Temperature Dependence of Proton Relaxation Times in Aqueous Solutions of Paramagnetic Ions

The nuclear magnetic relaxation times of protons in aqueous solutions of paramagnetic ions have been measured as a function of temperature. For Mn++ ions, the proton T1 and T2 were measured at 10, 20, and 28 Mc/sec, and for Co++, Cu++, and Gd+++ ions, 20 Mc/sec. The temperature effects observed can be explained by incorporating the proper temperature dependences of the several correlation times into the general Bloembergen-Solomon relaxation expressions. In the case of Mn++ solutions, which have a T1/T2 ratio of about 5.5 at room temperature and 20 Mc/sec, chemical as well as electron exchange and dipolar interactions are found to be important. Analysis of the data by means of the equations developed provides information concerning the chemical exchange of solvent protons with those in the hydrated Mn++ ion. An activation energy of 8.4 kcal/mole and other parameters for this process are evaluated. The temperature dependence of T1 and T2 in the Co++, Cu++, and Gd+++ solutions can be explained by the temperature dependence of the dipolar interaction alone. Varying the anion in Cu++ solutions had little effect on T1 or its temperature dependence; also, T1 for Cu++ solutions was found to vary inversely with concentration over the whole temperature range studied.