J . Phys. Chem. 1990, 94, 8614-8617
8614
Structural Effect on the Redox Thermodynamics of Poly(th1ophenes) Pascal Marque Corning Europe Inc., 7 bis avenue de Valuins, 7721 I Auon, France
and Jean Roncali* Laboratoire des MatPriaux MolEculaires. CNRS E R 241, 2 rue Henry Dunant, 94320 Thiais, France (Received: March 7, 1990)
The redox thermodynamics of poly(thiophene) (PT), poly(3-methylthiophene) (PMeT), and poly(3-nonylthiophene) (PNT) constructed from the in situ absorbance measurements performed have been analyzed by using the Nernst plots ( E vs log [O]/[R] at various doping levels. The apparent standard potential Eo’ and the initial slope of the Nernst plots of the oxidation process decrease in the order PT > PMeT > PNT. Concurrently, the redox process becomes progressively more complex with an increasing deviation from linearity above Eo’ and the appearance of two distinct oxidation stages for PMeT and PNT. Whereas hysteresis is evident for PT and PMeT, the redox process appears fully reversible in the case of PNT. Although the slope corresponding to initial step of the charging process decreases from PT to PNT, it remains of much larger magnitude than expected for a simple one-electron redox couple. These super-Nernstian behaviors are interpreted by (i) a decrease of the interactions among charged sites due to modifications in the populations of polarons and bipolarons caused by the structurally induced extension of the conjugation and by (ii) the overpotential related to the contribution of the mechanical strain associated with the insertion of the charge-compensating anion in the polymer matrix.
Introduction Electroactive conducting poly(thiophenes) (PTs) are presently subject to an intense research effort related to their potential use as electrode materials in various technological applications such as organic batteries,\ electrochromic devices: microelectrochemical transistor^,^ and selective modified electrode^.^ Although each of these applications implies a different conceptual approach, they all involve the electrochemical reversibility of the transition between the undoped neutral state and the doped conducting state. This transition, which is achieved by application of an adequate potential to the polymer electrode, involves the simultaneous transport of charges and ionic species in the polymer matrix. The final doped conducting state is characterized by the presence of positive charges associated with the charge-compensating anions on the polymer chains. Despite an apparent simplicity, the redox processes of conjugated polymers involve complex mechanisms due to the fact that the transition between the neutral and the oxidized state is accompanied by important modifications of the mass,5 volume,6 chain geometry,’ resistance,8 and capacitance9 of the polymer. Although the mechanisms of the doping/undoping processes of PTs have been already subject to several electro( I ) (a) Yamamoto, T. J . Chem. Soc., Chem. Commun. 1981, 187. (b) Kaneto, K.; Yoshino, K.; Inuishi, Y. Jpn. J . Appl. f h y s . 1983, 22, L 567. (c) Panero, S.;Prosperi. P.; Klaptse. B.; Scrosati, B.Electrochim. Acta 1986, 31, 1. 597. (2) (a) Garnier, F.; Tourillon, G.;Gazard, M.; Dubois, J. C. J. Elecrrwml. Chem. 1983, 148, 299. (b) Kaneto, K.; Yoshino, K.; Inuishi, Y. Jpn. J . Appl. f h y s . 1983, 22, L 412. ( 3 ) Thackeray, J. W.; White, H. S.; Wrighton, M. S. J. Am. Chem. Soc 1985, 89. 5133. (4) (a) Lemaire, M.; Delabouglise, D.; Garreau, R.; Guy, A,; Roncali, J. J . Chem. Soc.. Chem. Commun. 1988. 658. (bl Roncali. J.; Garreau R.; Delabouglise, D.:Garnier, F.; Lemaire, M. J . &em. Soc., Chem. Commun. 1989, 679. ( 5 ) Kaufman, J. H.; Kanazawa, K. K.; Street, G. B. f h y s . Rev. f e r ? . 1984, 53, 2461. (6) Tourillon, G.: Garnier, F. J . folym. Sci. f o l y m . f h y s . Ed. 1984, 22,
TABLE I: Data from Nernst Plots of PT,PMeT, and PNT
polymer PT PMeT PNT
Eo’(ox), V/Ag 0.75 0.62 0.48
sloPe(ox),
mV/log unit 230
160 120, 390, 220
slope(red), mV/log unit 300, 195 330, 120 390, 120
chemical studies,1° the thermodynamical aspect of these processes have attracted little attention until now. In this paper, a comparative analysis of the redox thermodynamics of three electrogenerated PTs, e.g., poly(thiophene) (F’T), poly( 3-methylthiophene) (PMeT), and poly(3-nonylthiophene) (PNT) has been carried out using the Nernst plots constructed from absorbance measurements performed in situ at various doping levels. It is shown that the introduction of alkyl substituents of increasing length leads to (i) a more complex redox behavior with the progressive appearance of two distinct oxidation stages and (ii) a decrease of the apparent standard potential Eo’ and of the deviation from Nernstian behavior. Although hysteresis appears in the charging/discharging process for PT and PMeT, in the case of PNT, the presence of the long alkyl chain on the polymer backbone leads to a significant increase of the reversibility. Although the slope of the Nernst plots decreases from PT to PNT, it remains in each case of much larger magnitude than expected for a simple monoelectronic transfer. These super-Nernstian behaviors are discussed in terms of changes in the interactions among charged sites resulting from the structurally induced extension of the conjugation and also by taking into account the contribution of the mechanical strain associated with the insertion of ionic species and solvent in the polymer matrix.
Experimental Section Thiophene and methyl-3-thiophene (Aldrich) were distilled twice under argon prior to use, and 3-nonylthiophene was prepared as already reported.” Solvents and electrolytes were purified according to the previously described procedures.’* Electropo-
33. (7) Heinze, J.; Storzbach, M.; Mortensen, J. Ber. Bunsen-Ges. f h y s . Chem.
1987, 91. 960.
(8) (a) Ofer. D., Wrighton, M. S.J. Am. Chem. Soc. 1988,110,4467. (b) Olmedo, L.; Chanteloube, 1.; Germain, A.; Petit, M.; Genies, E. M. Synth. Mer. 1989, 30, 159. (9) (a) Bull, A. R.; Fan, F. R.; Bard, A. J. J . Electrochem. Soc. 1982, 129, 1013. (b) Tanguy. J.; Mermilliod, N.; Hoclet, M. J . Electrochem. Soc. 1987. 134, 795.
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(IO) (a) Waltman, R. J.; Bargon, J.; Diaz, A. F. J . fhys. Chem. 1983.87, 1459. (b) Marque, P.; Roncali, J.; Garnier, F. J . Elecrrounul. Chem. 1987, 218, 107. (c) Sewagent, S.; Viel, E. Synth. Met. 1989, 31, 127. (d) Reynolds, J. R.; Shing-Guang Hsu; Arnott, H. J. J . folym. Sci. B 1989, 27, 2081. (e) Zotti, G.; Schiavon, G . Synth. Mer. 1989, 31, 347. ( I I ) Lemaire, M.; Garreau, R.; Garnier, F.; Roncali, J. New J. Chem. 1987. 11, 703. 0 1990 American Chemical Society
Redox Thermodynamics of Poly(thiophenes)
I t
.5l l ; ,
.a'
0
-1
PNT.I5*l6 As appears in Table I, the slope corresponding to the beginning of the oxidation process decreases from 230 mV/log unit for PT to 130 mV/log unit for PNT. The correlation of this sequence with that of the Eo' values indicates, as could be expected, that the oxidation of polymers with higher apparent standard potential requires more energy. The plots corresponding to the oxidation process appear linear for PT, whereas for PMeT, the slope shows a slight increase beyond log [O]/[R] = 1. The behavior of PNT appears more complex and three distinct regions can be distinguished that correspond to slopes of 13OmV/log unit up to log [O]/[R] = -0.2, 390 mV/log unit for -0.2 < log [O]/[R] +0.6 and 220 mV/log unit beyond log [O]/[R] = +0.6. These results that suggest the occurrence of distinct oxidation stages for PMeT and PNT appear consistent with the conclusions of a recent spectroelectrochemical analysis of substituted PTs in which we have shown that PNT differs from PMeT by a more intense absorption at 1.5 eV (which corresponds to the higher energy bipolaron band) at intermediate doping levels and by a more pronounced transition toward a metallic-like behavior at high doping level.I6 This latter transition occurs at 0.85 V/Ag, Le., the potential for which the slope decreases at log [O]/[R] = 0.6 (Figure IC). Figure 1 shows that the structure of the polymer affects also the undoping process which exhibits an increase of the initial slope and an increasing deviation from linearity around log [O]/[R] = 0 from PT to PNT. The comparison of the oxidation and reduction processes shows that hysteresis occurs for PT and PMeT whereas despite a greater complexity, the redox process is reversible for PNT which is consistent with the high symmetry of its cyclic ~oltammogram.'~~*'~ These results show that the polymer structure exerts a strong effect on the redox thermodynamics of PTs and that the grafting of long alkyl chains on the conjugated backbone leads simultaneously to a decrease of EO', to a significant increase of reversibility, and to the appearance of distinct oxidation stages during the charging process. Although the initial slope of the oxidation process decreases from 230 mV/log unit for PT to 130 mV/log unit for PNT, it remains in each case of significantly larger magnitude than the 59 mV/log unit expected for a simple monoelectronic redox couple. Super-Nernstian behaviors have been already observed in the redox processes of conducting polymers. Thus, recently, "fractional" electronic charge transfer has been proposed to account for the relationships between the open circuit potential and the doping level in polyacetylene cells.17 The non-Nernstian behavior of polypyrrole has been also interpreted by partial charge transferIs or by repulsive interactions between charged sites.Ig These super-Nernstian behaviors that seem to be inherent to the redox processes of conjugated polymers are probably related to the specificity of the mechanism of charge storage in these materials. As a matter of fact, and contrarily to nonconjugated redox polymers, the concentration and spatial localization of active sites in conjugated polymers depend on the oxidation state of the polymer. Furthermore, it has been shown that different charged species, Le., polarons and/or spinless bipolarons, can be simultaneously or successively involved in the mechanism of charge storage in conjugated polymers. These charged species differ by the energy required for their creation and by the minimal conjugation length required by their spatial extension.20 Thus in the case of short chain phenylene-vinylene oligomers, the charges are
:,=-
0
-2
The Journal of Physical Chemistry, Vol. 94, No. 23, 1990 8615
0
1
2
log[O]/[R]
Figure 1. Nernst plots recorded in 0. IO M LiCLO, in CH$N at various ratios of the concentration of oxidized and reduced sites ([O]/[R]): (a) on ITO. 0 , PT, (b) PMeT, (c) PNT. Deposition charge 60 mC oxidation, 0,reduction.
lymerizations were carried out in galvanostatic conditions (5 mA c d ) from a reaction medium involving 0.10 M monomer and 0.04 M Bu,NPF6 in nitrobenzene. The synthesis medium was degassed by argon bubbling prior to electropolymerization which was performed under an argon atmosphere. The films were grown on indium-tin oxide (ITO) coated glass electrodes with a deposition charge of 60 mC c d . After electrosynthesis, the films were electrochemically undoped, rinsed with CH,CN, and placed in a 1 X 1 cm quartz cell containing 0.10 M LiCIO, in dry CH3CN. A Pt ring allowing the passage of the analyzing beam was used as counter electrode and an Ag wire, the potential of which is very close to that of the Ag/AgCl electrode,I3 was used as a reference electrode. Absorbance measurements were performed at various values of the applied potential after equilibration of the film. The ratio of the concentration of oxidized and reduced sites [O]/[R] was determined by using [O]/[R] = (Ard - A ) / A - A,,), where A is the absorbance at a given applied potential, A,, the absorbance of the fully oxidized film, and Ard that of the fully reduced state.I4 Absorbances were measured at 500,520,and 540 nm for PT,PMeT, and PNT, respectively. These wavelengths correspond to the absorption maxima of the reduced polymers and to the maximum contrast between the doped and undoped states in the visible spectral region. Spectroelectrochemistry was performed on a Cary 219 spectrometer and the applied potential was controlled by a PAR 173 potentiostat. Results and Discussion
Figure la-c shows the plots of the polymer potential as a function of the logarithm of [O]/[R] for the three polymers. The comparison of these Nernst plots and the corresponding data in Table 1 shows that the apparent standard potential (E"') obtained from the plots of the oxidation process decreases from 0.75 V for PT to 0.48V for PNT. These results are in close agreement with previous electrochemical and spectroscopic analyses that have shown that the mean conjugation length increases from PT to (12) Roncali, J.; Gamier. F. New J . Chem. 1986, IO, 237. ( I 3) Genies, E. M.; Lapkoswki, M. J . Electroanul. Chem. 1987,236, 189. (14) Heineman, W. R.; Hawkridge, F. W.; Blount, H. N . In Electroonalyrical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, p I .
(15) (a) Roncali, J.; Gamier, F.; Lemaire, M.; Garreau, R. Synth. Mer. 1986, 15,323. (b) Roncali, J.; Garreau, R.; Yassar, A.; Marque, P.; Gamier, F.; Lemaire, M. J . Phys. Chem, 1987, 91, 6706.
(16) Roncali, J.; Marque, P.; Garreau, R.; Gamier, F.; Lemaire, M. Macromolecules 1990. 23. 1347. (17) Kaner, R. B.;'Porier, S.J.; Nairns, D. P.; MacDiarmid, A . G . J . Chem. Phys. 1989, 90, 5102. (18) Diaz, A. F.; Castillo, J . 1.; Logan, J . A.; Wen-Yaung Lee, J . Electroanal. Chem. 1981, 129, 115. (19) (a) Genies, E. M.; Pernaut, J. M. Synth. Met. 1984/85, IO, 117. (b) Nechtschein, M.; Devreux, F.; Genoud, F.; Vieil, E.; Pernaut, J. M.; Genies, E. Synth. Met. 1986, I S , 59. (20) Brtdas, J. L.; Themans, B.; Andre, J . M.; Chance, R. R.; Silbey, R. Synth. Met. 1984, 9, 265.
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The Journal of Physical Chemistry, Vol. 94, No. 23, 1990
stored essentially as polarons due to the limited extent of the conjugation length.2' Similarly, it has been shown that in polypyrrole, which has an average conjugation length limited to 4-5 monomer units,22polarons are the dominant charged species with one spin per charge up to ca. half of the maximum doping leveLigb In the case of PTs, the data are less conclusive since spin to charge ratios ranging from to 1 have been r e p ~ r t e d . ~ These ~.~~ discrepancies may arise from the different conditions used for the synthesis of the polymers that are known to exert a determining effect on the mean conjugation length and conductivity.'2q2s On the other hand, bipolarons have been shown to be the predominant charged species in extensively conjugated PTsZ6 On the basis of these observations, the decrease of the initial slope of the Nernst plots from PT to PNT could be explained by a progressive decrease of the Coulombic interactions between charged sites caused by a progressive decrease of the population of polarons to the benefit of that of bipolarons. This process being related to (i) the extension of the conjugation which allows as easier accomodation of bipolarons and (ii) an improved long-range order leading to a faster and more efficient recombination of polarons into bipolarons. Besides this "classical" interpretation in terms of interactions among charged sites, the super-Nernstian behavior of conjugated polymers can be interpreted from another point of view more specifically related to the electrochemical mechanisms of the redox process. It has been shown already that the doping of PTs is accompanied by an expansion of the polymer volume due to the electrostatically forced insertion of the charge-compensating anions in the polymer lattice. Thus, without considering either the swelling of the polymer by the solvent or the solvatation of the counterions, the charge-compensating anions are expected to occupy ca. 10% of the volume of the doped polymer. In fact, transmission electron microscopy has shown that the diameter of PMeT fibers increased from 25 to 80 nm upon doping6 This considerable increase of volume can be expected to affect the thermodynamics of the redox processes in two different ways. On the one hand, the expansion of the polymer volume will generate distortions in the polymer chains and hence modify the mean conjugation length. As already discussed, this modification will affect the balance of the polarons/bipolarons populations, the efficiency of the recombination of polarons into bipolarons, and hence the interactions between charged sites. On the other hand, it is likely that this volume expansion is accompanied by a significant increase of the internal pressure of the polymer matrix. This effect leads to a mechanical strain resulting in an additional contribution to the electrode potential. This problem has been recently analyzed by Evans et al. who have developed a theoretical model to account for the non-Nernstian behavior of poly(viny1ferrocene) films.27 In the simple case of a reversible isotropic stress/strain behavior, the electrode potential as a function of the fraction of oxidized sites f is expressed as E = Eo' + ( R T / n F ) In [ I
-fl + nVa2cKf/(lO4)Fz2(1)
where K is the elastic modulus of the polymer, c is the concentration of active sites, and V, and z are respectively the molar volume and the charge of the counterion. This model predicts that the mechanical work term associated with the elastic expansion of the polymer matrix upon insertion of the counterion increases with the elastic modulus, the concentration of sites, and the volume of the incorporated anion. In order to evaluate the validity of this model in our case. the difference ( A E ) between (21) Heinze, J.; Mortensen, J.; Mullen, K.; Schenk, R. J . Chem. SOC., Chem. Commun. 1987, 701. (22) Li, Y.: Qian, R. Synrh. Mer. 1989, 28, C127. (23) Colaneri, N . ; Nowak, M.; Spiegel, D.; Hotta, S.:Heeger, A. J. Phys. Reu. B 1987, 36, 7964. (24) Scharli. M.; Kiess, H.; Harbeke, G.; Berlinger, W.; Blazey, K. W.: Muller. K. A. Svnrh. Met. 1988. 22. 317. (25) Yassar.'A.; Roncali, Garnier, F. Macromolecules 1989, 22, 804. (26) Chung. T. C.: Kaufman, J. H.; Heeger, A . J.: Wudl, F. Phys. Reo. B 1984. 30. 702. (27)'(a)'Bowden, E. F.; Dautartas, M. F.; Evans, J . F. J . Electround. Chem. 1987.219.49. (b) Dautartas, M. F.; Bowden, E. F.; Evans, J. F. Ibid. 71. (c) Bowden. E. F.;Dautartas, M . F.: Evans, J . F. 1987, 219, 91.
Marque and Roncali the potential measured a t f = 1 and that expected for a simple Nernstian process, Le., 330 and 480 mV for PT and PNT, respectively, was assumed to result from the additional energy required to achieve the insertion of the counterions and the molar volume of the inserted anion was calculated with eq I . The concentrations of active sites, calculated from the doping levels and from the relationships between the deposition charge and the films thicknesses, were respectively 5.9 X and 1.4 X IO-' mol/cm3 for PT and PNT. The K value of 1.8 X 1 OIo dyn/cm2 was used for PT,28and since to our knowledge the K value of PNT has not been determined, the value of low-density polyethylene, Le., 0.5 X IO'O dyn/cm2 was used as a lower limit for that of PNT since the alkyl chain represents ca. 60% in weight of the polymer structure. These data yield V, values of 55 and 260 cm3 per mole of active sites for PT and PNT, respectively. Considering that the insertion of the counterion is accompanied by the insertion of a significant amount of solvent and ion pairs,29 the V, value calculated for PT shows a satisfying agreement with the molar volume of the perchlorate ion (39.4 ~ m ~ / m o I Several ) . ~ ~ factors must be considered to explain the large discrepancy observed in the case of PNT. First, it is clear that the assumed K value for PNT is underestimated; however, using the K value of PT still yields V, = 135 cm3/mol. Second, on the other hand, we have previously observed that PNT incorporates much larger amounts of solvent than PT and PMeT due to its more porous morphology.lSb Finally, another possible cause for this high V, value can involve the incorporation of a larger concentration of ion pairs in PNT. As a matter of fact, it has been shown that LiC104 undergoes significant contact ion pairing in acetonitrileM and such ion pairs have been already detected in PMeT.29 From this point of view it is probable that the low polarity of the environment constituted by alkyl chains in PNT allows the presence of larger ion pairs concentrations than in PT or PMeT. Thus, taking account of these various factors, the overall volume of species inserted in the PNT matrix can be expected to be much larger than that of the C10; ion alone. In summary, these results show that taking into account the contribution of the mechanical work associated with the electrostatically forced intrusion of the charge-compensating anions is worth considering and can contribute to accounting for the super-Nernstian behavior of conjugated polymers.
Conclusion The redox thermodynamics of three PTs have been analyzed by using the Nernst plots determined spectroelectrochemically. The decrease of ED' in the order PT > PMeT > PNT confirms the extension of the mean conjugation length associated with the introduction of alkyl substituents. The initial slope corresponding to the charging process decreases according to the same sequence whereas, for the substituted polymers, the slope increases above Eo'which suggests the Occurrence of successive distinct oxidation stages. In the case of PNT, this more complex behavior is associated with a significant improvement of the reversibility compared to PT and PMeT. Two different interpretations have been proposed to account for the super-Nernstian behavior observed with the three polymers. The first, which resorts to the classical Coulombic interactions between charged sites, implies that the extension of conjugation from PT to PNT results in an increase of the concentration of bipolarons which leads to a decrease of the Coulombic interactions between charged sites. The second takes into account the contribution to the electrode potential of the mechanical work required to achieve the expansion of the polymer lattice caused by the insertion of counterions and solvent molecules. Although this latter point of view highlights an aspect of the redox process of conjugated polymers which has not been considered until now, it is clear that it cannot be considered independently of its consequences on the mechanisms of charge (28) Osawa, S.;Ito, M.; Tanaka, K.: Kuwano, J . Synth. Met. 1987, 18, 145. (29) Baudoin. J. L.; Chao, F.: Costa, M. J . Chim. Phys. 1989, 86, 181. (30)Berman, H. A.; Stengle, T. J . Phys. Chem. 1979, 79, 1001.
J . Phys. Chem. 1990, 94, 8617-8620
storage and that both aspects are interdependent. As a matter of fact, the distortions of the conjugated system caused by the expansion of the polymer matrix are likely to affect the nature and the concentration of the species involved in the charge storage while on the other hand the geometrical relaxation associated with the formation of polarons and bipolarons can affect the overall mechanical contribution to the electrode potential. These results
8617
outline thus the necessity to develop a model more specifically adapted to the various aspects of the redox processes of conjugated polymers. Acknowledgment. We thank Corning Europe Inc. for meeting the Publication costs of this article. Registry No. FT, 25233-34-5; PMeT, 84928-92-7; PNT, 110851-64-4.
The Ion-Dipole Interaction Mechanism of Macrocyclic Ethers with 13C Dipole-Dipole Relaxation Time Measurements. 6+ Cakil Erk* Chemistry Department, Faculty of Art and Science, Istanbul Technical University, Maslak, 80626. Istanbul, Turkey (Received: February 13, 1990: In Final Form: May 9, 1990)
The stabilities of various stoichiometries of the 12-crown-4ether complexes with LiCl in methanol-d were estimated by an NMR method using I3C TI relaxation time measurements. The experimental coordination conditions are established by maintaining an identical host/guest concentration in order to achieve the actual compositions. The evidence was evaluated with the expression for the dependence of the mole fraction of the bound ligand, PAE,to the experimental values of the dipole-dipole relaxation time, TI*, PAE = ( l/Tlob"- I/TE)/(I/TAE- l / T E ) .The equilibrium constants K, for Li+ complex formation in methanol-d were obtained through the expression and regression of I/K,[Eo]"+ml = ( 1 - nP',Y(I - mPqm/P', where P ' = P A E / ( 1 + PAE(m- I)). Although the method presented is entirely different from the common analytical methods, the results are excellent in terms of agreement with earlier studies. The extension of a further degree of complexing ratios has also been proven with the simulation of TAE.
Introduction The nature of the ion-dipole interactions of macrocyclic ethers and their derivatives has been investigated by several methods.'" Recently, NMR spectroscopy dealing with the T I relaxation time measurements of the macrocyclic backbone of free and complexed ligands in solution has provided a new way of approaching this topic6-I4 The scope and the limitations of the method with respect to the study of protons were earlier discussed by Hertz9 and later reviewed by Deverel.l0 Similarly, alkali metal ion NMR spectroscopy research carried out by some laboratories has provided considerable information regarding ion and counterion mobility, as has been reported by Popov and co-workers,Il by Schneider and co-workers,l* and by Kowalewski and co-workers,13 respectively. On the other hand, as we recently pointed out, ion solvation or complexation of an electtostatical nature is not that simple, and the ability to predict uniform shell structure is not very likely, due to the ion-dipole exchange which takes places in different steps which possess similar rates, particularly for small ligands such as dioxane. Therefore, the study of the ion-dipole interaction is primarily a question of the degree of solvation which is difficult to estimate adequately. Accordingly, the complicated stereochemistry of the macrocyclic cavities of substances with multidentate nature such as antibiotics leads to a different binding tendency for different complexing sites4 In the case of diamagnetic cations associated with large molecules, the exchange process of ligand sites has been widely investigated.1° Nevertheless, while the thermal motions of such molecules have been shown to be considerably affected by the presence of ions, comparatively few data have been reported.I4 'Experimental work was conducted at the Chemistry Department, Faculty of Art and Science, Dicle University, Diyarbakir, Turkey. Part of this paper was submitted at the 9th Physical Organic Chemistry Conference of IUPAC, Re ensburg, Federal Republic of Germany, 1988. ?Professor of Organic Chemistry. Address for correspondence: Istanbul Teknik Universitesi, Fen-Ed. Fakultesi, Kimya Bolumu, Maslak, TR80626. Istanbul, Turkey.
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However, since less energy is required for the conformations of uniform cavity, the macrocyclic ethers could generate perfect models for solvation theories, but a single resonance observed from the exchange of ligand molecules in the vicinity of the ions can only be identified by the parameters of the chemical shift and the line width of that resonance. The chemical shift data contain information about all possible conformational sites. In fact, unless some assessment of the magnetic shielding in various environments is made, gaining more information will not be po~sible.'~-'~ With respect to alkali ion spectroscopy, the solute nuclear resonances of alkali-metal ions have been shown to undergo concentration-dependent chemical shift^.'^*^' In fact, 13Cnuclei studies are subject only to the solubility limitations of the ligands. In the vicinity of a cation, the macrocyclic ligand should not only be influenced by the ion-dipole interaction of the lone pair (1) Pedersen, C. J. Angew. Chem. Int., Ed. Engl. 1988, 27, 1021. (2) Cram, D. J. Angew. Chem., Int. Ed. Engl. 1988, 27, 1009. (3) Lehn, J. M. Angew. Chem., Int. Ed. Engl. 1988, 27, 90. (4) Christensen, J. J.; Sen, D.; Bradshaw, J. S.;Izatt, R. M.; Lamb, J. D.; Nielsen, S. A,; Chem. Reu. 1985, 85, 271. (5) Arnold, K.; Echegoyen, L.; Gokel, G. W. J . Am. Chem. Soc. 1987,109, 3713. (6) Erk, C. Fresenius 2.Anal. Chem. 1983, 316, 477. (7) Erk, C. Abstr. Magn. Reson. Med. 1984, I , 148. (8) Echegoyen. L.; Kaifer, A.; Durst, H.; Shultz, R. A.; Dishong, D. M.; Coli, D. M.; Gokel, G. W. J . Am. Chem. SOC.1984, 106, 5100. (9) Hertz, H. G. Prog. N M R Spectrosc. 1967, 3, 153. (10) Deverel, C. Prog. N M R Spectrosc. 1969, 4, 235. ( 1 1) Strasser, B. 0.;Shamsipur, M.; Popov, A. I. J . Phys. Chem. 1985, 89, 4822. (1 2) Cox, B. G.; Garcia-Rosas, J.; Schneider, H. J . Am. Chem. Soc. 1981, 103, 1054. (13) Eliasson, B.; Larsson, K. M.; Kowalewski, J. J . Phys. Chem. 1985, 89, 258. (14) Sutherland, I . 0. In Applications of N M R Spectrometry to Problems in Stereochemistry and Conformational Analysis; Takeuchi, Y ., Marchand, A. P., Eds.; VCH: Weinheim, FRG, 1988; pp 10-40. ( I 5) Orwille-Thomas, W. J. In Internal Rotation in Molecules; OrwilleThomas, W . J., Ed.; Wiley: London, 1974; pp 1-17. (16) Anet, F. A. L.; Anet, R. In Dynamic Nuclear Magnetic Resonance; Jackman, L. M., Cotton, F. A., Eds.; Academic: New York, 1975; p 543. (17) Forsen, S.: Drokenberg, T.; Wennerstrom, H . Q. Reu. Biophys. 1987, 19, 8 3 .
0 1990 American Chemical Society
