Binding Interaction and Raman Spectra of pâÏ Conjugated Molecules

Article pubs.acs.org/JPCC

Binding Interaction and Raman Spectra of p−π Conjugated Molecules Containing CH2/NH2 Groups Adsorbed on Silver Surfaces: A DFT Study of Wagging Modes Sha Tao, Li-Juan Yu, Ran Pang, Yi-Fan Huang, De-Yin Wu,* and Zhong-Qun Tian State Key Laboratory of Physical Chemistry of Solid Surfaces and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, Fujian, China ABSTRACT: The wagging mode is a characteristic out-of-plane bending vibration for a series of organic compounds containing −CH2/−NH2 groups, such as terminal olefins, p-substituted aniline derivates, and benzyl radicals. The SERS signal of the wagging mode is always sensitive to the interfacial interaction, displaying significant frequency shift and Raman enhancement. To understand the origin of the special SERS signal, density functional theory (DFT) calculation is performed to obtain harmonic vibrational frequencies and Raman intensities of equilibrium structures for the p−π conjugated molecules adsorbed on silver surfaces on the basis of the molecule−metallic cluster model. Our results showed that the frequency shift of the wagging mode is strongly dependent on the hybridization effect, the sp2 changing to sp3 hybridization causes a dramatic frequency shift for the wagging mode. Furthermore, our results also revealed the causes of the remarkable enhancement of this mode in SERS intensity. From the point of view of the frontier molecular orbital interaction and the change of electronic structures, the derivatives of polarizability tensor for the wagging coordinate are quite large appearing at the significant extent of the geometry deformation, closely associated with the p−π conjugation effect and the hybridization property, as well as the energy exchange of the frontier molecular orbitals. The last factor results in significant increases in the derivatives of polarizability tensor along with the direction of the wagging vibrations.

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INTRODUCTION Surface vibrational spectroscopy techniques, including surfaceenhanced Raman spectroscopy (SERS)1−3 and surfaceenhanced infrared spectroscopy,4,5 provide information for interfacial processes at the molecular level. More specifically, SERS offers advantages on providing information on surface adsorption configurations,6,7 molecule−metal interactions,3 and chemical reactions8,9 related to low-frequency peaks and avoiding the interference from the solvent molecules, such as water.7 For some cases, significant changes including relative intensity and peak shifts are observed in the surface vibrational spectra.10−23 These usually come from the interactions between probe molecules and surfaces or new species produced by surface induced chemical reactions.13,21−23 The experimental studies often analyze the spectral peaks by comparing the SERS signals with normal Raman spectra of these molecules in the gas phase or aqueous solution. In this case, the theoretical calculation on the analysis of surface vibrational spectroscopy plays a very important role on analysis of vibrational spectra. A series of studies showed that quantum chemistry calculations could clarify not only the assignment of vibrational fundamentals but also the enhancement mechanism of the SERS signals.8,13,24−26 Combining SERS with density functional theory (DFT) calculations, the SERS signals can be an indicator of sensitive surface probes to inspect the electronic structure of © 2013 American Chemical Society

the adsorption configurations and understand the complex SERS phenomena. The abnormal SERS spectra of the p−π conjugated molecules adsorbed on a metal surface were reported in previous studies. When the molecules containing CH2/NH2 groups, such as aniline, p-aminobenzonic acid, p-aminobenzonitrile, benzyl, and propylene radical adsorbed on silver films or electrodes, an intense and broad Raman band can be observed in their SERS spectra.13,21−23 The interpretation of the abnormal SERS bands was often neglected there. In the cases, the adsorption interaction takes place through the lone pair orbital of the amino group or the CH2 group. This causes the change in the hybridization property of the adsorbed groups companied with the p−π conjugation effect. To better understand the complex changes of SERS signals after the probe molecules adsorbed on the metal surface, it is necessary to investigate the relationship of the binding interaction and the Raman signal of the wagging mode by DFT calculations. Figure 1 shows the sketch of the CH2 wagging (ωCH2) mode of terminal olefins and the NH2 wagging mode (ωNH2) of aniline or its p-substituted derivatives. To present a whole underReceived: April 30, 2013 Revised: August 19, 2013 Published: August 21, 2013 18891

dx.doi.org/10.1021/jp4042777 | J. Phys. Chem. C 2013, 117, 18891−18903

The Journal of Physical Chemistry C

Article

the adsorption leads to the ωNH2 SERS band exhibiting very significant changes in the frequency and intensity. For example, the frequency significantly shifts to around 900 cm−1.8,24 The SERS spectra of o-aminobenzonitrile, m-aminobenzonitrile, and p-aminobenzonitrile displayed a strong and broad band at 900− 1000 cm−1, which was assigned to a mixed vibration of the inplane CH bending and the NH2 wagging.44 A recent study showed that a broad band near 850−1050 cm−1 was observed in SERS spectra of some substituted aromatic amine pollutants, like aniline, p-chloroaniline, p-methylaniline, and p-aminobenzoic acid adsorbed on silver sols.45 A further theoretical analysis for these interesting phenomena is required for unambiguous assignment. Although many theoretical studies reported for the adsorption interaction of these molecules on different metal surfaces, the vibrational spectral analysis was neglected. Such reports like a DFT calculation proposed a red shift for the CH2 wagging mode on the atop site and a blue shift on the short bridge site for ethylene adsorbed on Ag(110).46 Similarly, for 1,3-butadiene adsorbed on silver films, the blue shift of the wagging frequency was explained due to the molecule interacting with interfacial Ag ions or positively charged active sites through the π-bonded configuration.32 To our best knowledge, there is no theoretical study focusing on the Raman enhancement of the wagging vibrations until our recent studies reported.8,24 For p-aminothiophenol interacting with two silver clusters, DFT calculation showed that a relatively strong Raman peak at about 900 cm−1 was attributed to the NH2 wagging vibration coupled with the CH out-of-plane bending vibration of the benzene ring.8 This was further demonstrated in different aromatic amines adsorbed on noble metal clusters by theoretical investigation.24 For SERS of benzyl chloride reduced in a rough silver electrode, an abnormal intense Raman peak observed at the reduction potential was assigned to the wagging vibration of the adsorbed CH2 group based on DFT calculations.13 As mentioned above, it is necessary to clarify why the special NH2 and CH2 wagging vibrations show such significant frequency shifts and special SERS bands. This was interpreted partially because of the change of the hybridization effect from sp3 to sp2 during periodic vibrations of the wagging mode, however, the nonresonantly selective enhancement on the vibration seems to be a general feature for p−π conjugated molecules. All these lead us to do further systematical theoretical analysis on the wagging modes. In this work, the SERS of the wagging modes for aromatic amines and terminal olefins adsorbed on silver surfaces are investigated by using DFT calculations. Comparing with the experimental observations, we calculated the titled molecules interacting with neutral silver clusters and positively charged silver clusters. We further summarize the trend of the frequency shift and the Raman enhancement of the wagging vibration. The origin of the phenomena is analyzed in detail through inspecting the binding interaction, geometry deformation, polarizability tensor derivatives, and molecular orbital property.

Figure 1. Definition of wagging vibration modes.

standing on the SERS of the wagging mode for olefins and aniline derivatives adsorbed on metal surfaces, we addressed a brief review as follows. Different vibrational spectroscopy techniques have been used to characterize the adsorption state by concerning the ωCH2 mode. Compared with the wagging frequency in free terminal olefins, the ωCH2 SERS signals of adsorbed molecules on silver displayed different shifts dependent on surface coverage and surface adsorption sites.14−20,27−33 In previous studies, the blue shift is attributed to the surface adsorption sites with partially positively charged silver clusters.27,30−32 A red shift shoulder peak was also observed in the SERS spectra of ethylene adsorbed on silver films, while its assignment was often neglected though it generally displays a constant frequency. The SERS spectra of terminal olefin were first reported by Moskovits et al. for ethylene and propylene adsorbed on a Ag substrate.11,12 Three peaks were observed at 977, 955, and 917 cm−1 in the range of the wagging mode for ethylene adsorbed on the cold-deposited Ag film. Similar peaks at 971, 944, and 920 cm−1 were observed for ethylene matrices with a very low concentration of silver on a Ag substrate by Brings et al.10 The blue shifts for the ωCH2 mode were also observed for ethylene adsorbed on Ag surfaces precovered by chlorine or oxygen.27−29 In the case, the adsorption took place selectively at silver atoms on which a positive charge had been induced by the oxygen atom.34 In contrast to the blue-shift on silver, SERS of ethylene on copper and gold films measured by Otto et al. showed a large red shift of the ωCH2 mode.10,35−41 Similar changes of the wagging mode were observed for other terminal olefins adsorbed on Ag surfaces. The Raman spectrum of 1, 3-butadiene in solution containing silver ions showed that the wagging vibration shifted to a higher frequency by 25−44 cm−1.30−32 An intense and broad band was observed around 800 cm−1 in the SERS of benzyl chloride reduction at a silver electrode.13 The signal was assigned to the methylene wagging vibration of adsorbed benzyl radical and its anionic species.13 Besides, abnormal intense IR signals of the wagging vibration were also observed in propylene,14−17 acrolein, and acryonitrile18−20 adsorbed on low-index single crystal surfaces by surface enhanced infrared spectroscopy. For aromatic amine derivatives adsorbed on silver surfaces, an intense and broad SERS band can be observed at ∼950 cm−1. The band was not attributed to the NH2 wagging vibration in previous studies, but to the −NH2 rocking vibration in p-aminobenzoic (PABA) and p-aminobenzonitrile (PABN) adsorbed on silver electrodes.21−23 In their adsorption states, the p-position-substituted group of aniline should display strong adsorption binding to silver surfaces. For aniline, the NH2 wagging vibration is at around 660 cm−1, corresponding to the 0 → 3 transition from the double-well potential,42,43 and its harmonic fundamental was proposed to be 540 cm−1.50 While

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COMPUTATIONAL DETAILS To mimic the surface adsorption interaction of the p−π conjugated molecules containing the wagging vibration, we adopted a molecule−metallic cluster model. For aromatic amines, the model focuses on the interaction of the NH2 group binding to silver clusters. While for terminal olefins, there are two possible adsorption models, the weakly π-bonded 18892



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Figure 2. Optimized structures of molecules interacting with Ag4 cluster. (A) Ethylene, propylene, 1, 3-butadiene, acrolein and vinyl amine interact with metal via double bond. (B) p-Substituted aniline derivates interact with metal along the amino group approaching the surfaces. (C) Propylene free radical, propylene radical anion, benzyl radical, and benzyl radical anion interact with metal via methylene.

configuration and the di-σ-bonded configuration.47−51 The former was considered in this article because of that the πbonded adsorption on silver clusters is more stable thermodynamically than the latter one.46,47,51 The selected metallic clusters are not only to reflect the electronic structures of surface active sites but also to follow the general binding propensity, such as the energy close, the symmetry matching, and the maximum overlap. Recently, Metiu et al. reported that the desorption energy of propylene from neutral Ag4 cluster was larger than that from Agn (n = 1, 2, 3, 5).52 Our previous studies also showed a strong binding interaction between Ag4 and pyridine.26,53 Accordingly, we mainly chose the Ag4 cluster to describe the surface active site. Figure 2 presents the modeling structures of terminal olefins, p-substituted aniline derivates and radical molecules interacting with Ag4. Other silver clusters with the sizes at n = 2, 6, and 13 were also considered in this paper to study the influence of different adsorption sites on the vibrational spectra. Density functional calculations were carried out with hybrid exchange-correlation functional B3LYP.54,55 The basis sets for C, N, O, S, and H atoms of studied molecules were 6-311+G(d, p), which includes a polarization function to all five kinds of atoms and a diffuse function to C, N, O, and S atoms.56,57 For silver atom, electrons in the valence and inner shells were described by the basis set, LANL2DZ, and the corresponding relativistic effective core potentials, respectively.58,59 Full geometry optimizations and analytic frequency calculations were carried out by using Gaussian 09 package.60 For

comparison of theoretical and experimental vibrational frequencies, we chose a scaling factor of 0.981 for the calculated frequencies of the vibrational modes mentioned in this paper. Natural bond orbital (NBO) analysis61,62 has been used to investigate orbital bonding interactions when the wagging mode of adsorbed molecules moves along the normal coordinate. The molecule−Ag4 model is proper not only to describe molecular adsorption at active sites on rough silver surfaces but also to well predict vibrational spectra.24 The theoretical methods were used well to calculate the binding interaction, vibrational frequency, and Raman intensity in our previous work.25,26,53 Taking the solvent effect into account, we used the solute model of density (SMD) approach,63 which considered the nonelectrostatic terms and was recommended to well predict solvation Gibbs free energies (ΔG) of ions and molecules. We chose water with dielectric constant (ε = 78.3) as the solvent. The SMD model was also used to calculate vibrational spectra of molecule−Agn+ complexes. Since the wagging vibration is sensitive to surroundings, the hydrogen bonding interaction between the molecules containing NH2 has great effect on the ωNH2 vibration. Binding energies (BE) are estimated by subtracting the energies of molecule and silver clusters from the total energy BE = −(EMOL − M n − EMOL − E M n)

where n denotes the number of metal atoms in the silver cluster, EMOL−Mn, EAN, and EMOL denote the energies of the 18893



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complex, free molecule and metallic cluster, respectively. For making a correction from the full basis set associated with the entire system, the counterpoise correction of the basis set superposition error (BSSE) was considered in the binding energy in the present calculation.64,65 The correction decreases the overestimation of binding energies. Absolute Raman intensities are measured on top of the differential Raman scattering cross section from the Raman scattering factor, as published in our previous work.8 In this case, the Raman scattering factor (in Å4/amu) was calculated as ⎛ dα ⎞ 2 ⎛ dγ ⎞ 2 ⎟⎟ + 7⎜⎜ ⎟⎟ Si = 45⎜⎜ ⎝ dQ i ⎠ ⎝ dQ i ⎠

Table 1. Selected Parameters of Optimized Geometries, Bonding Energy (BE) and Wagging Frequency for Ethylene−Agn Complexes

C2H4− Ag2 C2H4− Ag4 C2H4− Ag6 C2H4− Ag13 a

(1)

C−Ag (Å)

C−C (Å)

BE (kcal/mol)

BEa (kcal/mol)

frequency (cm−1)

2.696

1.340

4.91

4.07

960/978

2.474

1.356

11.52

10.49

933/944

2.833

1.337

3.10

2.36

966/978

2.605

1.348

5.38

4.46

940/955

Considering BSSE correction.

where α̅ ′ = γ ′2 =

1 ′ + αyy ′ + αzz ′) (αxx 3

(2)

1 ′ − αyy ′ )2 + (αyy ′ − αzz ′ )2 + (αxx ′ − αzz ′ )2 {(αxx 2 ′ )2 + (αyz ′ )2 + (αxy ′ )2 ]} + 6[(αxz

(3)

HOMO π orbital matches well with the LUMO of Ag4 itself in symmetry and energy, which belongs to s-type orbital with the electron cloud mainly populated at the obtuse angle side of the rhombus. The NBO analysis showed the energy of the main orbital interaction is 9.08 kcal/mol. In previous studies, ethylene was proposed to chemically adsorb on metal surfaces in two different states, π or di-σ bonded states.47−51 The πbonded configuration is usually proposed the molecule adsorbed at a top site, while the di-σ bonding usually occurs at a short bridge site.46,66,67 The π-bonded configuration is preferentially considered as the stable one for the terminal olefins on metal surfaces, while for the di-σ bonded complexes, significant deformation of molecular geometries occurs.46,51,67 Therefore, we only consider the π-bonded configuration in the present paper. Table 2 lists the binding energies of different terminal olefins interacting with silver clusters. Our calculated results showed that the binding energies are less than 12 kcal/mol, in a fairly good agreement with the experimental values about 9−12 kcal/ mol reported in previous papers.14,17,68−71 This indicates that the adsorption interaction is weakly π-bonded chemisorption for ethylene derivatives on silver. Among these complexes, one can find that the vinylamine−Ag4 owns the largest BE value. This can be interpreted because of the p−π conjugation effect from the lone pair orbital in the amino group strongly conjugating with the π-orbital of the CC double bond. The charge transfer occurs from the lone pair orbital to the πorbital. The increase in the density of the π electron cloud results in the strengthening the interaction between the πorbital of olefin and the metallic orbital. On the contrary, for acrolein−Ag4 the aldehyde group is an electron-withdrawing group which decreases the electron density of the π-orbital on the CC bond. Thus the interaction is the weakest in acrolein−Ag4. The inversion angles of the CH2 group are also calculated, the out-of-plane bending angles less than 10 degrees demonstrate that these adsorbed molecules almost remain the sp2-hybridization. The process that involves charge transfer from a π-orbital of the adsorbate to an unoccupied metal levels leads to a decrease of the bond orders and an increase of the CC bond lengths. The CH2 wagging frequency of adsorbed olefins displays a red shift compared to that of the free molecules. Our calculation predicted that the Raman frequency of the ωCH2 mode has a red shift about 20 cm−1 for ethylene adsorbed at the neutral silver clusters. The large frequency shift is in accord with the observation in experimental Raman spectra.10,12 For example, for ethylene adsorbed on Ag films, a band at 917 cm−1 was observed in the SERS with a red shift about 32 cm−1

The polarizability derivative α′ represents (∂α/∂Q)0 in Gaussian output just provide the values for molecule at the equilibrium position in Cartesian coordinates. For investigating the relationship of the polarizability derivatives and the geometry, we calculated the variation of the six polarizability tensor components along with the normal coordinate of the wagging vibration in the molecule and molecule-metal complexes. Each value of the normal coordinate corresponds to a molecular structure in Cartesian coordinates. Thus we can understand easily that how the change of different polarizability tensor elements with the variation of geometric structures, electronic property and molecular orbital interaction in the process of the molecular vibration along the wagging mode. Then the derivatives (∂α/∂Q)0 are calculated from the slope of each polarizability tensor components along with the wagging coordinate at the equilibrium position. The normal coordinate was defined according to the mass weight coordinates, 3N

Qi =

∑( i

mi Δxi)2 (4)

where Δxi and mi represent the vibrational displacement vectors in Cartesian coordinates and the atomic mass of the ith atom, respectively. To make direct comparison with the SERS experiments, the simulated Raman spectra were presented in terms of the Lorentzian expansion with a line width of 10 cm−1 and an excitation wavelength of 514.5 nm was used.

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RESULTS AND DISCUSSION Terminal Olefins. The simplest terminal olefin ethylene has been considered as a prototype for the interaction of olefins with metal surfaces. The π-bonded property is preferred to the molecule interacting with silver clusters. Table 1 presents the binding energies of ethylene interacting with different silver clusters. Obviously, the binding energy of ethylene−Ag4 is the largest among different complexes. This indicates that the tetramer silver Ag4 can describe well the surface active site on silver surfaces. When ethylene interacts with Ag4 through the πbonded form, the main orbital interaction is between the HOMO orbital of ethylene and the LUMO orbital of Ag4. The 18894



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Table 2. Inversion Angle (α,degree), CC Bond Length (Å), Calculated Vibrational Frequency (ω, cm−1), Raman Activity (SR, Å4/amu), and Bonding Energies (BE) of Molecule−Ag4 (kcal/mol) species

α

C2H4 C2H4−Ag4

5.9

1.356

2.473

C3H6 C3H6−Ag4

6.8

1.331 1.355

2.473

C4H6−Ag4 C3H4Ot C3H4Ot-Ag4 C2H3NH2 C2H3NH2−Ag4(C−Ag) C2H3NH2−Ag4(N−Ag)

ω

C(N)−Ag

1.329

C4H6

a

CC

1.338 0 8.4 9.1 0 37.1 16.1 48.5 0

1.338 1.363 1.335 1.370 1.334

2.456

1.364 1.326

2.424 2.394

2.417

SR 958 956 944 933 925 923

2.58 0 66.09 24.21 2.88 74.56

921 920 932 901 979 915 813 (CH2) 544 (NH2) 808 (CH2) 901 (NH2) 874 (CH2)

0 14.13 22.28 439.20 2.02 141.71 8.39 3.86 64.87 58.86 2.12

BEa

BEb

BE (exptl)

11.52

10.49

9.56,34 8.7668

11.07

9.78

8.9−10.117 10.8−12.614

10.23

8.85

9.6−10.869

9.77

8.26

14.07 12.48

12.87 11.23

≤8.3770,71

Bonding energy in gas phase. bConsidering BSSE correction.

active sites of silver. The ωCH2 vibration has a blue shift to around 1000 cm−1. The smaller silver clusters used, the larger frequency shift is found for the wagging mode. The calculated binding energies for the four complexes decrease in an order as 32.76, 18.25, 16.95, and 9.08 kcal/mol respectively. For the smaller silver clusters, the binding energies are significantly larger than that of the corresponding neutral complexes. This is in accord with the experimental result that ethylene adsorbed on a precovered atomic oxygen silver surface27 and Raman spectra of 1,3-butadiene with silver ion.30,32 Table 3 lists selected geometric parameters, binding energies and wagging frequencies of the four structures C2H4−Agn+ (n =

compared to that in free molecule.12 Under very low concentration of silver, a band appeared at 920 cm−1.10 For propylene adsorbed on Ag surfaces, the ωCH2 band was observed at 909 cm−1 in the IR spectra, slightly lower than the frequency of 912 cm−1 in free molecule.14,17 This is in agreement with our calculated value at 910 cm−1 for propylene interacting with silver clusters. Similar phenomena were observed for 1-butene and styrene adsorbed on Ag surfaces.72,73 For 1,3-butadiene, which is the simplest conjugated alkene containing two wagging modes, our calculated results showed that the wagging frequency of the methylene close to the silver cluster decreases as the CC bond length increases. At the same time, the wagging frequency of the other methylene increases as the CC bond length shortens. Accordingly, we inferred that the wagging frequencies depend on the variation of the CC bond distances in olefin-neutral silver cluster complexes. Figure 3 presents simulated Raman spectra of ethylene binding to cationic Agn+ clusters (n = 1, 3, 7, and 13), in analogy to the case that ethylene adsorbs on positively charged

Table 3. CC Bond Length (Å), C−Ag+ Bond Length (Å), Calculated Vibrational Frequency (ω, cm−1), Bonding Energies (BE) of Molecule−Agn+(kcal/mol) species

CC

C−Ag+

frequencies

BEa

BEb

+

1.353

2.447

11.94

11.60

C2H4Ag3+

1.344

2.569

7.14

6.53

C2H4Ag7+

1.351

2.514

9.50

8.46

C2H4Ag13+

1.353

2.524

992 987 987 982 962 961 937 936

4.87

3.95

C2H4Ag

a b

Bonding energy in aqueous solution with the SMD model. Considering BSSE correction.

1, 3, 7, and 13) with the SMD model. The calculated results showed that the solvent effect significantly decreases the binding energies, which are 11.60, 6.53, 8.46, and 3.95 kcal/mol for the four complexes respectively. This is mainly due to the reduction of the electrostatic interaction from the solvent screening effect. After considering the solvent effect, there is a small difference in the BE values between C2H4−Ag4 and C2H4−Ag4+, suggesting that the two surface species may coexist on Ag surfaces. This is why different wagging peaks can be observed on silver films. Especially for C2H4−Ag13+, the BE is

Figure 3. Simulated Raman spectra of ethylene interacted with cationic Ag clusters (Ag+, Ag3+, Ag7+, and Ag13+). An incident wavelength of 514.5 nm and a line with of 10 cm−1 were used in the simulated Raman spectra. 18895



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Table 4. Inversion Angle (a, degree), C−NH2 Bond Length (Å), N−Ag Bond Length (Å), Vibrational Frequency(ω, cm−1), Raman Activity (SR, Å4/amu), and Bonding Energies (BE) of Molecule−Metal (kcal/mol) Calculated at the B3LYP/6-311+G** Levela species

a (deg)

C−N

C6H4NH2OCH3 C6H4NH2OCH3−Ag4 C6H4NH2CH3 C6H4NH2CH3−Ag4 C6H5NH2 C6H5NH2−Ag4 C6H4NH2Cl C6H4NH2Cl−Ag4 C6H4NH2Br C6H4NH2Br−Ag4 C6H4NH2CO2CH3 C6H4NH2CO2CH3−Ag4 C6H4NH2CO2H C6H4NH2CO2H−Ag4 C6H4NH2CN C6H4NH2CN-Ag4 C6H4NH2NO2 C6H4NH2NO2−Ag4

42.53 51.60 40.52 50.95 39.15 50.55 38.41 50.14 38.04 50.06 33.11 48.99 32.25 48.63 31.87 48.18 28.11 47.40

1.406 1.438 1.401 1.435 1.398 1.433 1.396 1.431 1.395 1.430 1.386 1.425 1.384 1.424 1.383 1.422 1.378 1.419

N−Ag 2.375 2.380 2.385 2.393 2.394 2.402 2.407 2.414 2.423

ω

SR

(dσ/dΩ)i

611 951 581 944 564 934 548 917 541 915 448 907 441 902 435 894 385 885

4.40 130.29 6.31 93.81 6.88 101.04 3.40 72.15 3.33 78.59 17.32 170.95 15.30 152.37 11.39 105.72 25.13 145.78

0.56 9.43 0.85 6.86 0.97 7.49 0.50 5.47 0.49 5.97 3.31 13.14 2.99 11.80 2.27 8.28 5.95 11.55

BEb

BEc

13.93

12.33

13.38

11.79

12.84

11.28

12.10

10.51

12.01

10.44

11.17

9.58

10.77

9.18

10.19

8.61

9.45

7.86

The differential Raman scattering cross section (10−30cm2/mol·sr) is calculated from the Raman scattering factors. bBonding energy in gas phase. c Considering BSSE correction. a

only 3.95 kcal/mol. The corresponding wagging frequency lowers to 937 cm−1. This is analogous to ethylene adsorbed on neutral silver surfaces, indicating that the solvent effect significantly influences the binding interaction between C2H4 and Ag13+. The blue shift frequency for ethylene adsorbed on small cationic silver clusters can be interpreted due to the small back-donation from the occupied orbital of the silver cluster to the antibonding orbital of the CC double bond. The NBO analysis indicates that the population in the antibonding orbital for C2H4−Ag4+ is ∼0.04e, which is obviously smaller than ∼0.12e in neutral complexes. Aniline and Its p-Substituted Derivatives. The previous study has investigated the effect of different adsorbate− substrate interaction on the SERS through typical doublefunctional aromatic amines.24 It is found that the amino group approaching a metal surface will exhibit unique Raman signals though the interaction between the para-substituted functional groups and metal clusters displays a strong binding interaction. Thus we pay our attention to the configuration of the molecules interacting with silver clusters via only amino group here. Table 4 presents binding energies, structural parameters, scaled frequencies, and Raman activities of aniline (AN) and its para-substituted derivatives through the NH2 group interacting with Ag4, including p-methoxyaniline (PMOA), p-methylaniline (PMA), p-chloroaniline (PCA), p-bromoaniline (PBA), paminobenzoic acid methyl ester (PABM), p-aminobenzoic acid (PABA), p-aminobenzonitrile (PABN), and p-nitroaniline (PNA). The binding energies are closely associated with the property of para-substituted functional groups. The electronwithdrawing (EW) groups decrease the charge distribution at the lone pair orbital of the NH2 group and weaken the binding interaction. In contrast, the electron-donating (ED) groups increase the charge distribution at the LP orbital of the NH2 group and strengthen the binding interaction. Figure 4 presents that the levels of the highest occupied molecular orbital (HOMO) become lower with the stronger ability of the

Figure 4. Energy levels (ranging from −11.63 to 0 eV) of molecular orbitals for aniline and p-substituted derivatives of aniline. HOMOs are indicated with asterisks. The LOMO of Ag4 is marked with red dotted line.

attracting electron substituent. So the energy gaps between the electron donor (HOMO of aromatic compounds) and the electron acceptor (LUMO of Ag4) are larger for the EW group substituted compounds than the ED-substituted compounds. This accounts for the binding energy increasing with the donor ability and decreasing with the withdrawing ability of parasubstituted functional groups. The BE values vary in a descending order: PMOA > PMA > AN > PCA > PBA > PABM > PABA > PABN > PNA. As shown in Figure 5A, the ωNH2 vibrational frequency strongly depends on the binding interaction between the amino group and metals. The wagging frequency exhibits a near linear relationship on the binding energies. In the EW-substituted compounds, the wagging frequencies are lower than that of aniline. On the contrary, the frequency increases in the ED substituted compounds. This is because the EW group enhances the p−π conjugation effect between the amino group and the benzene ring, while the ED group weakens it. The p−π conjugation interaction would further change the hybridization of the amino group. In the EW(ED)-substituted compounds the amino group is inclined to sp2 (sp3) 18896



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Figure 5. Dependence of amino wagging frequencies on bonding energy (A), N−Ag force constant (B), inversion angle (C), C−N bond length (black), and N−Ag bond length (red) (D) in Molecule−Ag4 complexes. The dotted line connecting two points presents the same molecule.

hybridization. Simultaneously, the inversion angle listed in Table 4 can reflect the extent of the sp3 hybridization. Figure 5B shows a near linear relationship of the wagging frequency on the inversion angle in free molecules and their corresponding silver complexes. This clearly shows that the wagging frequency increases as the extent of sp3 hybridization becomes large. Figure 5C and D show variations of the wagging frequencies with the force constants of the N−Ag bond and the C−N bond in different complexes. The substituent groups affect the C−N bond lengths in free molecules increasing as the enhancement of the electron donating ability of the substituent. When the N−Ag bond length becomes stronger, in agreement with increasing the N−Ag force constant, the wagging frequency increases and the C−N bond length becomes longer. Figure 6 presents simulated Raman spectra of aniline−Agn+ (n = 3, 7, and 13) complexes to describe aniline adsorbed on silver electrodes. Compared with aniline interacting with neutral silver clusters, the ωNH2 vibrational frequency further blue-shifted to about 1000 cm−1 in aniline−Agn+. The band is responsible for a wide band near 993 cm−1 on a silver electrode.21 It is noted that as the size of the silver clusters increases, the simulated Raman spectra are close to the SERS spectra. This indicates that the positively charged silver clusters with a large size are more appropriate to describe the Ag electrode surface. Evidence is given that SERS activity at Ag electrodes are associated with Ag+, most of the SERSgenerating adsorbates happen to be those capable of forming Ag+ complexes in water.74 After that Roy et al. reported that the “active sites” are actually small clusters which on Ag electrodes

Figure 6. Simulated Raman spectra of aniline adsorbed on cationic Ag clusters (Ag3+, Ag7+, Ag13+) in the gas phase. An incident wavelength of 514.5 nm and a line with of 10 cm−1 were used in the simulated Raman spectra.

have been identified as Ag4+,75 a recent study suggested that the active site should be Ag32+ on the basis of analysis of the SERS of pyridine in colloidal silver solution.76 Our calculated results indicated that some small positively charged silver clusters play important role on the SERS band of the wagging mode in aniline derivatives on silver surfaces. Benzyl Radicals. Table 5 presents calculated results of propylene free radical, propylene radical anion, benzyl radical, and benzyl radical anion interacting with silver clusters. For free propylene free radical and its anion, two C−C bonds are equivalent in chemical property. With regards to propylene free radical adsorbed on silver clusters, the C−C bond close to the 18897



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Table 5. Inversion Angle (a, Degree), C−C Bond Length (Å), C−Ag Bond Length (Å), Calculated Vibrational Frequency (ω, cm−1), Raman Activity (SR, Å4/amu), Scattering Cross Section (10−30 cm2/mol·sr), and Bonding Energies (BE) of Molecule− Metal (kcal/mol) species

a

C3H5 C3H5−Ag4 C3H5− C3H5−−Ag4 C6H5CH2 C6H5CH2−Ag4 C6H5CH2− C6H5CH2 −−Ag4 a

C−C

C−Ag

1.384 0 23.3 44.7 2.0 36.6 46.4

1.377 1.420 1.394 1.457 1.352 1.405 1.454 1.391 1.465

2.321

2.239

2.255 2.239

ω

SR

(dσ/dΩ)i

803 778 801 768 412 896 772 698 831 416 899

0.13 8.51 72.18 91.15 9.06 3354.86 10.03 6.84 3582.68 153.18 3879.01

0.05 3.34 27.20 36.29 8.36 1094.10 3.97 0.73 307.47 32.56 301.66

BEa

BEb

13.24

11.88

57.91

56.51

14.64

13.03

52.78

51.27

Bonding energy in gas phase. bConsidering BSSE correction.

silver cluster stretches. This causes a weakening C−C bond and a red shift in the wagging frequency. In the case, the CH2 inversion angle is predicted to be 23.3 degrees, indicating that its hybridization changes from sp2 to sp3 for the CH2 group binding to the silver cluster. The calculated results show that the decrease in the CH2 wagging frequency is due to the C−C stretching force constant rather than the change of the hybridization. When propylene free radical anion binds to silver clusters, the C−C bond close to the silver cluster stretches significantly, and the CH2 inversion angle increases to 44.7 degrees. Similar to the propylene free radical, the CH2 group with the sp3 hybridization has a large wagging frequency. Another C−C bond strengthens its double bond feature. When benzyl radical and its anion interact with silver clusters via the CH2 group, the inversion angles are 36.6 and 46.6 degrees, respectively. At the same time, the wagging frequencies increase dramatically. The wagging frequency is closely associated with the hybridization property. In the complexes of terminal olefins with silver clusters, the inversion angle of CH2 group has a slight change so that the hybridization of the CH2 group keeps at the sp2 state. In the case, the wagging frequency changes with the strength of the CC bond. However, for propylene and benzyl radicals binding to silver clusters, the frequency shift is mainly determined by the hybridization property of the CH2 group, similar to the case of aniline derivatives. Chemical Enhancement Effect. Comparing the calculated Raman intensity for free molecules and their silver clusters complexes (see Tables 2, 4, and 5), it is obvious that the intensity of the wagging mode has a great enhancement after the molecules adsorbed on silver surfaces. Especially for aromatic anilines and benzyl radicals, the Raman intensity is significantly enhanced by about 10−500-fold. Here our purpose is to make clear the cause of the Raman enhancement of the wagging mode. Figure 7 presents the variation of six polarizability tensor components along the wagging mode in aniline−Ag4 complexes. The six derivative values can be directly got from the curves in Figure 7. The Raman scattering factor calculated from Gaussian09 program can be reproduced completely by estimating these data from eq 2. Table 6 summarizes the related data of polarizability derivatives and Raman intensities of aniline, benzyl radical, benzyl radical anion and their silver-cluster complexes in the same way. It can be ′ value is quite larger than other five found that the αxx derivatives in aniline and the aniline-silver complex. So the

Figure 7. Polarizability components (unit in bohr3) along the amino wagging vibration in aniline−Ag4.

significant enhancement of Raman intensity mainly comes from the derivative αxx ′ . For benzyl and its anion, the values of αxx ′, α′yy, and α′zz are zero because of molecular planar configurations. In contrast, when they bind to silver clusters, the α′xx values increase significantly. So we can assume that the change of polarizability component αxx is the decisive factor for the chemical enhancement in Raman intensity of the wagging mode from the binding interaction. Figure 8 presents the polarizability tensor components αxx, αyy, and αzz varying along with the wagging vibration in these molecule−silver cluster complexes. Here, we mainly discuss the αxx relevant to the variation of hybridization property. From Figure 8, we can see that in the three complexes the αxx is larger in the sp2 hybridized state than that in the sp3 one. It just corresponds to the direction of the wagging vibration in standard orientation coordinate (see Figure 8). The previous study showed most of the molecular polarizability was contributed from the electrons in the outer valence orbitals and the higher energy orbitals are highly polarizable.77 Therefore, the polarizability significantly varies at the frontier molecular orbitals in the silver cluster complexes upon the wagging mode vibrating periodically. To understand the change of the polarizability tensor, we compared the energy levels of frontier occupied orbitals in different complexes. Figure 9 presents the variation of the MO energies ranging from LUMO+1 to some low occupied orbitals of the silver cluster complexes along the wagging coordinate. Here, we represented the second and the third HOMOs as the HOMO− 1 and HOMO−2, respectively. The bonding orbitals were HOMO−1 and HOMO−2 in AN−Ag4, the HOMO−1 in 18898



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Table 6. Six Derivatives of Polarizability Components αxx ′ , αyx ′ , αyy ′ , αzx ′ , αzy ′ , and αzz ′ , Isotropous Polarizability (Å2/amu1/2), Anisotropic Polarizability, and Raman Scattering Factor (SR, Å4/amu) of Wagging Mode for Aniline, Benzyl, and Their Metallic Complexes species

α′xx

α′yx

α′yy

α′zx

α′zy

α′zz

C6NH2 C6NH2−Ag4 C6CH2· C6CH2·-Ag4 C6CH2− C6CH2−-Ag4

0.75 2.83 0.00 16.84 0.00 −17.66

0.00 0.00 0.00 1.25 0.00 1.06

−0.13 −0.94 0.00 2.46 0.00 0.60

−0.10 0.02 0.57 0.00 −0.03 −0.35

0.00 0.00 0.00 0.00 2.73 0.15

0.04 0.18 0.00 0.77 0.00 −1.00

α̅ ′ 0.22 0.69 0.00 6.69 0.00 −6.02

γ′2

SR

0.68 11.26 0.97 238.72 22.29 310.71

6.88 100.39 6.76 3686.34 156.01 3806.80

Figure 8. Molecule−silver clusters complexes in their rectangular coordinates and dependence of the polarizability tensor components αxx, αyy, and αzz (unit in bohr3) on normal coordinate of wagging modes in aniline, benzyl radical, benzyl radical anion, and their silver complexes. The perpendicular red dotted lines denote plane structures of molecule parts.

change from sp3 to sp2. From Figure 9B, there is a cross point of an orbital energies, indicating the orbital interaction existing. In [benzyl−Ag4]−, when the CH2 group changes the hybridization from sp3 to sp2 with the wagging vibration, the HOMO−1 from the lone pair orbital interacting with the Ag4 drastically increases in the orbital energy. In the case, the order of the frontier orbital levels changes, especially the electron configuration of Ag4 varying in a large extent. As shown in Figure 9C, the orbital interaction works in a similar way contributing to the increase in the αxx polarizability in benzyl− Ag4. It was found that the polarizability of the silver cluster plays a vital role in contributing to the total polarizability of the complexes. So the change of the electron configuration is the primary cause in increasing polarizability when the Ag4 interacts with the lone pair orbital.

benzyl anion−Ag4 and benzyl−Ag4. It is obvious that for AN− Ag4 the HOMO−1 and HOMO−2 energy levels gradually raised as the hybridization changes from sp3 to sp2. For benzyl anion−Ag4, the energy level of the bonding orbital changes in order, raised from HOMO−1 to HOMO as the hybridization changes from sp3 to sp2. For benzyl−Ag4, the HOMO−1 energy level also gradually raised as the hybridization changes from sp3 to sp2. The lone pair electron delocalization effect and the increase of the orbital energy cause the enlargement of the orbital polarizabilities. The conjugation effect is strong in the sp2 configuration but becomes weak in the sp3 configuration. The strong adsorption interaction results in the decrease of the lone pair orbital energy and the orbital polarizability. As seen in Figure 9A, the energies of HOMO−1 and HOMO−2 orbitals for AN−Ag4 gradually raised when the hybridization of aniline 18899



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ethylene or aniline adsorbed on different silver clusters. So we conclude that the band positions of both ωCH2 and ωNH2 modes are sensitive to the adsorption sites. This leads to the observed broad band from a superposition of the SERS wagging peaks of different adsorption states. The factor should interpret the broad band observed in olefins, aniline, and benzyl adsorbed on silver surfaces. The second factor arises from the coupling of intramolecular vibrational modes of adsorbed molecules. The reason is mainly aimed at aromatic anilines. Table 7 presents scaled frequencies, Table 7. Scaled Frequencies (ω, cm−1), Raman Activity (SR, Å4/amu), and Assignments of Selected Vibrational Modes in Aniline-Ag4 Calculated by B3LYP Method C6H5NH2− Ag4 basis set

vibration mode

ω

SR

Q25

934

83.61

Q24

897

32.29

Q26 Q25

965 929

113.62 50.04

Q24

895

57.96

6-311+G**

Q25 Q24

934 880

101.04 22.28

6-311+ +G(3d,3p)

Q25

932

113.11

Q24

878

19.33

aug-cc-PVDZ

Q25 Q24

920 861

108.37 19.48

aug-cc-PVTZ

Q25

933

90.33

Q24

887

32.76

Q25

932

85.62

Q24

885

28.72

6-31G

6-31G* 6-31+G

aug-cc-PVQZ

Figure 9. Variation of molecular orbital energies (in atomic units) for aniline−Ag4 (A), benzyl radical anion−Ag4 (B), benzyl radical−Ag4 (C) along the normal coordinates. The lowest unoccupied molecular orbitals (LUMO) and the highest occupied molecular orbitals (HOMO) are indicated with red marks. The plots of HOMO−1 or HOMO−2 orbitals are showed in the curves. Zero value of normal coordinate corresponds to the equilibrium configurations.

a

PED (%)a ωNH2(46), γC2H(14), γC6H(14), γC4H(12) ωNH2(33), γC4H(24), γC2H(20), γC6H(20) ωNH2(65) ωNH2(35), γC2H(19), γC6H(19), γC4H(14) ωNH2(42), γC4H(19), γC2H(16), γC6H(16) ωNH2(66) γC2H(30), γC6H(30), γC4H(29), ωNH2(12) ωNH2(66) γC2H(31), γC6H(31), γC4H(28), ωNH2(12) ωNH2(66) γC2H(30), γC6H(30), γC4H(26), ωNH2(12) ωNH2(58), γC2H(9), γC6H(9) γC2H(27), γC6H(27), γC4H(26), ωNH2(20) ωNH2(58), γC2H(9), γC6H(9) γC2H(27), γC6H(27), γC4H(27), ωNH2(19)

ω, wagging; γ, out-of-plane.

Raman activities, and the vibrational analysis of aniline−Ag4 calculated by different basis sets. On the basis of normal-mode analysis of the optimized structures and the force constants calculated at the different theoretical approaches, the PED values presented in Table 7 for vibrational fundamentals are related to the wagging coordinates. The results show that the ωNH2 vibration strongly coupled with the out-of-plane C−H bending calculated by using the larger basis sets. These vibrations also display large Raman intensities because of the mixture of the wagging coordinate. For the modes with higher frequencies, the PED values of the amino wagging coordinate are dominant, indicating that this mode mainly comes from the amino wagging vibration. The modes of the lower frequency calculated by large basis sets can be attributed to a mixed vibration of the amino wagging coordinate and the out-of-plane bendings of the C4−H, C6−H, and C2−H bonds. This is another reason why the NH2 wagging band is very broad observed in SERS experiments. The third factor comes from congeneric species adsorbed on different surface sites. A superposition of the spectra of the

Reason of Broad Bands. It is an interesting problem that why the vibrational signal of the wagging mode is very broad in observed SERS spectra. This is generally interpreted before because of the interaction of adsorbed molecules with metal surfaces. On the basis of our calculated results, we can summarize as the three factors. The first factor is the different adsorption states contribute to the origin of the broad band. Surface-enhanced infrared spectroscopy in the ωCH2 vibration region were measured for ethylene, propylene, and acrylonitrile adsorbed on oxygen precovered Ag, which exhibits a broad features from at least four adsorbed states on surfaces.15,27,28,33 In this paper, we have simulated the Raman spectra of molecules adsorbed on silver clusters with different sizes and sites. As shown in Table 3, Figures 3 and 6, the peak position of the wagging mode changes strongly dependent on the adsorption sites when 18900



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National Basic Research Programs (No. 2009CB930703). D.Y.W is grateful for the support from Xiamen University (2010121020).

benzyl moieties, including benzyl radical and benzyl anion, shows that the overlap of the bands affords a broad peak between 850 and 900 cm−1.13 Our calculated spectra showed that there are the intense Raman bands of the ωCH2 mode at 831 and 899 cm−1 for adsorbed free benzyl radical and benzyl anion, respectively. This further showed the broad band from surface congeneric species as observed in the SERS experiment.13 So we can make a conclusion that the broad band may come from the contribution of the three different factors.

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(1) Haynes, C. L.; McFarland, A. D.; Van Duyne, R. P. SurfaceEnhanced Raman Spectroscopy. Anal. Chem. 2005, 77 (17), 338−346. (2) Stiles, P. L.; Dieringer, J. A.; Shah, N. C.; Van Duyne, R. P. Surface-Enhanced Raman Spectroscopy. Annu. Rev. Anal. Chem. 2008, 1, 601−626. (3) Wu, D. Y.; Li, J. F.; Ren, B.; Tian, Z. Q. Electrochemical SurfaceEnhanced Raman Spectroscopy of Nanostructures. Chem. Soc. Rev. 2008, 37 (5), 1025−1041. (4) Osawa, M. Dynamic Processes in Electrochemical Reactions Studied by Surface-Enhanced Infrared Absorption Spectroscopy (SEIRAS). Bull. Chem. Soc. Jpn. 1997, 70 (12), 2861−2880. (5) Aroca, R. F.; Ross, D. J.; Domingo, C. Surface-Enhanced Infrared Spectroscopy. Appl. Spectrosc. 2004, 58, 324−338. (6) Tian, Z. Q.; Ren, B. Adsorption and Reaction at Electrochemical Interfaces as Probed by Surface-Enhanced Raman Spectroscopy. Annu. Rev. Phys. Chem. 2004, 55, 197−229. (7) Tian, Z. Q.; Ren, B.; Wu, D. Y. Surface-Enhanced Raman Scattering: From Noble to Transition Metals and From Rough Surfaces to Ordered Nanostructures. J. Phys. Chem. B 2002, 106 (37), 9463−9483. (8) Wu, D. Y.; Liu, X. M.; Huang, Y. F.; Ren, B.; Xu, X.; Tian, Z. Q. Surface Catalytic Coupling Reaction of p-Mercaptoaniline Linking to Silver Nanostructures Responsible for Abnormal SERS Enhancement: a DFT Study. J. Phys. Chem. C 2009, 113 (42), 18212−18222. (9) Huang, Y. F.; Zhu, H. P.; Liu, G. K.; Wu, D. Y.; Ren, B.; Tian, Z. Q. When the Signal is Not From the Original Molecule to be Detected: Chemical Transformation of Para-Aminothiophenol on Ag During the SERS Measurement. J. Am. Chem. Soc. 2010, 132 (27), 9244−9246. (10) Brings, R.; Mrozek, I.; Otto, A. Relationship Between Resonant Raman-Scattering in Ag-C2H4 Complexes and SERS. J. Raman Spectrosc. 1991, 22 (2), 119−124. (11) Manzel, K.; Schulze, W.; Moskvits, M. Surface-Enhanced Raman Spectra of C2H2 and C2H4 Adsorbed on Silver Colloid. Chem. Phys. Lett. 1982, 85 (2), 183−186. (12) Moskovits, M.; Dilellla, D. P. Enhanced Raman Spectra of Ethylene And Propylene Adsorbed on Silver. Chem. Phys. Lett. 1980, 73 (3), 500−505. (13) Wang, A.; Huang, Y. F.; Sur, U. K.; Wu, D. Y.; Ren, B.; Rondinini, S.; Amatore, C.; Tian, Z. Q. In Situ Identification of Intermediates of Benzyl Chloride Reduction at a Silver Electrode by SERS Coupled With DFT Calculations. J. Am. Chem. Soc. 2010, 132 (28), 9534−9536. (14) Pawela-Crew, J.; Madix, R. J. Anomalous Effects of Weak Chemisorption on Desorption Kinetics of Alkenes: The Desorption of Propylene and Propane from Ag(110). J. Chem. Phys. 1996, 104 (4), 1699−1708. (15) Huang, W.; White, J. Propene Oxidation on Ag (111): Spectroscopic Evidence of Facile Abstraction of Methyl Hydrogen. Catal. Lett. 2002, 84 (3), 143−146. (16) Huang, W.; White, J. Transition From π-Bonded to di-σ Metallacyclic Propene on O-Modified Ag (111). Langmuir 2002, 18 (25), 9622−9624. (17) Huang, W. X.; White, J. M. Propene Adsorption on Ag(111): A TPD and RAIRS Study. Surf. Sci. 2002, 513 (2), 399−404. (18) Akita, M.; Osaka, N.; Itoh, K. Infrared Reflection Absorption Spectroscopic Study on Adsorption Structures of Acrolein on Polycrystalline Gold and Au(111) Surfaces Under Ultra-high Vacuum Conditions. Surf. Sci. 1998, 405 (2−3), 172−181. (19) Fujii, S.; Misono, Y.; Itoh, K. Surface-Enhanced RamanScattering Study on Temperature-Induced Adsorption Structure Change of Acrolein on Coldly Evaporated Silver Filmes. Surf. Science 1992, 277 (1−2), 220−228.

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CONCLUSION We have investigated the SERS of the wagging mode for terminal olefins, p-substituted anilines and benzyl radicals adsorbed on silver surfaces. Our theoretical analysis provides further interpretation for three aspects of the SERS feature of the wagging mode by using DFT calculations. First, we focus on the frequency shift. For the terminal olefins interacting with surfaces through the π-bonded form, we inferred that the redshift peaks of the wagging mode come from the molecules adsorbed on neutral silver atoms of the surfaces, while the blueshift peaks from the positively charged silver sites. For aromatic amines adsorbed on metal surfaces with the NH2 group, the frequency shifts of the wagging mode strongly depend on the hybridized property of the NH2 group. The results show the amino wagging frequency increases with the inversion angle increasing. Second, we explained the reason why the Raman intensity enhancement of the wagging mode is enhanced significantly. It is concluded that the significant change of the polarizability component αxx is a decisive factor contributing to the chemical enhancement after adsorption. The derivative of polarizability component is obviously larger than the other five ones. In molecule−silver complexes the αxx is larger when the molecular moiety is sp2 hybridized, and the value sharply decreases as the molecular moiety changing to a sp3 hybridization. This change is mainly affected by the variation of the electronic structure for adsorbed molecules and the silver cluster moiety. Finally, we accounted for the cause of the broad band in observed SERS spectra. Our calculated results show that the wagging vibration is very sensitive to the environment factors. The broad band may come from the contribution of molecules adsorbed on several different surface adsorption sites, the coupling of vibrational modes, and the coadsorbed congeneric species. Furthermore, similar to aniline and benzyl radical in free states, the wagging vibration displays very significant anharmonicity on silver surfaces as mentioned before. It is possible to observe the overtone and combination transitions contribution to the broad band from the wagging vibration and the related vibrational coupling. In summary, the spectral characteristic of the wagging vibration in different surface species is an interesting phenomenon in surface-enhanced Raman spectroscopy.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support of this work by the NSF of China (Nos. 21021002, 91027009, and 20973143) and 18901



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