O-17 NMR LINE BROADENING AND T-1, T-2 MEASUREMENTS ON CRO42- AND HCRO4-

The increase in the line width of the O-17 signal of CrO42- (Delta nu( CrO42-) = 11+/-2 Hz) on adding acid is attributed to a much broader HC rO4- signal (Delta nu(HCrO4-)). This broadening is shown to have acid- independent (Delta nu(0) = 67+/-5 Hz) and acid-dependent [Delta nu(1) = (1.0+/-0.1) x 10(8) Hz M(-1)] contributions. Direct T-1 and T-2 meas urements give T-1 approximate to 70 ms, T-2 approximate to 40 ms for C rO42- With the difference being attributed to spin coupling (T-2' = 90 ms). For HCrO4- T-1 = 8+/-3 ms, and the acid-catalysed Delta nu(1) co ntribution is attributed to rapid exchange of H+ in H2CrO4 (k approxim ate to 1 x 10(9) s(-1)) showing that spin exchange effects are importa nt for this ion.