPs. Silver also possesses the highest thermal and electrical conductivity among the elements in the periodic table. To disperse Ag-NPs in a solution, organic coating materials are needed, because Ag-NPs easily aggregate into a larger particle. Commonly used materials for this purpose include thiols,5-7 carboxylic acids,8-11 and amines.8,12-16 Thiol and carboxylic acid molecules adsorb on the silver surface as thiolates and carboxylic acid anions, respectively. On the other hand, amine can adsorb on the surface in either of two charged states, namely, neutral (-NH2) or cationic (-NH3+), depending on the pH of the synthesis solution. In the case of fine wiring and interconnects applications, it is essential to identify the charged state of amine on the Ag-NPs, because the state significantly affects the sintering temperature of Ag-NPs. Using 4-aminothiophenol, Gole et al. showed that the surface coverage of silver particles by the -NH3+ group reaches a maximum at pH around 9.10 Such amine-coated silver systems have been studied by using infrared (IR) and Raman spectroscopies. Liang et al. reported the surface-enhanced Raman spectra of p-aminobenzoic acid in yellow silver colloids.6 They assigned the 1615 and 1148 cm-1 peaks as * Corresponding author. E-mail:
[email protected]. † Hitachi, Ltd., Advanced Research Laboratory. ‡ Hitachi, Ltd., Materials Research Laboratory.
Figure 1. Structural model (consisting of 22 silver atoms) for the AgNP surface represented by the Ag(111) surface. The positions of the outer-layer atoms, shown in gray, were fixed during geometry optimization.
NH2 scissoring and wagging modes, respectively. Bardosova et al. measured Fourier-transformed (FT) IR spectra of pure docosylamine film and docosylamine formed over a subphase containing various materials.16 They reported that a typical peak for the pure amine is at 3349 cm-1, while it shifts to 3207 cm-1 when the film is formed over a subphase containing chloroplatinic acid. Sastry et al. measured FTIR spectra of a Langmuir-Blodgett film consisting of silver clusters and octadecylamines synthesized at pH 9.9 They showed that the N-H antisymmetric stretching frequency is at 3260 cm-1, while the corresponding frequency for the pure amine film is at 3331 cm-1. Manna et al. synthesized and characterized N-hexadecylethylnediamine-coated Ag-NPs in waterethanol solution.13 The N-H antisymmetric vibrational frequency for pure diamine is 3345 cm-1 and that for the NPs is 3340 cm-1. Chen et al. studied the formation mechanism of oleylamine-coated Ag-NPs.15 They observed peaks that are attributed to oleylamine as well as to a carbon-nitrogen double bond and a triple bond. In
10.1021/jp803195e CCC: $40.75 2008 American Chemical Society Published on Web 08/15/2008
13852 J. Phys. Chem. C, Vol. 112, No. 36, 2008
Tobita and Yasuda TABLE 1: Assignment of the Major Peaks (in cm-1) Appearing in Figure 3 peak
assignment
calcd
expt
1 2 3 4 5 6
NH symmetric stretching CH stretchinga NH2 scissoring CH2 scissoring CH2 wagging NH2 wagging
3419 2878 1622 1461 1286 814
3325 2752 1616 1437 1300 800
a Frequencies in methylene group that is adjacent to the amino group.
Figure 2. Molecular structures of (a) octylamine and (b) ascorbic acid. Each colored sphere represents a different atomic element: white, oxygen; black, nitrogen; dark gray, carbon; light gray, hydrogen.
Figure 4. Top views of (a) neutral and (b) cationic octylamine structures bound on the Ag(111) surface. Each colored sphere represents a different atomic element: black, nitrogen; white, silver; dark gray, carbon; light gray, hydrogen.
Figure 3. Vibrational spectra of octylamine: (a) IR, experimentally observed; (b) IR, calculated; (c) Raman, experimentally observed; and (d) Raman, calculated.
the above studies, the charged state of the amino group, neutral or cationic, in the spectra was not clearly discussed. The objective of the present study, therefore, is to characterize the vibrational spectra of the neutral and cationic amines, respectively, on a silver surface. For this characterization, experimental and theoretical methods were used in combination.
In the experiment, Ag-NPs coated by neutral octylamine were synthesized in toluene and extracted in powdery form. Using toluene solvent retains the charged state of the amine in the neutral form. The FTIR spectrum of the powdery sample was then measured. In the theoretical calculations, density-functional theory was used to obtain the vibrational spectra of the neutral and cationic octylamine, respectively, adsorbed on a silver surface. A comparison of the measured and calculated spectra shows that the synthesized particles are coated by the neutral amines as expected. It is also shown that the shift of antisymmetric N-H stretching vibrational frequency is larger in the case of the cationic amine than that in the case of the neutral amine. In regards to one of the IR spectra of amine-coated AgNPs measured prior to this study,9 the amount of frequency shift indicates that the amine is likely to take the cationic form.
Vibrational Characterizations of Silver Nanoparticles
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Figure 7. (a) Sum of calculated IR spectra for neutral octylamine bound on Ag(111), ascorbic acid, octanimine, and octanonitrile. (b) Experimentally observed IR spectrum of amine-coated Ag-NPs.
Figure 5. IR spectra of octylamine adsorbed on a silver surface: (a) neutral amine bound on Ag(111) surface (calculated); (b) cationic amine bound on Ag(111) surface (calculated); (c) octylamine adsorbed on Ag-NPs (experimentally observed). The inset of parts a and b shows magnified spectra for the range 1000 to 1700 cm-1.
Figure 6. Calculated IR spectrum of (a) ascorbic acid, (b) octanimine, and (c) octanonitrile.
As described above, differences in vibrational spectra of the neutral and cationic forms of an amine on a silver surface are clarified. It is shown in the following that the synthesized AgNPs are actually coated by the neutral amine as well as residual ascorbic acid and oxidized amines. Clear distinction of Ag-NPs coated by neutral or cationic amine is thus important in controlling the sintering temperature of Ag-NPs in fine-wiring and interconnects applications. 2. Methods and Materials Theoretical Calculations. It has been reported that the electron-diffraction pattern for Ag-NPs coated by organic
molecules coincides with the one for bulk silver.17 This finding indicates that the surface of Ag-NPs is composed of crystalline faces of a bulk-silver lattice, which is face-centered cubic. Lower energy crystalline faces appear preferentially at the surface according to Wulff’s theorem.18 The Ag-NP surface should thus largely consist of a closely packed and low-energy Ag(111) surface. In this work, therefore, a Ag-NP surface was modeled by an Ag22 cluster representing the Ag(111) surface, as shown in Figure 1. The model consists of 19 atoms in the first layer and three atoms in the second. A cluster model of this size is large enough to obtain converged vibrational frequencies with respect to the cluster size because vibrational frequencies of adsorbed organic molecules on a metal surface behave as a local property.17,19-21 To calculate the vibrational spectrum of a system, a corresponding geometry such that the energy of the system is at a stationary point of the potential energy surface must be known. In this study, such a stationary point was searched for under the following constraint: the position of organic molecules adsorbed on the Ag(111) surface, seven inner silver atoms in the first layer and the three silver atoms in the second layer, shown in white in Figure 1, were relaxed, while the positions of the remaining 12 silver atoms, shown in gray in Figure 1, were fixed. In the initial geometry, the nearestneighbor distance between silver atoms was 2.889 Å, i.e., the atomic distance in bulk silver.18 The organic molecules investigated in this study are the neutral and cationic forms of octylamine. Vibrational spectra of a single molecule of neutral octylamine (C8H17NH2) and cationic octylamine (C8H17NH3+) as well as oxidized amines such as octanimine (C7H15CHdNH) and octanonitrile (C7H15CtN) were investigated. An IR spectrum of ascorbic acid (C6H8O6), which is a reagent used in the NP synthesis, was also calculated, as described in the experimental section. The molecular structures of octylamine and ascorbic acid are shown in Figure 2. Calculations by density functional theory were performed by using a combination of the Becke’s exchange functional and the Perdew and Wang’s correlation functional (BPW91).22,23 A LANL2DZ basis set24 was used for silver atoms, and a 6-31+G(d,p) basis set25 was used for the first-row atoms. Among the tested combinations of functionals and basis sets, the combination of the BPW91 functional and LANL2DZ basis set was reported to most accurately reproduce molecular geometries, vibrational frequencies, and dissociation energies for gold-, silver-, and copper-containing molecules.26 Since vibrational peaks above 200 cm-1 are attributed to the motions of organic molecules, an all-electron basis set containing
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Tobita and Yasuda
TABLE 2: Summary of N-H Symmetric Stretching Frequencies of Free Amines and Amines Interacting with Metal Surfacea method FTIR calculation (Raman) FTIR calculation (IR, Raman) FTIR a
freq (cm-1)
material
3345 3340 (-5) 3419 3406 (-13) 3349 3207 (-42) 3332 3277 (-55) 3331 3260 (-71)
pellet of hexadecylethylenediamine pellet of hexadecylethylenediamine-coated Ag-NPs single molecule of octylamine octylamine molecule bound on the Ag(111) surface docosylamine film film of docosylamine over a subphase containing chloroplatinic acid single molecule of octylamine cation octylamine cation molecule bound on Ag(111) surface octadecylamine film film of octadecylamine-coated Ag-NPs
ref 13 this work 16 this work 9
Numbers in parentheses in the frequency column stand for frequency shifts.
polarization and diffuse functions was used for a better description of the amines and ascorbic acid. All the calculations were performed with the Gaussian03 program.27 Experimental Details. Silver nitrate (AgNO3), octylamine, ascorbic acid, toluene, and acetone were purchased from WAKO Chemical and used as received. AgNO3 (0.012 mol) and octylamine (0.18 mol) were added to 100 mL of toluene solution and the mixture was stirred at room temperature. Next, 0.01 mol of ascorbic acid, as a reducing agent, was added to the toluene solution with continued stirring. Within a few minutes, the reaction mixture turned brown. The stirring was continued for 2 h to complete the reaction. Adding 400 mL of acetone precipitated the Ag-NPs. The NPs were collected by damping the supernatant solution and evaporating the residual solvent around 320 K. The collected NPs were in powdery form and silver-blue. For pure octylamine and octylamine-coated Ag-NPs, IR spectra were measured with a JASCO, FT/IR-620 spectrometer with 32 scans per spectrum at 4-cm-1 resolution. The octylamine spectrum was measured by holding the sample in between plates made of KBr, because octylamine is liquid at room temperature. The spectrum for the NP powder was measured by the diffuse reflectance IR Fourier transform (DRIFT) technique. To record the spectra, a diffuse reflection attachment (JASCO DR-81) was added to the sampling compartment of the FTIR spectrometer. Raman spectra were measured with a Bruker RFS100 spectrometer, using a laser beam with an excitation wavelength of 1064 nm and a charge-coupled device (CCD) detector with 500 scans per spectrum at a resolution of 4 cm-1. The laser power used was 500 mW. 3. Results and Discussion The calculated and experimentally measured IR and Raman spectra of octylamine are shown in Figure 3. The positions of the major peaks determined by calculations and experiments agree within 5%. Assignments of the numbered peaks 1 to 6 in Figure 3 are given in Table 1. Peaks 1, 3, and 6 are related to the motions of the amino group. Peak 2, at 2752 cm-1, represents the C-H stretching mode of the methylene group adjacent to the amino group. This peak has a lower frequency than the typical C-H stretching mode of the methylene group in alkanes and has also been observed in an independent measurement.28 The calculated frequencies are slightly overestimated for all the peaks but peak 5. This is usually the case for ab initio calculations using the harmonic approximation. The use of an empirical single-valued scaling factor for a given combination of theory level and basis set, compiled on the NIST web page, would yield a better agreement between the calculated and measured peak positions.29 However, the validity of using such a factor for vibrational spectra of organic molecules
adsorbed on a metal surface has not yet been clarified. The spectra reported in this work are thus not scaled. Optimized structures of neutral and cationic octylamine, respectively, bound on the Ag(111) surface are shown in Figure 4. In neutral octylamine, the nitrogen atom is almost at an atop site, i.e., the nitrogen atom is placed right above a silver atom, as shown in Figure 4a. The lone-pair of the nitrogen atom directs to the silver atom. On the other hand, the nitrogen atom of cationic amine stabilizes at a hollow site, i.e., in the middle of a triangular region created by three adjacent silver atoms. Two of the three hydrogen atoms bound to the nitrogen are directed to the Ag(111) surface. Calculated IR spectra of neutral and cationic octylamine, respectively, bound on the Ag(111) surface as well as measured IR spectra of octylamine-coated Ag-NPs are shown in Figure 5. A comparison between the calculated and measured spectra reveals that the NPs are coated by neutral amine, as judged by peaks in two characteristic regions. The first region, shown shaded, is around 3000 cm-1, representing C-H stretching modes. In panels a and c of Figure 5, multiple sharp peaks are observed in this region. On the other hand, peak intensity in this region is much lower in Figure 5b; instead, there is a strong peak around 2800 cm-1. This peak is assigned as the N-H stretching mode of the NH3+ group.30-32 The second region, also shaded, is around 800 cm-1, representing NH2 wagging modes. No corresponding strong peak appears in Figure 5b. There are a few peaks that appear in Figure 5c but not in Figure 5a. Relatively weak peaks at 3370 and 1626 cm-1, labeled with stars in Figure 5c, are inferred to be related to carbon-nitrogen double-bond vibrations.15 The CdN bond could be formed during the amine oxidation to nitrile through imines on the silver surfaces.33 The nitrile-formation temperature of 370 K33 supports the possibility of the coexistence of amine, imine, and nitrile, since the highest temperature reached under our experimental conditions is around 320 K (when the residual solvent is evaporated). At this temperature, amine-to-imine and imine-to-nitrile conversion could occur only slowly. Moreover, peaks at 1742 and 480 cm-1 are not observed in Figure 5a. These two peaks may be attributed to vibration of the ascorbic acid that was used as the reducing agent in the NP synthesis. Besides the vibrational spectra, we add that thermogravimetry analysis (TGA) of the Ag-NPs showed exothermic peaks around 206, 373, and 407 °C. The existence of multiple peaks implies that there are multiple kinds of organic species contained in the Ag-NPs. To verify the above speculations, IR spectra of ascorbic acid, octanimine, and octanonitrile were calculated and compared with observed peaks (see Figures 5 and 6). In regards to the ascorbic acid spectrum, i.e., Figure 6a, peaks exist at 1781, 1721, and 511 cm-1, which agree well with the experimentally observed peaks at 1742 and 480 cm-1. These peaks are assigned as CdO
Vibrational Characterizations of Silver Nanoparticles and CdC stretching for the higher frequencies and OsH outof-plane bending for the lower ones. The starred and broad experimental peaks around 3370 and 1626 cm-1 are reproduced in the calculated octanimine spectrum, i.e., Figure 6b, at 3365 and 1665 cm-1. A corresponding peak to the experimental peak at 2155 cm-1 is observed in the calculated spectrum of octanonitrile, i.e., Figure 6c, at 2267 cm-1. From the above discussions, it is reasonable to conclude that the experimentally observed IR spectrum, i.e., Figure 5c, mainly consists of peaks from neutral octylamine and ascorbic acid as well as from a small fraction of oxidized octylamines. The sum of peaks in Figures 5a and 6a-c was calculated and is shown together with the experimentally observed spectrum in Figure 7. The two spectra resemble each other, except that peak intensity between 1000 and 1500 cm-1 in the calculated spectrum is weaker. The peaks in this region are assigned as C-H bending modes of the amines and C-O stretching and C-H and O-H bending modes of ascorbic acid. A probable reason for the weaker peak intensity is the absence of the closed packing effects of the alkyl chains of the amines in the calculation model. It is known that methylene out-of-plane vibrational modes have a transition dipole moment parallel to the alkyl chain axis.9 Our calculation model uses only one octylamine molecule adsorbed on a silver surface, leading to relatively weak intensities in the 1000 to 1500 cm-1 region. Observed peak frequencies for the N-H symmetric stretching mode are compared with those measured prior to this study.9,13,16 Five sets of frequencies are listed in Table 2. Each set shows the frequency of a free amine and of an amine adsorbed on a metal surface. The amount of frequency shift is given in parentheses of the frequency column. In all the cases, the frequency decreases as the amine adsorbs. The degree of frequency shift among the experiments largely differs. Especially, in ref 9, Sastry et al. reported the vibrational peak at 3260 cm-1 as a novel peak. This peak is likely to be the N-H stretching frequency of cationic octylamine bound on Ag(111) for the following three reasons. The first reason is the close agreement between the calculated frequency, 3277 cm-1, and the observed one, 3260 cm-1. The second reason is that the peak intensities obtained both from calculation and experiment are relatively weak (see Figure 5b and Figure 5 in ref 9). The third reason is that the Ag-NPs used in their experiment are prepared in a solution with pH 9.9 At this pH value, the amino group is more stable in the cationic form.10 4. Summary Octylamine-coated Ag-NPs were synthesized, and their FTIR spectrum was obtained. Moreover, IR spectra of neutral and cationic octylamine, respectively, bound on a Ag(111) surface were calculated by using density functional theory. The calculations show that there are characteristic differences between neutral and cationic octylamine, such as the position of the nitrogen atom, that is, an atop or hollow site, on the silver surface, and presence/absence of peaks around 3000 and 800 cm-1. The observed spectrum is likely to represent a mixture of neutral octylamine and ascorbic acid as well as oxidized octylamines with carbon-nitrogen double or triple bonds. On the other hand, a spectrum measured prior to this study shows the characteristic peak position of cationic octylamine. In the case of cationic amine, the frequency shift of the N-H stretching mode from free amine to amine adsorbed on a silver surface is larger than that in the case of neutral amine. These results demonstrate that a combined analysis of calculated and experimentally observed spectra is a powerful tool for understanding structures of organic molecules adsorbed on a metal surface.
J. Phys. Chem. C, Vol. 112, No. 36, 2008 13855 References and Notes (1) Baletto, F.; Ferrando, R. ReV. Mod. Phys. 2005, 77, 371. (2) Metal Nanoparticles: Synthesis, Characterization, and Applications; Feldheim, D. L., Foss, C. A., Eds.; CRC Press: Boca Raton, FL, 2002. (3) Moon, K.-S.; Dong, H.; Maric, R.; Pothukuchi, S.; Hunt, A.; Li, Y.; Wong, C. P. J. Electron. Mater. 2005, 34, 168. (4) Joo, S.; Baldwin, D. F. Electron. Compon. Technol. Conf. 2007, 219. (5) Sarathy, K. V.; Raina, G.; Yadav, R. T.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 1997, 101, 9876. (6) Malinsky, M. D.; Kelly, K. L.; Schatz, G. C.; Van Duyne, R. P. J. Am. Chem. Soc. 2001, 123, 1471. (7) Choi, S.-H.; Lee, S.-H.; Hwang, Y.-M.; Lee, K.-P.; Kang, H.-D. Radiat. Phys. Chem. 2003, 67, 517. (8) Liang, E. J.; Engert, C.; Kiefer, W. Vib. Spectrosc. 1993, 6, 79. (9) Sastry, M.; Mayya, K. S.; Patil, V.; Paranjape, D. V.; Hegde, S. G. J. Phys. Chem. B 1997, 101, 4954. (10) Gole, A.; Sainkar, S. R.; Sastry, M. Chem. Mater. 2000, 12, 1234. (11) Wang, T. C.; Rubner, M. F.; Cohen, R. E. Langmuir 2002, 18, 3370. (12) Patil, V.; Sastry, M. Langmuir 2000, 16, 2207. (13) Manna, A.; Imae, T.; Iida, M.; Hisamitsu, N. Langmuir 2001, 17, 6000. (14) Yamamoto, M.; Nakamoto, M. J. Mater. Chem. 2003, 13, 2064. (15) Chen, M.; Feng, Y.-G.; Wang, X.; Li, T.-C.; Zhang, J.-Y.; Qian, D.-J. Langumuir 2007, 23, 5296. (16) Bardosova, M.; Tredgold, R. H.; Ali-Adib, Z. Langmuir 1995, 11, 1273. (17) Delgado, J. M.; Rodes, A.; Orts, J. M. J. Phys. Chem. C 2007, 111, 14476. (18) CRC Handbook of Chemistry and Physics, 86th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2005. (19) Cluster Models for Surface and Bulk Phenomena; Pacchioni, G., Bagus, P. S., Parmigiani, F., Eds.; NATO ASI Series; Plenum: New York, 1992. (20) Gil, A.; Clotet, A.; Ricart, J. M.; Illas, F.; Alvarez, B.; Rodes, A.; Feliu, J. M. J. Phys. Chem. B 2001, 105, 7263. (21) Bauschlicher, C. W., Jr J. Chem. Phys. 1994, 101, 3250. (22) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (23) 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. (24) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (c) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (25) (a) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257. (b) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (c) Clark, T.; Chandrasekhar, J.; Schleyer, P. v. R. J. Comput. Chem. 1983, 4, 294. (26) Legge, F. S.; Nyberg, G. L.; Peel, J. B. J. Phys. Chem. A 2001, 105, 7905. (27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision D.02; Gaussian, Inc.: Wallingford, CT, 2004. (28) SDBSWeb: http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, June 10, 2008). (29) NIST Computational Chemistry Comparison and Benchmark Database, NIST Standard Reference Database No. 101 Release 12, Aug 2005, Editor: Russell D. Johnson III: http://srdata.nist.gov/cccbdb. (30) Infrared Determination of Organic Structure; Randall, H. M., Fuson, N., Fowler, R. G., Fangl, J. R., Eds.; D. Van Nostrand Company Inc.; New York, 1949. (31) Lenormant, H. J. Chim. Phys. 1952, 49, 635. (32) Vis, J. H.; Meinke, P. Can. J. Chem. 1969, 47, 1581. (33) Thornburg, D. M.; Madix, R. J. Surf. Sci. 1990, 226, 61.
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