Nuclear Magnetic Relaxation of 19F Due to Cu(II) in Aqueous Solutions of F− and HF

The effect of Cu + + on T1 and T2 of aqueous fluoride has been measured over a range of fluoride concentrations and from pH 0.5–6. A strong interaction due to electron overlap in the complex CuF+ dominates both the T1 and T2 effects. The T2 data have been treated by a three-site analog of the Swift–Connick equation, taking explicit account of T1 and T1e, the spin–lattice relaxation time of the unpaired electron, as well as allowance for chemical effects on the lifetime of the complex. The hyperfine tensor in CuF+ has an anisotropic part as well as an isotropic part, equal to 8 × 108 and 1 × 109 rad/sec, respectively. The former controls the T1 process with a correlation time determined by the tumbling motion, while the latter, the familiar scalar-contact interaction, controls the T2 process in a way modified by various correlation time effects. T1e seems too long to give a measurable effect on either process. The following results were obtained for the rate constants of the chemical processes which control the lifetime of the CuF+ complex: CuF+→Cu++ + F− k = 1 × 108 / sec, CuF+ + H+→Cu++ + HF k = 5 × 109 / sec·M, CuF+ + F−→F− + CuF+ k = 1 × 108 / sec·M, CuF+ + HF2−→HF2− + CuF+ k = 1 × 109 / sec·M. The last two are F− exchange reactions. These parameters all depend on the assumption that the CuF+ complex with the contact interaction has the molar-scale association constant K = 2. In addition to the processes which involve CuF+ there is one involving CuHF + + which dominates T1− 1 at the lowest pH. The contribution of this species to the relaxation processes is accounted for by the dipole–dipole interaction of the unpaired electron with the 19F, and an association constant little different from that of random collisions.