Variable-Temperature and Variable-Pressure 17O-NMR Study of Water Exchange of Hexaaquaaluminium(III) |
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Authors: | Deirdre Hugi-Cleary Lothar Helm André E. Merbach |
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Affiliation: | Institut de chimie minérale et analytique de l'Université de Lausanne, Place du Château 3, CH–1005 Lausanne |
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Abstract: | The transverse relaxation rate of H2O in Al(H2O) has been measured as a function of temperature (255 to 417 K) and pressure (up to 220 MPa) using the 17O-NMR line-broadening technique, in the presence of Mn(II) as a relaxation agent. At high temperatures the relaxation rate is governed by chemical exchange with bulk H2O, whereas at low temperatures quadrupolar relaxation is prevailing. Low-temperature fast-injection 17O-NMR was used to extend the accessible kinetic domain. The samples studied contained Al3+ (0.5 m), Mn2+ (0.2–0.5 m), H + (0.2–3.1 m) and 17O-enriched (20–40%) H2O. Non-coordinating perchlorate was used as counter ion. The following H2O exchange parameters were obtained: k = (1.29 ± 0.04) s−1, ΔH* = (84.7 ± 0.3) kJ mol−1, ΔS* = +(41.6 ± 0.9) J K −1 mol−1, and ΔV = +(5.7 ± 0.2) cm3 mol−1, indicating a dissociative interchange, Id, mechanism. These results of H2O exchange on Al(H2O) are discussed together with the available complex-formation rate data and permit also the assignment of Id mechanisms to these latter reactions. |
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