Molecular motion in liquid toluene from a study of 13C and 2D relaxation times

The 13C nuclear spin-lattice relaxation times for ring and methyl carbons in liquid toluene were studied from −95°C to +60°C at frequencies of 14 and 61 MHz. Data were taken for protonated as well as deuterated toluene. The results were analyzed in terms of three relaxation mechanisms: intramolecular dipole-dipole coupling, spin-rotation interaction, and anisotropic chemical shift. The last mechanism gives a significant contribution only to the relaxation rate of the ring carbons of the deuterated species at 61 MHz and low temperatures. A tentative value of Δσ = 295 ppm is obtained in this case. In order to separate the contributions of the dipole-dipole and spin-rotation interactions the 13C data are compared with deuteron relaxation times. Comparison of the 13C data in the protonated and deuterated form of toluene shows that the correlation times for the ring differ by 20% and an even larger effect of isotopic substitution is found for the methyl group. It is demonstrated that the fast internal motion of the methyl group cannot be studied quantitatively using deuteron or 13C intramolecular dipole-dipole relaxation rates alone because of the sensitivity of the results to the angle, varpi, the Z-axis of the electric field gradient, or the internuclear vector, respectively, forms with the C3 axis. Analysis of the relaxation rates due to spin-rotation interaction yields τj (internal), the correlation time of angular momentum of the internal motion directly. The correlation time of reorientation τc (internal) is calculated from τj (internal) using Gordon's extended diffusion model which is applied to a symmetric rotor with a fixed axis. It is found that both τj (internal) and τc (internal) are of the same magnitude as the correlation time of the free rotor. The ratio of correlation times of the overall and internal reorientation ranges from approximately 200 at the melting point to approximately 13 at +60°

Supramolecular aspects of polymer science: A challenge for solid state NMR

Recent developments in structural elucidation of supramolecular systems by advanced solid state NMR are described. Special emphasis is placed on hydrogen-bonded sytems and columnar stacks of aromatic moieties. In imidazole-based proton conductores spatially separated regions of high and low mobility are identified. In stacks of alkyl-substituted hexabenzocoronenes maximum charge carrier mobility is observed for crystal-like stacking of the discs

Stable isotope record and late Quaternary sedimentation rates at the Antarctic continental margin

Four cores from the Antarctic continental margin located between 50 and 200 km from the present-day ice shelf edge, were selected for sedimentological and mass spectrometer analysis. The first stable isotope records of the Southern Polar Ocean can be correlated in detail with global isotope stratigraphy. Together with magnetostratigraphic, sedi¬mentological and micropaleontological data, the record provides stratigraphic and paleoceanographic information back to the Jaramillo subchron (910 kyr).Although the isotope values have been altered by diagenetic processes in the sediments, which are poor in carbonate, an interpretation is possible via correlation with the sedimentological parameters. Oxygen isotope data give indications for a meltwater spike at the beginning of interglacials, when large scale melting of parts of the ice shelves took place. The synchronous record of the benthic and planktonic d13C-signals reflect continuous bottom water formation also during glacials.Primary productivity was strictly reduced during glacials due to continuous ice coverage in the Weddell Sea. The climatic improvement at the beginning of an interglacial is associated with peak values in biologic activity lasting for about 15 kyr.During one climatic cycle, mean sedimentation rates at the continental margin decrease with increasing distance from the continent from 5.2 to 1.3 cm/kyr. Maximum sedimentation rates of 25 cm/kyr at the beginning of an interglacial down to 0.6 cm/kyr during glacial periods have been calculated. The rate is mainly controlled by movements of the ice shelf edge and ice rafting