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 the17O‐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‐injection17O‐NMR was used to extend the accessible kinetic domain. The samples studied contained Al3+(0.5m), Mn2+(0.2–0.5m),H+(0.2–3.1m) and17O‐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−1mol−1, and ΔV = +(5.7 ± 0.2) cm3mol−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 Idmechanisms to these latter reactions.
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Hugi‐Cleary et al. (1985) studied this question.