On the T1/T2 Ratio for Protons in Aqueous Mn++ Solutions

The T1≠T2 anomaly for water protons in solutions containing Mn++ has been investigated. At 40 Mc (proton frequency), T1/T2=9.5(±0.5) for Mn++ solutions. When the Mn++ is complexed by the hexadentate ligand, ethylenediaminetetraacetate (MnY—2), or by the tetradentate ligand, nitrilotriacetate (MnX—1), T1 is increased slightly and T2 is increased markedly, so that T1=T2 and the anomaly disappears. For MnX—1, there are probably still 2 H2O molecules complexed to Mn++. EPR studies show that the paramagnetic relaxation time is reduced by complexing (from 3×10—9 sec for Mn++ to 3×10—10 sec for MnX—1). The results therefore support the suggestion by Bloembergen that the anomalous T2 broadening is caused by an AI·S coupling between the electron spin, S, of Mn++ and the proton spins, I, in the coordination sphere plus a long paramagnetic relaxation time. All features of the variation of T1 and T2 with frequency are explained rather well by consideration of the combined effects of the I·S interaction and the dipole interaction. The experimental results are Mn++ MnY−2 MnX−1MnX2−4NT1 M sec1.6×10−43.05×10−42.35×10−4NT2 M sec1.65×10−52.6×10−42.25×10−47.4×10−4T1/T29.5(±0.5)1.15(±0.12)1.05(±0.15)