Study of the Liquid-State Dynamics by Means of Magnetic Resonance and Dynamic Polarization

Abstract

The dynamics of the liquid state of two organic free-radical solutions has been studied by means of EPR, NMR, and DNP (dynamic nuclear polarization). In our spectrometer these effects can be measured simultaneously and over a wide range of temperatures. The aim was to study the gradual transition from the ordinary liquid state to a greatly supercooled liquid state, and in particular the change from the Overhauser-type DNP to the solid-type DNP. The samples were a 0.02-mole/liter solution of tanone in isopropanol and a 0.03-mole/liter solution of tetrachlorosemiquinone in tetraethyleneglyco. The EPR can be explained in terms of random rotational modulation of the anisotropic $g$ factor of the radical, with a correlation time which is an exponential function of reciprocal temperature. At the temperatures of interest, $T_{1e}$ and $T_{2e}$ differ by at least one order of magnitude. The NMR follows from the Torrey model, the protons relaxing through dipole interaction with the electron spin while their molecule is temporarily adsorbed in the solvation layer of a radical. The data can be fitted by a model with a dipole-dipole correlation function based on a rather broad distribution of correlation times, the mean value and temperature dependence being similar as in EPR. The DNP shows a typical gradual transition from the (even) Overhauser effect to the (odd) solid effect as the temperature is lowered. This happens when the correlation time and $T_{2e}$ cross. A theoretical analysis of the DNP on the basis of the liquid structure as revealed by EPR and NMR is given in the following paper.