Intermolecular scalar relaxation

In a computer simulation of fluoride ions in water, possible intermolecular scalar relaxation coupling mechanism was studied, modulated by water dynamics (exchange of water molecules in the first hydration sphere) due to modulation of the J coupling [81]. 

A. Laaksonen, J. Kowalewski, and B. Jdnsson, Intermolecular nuclear spin-spin coupling and scalar relaxation. A quantum-mechanical and statistical-mechanical study for the aqueous fluoride ion, Chem. Phys. Lett., 89 (1982), 412. 

Very few reports have been published on fluorine relaxation times in most cases the results reported for several different molecules indicate that the spin rotation mechanism is dominant at higher temperatures whereas the intermolecular dipole-dipole mechanism is not negligible at lower ones. The chemical shift anisotropy contribution can also be an important factor, whereas the intramolecular dipole-dipole mechanism and scalar coupling contribution seem to be negligible in F relaxation. Some fluorine relaxation times (Tj, T2 and Tj ) have been used for establishing dynamics in a copolymer of tetrafluoroethene and hexafluoropropene. 

Relaxation mechanisms in descending order of their contribution. dd = intra-, intermolecular dipole-dipole interaction, sr = spin-rotation, (int) = internal, sa = shielding anisotropy, pe = proton exchange, e = electron-nuclear (values normalized to a viscosity of 1 cP), sc = scalar coupling. 

In the gas phase, above the minimum, the F spin-lattice relaxation time is linear with density p of the gas, and the slope (TJp) is found to be consistent with Ti/p)ccT in cases where temperature-dependent measurements of F relaxation have been carried out. This implies that the relaxation is dominated by the spin-rotation mechanism in the gas phase. In the liquid phase, the results for several molecules indicate that spin-rotation mechanism is dominant, especially at higher temperatures and intermolecular dipole-dipole mechanism contributes to the relaxation at lower temperatures, e.g., below 200 K in FCIO3. Intramolecular dipole-dipole mechanism, chemical shift anisotropy and scalar coupling contributions are usually found to be negligible in F relaxation. Where intermolecular dipole-dipole and spin-rotation mechanisms have been used to interpret results using Arrhenius temperature dependence for both, the activation energies are usually not equal. Some typical values of F spin-lattice relaxation times are shown in Table 4. 

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