Anthony Frdiello and ~ o n a l dE. Schuster
California State College ot Los Angeles 10s Angeles 90032
I
A Proton Magnetic Resonante Coordination Number Study -
AICI3 in aqueous solvent mixtures
Physical chemistry experiments dealing with electrolytic solutions usually involve conductance, electromotive force, or transference number techniques. However, none of these methods provides a direct measure of the coordination number of an ion, even though this parameter has been the object of numerous investigations and is a valuable aid to the interpretation of data compiled by the above mentioned techniques. Nuclear magnetic resonance spectroscopy has been applied extensively to studies of electrolytic solutions (111) and to the determination of cation coordination numbers in water (12-17) and in nonaqueous solvents (18-25). These methods often require special instrumentation and isotopic sample enrichment (12-15), or involve measurements and mathematical analyses which are too detailed for a physical chemistry laboratory experiment (16, 17). Solvent exchange in nonaqueous solutions is frequently slow enough to permit the observation of signals arising from the protons of bulk molecules and molecules in the first coordination shell of a cation (18-25). Thus, coordination numbers can be evaluated directly and unambiguously by integrating the separate resonance peaks. Recent studies in our laboratory have allowed us to extend this direct integration technique to aqueous solutions of several cations, including Al(III), Be(II), and Ga(II1) (24, 25). The experiment to be described will involve a solvation study of AI(II1). Theory
At ambient tempcriltures, aqueous solutions of Al(111) exhibit oxygen-17 nuclear magnetic resonance signals for bulk water molecules and water molecules in the first solvation shell of this cation (12-15). This indicates that the exchange of water molecules between the two environments is proceeding with a half life of >0.0005 sec. With "O nuclear magnetic resonance equipment and isotopic enrichment of the water, integration of the separate resonance signals provides a direct measure of the coordination number of the Al(111). I n these same AI(II1) solutions, at room temperature, only one proton resonance signal is observed, even though the exchange of water molecules is rclatively slow. This results from a rapid proton exchange which averages the signals arising from the bulk and complexed water molecules. However, it has been demonstrated that if these concentrated AI(II1) solutions are cooled to -20 to -40°C, the proton exchange is slowed to such an extent that separate bulk and complexed water resonance signals can be observed (24). The same phenomenon arises in solutions of this ion in
aqueous solvent mixturcs (25). This is illustrated in the figure, wherein the two water proton peaks are clearly evident at -20°C for a solution of A1C13in a dimethylsulfoxide (DYIS0)-water mixture. Since the
The proton magnetic resononce spectrum of o 2M AlClr solution in o 10: 1 mole ratio mixture of water to dimethyl$ulfoxide IDMSOI, illustrating the signals due to bulk solvent IBE~o and BDMSO) ond ralvent in the AI(IIIIcoordination shell (Cnlo and Comol. The water ond DMSO signals were recorded at different rpectrol amplitudes on a V w i m A60 Spectrometer.
water proton resonance signals are separated by -250 cps, they can be directly integratcd to yield the Al(II1) coordination number. As illustrated for DAISO in the figure, several organic components also exhibit separate resonance signals in aqueous AIClasolutions. Thus, the contributions of water and the second solvent component to the first solvation shell of Al(II1) can be determined independently by integration. Materials
This experiment will require a supply of AlCIa. GH20, distilled water, reagent gradc acetone and dimethylsulfoxide, precision nmr sample tubes, fine grid nrnr chart paper, and access to a proton magnetic resonance spectrometer equipped with avariable temperature unit. The Experiment
Prior to the laboratory period, a saturated aqueous solution of A1Clr.6H20,about 3 M , should be prepared, the density measured, and the concentration accurately determined by a chloride ion titration with silver nitmte. Using these AIC13solution data arid the densities of thc nonaqueous components, a 1.0M solutionof AlCla in a 5:1 mole ratio mixture of H 2 0 to DAISO and a 0.7111 solution in a 5 : l mixture of H 2 0 to acetone, should be prepared by the students by dilution techniques. The concentrations of all constituents must he known to 1-2'%. A sample of each solution can then be Volume 45, Number 2, February 1968
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sealed in an nmr tube to prevent loss of solvent by evaporation. The approximate freezing point of each sample should be determined to avoid freezing the sample in the probe insert. The nmr spectrum of each sample should be recorded a t probe temperature (-35°C) and at successively lower temperatures in 15'C increments. The DMSO spectrum in the aqueous salt solution will consist of two resonance signals, as seen in the figure, with the peak corresponding to complexed DMSO displaced about 15 cps downfield from the bulk signal. The DMSO peaks should be integrated a t probe temperature since they exhibit viscosity broadening and overlap slightly at lower temperatures. Only one proton resonance signal will be observed for acetone over the entire temperature range. In each sample, as the temperature is lowered, the water proton signal will broaden considerably to a line width of about 100 cps at 0°C. At still lower temperatures, there appears a second broad resonance peak, displaced approximately 250 cps downfield from the original water signal. At this point, the temperature should be varied in 5°C increments to maximize the sharpness of the complexed water peak. The separate water signals can then be integrated. Since these peaks are quite broad and differ considerably in relative areas, the integration may he more difficult than the previous DSfSO measurements. With care the water signal integrations should be made with a precision of lo'%. The coordination number contribution of water in each sample and DMSO can be calculated from the integrated areas and the concentrations by: (area of complexed peak)* molesn (area of bulk and eomplexed peak)* iizaia In the acetone water system a value of six will be obtained for the water contribution to the AI(II1) solvation shell, since no signal for complexed acetone is ohserved. The water and DMSO values will he roughly five and one, respectively. The theoretical basis of the phenomenon of nmr should be presented in a recitation before or a t the start of the laboratory period and in the lecture portion of the physical chemistry course. It is more beneficial to have a qualified technician carry out the actual operation of the nmr spectrometer, with close observation by the students involved. Usually little is gained by having a student, with no prior experience, attempt to operate an nmr spectrometer and frequently much is lost in the form of a broken probe insert. Since spectrometer time is quite valuable, the experiment should be performed by groups of two or three students, who would tabulate the same experimental data, but make independent calculations. Discussion
The nmr results illustrate several important features, all of which should be discussed by the students in their reports. With a brief background in the factors which inhence the chemical shift of a nucleus, obtained in lecture, recitation, or by individual study, reasons should be proposed by the student for the appearance of the complexed solvent signals, water and DMSO, at lower applied magnetic field than the respective bulk signals. Since the interaction site in the water molecule is the unshared electron pair of the oxygen atom, the charge 92
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withdrawal produced by salvation of AI(II1) results in extensive deshielding of the nearby hydrogen nuclei. Thus, the signal of complexed water appears at lower field. The separation of the DMSO signals is far less, since the methyl protons are several bonds removed from the interaction site. The dependence of nmr signal shape on rate processes occurring in solution is demonstrated by the spectra recorded at various temperatures. At room temperature when only one water proton resonance signal is observed, the lifetime of a proton at n particular site may be approximated by r ~ 1 / 2 a A v= 0.0005 see. When the separate signals are observed the proton exchange has been slowed to such an extent that 7 increases by about 103. By recourse to references (12-Id), it can be verified that the process being observed is indeed a proton exchange and not the exchange of entire water molecules. The large line widths of the water signals are due to several factors including the increased viscosity of the solution at low temperatures. Also, the proton exchange process most likely has not been completely quenched. Studies at lower temperatures are prevented by viscosity broadening. The much greater linewidth of the signal of complexed water molecules may be the result of spin-spin or quadrupole interaction with the Al(II1). The coordination number results illustrate the feature of an nmr signal that the area is directlv nro~ortionalto the concent&ion of the species respdnsib~efor the resonance peak. The value of six for AI(III), obtained by relating the integrated peak areas to the concentrations of the species in solution, is the same as that obtained for this ion by other techniques, such as isotopic dilution ($6). The fact that no resonance signal is observed for complexed acetone may be attributed to one or a combination of several factors. First, if the exchange of acetone molecules between the hulk and complexed environments is very fast, only an average resonance signal will result. Second, the separation of the bulk and complexed acetone signals may not be large enough to permit signal resolution. Finally, acetone molecules may not solvate Al(II1) to any appreciable extent in the presence of water. Considering the value of six found for the contribution of water molecules to the solvation shell of AI(III), the last possibility is the most likely. The inability of acetone to compete with water for AI(II1) contrasts sharply with the intensive solvation by DMSO. Since acetone and DMSO have comparable dipole moments of 2.7 and 3.9 debye, respectively, this property cannot be very influential in determining solvating ability. The molecular property that should be considered is the relative basicity. Thus, DMSO, by several orders of magnitude, is more basic than acetone ($7). The difference in salvation behavior may reflect this fact. Conclusion
This experiment, which should be completed in one laboratory period, results in the student gaining a practical acquaintance with several aspects of the nmr technique. These include the concept of chemical shift and the factors which influence this parameter, the effect of a rate process on signal shape, the dependence of line
width on viscosity, magnetic interactions, and a rate process, and the direct relationship of peak area to concentration. What may prove to be the most interesting aspect of the experiment for the student is the evidence for the validity of the concept of an ion "solvation shell." Finally, differences in solvating ability of various organic compounds can he quantitatively estimated. Acknowledgments
This work was supported in part by Grant No. 14-010001-762 from the Office of Saline Water, US. Department of the Interior. The authors also wish to acknowledge the technical assistance of Rlr. A. Rlorriss and Mr. 0. R. Vanderstay. Background Reading
ROBERTS,J . D., "Nuclear Magnetic Resonance," RIcGra~v-HillBook Co., New Yorlt, 1959. JACKMAN, L. M.,"Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry," Pergamon Press, New York, 1959. POPLE, J. A,, SCHXEIDER, W. G., AND BERNSTEIN, H. J., "High-resolution Nuclear Magnetic Resonance," McGraw-Hill Book Co., New Yorlc, 1959, Ch. 15. This reference discusses the effects of electrolytes on the water proton resonance signal. HUNT,J. P., 'Wetal Ions in Aqueous Solution," W. A. Benjamin, Inc., New York, 1963. This reference discusses a variety of methods, including the nmr technique, for determining coordination numbers. Literature Cited (1) SHOOLERY, J. N., A N D ALDER,B., J . Chem. Phys., 23, 805 (1955). (2) HERTZ,H. G., A N D SPALTHOFF, W., Z . Elekt~ochem.,63, 1096 (19.59). B. P., AND ~ U L D H E RA_, G ,J . Chem. P h y ~ . 34, , (3) F.~HRIC\ND, 16'24 (1961). J. C.. J . Chem. Phvs.. (4) HINDY.AN. " .36.. 1000 (1962). . . i 5 j FRATIELI.& A.,'ANDDOUGL~SS, U. C., J . Chem. P h p , 39, 2017 (1063).
R. E., T ~ a n sFaraday . Soc., (6) CRAIG,R. A,, A N D RICHARDS, 59, 1972 (1963). (7) FRATIELLO,A,, A N D CHRISTIE,E. G., Trans Faraday Sac., 61, 306 (1965). C.,~ AND RICHARDS, R. E., Mol. Phys., 10, 551 (8) D E V E R E L (1966). . . (9) FRATIELLO, A., AND MILLER,D. P., Mol. Phys., 11, 37 (1966). (10) CLICK,R. E., STEWART, W. E., A N D TEWARI,K. C., J . Chem. Phyi., 45, 4049 (1966). (11) FRATTELLO, A,, LEE, It. E., MILLER,D. P., AND NISHADA, V . M., Mol. Phys., in press (1967). J. A,, LE'IIONS, J. F., Tnune, H., J . Chem. Phys., (12) JACKSON, 32, 653 (1960). (13) CONNICK, R. E., A N D FIAT,1). N., J . Chem. Phys., 39, 1349 (1963). . . (14) CONNICK, R. E., A N D R A T ,D. N., ,J. Chem. Phys., 44,4103 IlQfi&\ \--"",. (15) FIAT,D. N., AND CONNICK, R. E., J . Am. Chem. floe., 88, 4794 (1966). (16) MALINO~SKI, E. R., KN.\w, P.S., AND FEWER, B., J . Chem. Phys., 45, 4274 (1966). W. G., J. Chem. Phys., 44, 3567 (17) SWIFT,T. J., A N D SAYRE, (1RRfiI \-."-,.
(18) SWINEHART, J. H., AND TAURE,A,, J . Chem. Phys., 37, 1579 (1962). . . (19) NAK=~MURA, S., AND MEIBOOM, S., J . Am. Chem. Soc., 89, 1765 (1967). (20) Lws, A,, AND MEIeooM, S., J . Chen. Phys., 40, 1058; 1066 (1964). , (21) THOMAS, S., A N D REYNOLDS, W. L., J . Chem. Phys., 44, 3148 (1966). (22) FRATIELLO, A., MILLER,D. P., A N D SCHUSTER, R. E., Mol. P h p . , 12, 111 (1967). (23) FRATIELLO, A., AND SCHUSTER, R. E., J . Phys. Chem., 71, 1948 (1967). (24) SCHUSTER, R. E., .4ND FRLTIELLO, A., J . Chem. Phys., in press (1967). (25) FRATIELLO, A,, AND SCHUSTER, R. E., Tet. Lett., in press (1967). (26) BALDWIN, H. W., AND TAUBE,H., J. Chem. Phys., 33, 206 (1960). (27) ARNETP,E. M., "Progress in Physical Organic Chemistry." (Edilors: COHEN,S. G., STREITWEISER, A,, JR., A N D TXXT,R. W.), Interscienoe Publishers (divisinn of John Wiley & Sons, Ino.) New York, Vol. 1, p. 223 (1963).
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