J. Chem. SOC., Faraday Trans. 1, 1982, 78, 289-293 Proton Chemical Shift of the Aqua-aluminium Ion Al( H 20); + BY J. W. AKITT? Physicochimie des Solutions, ENSCP, Universite Pierre et Marie Curie, 1 1 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France Received 4th March, 1981 The variation of the proton chemical shift of the aqua-aluminium complex Al(H,O):+ has been measured for solutions of AI(CIO,), in a series of acetone+ water mixtures of different compositions from - 80 to + 32 OC. The variation of chemical shift with temperature depends strongly upon the acetone/water ratio and appears to be small in pure water. Evidence is obtained for ion pairing in the second sphere in the acetone-rich solutions and this is believed to account for the temperature effects observed.There is general agreement that the study of solvent proton chemical shifts, particularly for aqueous solutions, is likely to give information about the solvation of ions. It is now well-established that anions and cations affect the n.m.r. parameters quite different1y;l for example, low-temperature spectra of concentrated aqueous MII1 and some MI1 solutions show two resonances at low temperatures, one due to the water coordinated by the cation and one due to all the remaining water, including that interacting with the anion. Such spectra are used to obtain solvation numbers which are in this case the same as the coordination numbers., At temperatures above 0 OC only one water resonance is observed and the presence of the electrolyte reduces the temperature dependence of its chemical shift, which effect may also be used to estimate cationic hydration number^.^^ The calculation used assumes that the chemical shift of the water complexed to the cation is independent of temperature, an assumption which so far has been only partially tested in pure water though it is not correct for Mg(H,O)E+ in a~etone.~' The chemical shift of the water in those cationic complexes which can be measured is well-correlated with the calculated shift to be expected from the electric field of the However, the shift for Al(H,O);+ in MeCN differs from that obtained in water so that it appears that some medium effect is likely.s We report here measurements of the behaviour of the complex A1(H20),3+ in aqueous acetone over a range of compositions in an endeavour to clarify its behaviour.EXPERIMENTAL Proton n.m.r. spectra were obtained at 100 MHz on a Varian XL 100 12 WG in the C.W. mode. Probe temperatures were measured using a thermocouple placed in a sample tube for each temperature used. Samples were made up by weight using distilled water, hexadeuteroacetone from Spectrometrie Spin et Techniques and Al(C10,), .8H,O from Fluka (purum cryst.) as received. Initially DSS was used as internal reference but this formed a complex with the AIL" in those solutions which contained much acetone and had to be replaced by TMS. The shifts t On sabbatical leave from the School of Chemistry, University of Leeds, Leeds LS2 9JT, where all correspondence should be addressed. 289290 were corrected for the small difference between the two references and then to the ethane gas reference normally used in this type of work3* * using the relationshiplo.l 1 PROTON CHEMICAL SHIFT OF A1(H20)iS 6 = 6,,, - 0.88. Concentrated HC10, was added to the highly aqueous solutions to reduce the rate of proton exchange12 so as to increase the range over which the resonance of the bound water could be seen without perturbation of its position by exchange. RESULTS In those solutions which contained predominantly acetone, the bound-water resonance moved downfield with decreasing temperature at a maximum rate of 0.0056 ppm O C - l . Below ca. - 10 O C the resonance split into a poorly resolved doublet whose two components then moved in parallel but changed in relative intensity as the temperature decreased, the high-field component being the most intense at high temperatures and the low-field one becoming most intense by - 60 O C , fig.1. A sample containing an excess of perchlorate (as NaClO,) behaved similarly except that the high-field component now dominated the doublet at all temperatures. The chemical shift of the high-field component is plotted in fig. 2 where it can be compared with the results for the singlets obtained in the more moist solutions. The proportion of free water in the acetone was measured from the spectra and it was found that this did not vary with temperature and agreed with the assumption that all the A1 was present as A1(H20)i+. The composition of the solutions is summarised in table 1. Proton exchange between acetone and water takes place slowly in the acetone-rich solutions.n I I 1 FIG. I.-lH n.m.r. spectra at 100 MHz of the bound water in aqueous acetone solutions of AI(CIO,), at two temperatures, - 58 OC (upper) and - 38 OC (lower). The shift markers are at 10 ppm from TMS for each spectrum and the peak separation is 0.08 ppm. The left-hand spectra are for Al(ClO,), in a mixture with X = 0.95, the central ones for AI(ClO,), in a mixture with X = 0.86 and the right-hand ones for Al(CIO,), plus NaClO, with X = 0.95. Xis the mole fraction of acetone, based on the quantity of Eon-bound water in solution.J. W. AKITT 8.0 29 1 - + I 1 I t TABLE 1 .-COMPOSITION OF THE SOLUTIONS MEASURED mole fraction acetone, X , [H,O] free [All relative to free water /[A13+] /mol kg-I notes 0.95 0.95 0.86 0.48 0.17 0 0 0.15 - 4 4 0.18 NaClO, addeda 8 0.31 - 24 0.42 acid added 35 0.90 acid added 52 1.01 acid added 11.5 2.36 acid added a The ratio [C10;]/[A13+] is 3.0 in all cases except where NaC10, was added, where it had the value 7.5.292 PROTON CHEMICAL SHIFT OF Al(H,O);+ As the water content of the samples increased, the temperature dependence of the chemical shift decreased and became too small to measure below a mole fraction, X , of acetone equal to 0.2.The certainty with which the slope of the plots can be measured decreases as the acetone content is reduced, since the range over which chemical shifts can be measured is decreased. Exchange sets in at lower temperatures on the one hand and the solutions freeze at progressively higher temperatures.The trends are nevertheless unmistakeable in fig. 2. The chemical shift at a given temperature moves steadily upfield as Xis decreased until X = 0.2 when the rate of change increases. The chemical shift in pure water as solvent is close to that found previo~sly.~ DISCUSSION It is obvious from these results that the behaviour of Al(H,O);+ in pure water cannot be inferred from its behaviour in aqueous acetone.13 It appears reasonable to accept for this aqua-cation at least that the proton shift is temperature-independent in pure water and has a value of ca. 8.8 ppm downfield of gaseous ethane. It will, however, prove difficult to obtain equivalent data for other cations where either proton exchange is too fast or the hydrolysis reactions are such that acid addition does not seem to slow the exchange, although the existing data are now more happily based in the light of the present results.BEHAVIOUR IN PREDOMINANTLY ACETONE SOLUTION This differs so much from that in pure water that the interactions and the species present must be of quite different types. The chemical shift is a linear function of temperature between 0 and - 80 OC provided the mole fraction of acetone is high and of the order of 0.95. Two resonances with the same sensitivity to temperature are observed for bound water but whose relative areas change with temperature, while the low-field component is almost entirely suppressed by the addition of excess C10, as NaClO,, an indication that we are observing an anion-cation interaction. However, the proportion of bound water is in accordance with the existence at all times of only Al(H,O);+, a conclusion which is reinforced by the small changes which occur in the spectra for an increase in water content from X = 0.95 to X = 0.86.Thus we must be observing the formation of second-sphere ion pairs such as have recently been reported in DMSO + benzene.', The numbers of water molecules and ClO; ions are similar in these solutions and we can expect the second sphere to contain some water, some acetone and some anions, the exact proportions depending upon any specific preferences for the molecule concerned. An ion pair formed in such a mixture will have a strong dipolar electric reaction field which will cause the proton chemical shift of the first-sphere water to move downfield: that nearest to the anion because its field will have the predominant effect, that furthest away from the anion due to the reaction field.We note that the low-field component is favoured ( a ) by low temperatures and so high dielectric constant, &,15 and (b) by addition of small quantities of water, but is suppressed by the addition of NaClO,. This points to its being the I : I complex. The high-field line must then arise from a complex involving more anions, disposed in such a way that the resulting multipole reaction field is a little weaker than that of the dipolar field. The chemical shifts of both components move in parallel and sensibly linearly with temperature. The dielectric constant of acetone also changes almost linearly with temperature, from 21.4 at 20 OC to 31.0 at - 80 OC15 and, if we assume that aqueous acetone with X = 0.95 behaves similarly, then the shifts correlate strongly with E , as is to be expected for a reaction-field effect.J.W. A K I T T 293 On the other hand, when the full range of compositions is considered we see that the chemical shifts move upfield with increasing E . In this case we must be observing a decrease in the extent of ion pairing as more water is added. I thank Madame M. J. Pouet of Madame Simonnin’s laboratory for obtaining the spectra and wish to record my warm appreciation of the interest and hospitality shown by Prof. R. Schaal. See, for example, Faraday Discuss. Chem. SOC., 1978, 64. A. Fratiello, R. E. Lee, V. M. Nishida and R. E. Schuster, Znorg. Chem., 1971, 10, 2552. E. R. Malinowski, P. S. Knapp and B. Feuer, J . Chem. Phys., 1966, 45, 4274. J. W. Akitt, Faraday Discuss. Chem. SOC., 1978, 64, 102. F. Toma, M. Villeman and J. M. Thiery, J . Phys. Chem., 1973, 77, 1294. G. W. Stockton and G. S . Martin, J . Am. Chem. Soc., 1972, 94, 6291. J. W. Akitt, J . Chem. SOC., Dalton Trans., 1973, 42. Y. Reuben and J. Reuben, J . Phys. Chem., 1976,80, 2394. J. W. Emsley, J. Feeney and L. M. Sutcliffe, High Resolution NMR Spectroscopy (Pergamon Press, London, 1965), vol. 2, pp. 669 and 673. j M. C. R. Symons, Faraday Discuss. Chem. SOC., 1978, 64, 127. lo W. G. Schneider, M. J. Bernstein and J. A. Pople, J . Chem. Phys., 1958, 28, 601. l 2 J. W. Akitt, J. Chem. SOC., Dalton Trans., 1973, 1177. l3 J. W. Akitt, Faraday Discuss. Chem. SOC., 1978, 64, 130. l4 W. Libius, W. Graybkowska and R. Pastewski, J . Chem. SOC., Faraday Trans. I , 1981, 77, 147. l 5 International Critical Tables, ed. E. W. Washburn (McGraw-Hill, New York, 1929), vol. 6. (PAPER 1 /369)