MATCHED SPIN PROBES FOR THE STUDY OF THE OVERALL MOTION OF MODEL LUBRICANTS

For ESR studies of molecular rotation in liquids, it is desirable to m atch the spin probe as closely as possible to the solvent molecules, I n this work the solvents were model lubricants whose overall motion ha s been characterized from carbon-13 NIMR relaxation measurements. Two compounds were studied, the approximately cylindrical molecule 5-(n-oc tyl)naphthalene (ON) (a model low-traction fluid) and the approximatel y spherical molecule dicyclohexy-3,4-furan dicarboxylate (DCFDC) (a mo del high-traction fluid). For these two compounds the matched spin pro bes 3,3-tetrakis(trideuteriomethyl)isoindolin-2-yloxyl (OTMIOD) and ,5 -bis(cyclohexyloxycarbonyl)-1,3,2-dithiazol-2-yl (BCCD), respectively, were prepared, ESR measurements were made on OTMIOD in ON and in tolu ene at two frequencies, namely 9 GHz (X band) and 94 GHz (W band), and the importance of making the high-frequency measurements was demonstr ated for evaluation of the anisotropy parameters, The results are diff erent for the two solvents, thus proving the necessity for matching th e spin probe to the solvent, BCCD was shown to be tumbling isotropical ly in DCFDC, Activation energies were derived for the overall motions from tbe ESR and NMR measurements. These activation energies were comp ared with those for the viscosities of the solvents. The viscosities o f both ON and DCFDC were measured over a wide temperature range using an ESR method. (C) 1997 by John Wiley & Sons, Ltd.