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Nuclear magnetic resonance studies of chloroaluminate melts
J. inorg nucl. Chem.,
1978.Vol.40, pp. 387-388 PergamonPress. Printedin GreatBritain
NUCLEAR MAGNETIC RESONANCE STUDIES OF CHLOROALUMINATE MELTS UDO ANDERS and J. A. PLAMBECK* Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada (Received 13 December 1976; received for publication 2 May 1977)
Abstract--Nuclear magnetic resonance has been used to study the structures of fused (Na, K)AI2CI7 at 170°C. Sodium, aluminum and chlorine magnetic resonance, and proton magnetic resonance on substitution with NH4+, indicate the structure to be essentially ionic as M+ A12C17-. INTRODUCTION
The ternary A1C13-NaCI-KC1 system forms one of the lowest-temperature fused salts known. This permits its study in commercial NMR facilities rather than requiring the special apparatus necessary at higher temperatures[I]. Electrochemical studies ([2]; and references cited therein) have used the 66:20:14 mole % mixture corresponding to (Na, K)AI2CI7 stoichiometry. We report here an NMR study related to the structure of this melt. Other studies, and additional literature, are available elsewhere [3]. EXPERIMENTAL
The purification of AICI3 and other chemicals used has been described in detail elsewhere[2-4]. NMR samples were prepared by melting the components together inside a nitrogen-filledglove box. The liquid melt was transferred into NMR tubes with disposable pipets. Standard medium wall tubes were used for proton NMR while tubes of 10 mm O.D. were used for all wideline experiments. The proton NMR was observed at 100MHz using a 12in. magnet (Model HA-100, Varian Assoc., Palo Alto, California). The temperature was varied from 120 to 170°C using a Model 4333 variable temperature probe (Varian). The nuclear magnetic resonances of 23Na, 27A1and 35C1were studied at 170-+2°C. A Model V-4200B Wideline Spectrometer with a 12 in. magnet of approx. 14kG (Varian) was used for these measurements, in conjunction with a Model 5100B frequency synthesizer and Model 5110B driver (Hewlett-Packard) which synchronized a Model V-4210A oscillator (Varian). For audio frequency amplification a Parr JB-4 amplifier (Princeton Applied Research) was used which fed its output into a Varian G-14 strip chart recorder. A particular resonance frequency was found by first searching for it with the variable frequency oscillator. This frequency was then synchronized by the frequency synthesizer using a Model 140 A oscilloscope (Hewlett-Packard). The linewidths were recorded in the dispersion mode[5] and are given as peak-peak values. The linewidth calibration was performed with the frequency synthesizer.
Z3Na nuclear magnetic resonance indicated a single sharp resonance at 15.87 MHz whose linewidth was 8.8 -+ 0.5Hz in melts of both A12Cl7- and AIC14- stoichiometry. No chemical shift with stoichiometry was observable. This is a somewhat sharper line than that for aqueous sodium ion, 11.5 Hz. This, and the similar N-H coupling, indicate that the cations in this melt are in at least as ionic an environment as they are in aqueous solutions (Ref. [5], p. 1093). Nuclear magnetic resonance of 35C1gave a single very broad line in all melts, probably due to the high quadrupole moment of this nucleus. The resonance of 27A1at 14 kG and 15.67 MHz showed a single line whose width depended on composition, increasing with added A1C13. Such addition produced no observable chemical shift, as shown by use of melts of AICL- and A12C17- stoichiometry in two concentric tubes. Both the sharper and broader line had the same center, indicating either a single environment for aluminum in the different melts (unlikely) or a rapid exchange equilibrium. The aluminum linewidth data permit the estimation of the equilibrium constant for A12CI7-~A1CLI-+Ale13 in these melts at 170°C. Using these linewidths[7] and the known total molal concentrations of aluminum and chloride, we estimate a value of 6 x 10-2 for this constant. It is assumed that the difference in viscosity of these melts is negligible, as is indicated by the best available data[8]. No literature data appear to exist with which this value can be directly compared. Algebraic manipulation of equilibrium constants determined at 455°C [9] gives a value of approx. 10-5. Use of part of the results of that study with data of Smith[10] or Tremillon [l l] gives values of about 10-*. These data refer to widely different temperatures and melt compositions, and it is therefore not surprising to lind poor agreement among them and with our results. It is clear, Table 1. Results of NMR measurements
RESULTS AND DISCUSSION
The results of this study are summarized in Table 1. The cation for A12C17-was the mixture of Na + and K + in mole ratio 20:14, except for the proton magnetic resonance work in which NH4C1 was substituted for a small part of the NaCI. Proton magnetic resonance of such substituted samples of both A12C17- and NaAICI4 stoichiometry resulted in the expected well-defined triplet with N-H coupling constant of 54.1-+0.2 Hz; the coupling constant was independent of temperature from 120 to 170°C. The reported value[6] for NH4 + in water at 25°C is 53.2-+0.2Hz, approximately the same.
Nucleus 1H 23Na 27A1
3SCI
*Author for correspondence.
Compound NH4+ in AI2CI7NH4+ in AICI4 AI2CI7AICI4 AICI3+ MCI, 2.8: 1 AIC13+ MC1, 1.4:1 AIC13+ MC1, 1.0:1 Al(NO3)3, aq A12C17AIC14-
tN-H coupling. 387
Linewidth (Hz) 54.1 -+0.2t 54.1 -+0.2t 8.8 _+0.5 8.8 _+0.5 700_+30 550_+20 90-+ 5 42 -+4 7000-+500 5000_ 500
388
UDO ANDERS and J. A. PLAMBECK
however, that the value of this equilibrium constant is considerably less than unity, which indicates that the primary species existing in melts of 66:20:14 composition is indeed A12ClT-and that little free A1Cla exists. Such a conclusion is expected in view of the observed low vapor pressure of A1C13 above the ternary mixture [ 12] CONCLUSION This study indicates that fused alkali chloroaluminates of ml2C17-stoichiometry are essentially ionic, the entities in the melt being primarily M + and A12C17-. The amount of A1C13present at equilibrium is small and the A1C14-+ AIC13~AI2C17- equilibrium is rapid. Acknowledgements--The authors are grateful to the National research Council of Canada for support in the form of an Operating Grant (to J.A.P.) and a Science Scholarship (to U.A.). This work was also supported in part by the Defence Research Board of Canada. This paper was taken in part from the thesis (of U.A.) submitted to the Faculty of Graduate Studies, University of Alberta, Edmonton, in partial fulfillmentof the requirements for the Ph.D. degree. The authors are indebted to Dr. R. B. Jordan for helpful
discussions and to Mr. G. Bigam for assistance in the experimental work. REI~JIENCES 1. S. Hafner and N. H. Nachtrieb, Rev. Scientific Inst. 35, 680 (1%4); J. Chem. Phys. 40, 2891 (1964). 2. U. Anders and J. A. Plambeck, Can. J. Chem. 47, 3055 (1969). 3. U. Anders, Ph.D. Thesis, University of Alberta (1969). 4. D. A. Haines and J. A. Plambeck, Can. J. Chem. 46, 1727 (1968). 5. J. W. Emsley, J. Feeney and L. H. Sutcliffe,High Resolution Nuclear Magnetic Resonance, Vol. I, pp. 208--211.Pergamon Press, New York (1%5). 6. G. Fraenkel and Y. Asaki, J. Chem. Phys. 44, 4647 (1966). 7. H. E. Swift, C. P. Poole, Jr. and J. F. Itzel, Jr., J. Chem. Phys. 42, 2576 (1%5); J. Phys. Chem. 68, 2509 (1964). 8. J. A. Plambeck, Encyclopedia of the Electrochemistry of the Elements, VoL X: Fused Salt Systems, p. 235. Marcel Dekker, New York (1976). 9. R. H. Moore, J. R. Morrey and E. E. Voiland,J. Phys. Chem. 67,744 (1963);J. R. Morrey and R. H. Moore, ibid. 748. 10. G. P. Smith, Private communication. 11. B. Tremillon and G. Letisse, J. Electroanal. Chem. 17, 371 (1%8). 12. W. D. Treadwell and L. Terebesi, Heir. Chim. Acta 18, 103 (1935).
1978.Vol.40, pp. 387-388 PergamonPress. Printedin GreatBritain
NUCLEAR MAGNETIC RESONANCE STUDIES OF CHLOROALUMINATE MELTS UDO ANDERS and J. A. PLAMBECK* Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada (Received 13 December 1976; received for publication 2 May 1977)
Abstract--Nuclear magnetic resonance has been used to study the structures of fused (Na, K)AI2CI7 at 170°C. Sodium, aluminum and chlorine magnetic resonance, and proton magnetic resonance on substitution with NH4+, indicate the structure to be essentially ionic as M+ A12C17-. INTRODUCTION
The ternary A1C13-NaCI-KC1 system forms one of the lowest-temperature fused salts known. This permits its study in commercial NMR facilities rather than requiring the special apparatus necessary at higher temperatures[I]. Electrochemical studies ([2]; and references cited therein) have used the 66:20:14 mole % mixture corresponding to (Na, K)AI2CI7 stoichiometry. We report here an NMR study related to the structure of this melt. Other studies, and additional literature, are available elsewhere [3]. EXPERIMENTAL
The purification of AICI3 and other chemicals used has been described in detail elsewhere[2-4]. NMR samples were prepared by melting the components together inside a nitrogen-filledglove box. The liquid melt was transferred into NMR tubes with disposable pipets. Standard medium wall tubes were used for proton NMR while tubes of 10 mm O.D. were used for all wideline experiments. The proton NMR was observed at 100MHz using a 12in. magnet (Model HA-100, Varian Assoc., Palo Alto, California). The temperature was varied from 120 to 170°C using a Model 4333 variable temperature probe (Varian). The nuclear magnetic resonances of 23Na, 27A1and 35C1were studied at 170-+2°C. A Model V-4200B Wideline Spectrometer with a 12 in. magnet of approx. 14kG (Varian) was used for these measurements, in conjunction with a Model 5100B frequency synthesizer and Model 5110B driver (Hewlett-Packard) which synchronized a Model V-4210A oscillator (Varian). For audio frequency amplification a Parr JB-4 amplifier (Princeton Applied Research) was used which fed its output into a Varian G-14 strip chart recorder. A particular resonance frequency was found by first searching for it with the variable frequency oscillator. This frequency was then synchronized by the frequency synthesizer using a Model 140 A oscilloscope (Hewlett-Packard). The linewidths were recorded in the dispersion mode[5] and are given as peak-peak values. The linewidth calibration was performed with the frequency synthesizer.
Z3Na nuclear magnetic resonance indicated a single sharp resonance at 15.87 MHz whose linewidth was 8.8 -+ 0.5Hz in melts of both A12Cl7- and AIC14- stoichiometry. No chemical shift with stoichiometry was observable. This is a somewhat sharper line than that for aqueous sodium ion, 11.5 Hz. This, and the similar N-H coupling, indicate that the cations in this melt are in at least as ionic an environment as they are in aqueous solutions (Ref. [5], p. 1093). Nuclear magnetic resonance of 35C1gave a single very broad line in all melts, probably due to the high quadrupole moment of this nucleus. The resonance of 27A1at 14 kG and 15.67 MHz showed a single line whose width depended on composition, increasing with added A1C13. Such addition produced no observable chemical shift, as shown by use of melts of AICL- and A12C17- stoichiometry in two concentric tubes. Both the sharper and broader line had the same center, indicating either a single environment for aluminum in the different melts (unlikely) or a rapid exchange equilibrium. The aluminum linewidth data permit the estimation of the equilibrium constant for A12CI7-~A1CLI-+Ale13 in these melts at 170°C. Using these linewidths[7] and the known total molal concentrations of aluminum and chloride, we estimate a value of 6 x 10-2 for this constant. It is assumed that the difference in viscosity of these melts is negligible, as is indicated by the best available data[8]. No literature data appear to exist with which this value can be directly compared. Algebraic manipulation of equilibrium constants determined at 455°C [9] gives a value of approx. 10-5. Use of part of the results of that study with data of Smith[10] or Tremillon [l l] gives values of about 10-*. These data refer to widely different temperatures and melt compositions, and it is therefore not surprising to lind poor agreement among them and with our results. It is clear, Table 1. Results of NMR measurements
RESULTS AND DISCUSSION
The results of this study are summarized in Table 1. The cation for A12C17-was the mixture of Na + and K + in mole ratio 20:14, except for the proton magnetic resonance work in which NH4C1 was substituted for a small part of the NaCI. Proton magnetic resonance of such substituted samples of both A12C17- and NaAICI4 stoichiometry resulted in the expected well-defined triplet with N-H coupling constant of 54.1-+0.2 Hz; the coupling constant was independent of temperature from 120 to 170°C. The reported value[6] for NH4 + in water at 25°C is 53.2-+0.2Hz, approximately the same.
Nucleus 1H 23Na 27A1
3SCI
*Author for correspondence.
Compound NH4+ in AI2CI7NH4+ in AICI4 AI2CI7AICI4 AICI3+ MCI, 2.8: 1 AIC13+ MC1, 1.4:1 AIC13+ MC1, 1.0:1 Al(NO3)3, aq A12C17AIC14-
tN-H coupling. 387
Linewidth (Hz) 54.1 -+0.2t 54.1 -+0.2t 8.8 _+0.5 8.8 _+0.5 700_+30 550_+20 90-+ 5 42 -+4 7000-+500 5000_ 500
388
UDO ANDERS and J. A. PLAMBECK
however, that the value of this equilibrium constant is considerably less than unity, which indicates that the primary species existing in melts of 66:20:14 composition is indeed A12ClT-and that little free A1Cla exists. Such a conclusion is expected in view of the observed low vapor pressure of A1C13 above the ternary mixture [ 12] CONCLUSION This study indicates that fused alkali chloroaluminates of ml2C17-stoichiometry are essentially ionic, the entities in the melt being primarily M + and A12C17-. The amount of A1C13present at equilibrium is small and the A1C14-+ AIC13~AI2C17- equilibrium is rapid. Acknowledgements--The authors are grateful to the National research Council of Canada for support in the form of an Operating Grant (to J.A.P.) and a Science Scholarship (to U.A.). This work was also supported in part by the Defence Research Board of Canada. This paper was taken in part from the thesis (of U.A.) submitted to the Faculty of Graduate Studies, University of Alberta, Edmonton, in partial fulfillmentof the requirements for the Ph.D. degree. The authors are indebted to Dr. R. B. Jordan for helpful
discussions and to Mr. G. Bigam for assistance in the experimental work. REI~JIENCES 1. S. Hafner and N. H. Nachtrieb, Rev. Scientific Inst. 35, 680 (1%4); J. Chem. Phys. 40, 2891 (1964). 2. U. Anders and J. A. Plambeck, Can. J. Chem. 47, 3055 (1969). 3. U. Anders, Ph.D. Thesis, University of Alberta (1969). 4. D. A. Haines and J. A. Plambeck, Can. J. Chem. 46, 1727 (1968). 5. J. W. Emsley, J. Feeney and L. H. Sutcliffe,High Resolution Nuclear Magnetic Resonance, Vol. I, pp. 208--211.Pergamon Press, New York (1%5). 6. G. Fraenkel and Y. Asaki, J. Chem. Phys. 44, 4647 (1966). 7. H. E. Swift, C. P. Poole, Jr. and J. F. Itzel, Jr., J. Chem. Phys. 42, 2576 (1%5); J. Phys. Chem. 68, 2509 (1964). 8. J. A. Plambeck, Encyclopedia of the Electrochemistry of the Elements, VoL X: Fused Salt Systems, p. 235. Marcel Dekker, New York (1976). 9. R. H. Moore, J. R. Morrey and E. E. Voiland,J. Phys. Chem. 67,744 (1963);J. R. Morrey and R. H. Moore, ibid. 748. 10. G. P. Smith, Private communication. 11. B. Tremillon and G. Letisse, J. Electroanal. Chem. 17, 371 (1%8). 12. W. D. Treadwell and L. Terebesi, Heir. Chim. Acta 18, 103 (1935).