ARTICLE pubs.acs.org/JPCC
Conformations of Morpholine in Liquid and Adsorbed on Gold Nanoparticles Explored by Raman Spectroscopy and Theoretical Calculations Min Xie, Guichi Zhu, Yongjun Hu,* and Huaimin Gu MOE Key Laboratory of Laser Life Science & Institute of Laser Life Science, College of Biophotonics, South China Normal University, Guangzhou 510631, China
bS Supporting Information ABSTRACT: Morpholine is a typical six-membered saturated heterocycle with the molecular formula HN(CH2CH2)2O. In this work, the conformations of Morpholine in liquid and adsorbed on the surface of gold nanoparticles were studied by means of Raman spectroscopy and theoretical calculations. Ab initio calculations indicate that the energy of the chair conformers of Morpholine is ca. 7.5 kcal/mol lower than the skewboat conformers, which implies that the chair conformers would be favorable in liquid Morpholine. Comparison of the observed Raman spectra of liquid Morpholine, its solution, and the predicted spectra of the chair conformers (equatorial and axial) revealed that both of the chair conformers coexist in its liquid, and the content of the equatorialchair conformer may reduce in the solution. Considering the concentration-dependent Surface-Enhanced Raman Scattering (SERS) spectral profile, the surface selection rule, and the theoretical calculations, it has been inferred that at higher concentrations Morpholine is vertically chemisorbed on gold nanoparticles through the N atom of the ring, and the dominant conformation adsorbed is the axial-chair conformer. However, at the dilute concentrations, Morpholine is gradually flatly chemisorbed on the gold nanoparticles through the N atom, and the Morpholine may be deprotonated. Furthermore, the predicted spectra agree with the experimental ones very well, which confirms the results above.
1. INTRODUCTION Morpholine is a typical six-membered saturated heterocycle with the molecular formula HN(CH2CH2)2O. Due to the nucleophilic nitrogen lone pair, Morpholine often has been used as a chemical precursor or catalyst in many organic syntheses.1 Substituted Morpholines display anesthetic properties, and the Morpholine derivative, Amorolfine, is found to be used as an antifungal drug.1 Furthermore, the Morpholine ring structure is an important heterocycle present in many compounds of biological and pharmaceutical relevance.2 However, to our knowledge, no studies with regard to the conformational equilibrium and the concentration-dependent SERS of Morpholine have been preformed. Surface-Enhanced Raman Scattering (SERS) is a highly effective technique which is intensely enhanced by the analyte molecules adsorbed onto the rough metal surfaces or nanometersized metallic particles.3 5 It can provide information on the molecules adsorbed on the surface of silver, gold, or other noble metals, e.g., the adsorption orientation of molecules and the interaction mechanism of the molecules with the surfaces of substrates.5 7 Among the noble metals, gold nanoparticles are the most stable metal nanoparticles and better control the particle size and shape. Moreover, the gold nanoparticles usually have been used in various fields, such as material science, supramolecular chemisty, biology, catalysis, and nanotechnology.8 Theoretical calculations, especially the density functional theory (DFT) and Møller Plesset second-order perturbation r 2011 American Chemical Society
theory (MP2) calculations, usually have been used to optimize the structures and predict the energies and frequencies of molecules,9,10 ions,11 clusters,12 14 and molecule metal complexes.15,16 Recently, Raman spectroscopy combined with Quantum Chemical Calculations was applied to distinguish molecular conformers/ rotamers,17 to explore the conformational equilibrium in the condensed state,18 21 to assign the vibrational bands, and to interpret the adsorption orientation and interaction mechanism of the molecules absorbed on the noble metal surface.22 25 In this work, we first explore the conformational equilibrium of liquid Morpholine by Raman spectroscopy combined with the theoretical calculations (DFT/MP2). Then, the adsorption orientation and the interaction mechanism of Morpholine adsorbed on gold nanoparticles in different concentrations (10 1 10 8 M) have been investigated by the concentration-dependent SERS and theoretical calculations.
2. METHODS 2.1. Chemicals and Proceduce. Liquid Morpholine (>98% pure) was purchased from Aladdin-reagent company (Shanghai, China) and used as received. The gold nanoparticles were prepared according to Frens’ method26 and have been used in our previous Received: July 11, 2011 Revised: September 4, 2011 Published: September 14, 2011 20596
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Table 1. Geometrical Parameters and the Values of the Energy Difference ΔE Relative to the Equatorial-Chair Conformer of the Four Conformers of Morpholinea chair eq.
skew-boat ax.
eq.
ax.
bond lengths (Å)
Figure 1. Molecular structure of Morpholine and its four conformers. 27
work. In a typical process, 100 mL of 1 mM HAuCl4 aqueous solution was heated to boiling with vigorous stirring. Next, 5 mL of freshly prepared 38.8 mM trisodium citrate solution was added, and the solution color changed from colorless to deep red. The mixture was then kept boiling for 20 min. Finally, the solution was cooled to room temperature with continuous stirring. The resulting red gold nanoparticles were obtained with the diameter at ca. 40 nm. Different concentrations of Morpholine were added in the solution of gold nanoparticles. The final concentrations of Morpholine are 10 1 to 10 8 M. All aqueous solutions were prepared using ultrapure water (g18 MΩ, Elga water purification system, ELGA). 2.2. Instrumentation. The Raman spectra were measured with a microscopic Raman spectrometer (Nippon Optical System Co., Japan). The excitation source was from a He Ne laser (Melles Griot, U.S.) tuned at 632.8 nm with a power of 1 mW at the sample location. The typical accumulation time used in this study was 60 s. The concentration-dependent SERS spectra of Morpholine were carried out with the concentrations from 10 1 to 10 8 M. 2.3. Computational Details. The calculations of geometry optimization, harmonic frequencies, and Raman scattering activities of Morpholine molecules were performed with the Gaussian 03 package.28 Two calculation methods (MP2/B3LYP) have been used in the present study. The MP2 method, which accounts for the electron electron correlation, can obtain more reliable results for correlation energies than the B3LYP method.29 Therefore, the geometry optimization and correlation energies of Morpholine were predicted by MP2/aug-cc-pvdz. However, compared to the MP2 method, the B3LYP method is inexpensive, and the prediction of normal-mode frequencies is reliable.29,30 Thus, the geometries and the harmonic frequencies of the Morpholine conformers were optimized at the B3LYP/aug-ccpvdz level. In addition, the LANL2DZ basis set was used for the gold atoms in the case of calculating the Morpholine Au complexes. The Raman intensities of Morpholine are derived from the calculated Raman scattering activities of the normal modes as in our previous work.31
3. RESULTS AND DISCUSSION 3.1. Conformational Equilibrium in Liquid Morpholine. Morpholine, as a six-membered saturated heterocycle molecule, is well-known to have various conformations from the chairskew-boat and equatorial axial equilibria of its ring to the different possible orientations of the N H group. Figure 1 shows the chemical structure (with atom labeling) and four conformations of the Morpholine molecule.
C1 C10
1.525
1.529
1.537
1.523
C4 C7
1.525
1.529
1.539
1.527
C1 N
1.471
1.473
1.466
1.480
C4 N
1.471
1.473
1.480
1.481
C10 O C7 O
1.435 1.435
1.435 1.435
1.442 1.428
1.442 1.442
N H14
1.020
1.024
1.019
1.019
bond angles (degree) C O C
109.88
110.04
110.71
112.69
C N C
109.48
109.72
110.50
111.56
C1 N H14
109.70
108.55
110.39
109.12
C4 N H14
109.70
108.55
109.88
109.18
dihedral angles (degree) O C10 C1 N O C7 C4 N
59.63 59.63
55.97 55.97
37.12 35.19
67.73 61.74
C10 C1 N H14
179.23
67.72
169.23
154.73
C7 C4 N H14
179.23
67.72
153.66
95.27
ΔE
relative energies (kcal 3 mol 1) 0.00 1.04 8.01
7.53
a
Numeration of atoms is provided in Figure 1. The data are predicted at the MP2/aug-cc-pvdz level.
The geometrical parameters and the values of the energy deference ΔE of the four conformers of Morpholine (relative to the equatorial-chair conformer) are shown in Table 1, which are obtained from MP2/aug-cc-pvdz calculations. The four conformers in Figure 1 are considered as local minimum-energy conformations for the absence of imaginary harmonic frequencies. As shown in Table 1, the chair conformers are much more symmetric and stable (by ca. 7.5 kcal/mol) than the skew-boat conformers, which indicates the chair conformers could be favorable in the liquid Morpholine. However, the energy difference between two chair conformers (axial and equatorial) is only ca. 1 kcal/mol. Furthermore, the previous works1,32 identified that the boat conformers have no contributions to the IR spectrum and microwave spectrum of Morpholine, which indicated that the two chair conformers are the dominant conformations in the liquid. Thus, only the two chair conformers are considered in this paper. Figure 2 shows the observed Raman spectra of liquid Morpholine and its solution (1 M) and the calculated spectra of the two chair conformers in the ranges of 100 1800 and 2600 3400 cm 1. Wavenumbers of all the experimental Raman bands and the calculated Raman bands of the chair conformers are also listed in Table 2, together with the rough assignments of the bands. The assignments were done by the assistance of the Gauss-view program and the calculated potential energy distribution (PED). The frequency values were given without applying any scaling factor in the range of 100 1800 cm 1, and the scaling factor 0.959 was applied in the range of 2600 3400 cm 1.33 20597
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Figure 2. Raman spectra of (a) Morpholine, (b) its solution (1 M), and (c and d) calculated chair conformers in the ranges of 100 1800 and 2600 3400 cm 1.
As is shown in Figure 2 and Table 2, most of the Raman bands of the predicted conformers agree with the experimental ones very well. In the Raman spectrum of liquid Morpholine, it is found that the band at 1015 cm 1 corresponds only to the C C in-plane stretching of the axial-chair conformer, and the band at 1037 cm 1 is assigned to the C C in-plane stretching and N H out-of-plane bending of the equatorial-chair conformer. The bands attributed to the N H stretching of the axial conformer and equatorial conformer can be seen at ca. 3301 and 3338 cm 1, respectively. The results above reveal that both of the chair conformers may coexist in the conformational equilibrium of the liquid. Furthermore, the theoretical calculations reveal that the equatorial-chair conformer is more stable, and thus it may be more favorable in the liquid. By comparing the spectra of liquid Morpholine and its solution (1 M), most of the bands are related to C H vibrations and are blue-shifted in the solution (see Table 2). Particularly, the band related to C H stretching is blue-shifted by about 15 cm 1. The results imply that an interaction has occurred between Morpholine and water. The blue shifts suggest that the interaction between Morpholine molecules in the liquid is stronger than that between Morpholine and water in the solution. Additionally, only one N H stretching band can be seen at 3327 cm 1 in the Raman spectrum of Morpholine solution. On the basis of the previous analysis, this band can be attributed to the N H stretching of the axial conformer which has been seen at 3301 cm 1 in the Raman spectrum of pure Morpholine. It also can be found that the intensity of the band at 1037 cm 1, which corresponds to the equatorial-chair conformer, is decreased in the spectrum of the diluted Morpholine. Calculated results reveal that the energy difference between the axial and equatorial chair conformer in the SCRF-PCM solvation model is only ca. 0.6 kcal/mol, which implies the equatorial-chair conformer would not be dominant in the solution. The ratio of two conformers is thus different between those in the liquid Morpholine and its solution, and the content of the equatorial-chair conformer in the liquid may be reduced by dilution. 3.2. Concentration Dependence of SERS Spectra. The Raman spectrum of liquid Morpholine and the SERS spectra of Morpholine adsorbed on gold nanoparticles recorded at different concentrations (10 1 to 10 4 M) are presented in Figure 3. It is found that
the features of SERS spectra of Morpholine with the concentrations from 10 3 to 10 8 M are approximately the same (see Figure S1 of the Supporting Information). However, it can be seen that SERS spectra of Morpholine adsorbed on the surface of gold nanoparticles undergo remarkable variations in the two concentration ranges (i.e., 10 1 to 10 2 M and 10 3 to 10 8 M). The wavenumbers of SERS spectra of Morpholine at two concentrations (10 1 and 10 4 M) are presented in Table 2. In Figure 3 and Table 2, it can be seen that a new intense band at ca. 210 cm 1 appears in all of the SERS spectra, which can be assigned to Au N stretching.34 In addition, the new band at 636 cm 1 which attributed to C C N in-plane scissoring and another new band at 718 cm 1 which is related to C1 N C4 stretching appear in 10 1 and 10 4 concentration SERS spectra, respectively, and all of these new bands above are related to the N atoms. The appearance of those bands in the SERS spectra indicates that Morpholine may be adsorbed on the surface of gold nanoparticles through the N atom of the ring. As we know, there are two possibilities of the adsorption of Morpholine on the gold surface, physisorption and chemisorption. In the case of physisorption, the bands of the SERS spectra are almost the same as that of the free molecules with only slight changes in the bandwidths, while in the case of chemisorption the position of Raman bands and their intensities are dramatically changed.25,35 As we can see in Table 2, the positions of most bands, such as the bands at 420, 479, 833, 914, 1066, 1130, and 1309 cm 1, have changed in 10 1 and 10 4 concentration SERS spectra. The band at 420 cm 1 assigned to C1 N C4 and C7 O C10 out-of-plane bending is broadened in its bandwidth and red shifts to 372 cm 1 in the SERS spectra. The band at 914 cm 1 which attributed to C1 N C4 and C7 O C10 symmetric stretching is enhanced and red shifts to 874 cm 1. However, the 1130 cm 1 band corresponding to C C in-plane rocking and N H out-of-plane bending is greatly enhanced and blue shifts to 1176 cm 1. These bands above, which are greatly enhanced or shifted in frequency, are related to the N atom of the ring. That suggests that Morpholine may be chemisorbed on the surface of gold nanoparticles through the N atom of the ring, though the physisorption cannot be excluded. 3.2.1. Adsorption Orientation at Higher Concentrations. A previous SERS study showed that the orientation of the molecules 20598
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Table 2. Wavenumbers of the Experimental Normal Raman, SERS Bands, and the Corresponding Predicted (B3LYP/aug-ccpvdz) Raman Spectra of the Chair Conformers, with the Rough Assignments of the Bands NRS pure
SERS 1M
10
1
M
206(vs)
chair conformers 10
4
M
eq.
ν(Au N)
211(vs)
274(vw)
assignmenta
ax.
262
258
α(C1 N C4, C7 O C10)
420(m) 442(vw)
430(m)
372(s) 443(w)
372(m) 445(vw)
414 442
401 442
α(C1 N C4, C7 O C10) β(C1 N C4, C7 O C10)
479(m)
479(m)
493(s)
493(m)
481
488
δ(C10 C1 N, C7 C4 N,
586
595
δ(C10 C1 N, C7 C4 N) ip
785
779
ν(C1 N, C4 N)
841
836
ν(C1 C10, C4 C7, C7 O, C10 O) ip
912
906
ν(C1 N, C4 N, C7 O, C10 O)
1017
ax. ν(C1 C10, C4 C7) ip eq. ν(C1 C10, C4 C7) ip + α(N H)
(C4 C7 O, C1 C10 O) ip 636(m) 718(m) 833(vs)
827(vs)
914(w) 1015(m) 1037(m)
824(s) 874(s)
873(m)
1015(m) 1037(m)
1012(vs)
1012(s)
1040(m)
1066(w)
1066(w)
1098(m)
1090(m)
1130(w)
1135(w)
1039 1072
1045
γ(C1, C4) ip γ(C1, C4, C7, C10) op
1080 1112(w)
1113
1128
ν(C7 O, C10 O)
1176(vs)
1136
1159
γ(C1, C4, C7, C10) ip + α(N H)
1150(vw)
1158(vw)
1162
1149
ν(C1 N, C4 N)
1202(s)
1207(s)
1212
1202
τ(C1H2, C4H2, C7H2, C10H2)
1277(w) 1309(s)
1285(w) 1311(s)
1286 1317
1305
eq.τ(C1H2, C4H2) τ(C7H2, C10H2)
1303(s)
1298(m)
1357(w) 1386(w) 1390(vw)
1390(vw)
1447(s)
1450(s)
1450(s) 1610(w) 2120(vs)
2741(w)
2754(w)
2776(w)
2793(w)
2847(vs)
2866(vs)
1332
1328
τ(C1H2, C4H2)
1359
1362
ω(C7H2, C10H2)
1376
ax. ω(C1H2, C4H2)
1389
1395
ω(C7H2, C10H2)
1468
1455
1379(s) 1438(s)
δ(C1H2, C4H2)
1534(m)
?
1610(w) 2122(s)
? δ(H O H) water bending mode ? ν(Au H) ν(C H) ν(C H)
2870(vs)
2915(vw)
2931(w)
2920(vs)
2956(vs)
2974(vs)
2960(m)
3301(w) 3338(w)
3327(w)
2819(vs)
2819
2847(vs)
(2870)
2917(vs) 2947 3382
2838
ν(C H)
2907
ν(C H)
2960
ν(C H)
3340
ax. ν(N H) eq. ν(N H)
a
The assignments were done by the assistance of the Gauss-view program and PED calculation results. Abbreviations used: vs, very strong; s, strong; m, medium; w, weak; vw, very weak; α, out-of-plane bending; β, in-plane bending; ν, stretching; δ, scissoring; γ, rocking; ω, wagging; τ, twisting; ip, in-plane; op, out-of-plane.
adsorbed on the noble metal surface could be estimated on the basis of the surface selection rule, as predicted by Moskovits36,37 and Creighton.38 According to this rule, the vibrational mode with the transition moment perpendicular to the surface should have a large enhancement in Raman signal.25 Thus, for a molecule adsorbed flat on the metal surface, its out-of-plane vibrational modes will be more enhanced compared with its in-plane vibrational modes, and its in-plane vibrational modes will be more enhanced compared with its out-of-plane vibrational modes when it is adsorbed perpendicularly to the surface.23,39,40 As shown in Table 2 and Figure 3, at higher concentrations (10 1 and 10 2 M), the intensities of the ring in-plane deformation bands have been
enhanced. Especially, the C C in-plane stretching band of axialchair Morpholine at ca. 1015 cm 1 is greatly enhanced. The band at 479 cm 1, which attributed to C C N and C C O in-plane scissoring, is intense and blue shifts to 493 cm 1 in the higher concentration SERS spectra. These phenomena indicate that the Morpholine vertically adsorbed on the gold surface is dominant at higher concentrations (10 1 and 10 2 M). As mentioned above, the C C in-plane stretching band of axial-chair Morpholine at ca. 1015 cm 1 is greatly enhanced. However, the intensity of the band at 1037 cm 1, which is related to the C C in-plane stretching and N H out-of-plane bending of the equatorial conformer, is gradually reduced when concentrations 20599
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Figure 3. Raman spectrum of Morpholine and the SERS spectra of Morpholine adsorbed on gold nanoparticles recorded at different concentrations (10 1 to 10 4 M).
are decreased and finally disappeared at low concentrations. The band at ca. 1277 cm 1 corresponding to C1H2 and C4H2 twisting of the equatorial-chair conformer is absent in the SERS spectra. At the higher concentrations, if the equatorial-chair Morpholine is vertically adsorbed on the gold nanoparticles, the N H stretching would be greatly enhanced. However, the N H stretching bands could not been observed in the higher concentration Raman spectra (see Figure 3). Thus, these indicate that the axial-chair conformer would be dominant on the surface at the higher concentrations (10 1 and 10 2 M). In addition, the SERS effect is also sensitive to the distance between the adsorbed molecule and the substrate. As a consequence, the vibrations close to the surface can be enhanced more than the further vibrations.23,41,42 As shown in the 10 1 and 10 2 M SERS spectra of Table 2 and Figure 3, the band at 1066 cm 1, which is related to C1 and C4 in-plane rocking, is enhanced and red shifts to 1040 cm 1. The new intense SERS band, which corresponded to C1H2 and C4H2 wagging of the axial-chair conformer, appears at ca. 1380 cm 1, and the intense SERS band, which is assigned to C1H2 and C4H2 scissoring, appears at ca. 1450 cm 1. In contrast, the bands at ca. 1112, 1357, and 1390 cm 1, which corresponded to C O stretching and C7H2 and C10H2 wagging, are very weak or disappear in the SERS spectra. We note that C1 and C4 are close to the N atom and that C7 and C10 are close to the O atom. Those results above confirm the axial-chair Morpholine vertically adsorbed on the surface of gold nanoparticles through the N atom of the ring instead of the O atom at the higher concentrations. 3.2.2. Adsorption Orientation at Low Concentrations. At the lower concentrations, the bands at 493, 718, 873, 1012, 1298, 1379, 1438, 1534, and 1610 cm 1 can be observed in the spectra. The enhanced bands at 1298, 1379, and 1438 cm 1 correspond to the CH2 vibrations of the ring. Both of the enhanced bands at 372 and 718 cm 1 are related to out-of-plane vibrations. In addition, the most intense band at ca. 833 cm 1 in the pure liquid, which is related to the C C and C O in-plane stretching mode, is absent in the low concentration SERS spectra. Furthermore, the intensity of the band at 1176 cm 1 which is related to
C C in-plane rocking is gradually reduced when concentrations are decreased. These results suggest Morpholine may gradually lie down on the surface of gold nanoparticles by dilution. As we know, in the case of the axial-chair conformer flatly adsorbed on the surface of gold nanoparticles, the N H stretching band (at ca. 3301 cm 1) should be greatly enhanced. However, no band appears in that range of the low concentration SERS spectra. Thus, the Morpholine, which flatly adsorbed on the surface of gold nanoparticles, mainly could be the equatorialchair conformer when the analyte molecule concentrations are diluted, while in the case of the equatorial-chair conformer flatly adsorbed on the surface of gold nanoparticles, the band relative to N H out-of-plane bending which intensely appears at ca. 1176 cm 1 at the higher concentrations should be greatly enhanced. Nevertheless, as we can see in Figure 3 and Figure S1 (Supporting Information), the intensity of the band at 1176 cm 1 is gradually decreased and finally disappeared in the low concentration SERS spectra. In addition, the SERS band at 1037 cm 1 which is assigned to the C C in-plane stretching and N H out-of-plane bending of the equatorial-chair conformer cannot be found in the low concentration SERS spectra. Note that all of the bands (at ca. 1037, 1176, and 3301 cm 1) relative to the N H group were absent in the low concentration SERS spectra of Morpholine. The results imply that a deprotonated reaction may occur while the Morpholine is adsorbed on the gold surface at the low concentrations. Similar results have been observed in previous works.43,44 However, two bands at ca. 1534 and 1610 cm 1 can be seen in the SERS spectra of the Morpholine at the low concentrations, which also has been observed by Joo and co-workers.45,46 What is more, both of the bands cannot be observed in the Raman spectra of Morpholine liquid and the predicted spectra of any Morpholine Au complexes. We agree with Joo’s perspective that the band at ca. 1534 cm 1 could be related to the six-membered saturated heterocycle binding to the gold surface. According to the previous work,47 the weak water metal interactions would lead to a blue shift of the bending mode of water (at ca. 1595 cm 1 in the gas phase). Thus, the band at ca. 1610 cm 1 20600
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Figure 4. SERS spectra of (a) Morpholine at 10 1 M, (b) the calculated Morpholine Au complex, and (c) the calculated (Morpholine Au) 2H2O complex.
Figure 5. SERS spectra of (a) Morpholine at 10 4 M, (b) the calculated equatorial-chair Morpholine Au complex, and (c) the calculated deprotonated Morpholine Au complex.
may be attributed to the bending mode of water which is influenced by the gold surface.48 In addition, a very intense band at ca. 2120 cm 1 can be seen in all the SERS spectra of the Morpholine (see Figure S1 of the Supporting Information). This band may be related to the Au H stretching.48 51 3.2.3. Theoretical Calculations on Morpholine Au Complexes. To have insight into the orientation of the molecules adsorbed on gold nanoparticles, the comparisons have been performed between the experimental Raman spectrum and simulated Raman spectra of Morpholine Au complexes. Figure 4 shows the experimental SERS spectrum of Morpholine at 10 1 M and the simulated SERS spectra of Morpholine Au and (Morpholine Au) 2H2O complexes. In Figure 4, the calculated SERS spectra were obtained by the axial-chair conformer Morpholine vertically adsorbed on the gold nanoparticle, and the (Morpholine Au) 2H2O is the complex which has considered the interactions between the Morpholine and water at higher concentrations. The distances of the Au N band of the two complexes in Figure 4 are 2.39 and 2.36 Å, respectively. As we can see in Figure 4, most of the bands in the predicted SERS spectra of the (Morpholine Au) 2H2O complex (Figure 4c) fit those in the experimental SERS spectra (10 1 M) (Figure 4a) better than that of the Morpholine Au complex (Figure 4b). In Figure 4, there is only a weak band at ca. 931 cm 1 in the experimental spectrum. The band at 931 cm 1 (marked by the asterisk), which is related to N H bending, is intense in the SERS spectrum of the Morpholine Au complex. However, no band appears in the range of the spectrum of the (Morpholine Au) 2H2O complex. It indicates that the main conformer in the higher concentration may be the (Morpholine Au) 2H2O complex, and water plays an important role in the solution of Morpholine even at the higher concentration SERS spectra. What is more, it further confirmed that the dominant conformation vertically adsorbed on the surface of gold nanoparticles is the axial-chair conformer at higher concentrations. The experimental SERS spectra of Morpholine at 10 4 M and the simulated SERS spectra of the equatorial-chair conformer Morpholine Au and deprotonated Morpholine Au complexes have been presented in Figure 5. Considering the solvation shell
would be formed surrounding the (deprotonated) Morpholine molecules in the low concentration solutions, both equatorialchair conformer Morpholine Au and deprotonated Morpholine-Au complexes are calculated in the SCRF-PCM solvation model. The calculated SERS spectra were obtained by the Morpholine flatly adsorbed on the gold nanoparticle. The distances of the Au N band of these two complexes are 2.10 and 2.05 Å, respectively. In Figure 5, it is found that the bands in the predicted SERS spectra of the deprotonated Morpholine Au complex (Figure 5c) fit those in the experimental SERS spectra (10 4 M) (Figure 5a) better than that of the equatorial-chair conformer Morpholine Au complex (Figure 5b). As we have mentioned above, the band at 1176 cm 1 which is related to N H bending is gradually decreased and finally disappeared in the low concentration SERS spectra (see Figure 3), while the band which is related to N H bending intensely appears at 1200 cm 1 in the predicted spectrum of the equatorial-chair conformer Morpholine Au complex (see Figure 5b). However, no band appears in that range of the deprotonated Morpholine Au complex’s spectrum (see Figure 5c). These confirmed that Morpholine may be deprotonated on the surface of the nanoparticles at the low concentrations, and this deprotonated Morpholine would gradually lie down on the surface of gold nanoparticles while the concentration is decreased.
4. CONCLUSION Conformations of Morpholine in liquid and adsorbed on the surface of gold nanoparticles were explored by means of Raman spectroscopy and theoretical calculations. By theoretical calculations, it has been found that the chair conformers of Morpholine are much more stable than skew-boat conformers. Combined with Raman spectroscopy, the appearance of the bands at 1015 and 1037 cm 1 indicates that both of the chair conformers exist in the conformational equilibrium of liquid Morpholine. Concentration-dependent SERS of Morpholine adsorbed on the surface of gold nanoparticles have been measured. The appearances of the new bands are related to N atom vibration 20601
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The Journal of Physical Chemistry C in concentration-dependent SERS spectra, indicating that Morpholine is adsorbed on the gold nanoparticles through the N atom of the ring. At the higher concentrations, the intensities of in-plane vibrational bands are enhanced largely, and their wavenumbers are found shifted. The results suggest that Morpholine is vertically chemisorbed on gold nanoparticles and that the dominant conformation adsorbed on the surface is the axial-chair conformer. However, at dilute concentrations, the intensities of in-plane vibrational bands become weaker, and the intensities of out-of-plane bands are stronger, which indicates that Morpholine is gradually flat on the gold nanoparticles. The experimental results also reveal that Morpholine may be deprotonated at the low concentrations. The theoretical calculations further confirmed the results above.
’ ASSOCIATED CONTENT
bS
Supporting Information. The normal Raman spectrum and SERS spectra measured at different concentrations (Figure S1). This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: 86-20-85217070. Fax: 86-20-85216052. E-mail: yjhu@ scnu.edu.cn.
’ ACKNOWLEDGMENT This work has been supported by NSFC (No. 20973067, 11079020) and Guangdong-NSF (No. 7005823) grants, the scientific research foundation for the returned overseas Chinese scholars, State Education Ministry, the foundation for introduction of talents by the universities in Guangdong Province, and the project under scientific and technological planning by Guangzhou City. The authors appreciate Mr. Liubin Zhao of the Xiamen University for the PED calculations. ’ REFERENCES (1) Oliver, T. A. A.; King, G. A.; Ashfold, M. N. R. Chem. Sci. 2010, 1, 89–96. (2) Assaf, G.; Cansell, G.; Critcher, D.; Field, S.; Hayes, S.; Mathew, S.; Pettman, A. Tetrahedron Lett. 2010, 51, 5048–5051. (3) Moskovits, M. Rev. Mod. Phys. 1985, 57, 783–826. (4) Daniels, J. K.; Chumanov, G. J. Phys. Chem. B 2005, 109, 17936–17942. (5) Biswas, N.; Kapoor, S.; Mahal, H. S.; Mukherjee, T. Chem. Phys. Lett. 2007, 444, 338–345. (6) Corredor, C.; Teslova, T.; Ca~namares, M. V.; Chen, Z. G.; Zhang, J.; Lombardi, J. R.; Leona, M. Vib. Spectrosc. 2009, 49, 190–195. (7) Cardini, G.; Muniz-Miranda, M. J. Phys. Chem. B 2002, 106, 6875–6880. (8) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293–346. (9) Sapic, I. M.; Bistricic, L.; Volovsek, V.; Dananic, V.; Furic, K. Spectrochim. Acta A 2009, 72, 833–840. (10) Guirgis, G. A.; Mazzone, P. M.; Pasko, D. N.; Klaeboe, P.; Horn, A.; Nielsen, C. J. J. Raman Spectrosc. 2007, 38, 1159–1173. (11) Umebayashi, Y.; Fujimori, T.; Sukizaki, T.; Asada, M.; Fujii, K.; Kanzaki, R.; Ishiguro, S. J. Phys. Chem. A 2005, 109, 8976–8982. (12) Yao, J.; Im, H. S.; Foltin, M.; Bernstein, E. R. J. Phys. Chem. A 2000, 104, 6197–6211. (13) Lin, K.; Zhou, X. G.; Luo, Y.; Liu, S. l. J. Phys. Chem. B 2010, 114, 3567–3573.
ARTICLE
(14) Shirhatti, P. R.; Wategaonkar, S. Phys. Chem. Chem. Phys. 2010, 12, 6650–6659. (15) Pakiari, A. H.; Jamshidi, Z. J. Phys. Chem. A 2007, 111, 4391–4396. (16) Wu, D. Y.; Liu, X. M.; Huang, Y. F.; Ren, B.; Xu, X.; Tian, Z. Q. J. Phys. Chem. C 2009, 113, 18212–18222. (17) Balabin, R. M. J. Phys. Chem. A 2009, 113, 1012–1019. (18) Fujii, K.; Fujimori, T.; Takamuku, T.; Kanzaki, R.; Umebayashi, Y.; Ishiguro, S. J. Phys. Chem. B 2006, 110, 8179–8183. (19) Fujii, K.; Fujimori, T.; Takamuku, T.; Kanzaki, R.; Umebayashi, Y.; Ishiguro, S. J. Phys. Chem. B 2007, 111, 12829–12833. (20) Asada, M.; Mitsugi, T.; Fujii, K.; Kanzaki, R.; Umebayashi, Y.; Ishiguro, S. J. Mol. Liq. 2007, 136, 138–146. (21) Fujimori, T.; Fujii, K.; Kanzaki, R.; Chiba, K.; Yamamoto, H.; Umebayashi, Y.; Ishiguro, S. J. Mol. Liq. 2007, 131 132, 216–224. (22) Hu, G. S.; Feng, Z. C.; Han, D.; Li, J.; Jia, G. Q.; Shi, J. Y.; Li, C. J. Phys. Chem. B 2007, 111, 8632–8637. (23) Hu, G. S.; Feng, Z. C.; Li, J.; Jia, G. Q.; Han, D. F.; Liu, Z. M.; Li, C. J. Phys. Chem. B 2007, 111, 11267–11274. (24) Hu, G. S.; Han, D. F.; Jia, G. Q.; Chen, T.; Feng, Z. C.; Li, C. J. Raman Spectrosc. 2009, 40, 387–393. (25) Biswas, N.; Thomas, S.; Sarkar, A.; Mukherjee, T.; Kapoor, S. J. Phys. Chem. C 2009, 113, 7091–7100. (26) Frens, G. Nat. Phys. Sci. 1973, 241, 20–22. (27) Zhu, G. C.; Hu, Y. J.; Gao, J.; Zhong, L. Anal. Chim. Acta 2011, 697, 61–66. (28) 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. et al. Gaussian 03, revision B.05; Gaussian, Inc.: Pittsburgh, PA, 2004. (29) Shirhatti, P. R.; Wategaonkar, S. Phys. Chem. Chem. Phys. 2010, 12, 6650–6659. (30) M€uller-Dethlefs, K.; Hobza, P. Chem. Rev. 2000, 100, 143–168. (31) Xie, M.; Qi, Y. J.; Hu, Y. J. J. Phys. Chem. A 2011, 115, 3060–3067. (32) Capparelli, A. L.; Mara non, J.; Sorarrain, O. M.; Filgueira, R. R. J. Mol. Struct. 1974, 23, 145–151. (33) Veerman, A.; Lee, H. M.; Kim, K. S. J. Chem. Phys. 2005, 123, 084321. (34) Jang, N. H. Bull. Korean Chem. Soc. 2002, 23, 12. (35) Campion, A.; Kambhampati, P. Chem. Soc. Rev. 1998, 27, 241–250. (36) Moskovits, M. J. Chem. Phys. 1982, 77, 4408–4417. (37) Moskovits, M.; Suh, J. S. J. Am. Chem. Soc. 1985, 107, 6826–6829. (38) Creighton, J, A, Surf. Sci. 1983, 124, 209–219. (39) Gao, X.; Davies, J. P.; Weaver, M. J. J. Phys. Chem. 1990, 94, 6858–6864. (40) Creighton, J. A. Spectroscopy of Surfaces Advances in Spectroscopy; John Wiley & Sons: New York, 1988. (41) Tsen, M.; Sun, L. Anal. Chim. Acta 1995, 307, 333–340. (42) Cotton, T. M.; Uphaus, R. A.; Mobius, D. J. Phys. Chem. 1986, 90, 6071–6073. (43) Wrzosek, B.; Bukowska, J. J. Phys. Chem. C 2007, 111, 17397– 17403. (44) Geese, B.; McNaughton, D. J. Phys. Chem. B 2002, 106, 1461–1470. (45) Joo, S.-W.; Kang, W.; Oh, J. J. J. Raman Spectrosc. 2008, 39, 1444–1449. (46) Kang, H.; Noh, J.; Ganbold, E.-O.; Uuriintuya, D.; Gong, M.-S.; Oh, J. J.; Joo, S.-W. J. Colloid Interface Sci. 2009, 336, 648–653. (47) Morgenstern, K.; Nieminen, J. Phys. Rev. Lett. 2002, 88, 066102. (48) Li, J. F.; Huang, Y. F.; Duan, S.; Pang, R.; Wu, D. Y.; Ren, B.; Xu, X.; Tian, Z. Q. Phys. Chem. Chem. Phys. 2010, 12, 2493–2502. (49) Zhai, H. J.; Kiran, B; Wang, L. S. J. Chem. Phys. 2004, 121, 8231. (50) Andrews, L.; Wang, X. F.; Manceron, L.; Balasubramanian, K. J. Phys. Chem. A 2004, 108, 2936–2940. (51) Li, J.; Liang, J.; Korzeniewski, C.; Pons, S. Langmuir 1987, 3, 470–476. 20602
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