J. Phys. Chem. B 2000, 104, 11001-11005
11001
Hydration of the CH Groups in Dimethyl Sulfoxide Probed by NMR and IR Kazuko Mizuno* Center for Instrumental Analysis, Fukui UniVersity, Fukui 910-8507 Japan
Shingo Imafuji, Torayuki Ochi, Tomoko Ohta, and Shiro Maeda Department of Applied Chemistry and Biotechnology, Fukui UniVersity, Fukui 910-8507 Japan ReceiVed: March 22, 2000; In Final Form: August 29, 2000
1H and 13C NMR of dimethyl sulfoxide (DMSO)/H O mixtures were measured, together with the IR of DMSO/ 2 D2O mixtures, to study the effect of the polar SdO group on hydration of the CH groups. Chemical shifts were determined by the external double reference method, which provides the in situ volume magnetic susceptibility indispensable to the correction of the chemical shifts. The chemical shift of the water protons as the measure of the polarization of the water in the mixtures, δH2O, increases from 3.6 ppm at the water mole fraction XH2O ) 0.05 to 4.8 ppm, the value for pure water, at XH2O ) 0.80. It exceeds 4.8 ppm in the region of XH2O > 0.80 at 23.3 °C, indicating the presence of anomalously polarized water molecules, socalled hydrophobic hydration. The frequencies of the CH stretching vibration bands for (DMSO)/D2O mixtures, ν(CH), increase with increasing XD2O, implying the progressive depolarization and contraction of the CH bonds. ν(CH) values take maxima at XD2O ) 0.96. The chemical shift of the CH proton increases very slightly with increasing XH2O, whereas that of the CH carbon decreases, suggesting the polarization of the CH bonds contrary to the depolarization in them as shown by the blueshifts of the ν(CH) values. The pushball hydration model previously presented is applied to interpret the results; the electron of CH hydrogen is pushed toward the carbon atom due to dispersion interaction with the electrons of water oxygen. The pushing effect probed by the blueshifts of ν(CH) can be related to the increase in the polarization of the water molecules probed by δH2O. The redshifts in ν(CH) in the water rich extreme may be ascribed to a partial polarization of the CH bond resulting from hydrogen bonding interaction with highly polarized water molecules, in addition to the dispersion interaction. The role of the SdO group in the hydration of the CH groups is discussed in comparison with the roles of the hydrophilic groups of acetone and tert-butyl alcohol.
Introduction A variety of experimental techniques and theoretical calculations have been devoted to the elucidation of liquid structures in aqueous mixtures of organic compounds,1 especially to finding the iceberg structure proposed by Frank and Evans as a water structure model in the vicinity of apolar solutes.2 Despite many supports, doubts about the validity of the model have been also growing.3 Other than discerning the water structure around apolar solutes, there is another unsolved problem concerning the hydration of alkyl groups in water soluble solutes; why do the frequencies of the symmetric and asymmetric CH stretching vibrational modes of solutes, ν(CH), increase on dilution with water? The blueshift indicates an electronic depolarization of the CH bond, whereas solvation by a polar solvent such as water usually gives rise to an increase in the intramolecular polarization of the solute molecule. Consequently, the blueshift is contrary to expectation and should be closely related to the mechanism of the hydration of the CH group and the water structure in the vicinity of the CH group. Carius et al. observed that the vibrational frequencies of the Raman bands characteristic of the strong polar groups of acetone, dimethyl sulfoxide (DMSO), and hexamethylphosphoricacidtriamide (HMPT) show a drastic decrease with increasing water concentrations, whereas the ν(CH) values increase monotonically.4 They reported also that the shifts of ν(CH) are dependent on the polarity of the polar group of acetonitrile,
acetone, THF, DMF, DMSO, and HMPT. They interpreted the blueshifts in terms of the decrease in the molecular symmetry resulting from the hydrogen bonding interaction of the polar groups with water. Thomzik et al. measured the frequency shifts of both the CH and the polar groups in n-propylamine, piperidine, morpholine, and 1,4-dioxane on mixing with water, and obtained similar results to those obtained by Carius et al.5 Kamogawa et al. observed the blueshifts of the ν(CH)s of ethanol in Raman spectra on mixing with water, and found concentration dependence of the blueshifts to be very similar to that of the partial molar volume of ethanol.6 Thus they showed that the blueshifts of ν(CH) are closely related to the change in the liquid structure. They observed also the blueshifts of the ν(CH) of 1,4-dioxane in Raman spectra, while zero and redshifts were observed for its CC and CO stretching modes, respectively.7 The blueshifts were not attributed to the secondary effect due to the hydrogen bonding at the oxygen atom but to the hydration of the CH group. These reports suggest that not only the solute but also the water molecule in the mixtures should be investigated to clarify the origin of the blueshifts. Since the 1H chemical shift is proportional to the electron density about a proton, the chemical shift of the water protons, δH2O, for an aqueous mixture of a compound is taken as a measure of the polarization of the water molecules averaged over the mixture; the larger is δH2O, the stronger is the polarization of the water molecules. Thus in a
10.1021/jp001079x CCC: $19.00 © 2000 American Chemical Society Published on Web 11/03/2000
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previous work, we measured IR spectra of deuterium oxide (D2O) mixtures with acetone, together with 1H and 13C NMR spectra of aqueous mixtures with acetone.8 The obtained results may be summarized as follows: (1) the ν(CH) values of acetone increase with increasing mole fraction of D2O, XD2O, and have maxima at XD2O ) 0.96; and (2) the δH2O increases from 3.0 ppm at the water mole fraction XH2O ) 0.05 to 4.9 ppm at XH2O ) 0.98 and exceeds 4.8 ppm, the δH2O for pure water, in the region of XH2O > 0.96 at 23 °C. We concluded from the results that there are at least two kinds of hydration states of the CH groups and that an electronic redistribution in acetone and an anomalous polarization of surrounding water molecules give rise to the hydration cluster in the water rich region. For aqueous tert-butyl alcohol mixtures also, we obtained almost the same results and conclusion.9 In the present work, we measured IR and NMR spectra of aqueous mixtures of DMSO with a more polar group, SdO, than CdO in acetone. It is important to study the effects of a polar group in solutes on the concentration dependence of both ν(CH) and δH2O, to elucidate the anomalous behavior of apolar solutes on mixing with water.3 The obtained results are consistent with the hydration mechanism of the CH groups presented in the previous papers.8,9 Experiments and Methods 1. Materials. Water was distilled twice after deionization. Spectrograde DMSO was used. Deuterium oxide (99.9%) and DMSO-d6 were from Aldrich. 2. IR Measurements. IR spectra of DMSO/D2O mixtures and DMSO-d6/H2O mixtures were measured with a JASCO FTIR/620 spectrometer at 23, 48, and 62 °C using a liquid cell of calcium fluoride windows, which was mounted on a water jacket holder to heat the sample solution. IR spectra of DMSO/H2O mixtures were also measured at 22 °C. The resolution was 2 cm-1 and the data were encoded every 0.5 cm-1. 3. NMR Measurements. 1H and 13C NMR spectra of DMSO/ H2O mixtures were measured using a JEOL EX-400 NMR spectrometer operating at 400 MHz for 1H and 100 MHz for 13C at 1.0, 23.3, and 48.5 °C with an accuracy of (0.1 °C. The temperature was calibrated with a thermocouple inserted into the sample tube prior to the measurements. Chemical shifts were determined by the external double reference method9-11 using a homemade external reference tube, a capillary of 2 mm in diameter with a blown-out sphere of 4 mm in diameter at the bottom, which was filled with tetramethylsilane (TMS) to a height of 6 cm and set at the center of the sample tube of 5 mm in diameter. By this method, observed chemical shifts can be corrected precisely for the bulk volume magnetic susceptibilities of the sample and the reference solutions, χV(sam) and χV(ref).12 We applied this method to study the temperature dependence of the chemical shifts. When a capillary with a blown-out sphere at the bottom is used as a container of the reference solution, the NMR spectrum has two peaks for the reference substance in the sphere and in the cylinder.13 The difference in the chemical shift between the two peaks, ∆δref, is expressed in units of ppm
∆δref ) (kc - ks) {χV(sam) - χV(ref)} × 106 ) k {χV(sam) - χV(ref)} × 106
(1)
where kc and ks are the shape factors for the cylinder and the sphere which amount to -4π/3 and 0 for ideal shapes, respectively.
Figure 1. NMR spectra at temperatures T and Tr, where T > Tr, and a chemical shift δcor(T)Tr on a unified scale at a reference temperature Tr.
Taking into account χV(sam) and χV(ref), the chemical shift of a sample solution, δcor, referred to the reference substance in the cylinder is
δcor ) δobs - (4π/3){χV(sam) - χV(ref)} × 106
(2)
where δobs and δcor are the observed and the corrected chemical shifts, respectively.13 Substituting eq 1 into eq 2, we obtain
δcor ) δobs - (4π/3k)∆δref
(3)
which was used to correct the chemical shifts in situ for the bulk volume magnetic susceptibilities. As long as a measuring temperature is kept constant, all the data are referred to a peak of a reference substance at the same temperature. Consequently, it is possible to compare rigorously the data taken under different conditions, such as concentration and a kind of solvent. However, this is not the case for data measured at different temperatures. For a better understanding of the measurement method used to determine the temperature dependence of chemical shifts, NMR spectra at Tr and T are illustrated schematically in Figure 1, where Tr is the reference temperature and δsampleTr and δsampleT are peaks of a sample at Tr and T, respectively. A couple of peaks for the reference substance, δcy and δsp, are supposed to shift to the lower field with increasing temperature, the same as the case of TMS. To unify the scale of chemical shifts at T with the scale at Tr, δsampleT must be referred to 0 ppm at Tr, as shown in Figure 1. Then, δcor(T), referred to 0 ppm at T, is changed into δcor(T)Tr, referred to 0 ppm on a unified scale, which is expressed by the equation
δcor(T)Tr ) δobs + ∆δcy (T) - (4π/3k)∆δref (Tr)
(4)
where ∆δcy(T) is the difference in the δcy between Tr and T, or (δcyT - δcyTr), which is precisely determined in each measurement. Thus, the external double reference method allows us to study the temperature dependence of chemical shifts on a unified scale, which are not easily determined by conventional methods. In the present paper, chemical shifts are referred to liquid TMS at 23.3 °C. The k for the homemade capillary tube was determined by measuring ∆δref in eq 1 for water, MeOH, EtOH, iPrOH, and acetone and plotting the obtained ∆δ ref against the χV(sam) for each liquid listed in the literature.14 The plots gave a straight line, and the obtained k value, -4.09 ( 0.06, is close to the shape factor for the ideal sphere and cylinder, -4π/3 ()-4.19). The digital resolutions of the chemical shifts were
Hydration of CH in DMSO by NMR, IR
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Figure 3. Concentration dependence of chemical shifts for DMSO/ H2O mixtures at 1.0, 23.3, and 48.5 °C: (A) δH2O, (B) δCH3, and (C) δCH3.
Figure 2. Concentration dependence of IR frequencies at 23 °C: (A) νa(CH) and νs(CH) in DMSO/D2O mixtures, (B) νa(CD) and νs(CD) in DMSO-d6/H2O mixtures, and (C) ν(SO) in DMSO/H2O mixtures plotted vs mole fraction of D2O or H2O.
0.0012 and 0.0073 ppm, and the total experimental errors were within (0.002 ppm and (0.02 ppm for 1H and 13C, respectively. Results 1. IR Study. An IR spectrum of DMSO has two narrow absorption bands of asymmetric and a symmetric CH stretching modes at 2997 and 2914 cm-1, respectively. Besides these, there exists a band with a strong absorption intensity at 1042 cm-1 assigned as a SdO stretching mode. Concentration and temperature dependences of the wavenumbers of the CH bands, denoted as νa(CH) and νs(CH) hereafter, were studied for DMSO/D2O mixtures to avoid the spectral overlap of the CH and H2O bands. The νa(CH) and νs(CH) observed at 23 °C are plotted against the mole fraction of D2O, XD2O, in Figure 2 (A). Concentration dependences of the νa(CD) and νs(CD) for DMSO-d6/H2O mixtures were also measured as plotted as a function of XH2O in Figure 2 (B) and the wavenumber of the
SdO stretching mode, ν(SO), for DMSO/H2O mixtures is shown in Figure 2 (C). The νa(CH) and νs(CH), abbreviated as ν(CH) below, and νa(CD) and νs(CD) denoted as ν(CD) below, increase with increasing water fraction, while ν(SO) decreases as previously reported in the Raman study.4 The redshift in ν(SO) is attributed to a hydrogen-bonding interaction of the Sd O oxygen with water. It is remarkable that ν(CH) decreases in contrast to the almost constant ν(CD) in the water rich extreme. The temperature dependence of ν(CH) provides information about the contribution of a hydrogen-bonding interaction to the blueshifts. We observed no appreciable change in ν(CH) and ν(CD) on increasing the temperature from 23 to 48 and 62 °C. 2. NMR Study. Figure 3 (A) shows the concentration dependence of δH2O for DMSO/H2O mixtures at 1.0, 23.3, and 48.5 °C. The δH2O at XH2O ) 0.05 is ca. 3.6 ppm, indicating much less polarization of the water molecules in the DMSO rich region than in pure water. δH2O increases to regain the value for pure water at XH2O ) 0.80. In the region of XH2O > 0.80, δH2O exceeds the value, indicating the formation of water more polarized than pure water. On heating, δH2O becomes smaller at an almost constant rate up to XH2O ) 0.7. But the temperature effect on δH2O becomes stronger in XH2O > 0.7, suggesting that the hydrogen-bonding character of the water molecules is intensified in XH2O > 0.7 compared with the region of XH2O < 0.7.
11004 J. Phys. Chem. B, Vol. 104, No. 47, 2000 Figure 3 (B) (C) shows the concentration and temperature dependence of 1H and 13C chemical shifts for the methyl form of DMSO, δCH3 and δCH3, respectively. The net change in δCH3 is limited to only 0.2 ppm, but δCH3 varies in a complicated manner with concentration and temperature. δCH3 increases for XH2O < 0.4, remains almost unchanged for 0.4 < XH2O < 0.7, then decreases in the water rich region of XH2O > 0.7. Based on the excess partial molar enthalpies, entropies, Gibbs energies, and volumes, Koga et al. identified three different mixing schemes for these composition ranges.15 The correspondence of the three composition ranges for δCH3 to the thermodynamic properties is remarkable. On the other hand, δCH3 decreases monotonically on dilution, indicating an increase in the electron density about the methyl carbon. δCH3 and δCH3 become larger with increasing temperature contrary to δH2O, implying that the type of intermolecular interaction of the CH group is not hydrogen bonding but dispersion. What is of particular importance is that a temperature dependence of δCH3 in XH2O > 0.7 differs from that in XH2O < 0.7. In XH2O < 0.7 δCH3 at 1.0 °C is smaller than those at higher temperatures, whereas in XH2O > 0.7 δCH3 is almost temperature independent in the water rich extreme, indicating that the CH hydrogen takes part in an interaction of hydrogen bonding character in addition to the dispersion interaction. Discussion The Mechanism of the Blueshifts in ν(CH). We first compare the concentration dependence of ν(CH) for DMSO/ D2O mixtures with that of ν(CD) for DMSO-d6/H2O mixtures. As seen from Figure 2 (A) and (B), ν(CH) values have maxima, while ν(CD) values remain the highest in the water rich extreme. ν(CD) for pure DMSO-d6 reduces to 0.75 times the ν(CH) for pure DMSO, respectively, due to the isotopic shift. The net blueshifts in ν(CD) during the dilution, however, amount only to 0.65 times of those expected for the isotopic shifts in ν(CH). Tamura et al. reported that hydrogen bonding interactions in DMSO/D2O mixtures are stronger than those in DMSO/H2O mixtures.16 Then it follows that the blueshifts in ν(CD) will be less than in ν(CH) and the difference in the concentration dependences between ν(CD) and ν(CH) in the water rich region may be related to the weaker hydrogen bonding interactions and a reduced polarization of the water molecules in DMSOd6/H2O mixtures than in DMSO/D2O mixtures. We now focus our discussion on a mechanism of the blueshifts in ν(CH) and ν(CD), taking into account the concentration dependence of δCH3 and δCH3 together. As shown in Figure 3 (C), δCH3 decreases on dilution with water, indicating a progressive increase in the electron density about the CH carbon. Such an increase cannot be associated with the hydrogen bonding interaction between the SdO oxygen and water, which should have brought an increase in δCH3 because of electron withdrawing by the oxygen. Consequently, the decrease should be attributed to a change in the hydration state of the CH groups. As shown in Figure 3 (B), δCH3 increases very slightly with increasing XH2O up to 0.7. This, together with the decrease in δCH3, suggests the progressive polarization of the CH bonds, contrary to the depolarization indicated experimentally by the blueshifts in ν(CH). These results are the same as those obtained for aqueous acetone8 and aqueous tert-butyl alcohol mixtures;9 with increasing XH2O, the electron density increases both around the CH carbon nuclei and on the CH bond, whereas it decreases slightly around the CH proton. The only possible explanation is that the electron of the CH hydrogen atom is pushed toward the
Mizuno et al.
Figure 4. Pushball hydration (H2O>>>) model in aqueous DMSO mixtures.
CH bond by the water oxygen atom due to an electronic repulsion and/or dispersion interaction between the hydrogen and water oxygen atoms, which should contract the CH bond giving rise to the blueshifts observed. We named this mechanism pushball hydration in our previous work.9 Figure 4 illustrates pushball hydration denoted by >>>. Water molecules are connecting intermolecular CH and SdO groups in Figure 4, though intramolecular bridges due to pushball hydration may also be possible. What should be noted is the polarization of the CH bond as indicated by Cδ-Hδ+, compared with those in pure DMSO. Symons et al. concluded that there is a formation of 1:2 complex between the SdO oxygen and water.17 Although only 1:1 hydrogen bondings are drawn in Figure 4, formation of the 1:2 complex may be also possible. Figures 2 and 3 (A) show that the blueshifts in ν(CH) and ν(CD), the redshift in ν(SO), and the polarization of the water molecules vary concomitantly with increasing water fraction up to XH2O ) 0.98 and XD2O ) 0.93, indicating that these are correlated with each other. In other words, the increase in water concentration brings about not only the formation of linking hydrogen bonds OH2(‚‚‚OH2)n(n ) 0,1,2,...) due to the so-called cooperative effect of hydrogen bonding, but also the increasing redshift of ν(SO). Consequently, the blueshifts are supposed to be caused by the pushing effect of the water molecules which are polarized more and more with increasing XH2O. Then it follows that the formation of a hydration cluster of DMSO in the water rich region in which the CH groups are hydrated by the surrounding water molecules may result in the anomalously polarized water molecules, as verified by δH2O being larger than that of pure water. In the present model, we lay particular stress on the increase in the intramolecular polarization of the DMSO molecule, as characterized by the alternative arrangement of partially negative and positive ion atoms as indicated by Oδ-d Sδ+-Cδ--Hδ+, which seems to result in stable hydration clusters. The decreases in ν(CH) and the invariability of ν(CD) in the water rich extreme, on the other hand, may be explained in terms of a weak polarization of the CH bond induced by the anomalously polarized water molecules as probed by the extraordinarily large δH2O. In other words, the interaction between the CH hydrogen and the surrounding water molecules may have some hydrogen bonding character as described above. The present model gives a consistent account of the temperature dependence of δCH3, the difference in the extent of blueshifts between ν(CH) and ν(CD), and the difference in concentration dependence in the water rich region. Although the blueshifts in the CH stretching modes have been reported for many organic compounds in aqueous mixtures,4-7 no attention has been paid so far to the relationship between the blueshifts and the increasing polarization of the water molecules. The present model gives an explanation of the concentration dependences of ν(CH) and ν(CD) over the whole concentration range in terms of the varying polarization of the water molecules. Comparison of δH2O Among Aqueous Mixtures of Acetone, TBA, and DMSO. We have measured NMR and IR
Hydration of CH in DMSO by NMR, IR
J. Phys. Chem. B, Vol. 104, No. 47, 2000 11005 because the polarization of the water in the aqueous mixtures of alkanol from methanol to TBA varies with concentration and increases with the size of the alkyl group.9 These results lead to a suggestion that the polarization of water molecules in biological systems should vary over a wide range, since they are composed of many kinds of molecules with hydrophobic parts of various sizes and hydrophilic groups of various polarity. The present results imply that the wide range of polarization of water molecules makes it possible for water to accommodate various biological systems including even a hydrophobic region such as the internal parts of globular proteins. Conclusions
Figure 5. Concentration dependence of δH2O for aqueous binary mixtures of DMSO, acetone, and tert-butyl alcohol at 23.3 °C.
spectra of aqueous binary mixtures of acetone and tert-butanol(TBA), and found the same increasing δH2O and blueshifts in ν(CH) as observed in the present study of aqueous DMSO mixtures. Since all the chemical shifts were measured by the external double reference method and referred to liquid TMS at 23.3 °C, δH2O for the three aqueous mixtures can be compared with each other as shown in Figure 5. Apparently the two hydrophilic groups, SdO and CdO, affect the liquid structure differently; DMSO, with a stronger electron-donating group in hydrogen bonding than acetone, induces more polarization in the surrounding water molecules. Even in the water rich region, the SdO group in DMSO induces the anomalous polarization of the water molecules more strongly than the CdO group in acetone, reinforcing the role of hydrophilic groups in the formation of hydration clusters and in inducing the anomalous polarization of water molecules mentioned above. As for aqueous TBA mixtures, δH2O in the TBA rich region, 4.3 ppm, is larger than δH2O for DMSO and acetone systems despite the larger hydrophobic part in TBA. This fact reveals that self-associating OH groups in TBA molecules are more effective than SdO of DMSO in polarizing water, and that the size of the hydrophobic part is not the dominant factor to determine the δH2O value even in the solute rich region. However, the present results do not exclude the possible role of the hydrophobic part in determining the polarization of the water molecules in aqueous mixtures of organic compounds,
In aqueous DMSO mixtures, the polarization of the water molecules probed by δH2O varies with concentration from 3.6 ppm at XH2O ) 0.05 to 4.95 ppm in the water rich region. The concentration dependence of ν(CH) and ν(CD) can be related with that of the polarization of the water molecules. Thus, the pushball hydration model proposed for the hydration of the CH groups in acetone and tert-butyl alcohol also applies to the methyl groups of DMSO. The strong hydrophilic >SdO group in DMSO has been found to be a key site for inducing the anomalous polarization of water molecules in the water rich region and plays a dominant role in the formation of the hydration cluster of DMSO. Acknowledgment. We would like to express our sincere thanks to Professor Y. Shindo, Professor S. Ikeda, and Professor M. Mekata of Fukui University for helpful and stimulating discussions. References and Notes (1) Franks, F.; Desnoyers, J. E. Water Science ReViews 1; Franks, F., Ed.; University Press: Cambridge, U.K., 1985; p 171. (2) Frank, H. S.; Evans, M. W. J. Chem. Phys. 1945, 13, 507. (3) Blokzil, W.; Engberts, J. B. F. N. Angew. Chem., Int. Ed. Engl. 1993, 32, 1545. (4) Carius, W.; Mockel, K.; Schroter, O.; Thomzik, D. Z. Phys. Chem. Leipzig 1982, 263, 209. (5) Tomzik, D.; Schroder, O.; Walter, I.; Mockel, K. Z. Phys. Chem. Leipzig 1990, 271, 347. (6) Kamogawa, K.; Kitagawa, T. J. Phys. Chem. 1986, 90, 1077. (7) Kamogawa, K.; Kitagawa, T. Chem. Phys. Lett. 1991, 179, 271. (8) Mizuno, K.; Ochi, T.; Shindo, Y. J. Chem. Phys. 1998, 109, 9502. (9) Mizuno, K.; Kimura, Y.; Morichika, H.; Nishimura, Y.; Shimada, S.; Maeda, S.; Imafuji, S.; Ochi, T. J. Mol. Liq. 2000, 85, 139. (10) Momoki, K.; Fukazawa, Y. Anal. Chem. 1990, 62, 1665. (11) Momoki, K.; Fukazawa, Y. Anal. Sci. 1994, 10, 53. (12) Becker, E. D. High-Resolution NMR: Theory and Chemical Applications, 2nd ed.; Academic Press: New York, 1980; pp 38-39, 5051. (13) Mulay, L. N.; Haverbusch, M. ReV. Sci. Instrum. 1964, 35, 756. (14) Handbook of Chemistry and Physics, 67th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1994; p 8-57. (15) Lai, J. T.; Lau, F. W.; Robb, D.; Westh, P.; Nielsen, G.; Trandum, C.; Hvidt, A.; Koga, Y. J. Solution Chem. 1995, 24, 89. (16) Miyai, K.; Nakamura, M.; Tamura, K.; Murakami, S. J. Solution Chem. 1997, 26, 973. (17) Symons, M. C. R.; Eaton, G. J. Chem. Soc., Faraday Trans. 1, 1985, 81, 1963.
