Magnetic relaxation of chloride ions in aqueous solutions

JOURNAL

OF MAGNETIC

RESONANCE

27,153-155

(1977)

Magnetic Relaxation of Chloride Ions in Aqueous Solutions Recently a rapidly increasing number of papers on magnetic resonanceof the alkali metal and halide ion nuclei have appearedin the literature (I, 2). The growing interest in structure and m icrodynamic behavior in liquids and especially in systemsof biological interest may be responsible for this development. Advances in the theoretical description of the quadrupolar relaxation of thesenuclei (3,4), as well as improvements in instrumentation, may also play an important role. In this connection, knowledge of the relaxation rates of the ionic nuclei in simple aqueous electrolyte solutions is of fundamental importance. Therefore these systems were investigated in our laboratory some years ago (5, 6) and we learned at that tim e that relaxation rates measured by pulse techniques were more reliable than those derived from linewidth measurements.The latter may easily be influenced by instrumental broadening. However, owing to the weak NMR signals of 35C1and 37C1it was difficult to apply pulse techniquesin studying the 35C1relaxation. Therefore the data for aqueous alkali chloride solutions were obtained from linewidth measurementsusing a Varian DP-60 wide-line spectrometer(5). In Ref. (5), in this way, an extrapolation value for zero salt concentration (l/T,)!‘,, = 42 set-’ was found which was in agreementwith linewidth data reported by other authors (7). Recently Reimarssonet al. (8) measured the 35C1(and 37C1)spin-lattice relaxation in aqueousNaCl solutions at 2S°C and 8.82 MHz, finding an extrapolation value for 35C1of 25 set-l. Since in these systems the “extreme-narrowing” condition is fulfilled (l/T, = l/Z’J this result indicated that the previous 35C1linewidth measurementswere influenced by experimental broadening. Therefore we decided to remeasureall alkali chloride solutions with a pulsed spectrometer at a frequency of 14 MHz and a correspondingfield strength of 3.35 tesla which is now attainable in our laboratory. The magnet used was a Bruker B-E55 electromagnet and the spectrometerwas a Bruker m o d e l SXP 4-100 with a home-madeprobe head for lo-mm samples.Values of T, were determinedby the 90°-r-90” sequence. The temperature was m a intained constant at 25 + 0.5”C by p u m p ing water through the probe head. A signal-to-noiseratio of at least 30 : 1 was obtained by applying an electronic signal averager.The salts were purchasedfrom Merck A.G. Darmstadt. The concentrations are given in the m o lality scale (moles salt per kilogram of H,O). The experimentalerror is estimatedto be f5%. Our results, given in F ig. 1, show, indeed, that the relaxation rates are considerably smaller than those obtained from previous linewidth measurements.As an example the broken line shows the old results for CsCl; obviously, below l/T, z 70 set-’ line broadeningoccurred. Our new extrapolation value for in&rite dilution is (l/T,)Ps,, = 28 set-‘, confirming the above-mentionedresult of Reimarsson et al. (8) which is only slightly smaller, according to the higher temperatureof 28OC. Taking into account the temperaturedependencegiven in Ref. (a), both results agreewithin about 7 % , which is satisfactory, if one keepsin m ind an experimentalerror of + 1 0 % in Ref. (8). Copyright 0 1977 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain

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As Reimarsson et al. (8) pointed out, this extrapolation value agreesquite well with the prediction of the electrostatic theory, (l/T,)%,, = 40 see-‘, as given by Hertz (3), taking into account the uncertainty in the values of the Sternheimerfactors. Becauseof the smaller extrapolation value, the new results at finite ion concentrations show a stronger relative increase of the relaxation rates with increasing salt concentration than reported previously, which means that the point charge contributions (ion-ion contributions) to the electrical field gradient at the center of the Cl- ion are higher than those calculated in Ref. (5) on the basis of the old data. A detailed calculation, which could be easily done, is not given here.

FIG. 1. 15C1relaxation rates l/T, for solutions of CsCl 0, LiCl W, RbCl 0, NaCl 0, KC1 0, and NH,Cl A. The broken line represents results for CsCl solutions reported in Ref. (5). Concentrations are given in the molality scale (moles per kilogram of solvent). The error bar corresponds to +5%.

Knowing the correct values of the relaxation rates, we remeasuredthe samples at 5.8 MHz by the wide-line technique, using the Varian DP-60 spectrometer, obtaining relaxation rates of the order of those reported in Ref. (5). Several attempts have been made to correct wide-line data for instrumental broadening (9, 10). We analyzed our data in two ways. First we corrected our results for saturation effects and for errors caused by sinusoidal modulation as discussed by Gillen and Noggle (9). We then deconvoluted the measuredlinewidths numerically from the known field inhomogeneity, assuming a Lorentzian lineshape of the undisturbed signal and a Gaussian shape of the field inhomogeneity. Both corrections together lead to a value of ( 1/T,)05c,= 34 set-1; thus it is seen that even after one performs careful corrections linewidth measurements are to be regarded with caution.

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The problem demonstrated here by the example of 35C1relaxation rates of Cl- ions is also given to a much greater extent for the 35C1relaxation of ClO,- ions, where the linewidth is normally much smaller than in Cl- ions. Reimarsson et al. (8) also reported the relaxation rate of 35C1in ClO,- at infinite dilution as 3.6 set-’ at 28OC, which is in agreement with data from this laboratory (II). Again these values are much smaller than those which were recently published by other groups (12, IS), derived from linewidth measurements,which obviously were influenced by instrumental broadening. Finally we can state that the quadrupolar relaxation rates of Cl- and Br- and of all alkali nuclei in aqueous solutions have now been measured by pulse techniques, and therefore these results may be regarded as reliable. In the case of l*‘I, where for I- ions only linewidth data are known (5), we do not expect pulse measurements to yield different results, as the l*‘I linewidths in aqueous solutions are very broad (> 1000 mG) and instrumental broadening should not occur. Some test measurements using pulse techniques, carried out in our laboratory, confirmed this expectation. ACKNOWLEDGMENTS We thank Dr. B. Lindman for sending us Ref. (8) prior to publication. Furthermore, Professor H. G. Hertz for many helpful discussions.

we are grateful to

REFERENCES 1. B. LINDMAN AND S. FOR&N, in “NMR-Basic Principles and Progress” (P. Diehl, E. Fluck, and R. Kosfeld, Eds.), Vol. 12, Springer-Verlag, Heidelberg, 1976. 2. M. HOLZ AND M. D. ZEIDLER, in “Nuclear Magnetic Resonance,” Specialist Periodical Reports, Vol. 6, The Chemical Society, London, 1977. 3. H. G. HERTZ, Ber. Bunsenges. Phys. Chem. 77,531 (1973). 4. H. G. HERTZ,B~~. Bunsenges. Phys. Chem. 77,688 (1973). 5. H. G. HERTZ, M. HOLZ, R. KLUTE, G. STALIDIS,AND H. VERSMOLD,Ber. Bunsenges. Phys. Chem. 78, 24 (1974). 6. H. G. HERTZ, M. HOLZ, G. KELLER, H. VERSMOLD, AND C. YOON, Ber. Bunsenges. Phys. Chem. 78, 493 (1974). 7. C. DEVERELL AND R. E. RICHARDS, Mol. Phys. 16,421 (1969). 8. P. REIMARSSON,H. WENNERSTROEM,SVEN ENGSTRGM, AND B. LMDMAN, to appear. 9. K. T. GILLEN AND J. H. NOGGLE, J. Mugn. Resonance 3,240 (1970). 10. 0. HAWORTH AND R. E. RICHARDS, in “Progress in NMR” (J. W. Emsley, J. Feeney, and L. H. Sutcliffe, Eds.), Vol. 1, Pergamon, Oxford, 1966. II. M. CONTRERAS, Thesis, Karlsruhe, 1976. 12. M. S. GREENBERG AND H. I. POPOV,J. Solution Chem. 5,653 (1976). 13. Y. M. CAHEN, P. R. HANDY, E. T. ROACH, AND A. I. POPOV,J. Phys. Chem. 79,80 (1975). MANFRED HOLZ HERMANN WEING~~RTNER

Institutjiir Physikalische Chemie und Elektrochemie der Universitcit Karlsruhe, Germany Received April 4,1977