Roles of Metal Complex and Hydrogen Bond in Molecular Structures

metal ions (Ag+, Zn2+, Ca2+, Cd2+, Ni2+, and La3+) at their intrinsic pH values are investigated by use of. FTIR spectroscopy. The results indicate th...
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J. Phys. Chem. B 2000, 104, 10047-10052

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Roles of Metal Complex and Hydrogen Bond in Molecular Structures and Phase Behaviors of Metal N-Octadecanoyl-L-alaninate Langmuir-Blodgett Films Xuezhong Du† and Yingqiu Liang* Institute of Mesoscopic Solid State Chemistry and State Key Laboratory of Coordination Chemistry, Nanjing UniVersity, Nanjing 210093, People’s Republic of China ReceiVed: June 2, 2000; In Final Form: August 10, 2000

The N-octadecanoyl-L-alanine Langmuir-Blodgett (LB) films deposited from aqueous subphases containing metal ions (Ag+, Zn2+, Ca2+, Cd2+, Ni2+, and La3+) at their intrinsic pH values are investigated by use of FTIR spectroscopy. The results indicate that hydrogen bond and metal complex play an important role in the molecular structures and phase behaviors of the metal complex LB films. The condensing effects of Ag+ and Zn2+ ions give rise to an increase in the intermolecular hydrogen-bonding interactions between adjacent molecules. A preference of chelating bidentate is formed between these metal ions and carboxylate groups. The hydrocarbon chains in silver and zinc complex LB films take a biaxial orientation. The two LB films exhibit the thermal behavior of “glass transition”, which results from a gradual thermal dissociation of the enhanced intermolecular hydrogen-bonding interactions over a wide temperature range. While, the expanding effects of Ca2+, Cd2+, Ni2+, and La3+ ions lead to the occurrence of intramolecular hydrogen bond. A monodentate/unsymmetric ionic coordination is formed between these metal ions and carboxylate groups. The hydrocarbon chains in these LB films are uniaxially oriented at the angle 40-50° with regard to the respective film normals. These LB films show the phase behavior of “thermotropic liquid crystal” more than one phase process. It is regarded that the five-membered ring structures formed through intramolecular hydrogen bonds may have the similar effects to the mesogenic units in usual liquid crystals.

Introduction The studies of chirality-dependent intermolecular forces in two-dimensional self-assemblies are of tremendous importance in many biological processes. Emphasis has been centered upon the investigations of monolayers consisting of N-acyl amino acid derivatives since amino acids are known to play an important role in membrane sections.1-4 Fluorescence microscopy provides an efficient method to visualize chiral discrimination effects in the morphologies of amphiphilic monolayers if chiral symmetry breaking is manifested in the shape of micrometer-sized domains of the ordered phase curving in either direction or showing dendritic growth.5 However, these observations are confined to macroscopic scales and can determine neither molecular characteristics such as the orientation order of hydrocarbon chains nor the structure of headgroups of the film-forming compounds.2,3 FTIR spectroscopy appears to be suitable for the investigation of orientation order and molecular structure of well-defined organization by the LB technique.6 The N-octadecanoyl-L-alanine monolayers at the air-water interface and the corresponding LB films have been recently studied in detail,7-9 where the enantiomeric molecules assemble regularly through an extended intermolecular hydrogen-bonding network, and twist from neighbor to neighbor to develop chirality of the aggregate in the two-dimensional crystalline array.7,8 The chiral effects and intermolecular hydrogen-bonding interactions between hydrophilic headgroups correspondingly strengthen the interactions between hydrophobic chains, so that * To whom correspondence should be addressed. † Current address: Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China. E-mail: [email protected].

the transition temperature of the N-octadecanoyl-L-alanine LB film is nearly twice as high as that of the same-chain-length octadecanoic acid LB film.7 After metal ions penetrate into the N-octadecanoyl-L-alanine LB films through structure defects, they are selectively exchanged with carboxylic acid headgroups, and the ion exchange is preferred to proceed toward the decrease in the intermolecular distance in the LB films.10 The ion exchange in the N-octadecanoyl-L-alanine LB films are controlled by the structure of intermolecular hydrogen-bonding network.10 Previous studies showed that the presence of bivalent cations in the aqueous subphase may give rise to considerable compression or expansion of the film-forming molecules as well as to an increase or decrease in chiral discrimination in the respective monolayers depending on the kind of cation.11,12 However, the relative contributions of the different effects that may be of importance to chiral discrimination, i.e., electrostatic interaction, hydrogen bonding, and complex formation by counterions in the subphase, are not yet clear. The objective of this paper is to gain a deeper insight into these questions by systematically altering metal ions. The behaviors of the monolayers at the air-water interface and the structures and phase behaviors of the corresponding LB films have been studied through π-A isotherms and FTIR spectroscopy. Experimental Sections 1. Materials and Film Fabrication. The preparation of N-octadecanoyl-L-alanine samples and the fabrication of LB films were recently described in detail.7 The chemical reagents used were of analytical grade, and the water used was doubledistilled after a deionized Milli-Q exchange. The pH value of

10.1021/jp002016h CCC: $19.00 © 2000 American Chemical Society Published on Web 10/10/2000

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Figure 1. π-A isotherms of N-octadecanoyl-L-alanine enantiomeric monolayers on pure water and ion-containing subphases at their intrinsic pH values: pure water (pH 6.0), Ag+ (pH 5.8), Zn2+ (pH 6.8), Ca2+ (pH 5.6), Cd2+ (pH 5.9), Ni2+ (pH 5.8), La3+ (pH 6.1). Subphase temperature: 25 °C. Compression rate: 10 mm/min.

pure water was adjusted to 3.0 by the addition of HCl, but the pH values of ion-containing subphases (10-3 mol/L) were their intrinsic values without the addition of HCl or NaOH: AgNO3 at pH 5.8, ZnCl2 at pH 6.8, CaCl2 at pH 5.6, CdCl2 at pH 5.9, NiCl2 at pH 5.8, and LaCl3 at 6.1, unless otherwise stated. 11monolayer LB films were deposited from pure water and the ion-containing subphases onto CaF2 substrates by the vertical method (dipping/lifting rate 2 mm/min) at the fixed surface pressure 40 mN/m with a transfer ratio of 0.9-1.0 at 25 °C. 2. Spectrum Measurement. FTIR spectra were recorded on a Bruker IFS 66V spectrophotometer equipped with a DTGS detector. A KRS-5 polarizer was used for the polarization measurements of the LB films. Typically, 1000 interferograms were collected to obtain a satisfactory signal-to-noise ratio at the resolution 4 cm-1. Temperature control was achieved with an automatic controller (Graseby Specac. Inc.) through a copper-constantan thermocouple with an accuracy of (1 °C. After the LB films were heated to the desired temperatures, 15 min was spent for a thermal equilibrium. Results and Discussion 1. Monolayers at the Air-Water Interface. Figure 1 shows the π-A isotherms of N-octadecanoyl-L-alanine monolayers on pure water and ion-containing subphases, respectively. The enantiomeric monolayer on pure water subphase exhibits a transition from a tilted liquid crystalline phase to a tilted twodimensional crystalline phase with the area per molecule 0.23 nm2. The monolayer behavior suggests the preference of homochiral effect and intermolecular hydrogen-bonding interaction in the two-dimensional condensed phase.7 As indicated by the isotherms in Figure 1, the monolayers on the ion-containing subphases can be roughly classified into two groups: one exhibits the characteristics of compressed monolayers in the presence of Ag+ and Zn2+ ions, and the other takes the features of expanded monolayers in the presence of Ca2+, Cd2+, Ni2+, and La3+ ions. On the Ag+-containing subphase, the π-A isotherm shows extremely compressed characteristics with the limiting area 0.13 nm2, even below the theoretical value (0.20 nm2) of a saturated hydrocarbon chain. A comparable observation was encountered with the enantiomeric and racemic monolayers of N-hexadecanoylalanine in the presence of Pb2+

Du and Liang ions.11 It was considered to be due to the formation of threedimensional structure of the compressed monolayers.11 On an aqueous 10-3 mol/L ZnCl2 solution, a somewhat expanded feature occurs at lower surface pressures. A peak near 0.22 nm2 in the isotherm indicates a phase transition point from an expanded phase to a condensed phase, the possibility of which was suggested for the N-hexadecanoyl-L-alanine monolayer.12 Upon further compression, the isotherm exhibits a very steep portion with a molecular area of 0.21 nm2, comparable to the cross-sectional area 0.20 nm2 of a saturated chain. A similar phenomenon was observed in the N-hexadecanoylalanine monolayers on the aqueous ZnCl2 solution with a preferential homochiral interaction between 0.4 nm2/molecule and the collapse point.11 The monolayer behaviors suggest that the condensing effects of Ag+ and Zn2+ ions may give rise to an increase in the intermolecular hydrogen-bonding interaction as well as homochiral effect. However, in the cases of Ca2+, Cd2+, Ni2+, and La3+ ions, the π-A isotherms display very expanded characteristics, in contrast to the condensed features of saturated fatty acid monolayers on the ion-containing subphases.13,14 The expanding effects of these ions would lead to the occurrence of intramolecular hydrogen bond and the decrease/disappearance of homochiral effect. The hydrocarbon chains in the monolayers may be more tilted than those on pure water subphase. The above microscopic interactions, inferred from the macroscopic order of the π-A isotherms, can be confirmed through infrared spectroscopic studies of the corresponding LB films. 2. Molecular Structures of LB Films. Figure 2 shows the FTIR spectra of 11-monolayer N-octadecanoyl-L-alanine LB films deposited from pure water surface and aqueous subphases containing Ag+, Zn2+, Ca2+, Cd2+, Ni2+, and La3+ ions, respectively. All of the vibrational bands of the pure enantiomer LB film have been assigned in detail.7 It is constituted by carboxylic acid dimers [ν(CdO) at 1705 cm-1 and ν(C-O)/ δ(OH) at 1252 cm-1] with an almost all-trans chain conformation [νa(CH2) at 2918 cm-1, νs(CH2) at 2850 cm-1, presence of the band progression arising from the CH2 wagging vibration]. The singlet peak (at 1472 cm-1 for Ag+-containing and pure water subphases) of CH2 scissoring mode indicates that the alkyl chains in the LB film is in a triclinic crystal form.15 The absorption bands at 3324, 1646, and 1539 cm-1 are attributed to amide A, I, and II bands, respectively, and these band positions suggest that the enantiomeric molecules are engaged in an extended intermolecular hydrogen-bonding network through neighboring amide groups.7,8 In the presence of metal ions in aqueous subphases, the ν(CdO) and ν(C-O)/δ(OH) bands of carboxylic acid dimers are completely vanished. Some new absorption bands appear in the region 1600-1500 cm-1 and are attributed to the asymmetric stretching of carboxylate groups. These observations indicate that the metal ions at the intrinsic pH values of the aqueous solutions can completely dissociate the carboxylic acid headgroups of the enantiomeric monolayers at the air-water interface. From the profiles of the absorption bands, these spectra may be also classified into two groups as shown in the π-A isotherms. In the spectrum of silver N-octadecanoyl-L-alaninate LB film, a strong band at 1510 cm-1 and a very weak band at 1401 cm-1 are assigned to the asymmetric and symmetric carboxylate stretching vibrations [νa(COO) and νs(COO)], respectively, which coincides with the assignment of 1510- and 1400 cm-1 bands in the infrared spectrum of the n-hexadecanoic acid self-assembled monolayer by chemisorption onto the airexposed silver substrate.16 The frequency separation between

Metal N-Octadecanoyl-L-alaninate LB Films

J. Phys. Chem. B, Vol. 104, No. 43, 2000 10049

Figure 2. FTIR spectra of 11-monolayer N-octadecanoyl-L-alanine LB films deposited from pure water and ion-containing subphases.

Figure 3. Pair of tightly packed (a) silver and (b) zinc N-octadecanoylL-alaninate monolayers.

νa(COO) and νs(COO) is usually as a diagnostic tool to gain an insight into the respective coordination types.17,18 A separation of 109 cm-1 suggests that a chelating bidentate complex is preferably formed between silver and carboxylate (Figure 3a), which is consistent with the favorable geometry of carboxylate to bind symmetrically to silver surface in the n-hexadecanoic acid monolayer.16 The amide A band undergoes a frequency shift from 3324 to 3320 cm-1, indicating that the intermolecular

hydrogen-bonding interaction is only slightly increased by complex formation. Compared with the pure enantiomer LB film, the νa(CH2), νs(CH2), and δ(CH2) frequencies of the silver N-octadecanoyl-L-alaninate LB film remain unchanged. This may be the reason that ion exchange is only partial even under a long time after Ag+ ions penetrate into the N-octadecanoylL-alanine LB film.10 In the spectrum of zinc N-octadecanoyl-L-alaninate film, a very strong band at 1556 cm-1 is assigned to the νa(COO) stretching mode. The amide A band is further shifted down to 3310 cm-1 and the amide B band [first harmonic of δ(NH)] is shifted up to approximately 3075 cm-1, which suggest that the intermolecular hydrogen-bonding interaction is further increased in comparison with that in the silver complex LB film. In addition, the νs(CH2) frequency is reduced to 2848 cm-1, and the νa(CH2) and νs(CH2) absorption bands are intensified as compared with those in the pure enantiomer LB film. It is shown that the condensation effect of Zn2+ ions gives rise to an increase not only in the intermolecular hydrogen-bonding interaction but also in the interchain interaction. In the region 1470-1450 cm-1 appears a broad band contour, the second derivative spectrum of which shows two peaks at 1468 and 1458 cm-1 with comparable intensities. The former is due to the δ(CH2) bending mode, indicative of the chain packing of hexagonal subcell structure,19 and the latter is attributed to the νs(COO) stretching vibration, which is consistent with the assignments of the 1456 cm-1 band of zinc acetate.18 A separation of 98 cm-1 between 1556 and 1458 cm-1 indicates a preference of chelating bidentate (Figure 3b), in accordance with the coordination type of solid zinc acetate.18

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Du and Liang

Figure 4. Probable structures of calcium, cadmium, nickel, and lanthanum N-octadecanoyl-L-alaninate monolayers.

The polarized FTIR spectra of both silver and zinc complex LB films exhibit infrared anisotropy at the normal incidence, implying that the long hydrocarbon chains in the two films take a biaxial orientation. The pairwise intermolecular interactions between the groups attached to the chiral carbons of the neighboring molecules depend on the size of attached groups, their distance separation, and their relative orientation.20 In the two films, the enhanced intermolecular hydrogen-bonding interaction would give rise to an increase in interchain interaction and a decrease in distance separation. The minimum-energy configuration of the same enantiomers favors a twisted angle between them, the twist from neighbor to neighbor thus leads to chirality of the aggregate, which is schematically illustrated in Figure 3. However, completely different cases are encountered in the spectra of calcium, cadmium, nickel, and lanthanum N-octadecanoyl-L-alaninate LB films: (i) very broad bands consisting of multiple components appear in the region 1700-1400 cm-1; the bands around 1650-1640 and 1590-1560 cm-1 are assigned to amide I bands and νa(COO) stretching modes, respectively; the bands at 1467 cm-1 are due to the δ(CH2) bending modes; the bands near 1455 and 1417 cm-1 comprise the contribution of νs(COO) stretching modes since the bands near 1417 cm-1 are much broader than that at 1416 cm-1 in the pure enantiomer LB film; (ii) the amide A bands are substantially weakened and broadened in the region 3280-3270 cm-1; (iii) the other absorption bands are also reduced to a certain extents in comparison with those in the pure enantiomer LB film. These spectral changes imply that intramolecular hydrogen bonds may be formed between the carboxylate and amide groups, and that the alkyl chains should be more tilted than those in the pure enantiomer LB film. It is likely that an intramolecular hydrogen bond is formed via a five-membered ring structure between the NH bond of amide group and the carbonyl group of carboxylate. This kind of hydrogen bond results in a lower ν(NH) frequency, and the IR band is sometimes extremely broad,21 which is responsible for the band overlap in the region 1700-1500 cm-1. It is observed that the stronger the intramolecular hydrogen bond, the lower the intensity recorded in this region is.21 A preference of intramolecularly hydrogen-bonded monodentate (Figure 4a) or unsymmetric ionic coordination (Figure 4b) is thus tentatively attributed to the coordination types of the metal complexes. The frequency separation in the “H-bonded monodentate/ionic

coordination” between νa(COO) and νs(COO) should be lower than that in the pure monodentate/ionic coordination.11 The formation of intramolecular hydrogen bonds will be further confirmed by the specific phase behaviors of the LB films in the following section. However, no infrared dichroism can be found for calcium, cadmium, nickel, and lanthanum complex LB films at the normal incidence, which means that the hydrocarbon chains in these films are uniaxially distributed around the respective normals. According to Vandevyver’s convention,22 the hydrocarbon chains in the films are calculated to tilt at an angle of 44.8° for calcium complex, 43.0° for cadmium complex, 45.7° for nickel complex, and 39.9° for lanthanum complex. These LB films would exhibit no chirality as the films of achiral molecules with a uniaxial orientation. All these differences in the film structure would give rise to a change in the basic physicochemical property of the films. In the following section, special attention is focused on the phase behaviors of these LB films in order to seek insight into the roles of hydrogen bond and metal complex in phase behavior and the relationship between molecular structure and phase behavior. 3. Phase Behaviors of LB Films. The phase behaviors of LB films are correlated with molecular structure and interchain interaction. It is well-known that the ν(CH2) frequencies are very sensitive to the conformation of hydrocarbon chains.23,24 When the chain is highly ordered (trans-zigzag conformation), the bands appear at 2918 and 2850 cm-1, respectively, and their upward shifts are indicative of an increase in the conformational disorder; i.e., the number of gauche conformers in the chain increases. FTIR spectroscopy can precisely monitor these subtle conformational changes in the chains, the transition temperatures of LB films can thus be obtained. Figure 5a shows the temperature dependence of the νa(CH2) frequencies of the N-octadecanoyl-L-alanine LB films deposited from pure water subphase and the aqueous solutions containing Ag+, Zn2+, Ca2+, Cd2+, and Ni2+ ions, respectively, together with a change in the νa(CH2) frequency of octadecanoic acid LB film for a reference. At room temperature, the νa(CH2) frequencies of these LB films appear to be near 2918 cm-1, indicative of almost ordered all-trans conformation in the alkyl chains.23,24 For the pure enantiomer LB film, the νa(CH2) frequency gradually shifts upward with temperature before the film is heated to 120 °C, then an abrupt increase in νa(CH2)

Metal N-Octadecanoyl-L-alaninate LB Films

J. Phys. Chem. B, Vol. 104, No. 43, 2000 10051 increase in temperature, the νa(CH2) frequency of the calcium complex LB film shows two remarked changes around 100 and 160 °C again, and the final film is in a highly disordered state above 170 °C. The LB film undergoes three phase-transition processes from a highly ordered state to a melted disordered state through two mesophases from room temperature to 170 ˚C. As seen from Figure 5a, the second transitions of cadmium and nickel complex LB films take place at approximately 110 and 160 °C, respectively. These specific phase behaviors suggest that these LB films exhibit the feature of “thermotropic liquid crystal”, which is the property of molecules composed of flexible side chains incorporated with different functional mesogenic units.26 Owing to the asymmetry of chiral carbons, the fivemembered ring structures of intramolecular hydrogen bonds containing chiral carbons may act as rigid or semirigid groups and have the similar effects to the mesogenic units in usual liquid crystals.26 These LB films thus display the phase behaviors of thermotropic liquid crystal with more than one transition process. The phase behaviors of the metal Noctadecanoyl-L-Lalaninate LB films show a clear dependence of molecular structure of hydrogen-bond and complex formation. Conclusions

Figure 5. Temperature dependence of (a) νa(CH2) and (b) ν(NH) frequencies of 11-monolayer N-octadecanoyl-L-alanine LB films deposited from pure water and ion-containing subphases. Change in the νa(CH2) frequency () of octadecanoic acid LB film for a reference.

frequency occurs at the order-disorder transition temperature (122 °C) of the LB film, and subsequently a constant value of 2924 cm-1 is nearly remained above this temperature. Compared with the transition temperature 65 °C of octadecanoic acid LB film,25 in which a sudden transition process takes place, the enhanced thermal stability in the pure enantiomer LB film is considered to result from the intermolecular hydrogen-bonding interactions and the homochiral effects between the enantiomeric headgroups.7 In the cases of the silver and zinc complex LB films, the νa(CH2) frequency changes are similar to that of the pure enantiomer LB film below 120 °C. However, when the temperature is elevated above 120 °C, the νa(CH2) frequencies maintain a monotonic increase, instead of a sudden change, until the alkyl chains reach a highly disordered state. The two LB films show the thermal behavior of “glass transition”, especially for the zinc complex LB film. This could be owing to a gradual dissociation of the enhanced intermolecular hydrogen-bonding interaction over a wide temperature range (seen Figure 5b), in contrast to a complete dissociation of the relatively weak intermolecular hydrogen-bonding interaction in the pure enantiomer LB film around the transition temperature (122 °C). The interchain interaction, induced by the enhanced intermolecular hydrogen-bonding interaction, plays a determinative role in phase processes of the LB films. For the other group of calcium, cadmium, and nickel complex LB films, the νa(CH2) frequencies increase with temperature and rapidly reach 2921-2922 cm-1 in the range 60-70 °C. The temperatures are very comparable to the transition temperature (65 °C) of octadecanoic acid LB film,25 and much lower than the order-disorder transition temperature (122 °C) of the pure enantiomer LB film, in which the intermolecular hydrogenbonding network is formed. These observations provide powerful evidence for the formation of intramolecular hydrogen bonds in these films. It is reasonable to consider that the intramolecular hydrogen bonds are formed between the N-H bonds of amide groups and the carbonyl groups of carboxylates. Upon a further

The studies of the N-octadecanoyl-L-alanine monolayers at the air-water interface in the ion-containing subphases (Ag+, Zn2+, Ca2+, Cd2+, Ni2+, and La3+) and the corresponding LB films indicate that hydrogen bond and metal complex play an important role in the molecular structures and phase behaviors of the metal complex films, and that the special phase behaviors of the LB films are closely related to the molecular structures of the LB films. (1) The condensing effects of Ag+ and Zn2+ ions give rise to an increase in the intermolecular hydrogen-bonding interactions between adjacent molecules. A preference of chelating bidentate is formed between these metal ions and carboxylate groups. The hydrocarbon chains in silver and zinc complex LB films take a biaxial orientation. The two LB films exhibit the thermal behavior of “glass transition”, which may result from a gradual thermal dissociation of the increased intermolecular hydrogen-bonding interactions over a wide temperature range. (2) The expanding effects of Ca2+, Cd2+, Ni2+, and La3+ ions lead to the formation of intramolecular hydrogen bond. A H-bonded monodentate/unsymmetric ionic coordination may be formed between these metal ions and carboxylate groups. The hydrocarbon chains in these LB films are uniaxially oriented at the angle 40-50° with regard to the respective film normals. These LB films show the phase behavior of “thermotropic liquid crystal” more than one phase process. The five-membered ring structures formed through intramolecular hydrogen bonds are considered to have the similar effects to the mesogenic units in usual liquid crystals. Acknowledgment. The work was financially supported by the National Natural Science Foundation of China (NSFC, Grant 29873022) and a major research project grant from the State Science and Technology Commission of China. References and Notes (1) Heath, J. G.; Arnett, E. M. J. Am. Chem. Soc. 1992, 114, 4500. (2) Stine, K. J.; Uang, J. Y.-J.; Dingnam, S. D. Langmuir 1993, 9, 2112. (3) Parazak, D. P.; Uang, J. Y.-J.; Turner, B.; Stine, K. J. Langmuir 1994, 10, 3787. (4) Gericke, A.; Hu¨hnerfuss, H. Langmuir 1994, 10, 3782. (5) MoConnel, H. M. Annu. ReV. Phys. Chem. 1991, 42, 171.

10052 J. Phys. Chem. B, Vol. 104, No. 43, 2000 (6) Schrader, B. Practical Fourier Transform Infrared Spectroscopy; Ferraro, J. R., Krishnan, K., Eds.; Academic Press: San Diego, CA, 1990. (7) Du, X.; Shi, B.; Liang, Y. Langmuir 1998, 14, 3631. (8) Du, X.; Liang, Y. Chem. Phys. Lett. 1999, 313, 565. (9) Du, X.; Shi, B.; Liang, Y. Spectrosc. Lett. 1999, 32, 1. (10) Du, X.; Liang, Y. Langmuir 2000, 16, 3422. (11) Hu¨hnerfuss, H.; Neumann, V.; Stine, K. J. Langmuir 1996, 12, 2561. (12) Hoffmann, F.; Hu¨hnerfuss, H.; Stine, K. J. Langmuir 1998, 14, 4525. (13) Gericke, A.; Hu¨hnerfuss, H. Thin Solid Films 1994, 245, 72. (14) Simon-Kutscher, J.; Gericke, A.; Hu¨hnerfuss, H. Langmuir 1996, 12, 1027. (15) Holland, R. F.; Nielsen, J. R. J. Mol. Spectrosc. 1962, 9, 436. (16) Tao, Y.-T.; Lin, W.-L.; Hietpas, G. D.; Allara, D. L. J. Phys. Chem. B 1997, 101, 9732. (17) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; John Wiley & Sons: New York, 1986.

Du and Liang (18) Tackett, J. E. Appl. Spectrosc. 1989, 43, 483. (19) Snyder, R. G. J. Mol. Spectrosc. 1961, 7, 116. (20) Nadi, N.; Bagchi, B. J. Phys. Chem. A 1997, 101, 1343. (21) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: San Diego, CA, 1991. (22) Vandevyver, M.; Barraud, A.; Teixier, R.; Maillard, P.; Gianotti, C. J. Colloid Interface Sci. 1982, 85, 571. (23) Snyder, R. G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta Part A 1978, 34, 395. (24) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (25) Zhang, Z.; Liang, Y.; Tian, Y.; Jiang, Y. Spectrosc. Lett. 1996, 29, 321. (26) Kusumizu, S.; Kato, R.; Yamada, M.; Yano, S. J. Phys. Chem. B 1997, 101, 10666.