HIGH-PRESSURE C-13 MMR STUDY OF THE MOTIONAL DYNAMICS OF LIQUID BIS(2-ETHYLHEXYL) PHTHALATE

Natural abundance carbon-13 NMR spin-lattice relaxation times (T-1), spin-spin relaxation times (T-2), and nuclear Overhauser enhancements (NOE) of individual carbons in the complex Liquid of bis(2-ethylhexyl) phthalate (DEHP) were measured as a function of temperature from -40 to +70 degrees C and of pressure from 1 bar to 3 kbar. In addition, an unambiguous C-13 peak assignment was carried out by using both 2D hydrogen-carbon chemical shift correlation and specific relaxation of individual carbons in the molecule. The temperature and pressure effects on the relaxation behavior of DEHP were successfully interpreted in terms of a theoretical model assuming a Cole-Davidson distribution of correlation times. To evaluate the motional consequences of the presence of one or two ethylhexyl carboxylate side chains attached to the benzene ring, the relaxation behavior of DEHP was compared to the relaxation data obtained earlier for 2-ethylhexyl benzoate (EHB). The presence of the additional side chain made the reorientational motion of DEHP less anisotropic than that of the monoester analogue of EHB. The mobility gradient along the alkyl side chains was found to be the same for DEHP and EHB.