¹⁷O N.M.R. Line Broadening and T₁, T₂ Measurements on CrO₄^2- and HCrO₄^-

Abstract

The increase in the line width of the ¹⁷O signal of CrO₄²- (ΔvCrO₄^2- = 11±2 Hz) on adding acid is attributed to a much broader HCrO4- signal ΔvHCrO₄). This broadening is shown to have acid-independent (Δv0 = 67±5 Hz) and acid-dependent [Δv1 = (1.0±0.1) × 10⁸ Hz M-1] contributions. Direct T1 and T2 measurements give T1 ≈ 70 ms, T2 ≈ 40 ms for CrO₄²- with the difference being attributed to spin coupling (T2' = 90 ms). For HCrO₄^- T1 = 8±3 ms, and the acid- catalysed Δv1 contribution is attributed to rapid exchange of H,⁺ in H₂CrO₄ (k ≈ 1×10⁹ s^-1) showing that spin exchange effects are important for this ion.