Water Exchange on Hexaaquavanadium(III): a Variable‐Temperature and Variable‐Pressure¹⁷O‐NMR Study at 1.4 and 4.7 Tesla

Water exchange on hexaaquavanadium (III) has been studied as a function of temperature (255 to 413 K) and pressure (up to 250 MPa, at several temperatures) by 17 O‐NMR spectroscopy at 8.13 and 27.11 MHz. The samples contained V 3+ (0.30–1.53 m), H + (0.19–2.25 m) and 17 O‐enriched (10–20%) H 2 O. The trifluoromethanesulfonate was used as counter‐ion, and, contrary to the previously used chloride or bromide, CF 3 SO is shown to be non‐coordinating. The following exchange parameters were obtained: k = (5.0 ± 0.3) · 10 2 s −1 , Δ H * = (49.4 ± 0.8) kJ mol −1 , Δ S * = −(28 ± 2) JK −1 mol −1 , Δ V * = −(8.9 ± 0.4) cm 3 mol −1 and Δβ* = −(1.1 ± 0.3) · 10 −2 cm 3 mol −1 MPa −1 . They are in accord with an associative interchange mechanism, I a . These results for H 2 O exchange are discussed together with the available data for complex formation reactions on hexaaquavanadium(III). A semi‐quantitative analysis of the bound H 2 O linewidth led to an estimation of the proportions of the different contributions to the relaxation mechanism in the coordinated site: the dipole‐dipole interaction hardly contributes to the relaxation (less than 7%); the quadrupolar relaxation, and the scalar coupling mechanism are nearly equally efficient at low temperature (∼ 273 K), but the latter becomes more important at higher temperature (75–85% contribution at 360 K).