Variable‐Temperature and Variable‐Pressure¹⁷O‐NMR Study of Water Exchange of Hexaaquaaluminium(III)

Abstract

Abstract The transverse relaxation rate of H 2 O in Al(H 2 O) has been measured as a function of temperature (255 to 417 K) and pressure (up to 220 MPa) using the 17 O‐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 H 2 O, whereas at low temperatures quadrupolar relaxation is prevailing. Low‐temperature fast‐injection 17 O‐NMR was used to extend the accessible kinetic domain. The samples studied contained Al 3+ (0.5 m ), Mn 2+ (0.2–0.5 m ), H + (0.2–3.1 m ) and 17 O‐enriched (20–40%) H 2 O. Non‐coordinating perchlorate was used as counter ion. The following H 2 O exchange parameters were obtained: k = (1.29 ± 0.04) s −1 , Δ H * = (84.7 ± 0.3) kJ mol −1 , Δ S * = +(41.6 ± 0.9) J K −1 mol −1 , and Δ V = +(5.7 ± 0.2) cm 3 mol −1 , indicating a dissociative interchange, I d , mechanism. These results of H 2 O exchange on Al(H 2 O) are discussed together with the available complex‐formation rate data and permit also the assignment of I d mechanisms to these latter reactions.