Frequency Dependence of Proton Spin Relaxation in Aqueous Solutions of Paramagnetic Ions

Magnetic resonance relaxation times T1 and T2 were measured for protons in aqueous solutions of Cr(H2O)6+++, Mn(H2O)x++, Co(H2O)x++, and Nd(H2O)x++ in the frequency range 2.7 to 28.7 Mc/sec. T1 was observed to decrease with decreasing frequency for the first two ions, but not the last two, while T2 was essentially constant for each ion. The abnormally large values of T1/T2 which have been previously reported appear as a result of the effect at high frequencies. Those observations are interpreted in terms of a theoretical model of the magnetic interaction between ion and proton in which a random thermal process is presumed to occur with a characteristic correlation time, τe, of 10—8 to 10—11 sec.