J. Phys. Chem. 1981, 85, 3697-3699
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Figure 6. Time dependence of the order parameter (S)of methyl orange In the absence of magnetic field.
an estimate of the extension of the aggregates may be obtained. The amphiphile diffusion coefficient along the rodlike aggregates was determined for a magnetic field macroscopically aligned sample. This sample was then oriented at the magic angle between the director and Bo as described previously.8 The following results were obtained for the two nematic systems studied: the lateral m2 s-l for potassium diffusion along the rod DL = 5 X laurate and DL= 8 X 10-l' m2 s-l for sodium decyl sulfate. These values compare well with the lateral diffusion
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coefficients previously determined for corresponding lamellar mesophases. From this investigation it can be concluded, since no restricted diffusion was observed, that the aggregates must be quite long. By means of the relation ( x 2 ) = 2Dt, their length can be estimated to be longer than -2000 nm. Combined, all of these experimental findings are compatible with a phase structure consisting of long rodlike aggregates. The rod orienta in the magnetic field with the symmetry axis parallel to the applied magnetic field. The polarized absorption spectra of various chromophores solubilized in the lyotropic nematic are shown in Figure 4 and the the spectrum of the matrix itself in Figure 5. As can be inferred from the latter figure, the mesophase absorbs light strongly at wavelengths below -200 nm; Le., dissolved chromophores cannot be studied in this region. The time dependence of the order parameter, S, of methyl orange in an aligned sample, without an applied magnetic field, is shown in Figure 6. The observation of a constant S for at least 100 h means that the randomization proceeds extremely slowly. Therefore, this lyotropic nematic is a very convenient matrix for light spectroscopic studies since it also has a high transmission over a wide range of wavelengths. Acknowledgment. This work was supported by the Swedish Natural Science Research Council.
Unusually Poor Solvation of Ca2+ by Hexamethylphosphoramide Luis Echegoyen, Iieana Nieves, James Thompson, F6lix Rosa, Department of Chemistry, Universw of Puerto Rico, Rlo PMras, Puerto Rico 0093 1
and Rosarlo Concepcien Universldad Cat8ilca Madre y Maestra, Natural Sciences Department, Santiago, Domlnlcan Republic (Received: April 27, 198 1; In Final Form: July 30, 198 1)
The perchlorate salts of Mg2+,Ca2+,and Ba2+have been studied by 36ClNMR spectroscopy in hexamethylphosphoramide solution. The resonance line widths measured for these salts are After this salt is added the ESR spectrum corresponds to that of an ion pair where both U N and U H have drastically reduced values.
Introduction Hexamethylphosphoramide (HMPA) is known to be a very good solvent for cations, eliminating, almost completely, ionic association in this medium.l All alkali metal salts have been shown to be essentially 100% dissociated in this medium.2 This property has rendered HMPA as a very useful solvent for carrying out nucleophilic organic reactions as well as an excellent medium to study free anions in solution, particularly anion radicah3v4 (1) Normant, H. Angew. Chem., Int. Ed. Engl. 1967,6, 1046. (2) Ozari, Y.; Jagur-Grodzinski, J. J. Chem. Soc., Chem. Commun. 1974, 295.
0022-3654/81/2085-3697$01.25/0
Divalent cations have been shown to be strongly complexed by HMPA.6 The work of Debolster has shown that tetrahedral solid complexes are formed between HMPA and M(C104)2,where M = Mg, Ca, and Ba.6 The general formula of these complexes is M(HMPA)4(C104)2.Measuring the C1-0 stretching and bending frequencies these (3) Levin, G.; Jaaur-Grodzinski. . J.;. Szwarc, M. J. Am. Chem. SOC. 1970,92,2268. (4) Szwarc, M.; Staples, T. L. J. Am. Chem. Soc. 1970, 92, 5022. (5) DeBolster. M. W. G.:. Wieeerink. , F. J.:. Groeneveld. W. L. J. Inom. Nucl. Chem. 1973,35, 89. (6) DeBolster, M. W. G.; Vermaas, A.; Groeneveld, W. L. J. Inorg. Nucl. Chem. 1973,35,83. 1
0 1981 American Chemical Society
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authors concluded that the perchlorate anion interacts with the metal center in the calcium and barium cases, giving rise to pseudo-octahedral complexes.' This observation suggests that the four HMPA molecules cannot fully solvate these two cations and that perhaps ion pairing should be present in solutions of these salts in HMPA, but probably not in magnesium perchlorate solutions. Their work indicates stronger interadion between the perchlorate anion and the alkaline earth cation as the size of the cation increases. The technique of %C1NMR of ClO; in solution has been shown to be very powerful and sensitive to detect the degree of interaction between the anion and cationsS8 Stengle and co-workers showed that the 35ClNMR line width is very sensitive to the quadrupolar relaxation rate of the nucleus, which in turn is intimately related to the degree of ionic association of the This technique was utilized in the present work to assess the degree of ion pairing between divalent cation and the perchlorate anion in HMPA solutions. ESR spectra of anion radicals are also very sensitive to ion pairing perturbations and have been utilized extensively to quantitatively study the nature and strengths of these interactions.l0 When HMPA is used as the solvent, and the anion radicals are generated by alkali metal reduction (particularly sodium1'), the resulting species have been shown to be essentially free of ionic interactions.12 Additions of salts to these HMPA anion radical solutions result in the formation of ion pairs.13 Sometimes the new species produced are in fast equilibrium with the free anion radicals and only a time-average species is observed,14while sometimes all species are time resolved on the ESR time scale and afford simultaneous spectral observation.12 We present here the use of the ESR technique together with %C1NMR to determine the degrees of ion pairing of alkaline earth perchlorate salts in HMPA.
Experimental Section 2-(Dicyanomethylene)-1,3-indadione(I) was prepared 0
I
by the procedure described by Fatiadi16 and purified by vacuum sublimation prior to use. Solutions M) of I in dry HMPA12 were reduced with sodium to yield ESR spectra composed of two hyperfine splittings: a2N = 1.61 G and a4H = 0.57 G. These values agree well with those reported previ~usly.~~J' Additions of perchlorate salts to the anion radical solutions were accomplished as described elsewhere.12 (7) DeBolster, M. W. G.; Groeneveld, W. L., Recl. Trau. Chem., Pays-Bas 1972,91,171. (8)Berman, H. A.; Stengle, T. R. J. Phys. Chen. 1976, 79, 1001. (9) Berman, H. A.; Yeh, H.J. C.; Stengle, T. R. J. Phys. Chem. 1975, 79. 2551. (10) Stevenson, G. R.; Echegoyen, L. J. Phys. Chem. 1973,77,2339. (11) Stevenson, G. R.; Martir, W.; Alegria, A. E. J. Am. Chem. Soc. 1976.98.7955. (12) Stevenson, G. R.; Echegoyen, L.; Lizardi, L. R. J. Phys. Chem.
1972, 76, 1439. (13) Stevenson, G. R.; Alegria, A. E. J.Phys. Chem. 1975, 79, 1042. (14) Stevenson, G. R.; Alegria, A. E. J. Phys. Chem. 1976, 79, 361. (15) Fatiadi, A. J. Synthesis 1978, 165. (16) Chatterjee, S. Science 1967, 157, 314. (17) Chatterjee, S. J. Phys. B 1969, 725.
Echegoyen et ai.
35Cl NMR spectra were obtained at 8.72 MHz on a JEOL FX-9OQ spectrometer. 40' pulses were applied to the samples with a spectral window of 2000 Hz and 8K data point transforms. Tubes were 5 mm in diameter and a total of 300 scans were necessary for the magnesium and barium perchlorate salts while 8000 scans were stored for the calcium perchlorate solution. All NMR samples were 0.08 M in salt and were prepared under vacuum with dry HMPA. A pulse delay of 20 s was used for each salt solution studied. ESR spectra were recorded with the X-band of a Varian E-9 spectrometer. Solutions were prepared under vacuum torr) and their spectra recorded immediately.
Results and Discussion 35ClNMR line widths of perchlorate anions in solution are very sensitive to the symmetry of the electric gradient at the n u c l e ~ s .In~ the absence of ionic interactions the perchlorate anion processes a tetrahedral geometry which gives rise to a spherical electronic environment at the chlorine nucleus. This situation results in 3sClNMR line widths of less that 5 Hz.9 If the symmetric environment is distorted through the interaction of the anion with a cation in solution the electric field generated at the nucleus causes fast quadrupolar relaxation and a consequent increase of the line widtheg Line widths of up to 1000 Hz have been observed for systems where strong ionic association exists. The 36ClNMR spectra for the three salts studied here were obtained under identical conditions in HMPA. There is no evidence of ionic association in the case of magnesium perchlorate, where the observed line width was 2.5 Hz. This observation seems to confirm the solid-state behavior observed by DeBolster where no evidence was obtained of interaction between the perchlorate anion and the magnesium cation. The corresponding 35ClNMR spectrum of the barium perchlorate solution has a line width of 30 Hz, indicating some degree of ionic association. Again, this observation confirms the work of DeBolster. The surprising result was obtained when studying the calcium perchlorate solution by 35Cl NMR. In this case the observed line width was -950 Hz, a value indicating very strong interaction between the anion and the cation. This result does not follow the predicted behavior extrapolated from the solid-state data. In order to confiim this behavior tripyrrolidinephosphoramide (TPPA), a solvent with similar characteristics as HMPA, was used. This solvent has been shown to be even better than HMPA to solvate alkali metal cations although its structure is very similar.18 When the alkaline earth perchlorates were studied in TPPA, the 36ClNMR line widths observed were virtually identical with those obtained in HMPA, thus confirming the behavior of the calcium perchlorate. Chemical shifts were referenced to external 0.1 M NaC104 in D20. All three salts have the same chemical shift value, 6.2 ppm, although the accuracy of the measurement is very poor for Ca(C104)2due to the large line width observed. In order to further confirm the discontinuous behavior observed for calcium perchlorate in HMPA the ESR technique was used to investigate the ion pairing interactions between the alkaline earth cations and the 2-(dicyanornethylene)-1,34ndadioneanion radical (DCMI-.). In this situation, instead of monitoring the changes of the diamagnetic perchlorate anion, the ESR spectral changes of the anion radical are monitored. These changes reflect (18) Ozari, Y.; Jagur-Grodzinski, J. J. Chem. Soc., Chem. Commun. 1974,295.
Solvation of Ca2+ by HMPA
Figure 1. ESR spectra of the anion radical of 2-(dicyanomethylene)-1,3-indadionein HMPA at room temperature (1.5 X 104M): (a) immediately after alkali metal reduction; (b) containing 0.1 M Ba(CI0,)2 salt; (c) containing 0.02 M Ca(C10,)2.
the nature and strength of the interactions. Several anion radicals which exists essentially free of ionic interactions in HMPA solutions can be forced to associate with cations by addition of salts.13 Most of the anions that have been studied possess polar groups where the charge and spin density tend to concentrate, thus making them highly susceptible to form strong ion pairs. This high tendency to associate means little selectivity toward the cations used. The DCMI-. possesses four polar groups which disperse the charge and spin density over most of the molecule. This system should consequently be more selective toward different cations upon complexation to form ion pairs. The ESR spectrum of DCMI-*is presented in Figure la. The small total line width observed indicates considerable spin delocalization of the five-membered ring of the molecule, thus on the C=O bonds. The relatively large nitrogen hyperfine coupling constant (1.61 G)also indicates spin density on the cyano groups. Additions of lithium, sodium, potassium, magnesium, and barium perchlorates
The Journal of Physical Chemistry, Vol. 85, No. 24, 198 1 3899
to the solution exhibiting spectrum l a do not result in any appreciable spectral changes (Figure lb). These salts were added in large quantities (-0.1 M) yet no spectral evidence of ionic interaction was observed. These cations therefore interact stronger with the solvent (HMPA) than with the anion radical. On the other hand, addition of small amounts of calcium perchlorate (0.01 M) results in dramatic spectral changes (Figure IC).Furthermore, addition of 18-crown-6 to the solution exhibiting spectrum IC resulted in the reappearance of the original ESR spectrum. The coupling constants observed for spectrum ICare a2N = 0.82 G and a4H = 0.18 G. Since both hyperfine coupling constants have drastically reduced values, strong interaction of Ca2+with the oxygens on the anion radical is evident. The calcium cation, being poorly solvated by HMPA, is available for complexation with the anion radical. The fact that spectrum ICcorresponds to that of an ion pair is further confirmed by the observation of g tensor anisotropy.13 Variable-temperatureESR spectra of sample ICfailed to reveal any line width alternation effect between 5 and 100 "C. Both the NMR and the ESR data indicate that calcium perchlorate is not well solvated in HMPA solution. It is not obvious why this discontinuous behavior exists in this series of salts. Clearly, the primary solvation sheath cannot be responsible for the observed results because the barium cation should exhibit a higher degree of ion pairing than the calcium cation. From the solid state work of DeBolster it is known that all alkaline earth cations interact strongly with four HMPA molecules thus a higher degree of interaction between the cation and the perchlorate anion is predicted as the atomic number of the cation increases. This is not the observed result. The charge-to-size ratio of the calcium cation seems to be such that solvation stabilization is minimized when compared to those of magnesium and barium. Conductance measurements on solutions identical with those studied by NMR showed very little variation from one salt to the other. The values are essentially the same for all salts with a slightly smaller value for the Ca(C104)2solution. These data suggest that the calcium ion-paired species probably involves a solvent-separated pair. Acknowledgment. Acknowledgments are made to the donors of the Petroleum Research Fund, administered by the American Chemical Society the National Science Foundation (Grant CHE-7915201),the National Institute of Health (Grant RR-8102-07),and the Research Center of the Universidad Catdlica Madre y Maestra for financial support. The authors gratefully acknowledge the National Science Foundation (CHE-791462) for funds used to purchase the JEOL FX-9OQ NMR spectrometer.
