340
J. Phys. Chem. 1980, 84, 340-340
Sodium Quadrupolar Splitting in a Nonaqueous Liquld Crystalilne Solvent
Sir: Nuclear magnetic resonance spectra of alkali metal cations and halide anions in lyotropic liquid crystals consist of 21 lines (I = spin of the measured nucleus). These splittings are due to the quadrupolar interaction which exists even for symmetric nuclei in ordered media.' In lyotropic liquid crystals the ions are part of the system which consists of water, electrolyte molecules, and sometimes a nonelectrolyte component. The origin of the nonvanishing quadrupolar interactions or electric field gradients (efg) is still unclear. Two major approaches have been suggested: the first is based on electrostatic considerations2 (asymmetric hydration or charge distribution) whereas the second assumes that any symmetric atom, molecule, or ion is actually distorted by the asymmetric intermolecular potential in ordered system^.^ Recently the phenomenum of distortion in a cubic molecule has been deduced also from its linear dichroism spectrum? In order to gain more insight into this problem we sought a system where quadrupolar splittings of an alkali ion could be observed in a nonaqueous medium. In this communication we report the observation of 23Na+quadrupolar splitting in (2,2,2)NaSCN cryptate complex, which dissolves well in chloroform and other organic solvents. The liquid crystal was a solution of poly(y-benzylL-glutamate) (PBLG) in CDC13 The sample consisted of 0.180 g of PBLG (mol wt ca. 130000 obtained from Miles-Yeda Ltd.), 0.80 mL of CDC13,and 0.020 g of (2,2,2)NaSCN. The (2,2,2)NaSCN complex was prepared from (2,2,2) (4,7,13,16,21,24-hexaoxa-l,l0-diazabicyclo[8.8.8]hexacosane, C18H36N206)by equilibration with NaSCN in a chloroform s ~ l u t i o n . ~The sample was equilibrated for a few days in the refrigerator and for about 2 h in the magnetic field before full alignment was achieved. Other samples which contained more (2,2,2)NaSCN produced incomplete alignment or polycrystalline samples. NMR measurements were performed on a Bruker 322 S pulsed spectrometer interfaced with a Nicolet 1180 computer for accumulation and Fourier transformation. Deuterium sDectra were taken at 13.82 MHz and sodium spectra at 2i.81 MHz. All measurements were taken at room temperature (ca. 22 OC). The spectra of D and Na are shown in Figure 1. The deuterium spectrum ( I = 1) consists of a doublet with a splitting of 465 Hz, which is typical for the PBLGCDC13 system at this concentration and temperature. The sodium ( I = 3/2) spectrum consists of a triplet with an average splitting between adjacent lines of 792 Hz. This splitting is considerably smaller than sodium splittings observed in many lyotropic systems.l Sodium ions are located in the center of large cryptate cage molecules.6 The cryptate itself is certainly partially aligned, similarly to all asymmetric solutes in liquid crystal^.^ The cage may also suffer some slight distortions from its regular geometry. It would of course be desirable to evaluate its order parameter, from either the proton or nitrogen mectra, but this turns out to be rather comdicated: The reason is that the cryptate and PBLG spectra are superimposed and furthermore that the cryptate modifies the PBLG spectrum through its effect on its order parameter. A possible solution to this problem could be the use of a deuterated cryptate.
0022-3654/79/2084-0340$01 .OO/O
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Flgure 1. Deuterium (upper trace, 10 scans) and sodium (lower trace, 4000 scans) NMR spectra in the PBLG-CDC13-(2,2,2)NaSCN system.
Sodium spin relaxation in isotropic solutions of (2,2,2)NaSCN8is very likely due to quadrupolar interaction. This, however, as for Na+ relaxation in any aqueous solution, does not necessarily imply a distortion which is responsible for the splittings observed uniquely in ordered media. Also, the value of the quadrupole coupling constant for Na+ in a liquid crystal may be quite different from the value derived from relaxation studies in isotropic solvents. The relatively narrow lines for Na+ in our system (Figure 1) indicate that its rotational motion is not severly restricted (line widths are comparable to those observed in a (2,2,2)NaSCN-CDC13 solution). We are therefore observing a motionally averaged distortion and not an effect due to static binding. We must therefore conclude that the nonaveraged efg which operates on the Na+ ions and results in the observed quadrupolar splittings is caused by its being a solute in an ordered media. Our observation shows that quadrupolar splittings in alkali metal cations spectra are not unique to electrolyte solutions. This provides a strong and more direct support to the assumption that electrostatic interactions play a minor role in producing the observed quadrupolar splittings of ions in lyotropic liquid crystals.
This research was supported by a grant from the United States-Israel Binational Science Foundation (BSF), Jerusalem, Israel, References and Notes (1) A. Loewenstein, M. Brenman, and R. Schwarzmann, J. phys. Chem., 82, 1744 (1978), and references cited therein. (2) H. Wennerstrom, G. Lindblom, and B. Lindman, Chem. Scr. 6, 97 (1974). (3) A. Loewenstein and M. Brenman, Chem. phys. Lett., 58,435 (1978), and references cited therein. (4) B. Samori, J. Phys. Chem., 83, 375 (1979). (5) We are very grateful to Dr. J. P. Kintzinger and Professor J. M. Lehn who supplied us with this material. (6) 8. Metz, D. Moras, and R. Weiss, Chem. Commun., 444 (1971); D, Moras, Thesis, UniversitO Louis Pasteur de Strasbourg, 1971. (7) J. W. Emsley and J. C. Lindon, "NMR Spectroscopy Using Liquid Crystal Solvents", Pergamon Press, New York, 1975. (8) J. P. Klntzinger and J. M. Lehn, J. Am. Chem. SOC.,96, 3313 (1974).
f ~ ~ ~ ~ I ~ $ , $of$Technology e Haifa, Israel Recelved May 1, 1979
0 1980 American
Chemical Society
A. Loewenstein" M. Brenman
