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An exchange interaction is postulated between the electron spin and proton spin in adjacent water-molecules in paramagnetic solutions. In combination with the long electron spin relaxation times for Mn++ and Gd+++, an explanation is given why T1/T2 for protons is much larger than unity in solutions of these particular ions in high magnetic fields, whereas T1 is about equal to T2 in other solutions or in low fields.
N. Bloembergen (Thu,) studied this question.