Raman Spectroscopic Study on the Coordination Behavior of Rare

Oct 1, 2008 - Department of Applied Chemistry, National Defense Academy, 1-10-20, Hashirimizu, Yokosuka, Kanagawa,. 239-8686, Japan. ReceiVed: ...
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J. Phys. Chem. B 2008, 112, 13355–13358

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Raman Spectroscopic Study on the Coordination Behavior of Rare Earth Ions in N-Methylacetamide Takahiro Takekiyo, Yukihiro Yoshimura,* Yohei Ikeji, Naohiro Hatano, and Toshio Koizumi Department of Applied Chemistry, National Defense Academy, 1-10-20, Hashirimizu, Yokosuka, Kanagawa, 239-8686, Japan ReceiVed: March 3, 2008; ReVised Manuscript ReceiVed: August 13, 2008

The coordination behavior of rare earth (Ln3+) ions in N-methylacetamide (NMA) solution has been investigated at room temperature by Raman spectroscopy. The behavior of the symmetric Raman Ln-Cl stretching (νLn-Cl) band, and amide I (νAI), and III (νAIII) bands of NMA with the rare earth series is discussed in conjunction with the change in the coordination structure occurring in the middle of the rare earth series. A competition for a coordination equilibria between a Cl- ion and an NMA molecule from the rare earth chloride-NMA complex might occur in the middle rare earth region. It is demonstrated that the change in the coordination structure of Ln3+ ions in NMA is due to an elimination of an NMA molecule. 1. Introduction It is well-known that the decrease in the ionic radius of a lanthanide ion (Ln3+) owing to lanthanide contraction induces irregularities for numerous thermodynamic and transport properties of aqueous rare earth electrolytes (LnX; X ) anion) solutions.1-3 These irregularities mean that the coordination number of Ln3+ ions in aqueous LnX solutions changes from nine for the light Ln3+ ions (La3+-Sm3+) to eight for the heavy Ln3+ ions (Tb3+-Lu3+).4-7 A change in the coordination number for the Ln3+ ions in the middle region (Gd3+-Ho3+ or Er3+) of the rare earth series has also been studied in organic LnCl3 solutions with solvents such as alcohol, N,N-dimethylacetamide (DMA: CH3CON(CH3)2), and N,N-dimethylformamide (DMF: HCON(CH3)2).8-11 Typically, the coordination behavior of Ln3+ ions with amide compounds such as DMF and DMA having chain and ring structures12-18 has been investigated experimentally,10,11,19,20 and the addition of LnCl3 induces change in the solution structure of amide compounds. According to Raman, NMR, and extended X-ray absorption fine structure (EXAFS) studies by Ishiguro et al.,10,12,19,20 the Ln3+ ion has an eight-coordinate structure, [Ln(DMF)8]3+, in DMF for the entire rare earth series. On the other hand, in DMA, the La3+ ion has mainly an eightcoordinate structure, [La(DMA)8]3+, whereas the Lu3+ ion has a seven-coordinate structure, [Lu(DMA)7]3+. However there have been no reports on the coordination structure of Ln3+ ions in neat N-methylacetamide (NMA: CH3CONHCH3), which has a similar structure to DMA. It is interesting to determine whether the coordination structure of Ln3+ ions changes in a protic solvent such as NMA. Raman spectroscopy is one of the methods that is most often used as a tool for studying chemical composition, bonding, structure, phase, localization, size, induced stress, and reaction mechanisms with the modern Raman instrument.21-23 The exploration of spectra measurable using this technique can provide significant information about secondary and tertiary structures of peptides and proteins24-27 as well as coordination chemistry.28,29 * Corresponding author. Tel: +81-46-841-3810. Fax: +81-46-844-5901. E-mail: [email protected].

The frequency shift related to the Ln3+ ion for the Ln-Cl stretching mode (νLn-Cl) in alcohol LnCl3 solutions9 shows z-shaped behavior with the rare earth series, whereas the Ln-Cl stretching mode (νLn-OH) in aqueous solutions4-7,30 shows s-shaped behavior, meaning that these frequency behavior might depend on the properties of solvent and reflect the difference in the coordination structure of the Ln3+ ion in the solutions. However, details on these two types of frequency behavior are still unclear. It is thus important to examine which frequency behavior the present system shows. In this study, we have measured Raman νLn-Cl, amide I (νAI), and amide III (νAIII) spectra in LnCl3 · 15NMA solutions to investigate the coordination behavior of Ln3+ ions to NMA. Our results show that the coordination structure of Ln3+ ion in the NMA solution changes due to an elimination of an NMA molecule in the middle of rare earth series. 2. Materials and Methods NMA was purchased from Wako Junyaku Co. Anhydrous rare eath chlorides (LnCl3; Ln3+ ) La3+-Lu3+) were obtained from Soekawa Chemical Co. The concentration of all the solutions are set to be LnCl3 · 15NMA. Raman spectra were measured by a JASCO NR-1800 Raman spectrophotometer equipped with a single monochromator and a charge coupled device (CCD) detector. The exposure time for each run and spectral resolution were 300 s and 4.5 cm-1, respectively. The 514.5 nm line from a Lexel Ar+ ion laser was used as an excitation source with a power of 200 mW. The Raman spectra were measured at room temperature. The obtained spectra were fitted with GaussiansLorentzian mixing functions using the GRAMS/386 software (Galactic Industries Corp., Ltd.) to analyze the Raman Ln-Cl stretching, amide I, and amide III bands of NMA. Both Raman spectroscopy and density functional theory (DFT) calculation have been used as investigation methodology for structures and behavior.31,32 Therefore, we performed the calculation of vibrational spectra for NMA and LaCl3-NMA. DFT calculations were carried out using the GAUSSIAN 98 program.33 The geometry optimization and the frequency calculation were performed for NMA and LaCl3-NMA at the B3PW91/LAN2DZ level.

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Figure 1. (a) Typical Raman spectra in the region from 150 to 370 cm-1 and (b) variation of νLn-Cl frequency of neat NMA and LnCl3 · 15NMA solutions (Ln ) La, Eu, and Lu). The “str” and “ipb” represent the stretching and in-plane bending modes, respectively.

3. Results and Discussion Figure 1a shows the typical Raman spectra of LnCl3 · 15NMA solutions from 150 to 370 cm-1. From the calculated spectra, the peaks at 286 cm-1 for NMA and 295 cm-1 for LaCl3-NMA are assigned to the “free” and “bound” N-H in-plane bending (N-H ipb) modes, respectively. The peaks at 197 cm-1 for NMA and 207 cm-1 for LaCl3-NMA are assigned to the “free” and “bound” CdO in-plane bending (CdO ipb) modes, respectively. These assignments are consistent with the previous results of NMA.34,35 The calculated Ln-Cl stretching (νLn-Cl) band of LaCl3 · 15NMA was observed at 255 cm-1, and appeared between the N-H ipb and CdO ipb of NMA. No peak was observed in the calculated spectra of NMA. In the observed Raman spectrum, the peak at 231 cm-1 of the LaCl3 · 15NMA solution was observed by second derivative spectra (data not shown), and this peak was observed between the N-H ipb and CdO ipb of NMA. The peak position is in accord with the νLn-Cl band in the calculated spectra. As mentioned in the previous reports,8,9 the νLn-Cl band of the LnCl3 · 20ROH solutions (ROH ) MeOH, EtOH, and n-PrOH) is observed at ∼230 cm-1, which is near the peak at 231 cm-1 of the LaCl3 · 15NMA solution. On the other hand, the Raman Ln3+-OdC stretching (νLn-OdC) band of a Ln3+-amide complex such as LnCl3 in DMF is observed at 490 cm-1. 36,37 The peak position at 231 cm-1 of the LaCl3 · 15NMA solution is far from the νLn-OdC. On the basis of the calculated spectra and previous results, we assigned the peak at ∼230 cm-1 to the νLn-Cl band. We found that it is difficult to analyze the coordination behavior in LnCl3 · 15NMA solution using the N-H ipb and CdO ipb because of the very weak Raman intensity. Thus, we applied the νLn-Cl frequency to analyze the coordination behavior of Ln3+ ion in NMA solution. Figure 1b shows the νLn-Cl frequency variation of the LnCl3 · 15NMA solutions as a function of the ionic radius of the Ln3+ ion. According to a calorimetric study of LnCl3 in DMF by Ishiguro et al.,10 the formation of [LaCl]2+ and [LaCl2]+ occurs extensively, but that of [LaCl3] does not. In the case of Ce3+, Pr3+, and Nd3+ ions, [LnCl3] complexes form extensively, as do [LnCl]2+ and [LnCl2]+, but [LnCl4]- is scare. They showed that the Ln3+-DMF and Ln3+-Cl-interactions may be enhanced with the decrease in ionic radius of the Ln3+ ions, and this irregular variation may be due to the varying coordination geometries of LnCl3-DMF solutions. In the present system, NMA is a protic solvent having a high relative dielectric constant, which might form hydrogen-bonded clusters among

Takekiyo et al. solvent molecules. Thus it is likely that protic solvents such as NMA give less stability for metal-ion complexation than aprotic solvents such as DMF. This implies that the [LnCl3] complex is not a main species, and perhaps [LnCl2]+ and [LnCl]2+ species are present in equilibrium. According to the previous Raman study by Kanno and Yoshimura,8,9 the peak position at ∼230 cm-1 of the νLn-Cl frequency for the light Ln3+ ions in LnCl3 · 20ROH solutions is attributed to the eight-coordination structure. The peak position of the νLn-Cl frequency in LnCl3 · 15NMA solutions is nearly the same as in the LnCl3 · 20ROH solutions. It was also reported that the coordination number of Ln3+ ions in DMA changes from eight for the light Ln3+ ions to seven for the heavy Ln3+ ions.10-14 On the basis of these compiled results and considering the similarities of molecular structure and size between NMA and DMA, we tentatively assume that the coordination structure of the Ln3+ ion of LnCl3 · 15NMA solutions in the light Ln3+ region may take eight-coordination, described as [LnClx(NMA)y]z+ (e.g., x + y ) 8; x ) 1 and z ) 2 or x ) 2 and z ) 1) for the average LnCl3-NMA complex in the present system. It is interesting to point out that the observed νLn-Cl frequency of the LnCl3 · 15NMA solutions shifts to a higher frequency from 231 cm-1 for the La3+ ion to 256 cm-1 for the Lu3+ ion (∆νLasLu ) 25 cm-1) with showing “extended z-shaped” behavior. The νLn-Cl frequency jump in the middle region (Sm-Ho) is about 10 cm-1. It was concluded that the irregular behavior of the Raman frequency shift5-7 and thermodynamic properties 38-41 throughout the rare earth series is caused by the change in the total coordination number of the Ln3+ ions. A similar νLn-Cl frequency change with showing extended z-shaped behavior was reported in LnCl3 · 20ROH solutions by Kanno and Yoshimura.8,9 The νLn-Cl frequency of LnCl3 · 20ROH solutions shifts to a higher frequency (∆νLasLu)36 cm-1), and the νLn-Cl frequency jump in the middle region is about 10 cm-1. They reported that the large νLn-Cl frequency shift and νLn-Cl frequency jump in the middle region indicate a decrease in the coordination number of Ln3+ ion in the LnCl3 · 20ROH solutions by an elimination of an alcohol molecule. The ∆νLasLu in the LnCl3 · 15NMA solutions is smaller than that in the LnCl3 · 20ROH solutions, but, importantly, the νLn-Cl frequency jump in the middle region of LnCl3 · 15NMA solutions is almost consistent with that of the LnCl3 · 20ROH solutions. On the basis of these results, the coordination number of the Ln3+ ion in LnCl3 · 15NMA solutions might decrease by one in the middle (Sm-Ho) region. The νLn-Cl frequency change of the LnCl3 · 15NMA solutions should arise from following either of two possible contributions. One is that the electrostatic interaction between the Ln3+ ion and the chloride ion (and inevitably CdO group) becomes stronger by an elimination of an NMA molecule from the [LnClx(NMA)y]z+ complexes. Another one is simply an elimination of the Cl- ion from the [LnClx(NMA)y]z+ complexes. To test which contribution is dominant to the coordination structure of the Ln3+ ions in LnCl3 · 15NMA solutions, we examined the coordination behavior of NMA using Raman amide I and III bands. It is known that the Raman amide I and III modes of NMA mainly result from the CdO stretching and N-H bending modes, respectively.42-44 These modes are useful to investigate the behavior of the CdO and N-H groups in the solutions. Figure 2 shows the typical Raman amide I and III spectra of LnCl3 · 15NMA solutions. Each Raman amide I and III spectrum except pure NMA consists of two Raman bands, which are at ∼1655 cm-1 and ∼1590 cm-1 for the amide I band, and at

Coordination Behavior of Rare Earth Ions in NMA

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Figure 2. Typical Raman (a) amide I and (b) amide III region spectra in neat NMA and LnCl3 · 15NMA solutions (Ln ) La, Eu, and Lu). The “CCH3 sb” represents the CCH3 symmetric bending mode.

∼1310 cm-1 and ∼1330 cm-1 for the amide III band. From the calculated spectra using the DFT method at the B3PW91/ LANL2DZ level, the amide I band of “free” NMA appears at a higher frequency than that of “bound” NMA. On the other hand, the amide III band of “free” NMA appears at a lower frequency than that of “bound” NMA. Actually, the Raman intensities at ∼1590 cm-1 for the amide I band and at ∼1330 cm-1 for the amide III band increase as the concentration of LnCl3 increases (data not shown). Therefore, the Raman bands at ∼1590 cm-1 for the amide I mode and at ∼1330 cm-1 for the amide III band are assigned to the NMA molecules bound to LnCl3 (νAI,(b) and νAIII,(b), respectively). The Raman bands at ∼1655 cm-1 for amide I mode and at ∼1310 cm-1 for amide III mode are assigned to the “free” NMA molecules (νAI,(f) and νAIII,(f), respectively). The peak at ∼1380 cm-1 is assigned to the CCH3 symmetric bending mode of NMA.33 Figure 3 shows the νAI frequency shifts and intensity ratio (RAI ) Ifree/Ibound) between the “free” and “bound” amide I bands of the LnCl3 · 15NMA solutions as a function of ionic radius of Ln3+ ion. We see that the νAI,(b) shifts to a higher frequency with decreasing ionic radius of the Ln3+ ion, and the νAI,(f) hardly shifts. This clearly shows that the intermolecular electrostatic interaction between the CdO groups and the Ln3+ ion becomes stronger. An interesting feature is that the value of RAI of the amide I band in LnCl3 · 15NMA increases in the La-Nd region, but the increase seems to become mild, just a little in the Sm-Eu region, then the value of RAI increases after the Tb region again. A similar behavior is also observed in the intensity ratio (RAIII ) Ifree/Ibound) between the “free” and “bound” of the amide III band, as shown in Figure 4. The value of RAIII increases in the La-Nd region, and becomes almost constant in the Sm-Eu region. Again, RAIII increases drastically in the Tb-Ho region. On the basis of these results, we conclude that the coordination structure of the Ln3+ ion in the NMA solution changes by an elimination of an NMA molecule in the middle of the rare earth series, although, unfortunately, we cannot determine the definite coordination structure of the Ln3+ ion only from the Raman results. Finally, we mention the z-shaped behavior of the νLn-Cl frequency shift in the middle Ln3+ region. We suppose that the z-shaped behavior arises from a competition of a coordination

Figure 3. (a) The frequency shift of the amide I band in the LnCl3 · 15NMA solution as a function of the ionic radius of the Ln3+ ion. The close and open circles represent the “free” and “bound” amide I bands, respectively. The straight solid and dashed lines are drawn by a least-squares fit. (b) The change in the intensity ratio (RAI ) Ifree/ Ibound) between the “free” and “bound” amide I bands in the LnCl3 · 15NMA solution as a function of the ionic radius of the Ln3+ ion.

Figure 4. The change in the intensity ratio (RAIII ) Ifree/Ibound) between the “free” and “bound” amide III bands in the LnCl3 · 15NMA solution as a function of the ionic radius of the Ln3+ ion.

equilibria between a Cl- ion and a solvent molecule from the Ln3+ complex. In view of the above results along with previous ones for LnCl3 · 20ROH solutions and assuming that the average inner-sphere complexes for light rare earth ions in LnCl3 · 15NMA solutions are described as [LnClx(NMA)y]z+,

13358 J. Phys. Chem. B, Vol. 112, No. 42, 2008 these results in the NMA solutions can be well explained in terms of solvation equilibria between eight- and sevencoordinate complexes in conjunction with the formation of higher-chloro complexes as follows (e.g., x + y ) 8; x ) 1 and z ) 2):

That is, there are two kinds of equilibria in the LnCl3 · 15NMA solutions; one is the equilibrium between eight- and sevencoordinate species, and the other is the equilibrium between higher- and lower-chloro complexes for the respective eightand seven-coordinate species. We imply that these complicated equilibria in the coordination number change region cause the z-shaped behavior. On the other hand, in the absence of the competition in the equilibrium, i.e., such as in the aqueous solutions,4-7,28 the s-shaped behavior in the middle Ln3+ region can be observed. 4. Conclusion The Raman Ln-Cl stretching (νLn-Cl), amide I (νAI), and amide III (νAIII) spectra in LnCl3 · 15NMA solutions have been measured as a function of the ionic radius of the Ln3+ ion. We have discussed the change in the coordination structure of Ln3+ions in NMA. On the basis of the analyses of the νLn-Cl, νAI, and νAIII bands, the coordination structure of the Ln3+ ion in NMA changes into another coordination structure in the middle of the rare earth series. The change in the coordination structure of the Ln3+ ion in NMA arises from the competition for a coordination equilibria between a Clion and an NMA molecule from the [LnClx(NMA)y]z+ complexes. We find that the behavior on the frequency shift of the solutions (i.e., s-shaped or z-shaped behavior) clearly reflects the difference in the coordination structure change of the Ln3+ ion. According to the results of LnCl3 · 20ROH · XLiCl solutions (ROH ) MeOH, EtOH; X ) 1-4),45,46 the increase of the ligand concentration X of Cl-ion in the solutions shows the anomalous coordination behavior in the middle rare earth region; the νLn-Cl frequency increases with increasing ligand concentration X, which apparently means that the lower-chloro complexes become dominant despite the increase of salt concentration. We expect that similar anomalous behavior is also observable in LnCl3 · 15NMA solutions, as the z-shaped behavior on the frequency shift is observed as well. More continuous experimental studies such as ligand concentration, temperature effects, and so forth will give us further knowledge for understanding the coordination equilibrium of the Ln3+ ion in a protic solvent such as NMA. References and Notes (1) Spedding, F. H.; Pikal, M. J.; Ayers, B. O. J. Phys. Chem. 1966, 64, 2440. (2) Spedding, F. H.; Cullen, P. F.; Habenschuss, A. J. Phys. Chem. 1974, 78, 1106. (3) Kanno, H.; Akama, Y. Chem. Phys. Lett. 1980, 72, 181. (4) Kanno, H.; Hiraishi, J. Chem. Phys. Lett. 1980, 75, 553. (5) Kanno, H.; Hiraishi, J. J. Phys. Chem. 1982, 86, 1488.

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