Spin-lattice relaxation times of P1 centers in a suite of two natural type Ib, two synthetic type Ib, and one natural type Ia diamonds were measured at 9.6 GHz as a function of temperature in the range 300 K>T>4.2 K. An analysis of the results revealed that for three of the diamonds (two synthetic type Ib and the natural type Ia) spin-orbit phonon-induced tunneling is the main relaxation mechanism. In the case of the Ia diamond cross-relaxation takes place between P1 and P2 centers. In the natural type Ib samples a much more effective relaxation mechanism dominates at lower temperatures. Electron spin resonance spectra of the latter samples revealed the presence of N3 centers. It seems that the more effective relaxation mechanism is associated with the N3 centers and that the P1 centers relax via the N3 centers to the lattice at these temperatures.
A detailed NMR investigation, supported by DSC and X-ray diffraction measurements, of the combwax of the African bee apis mellifera adansonii is reported. Proton spin-lattice relaxation times in the laboratory and rotating frames, as well as the proton spin-spin relaxation time, have been measured as a function of temperature (333 K>T>110 K). The liquid content of the wax has also been determined as a function of temperature by employing a simple pulse technique. The motional parameters associated with the reorientations of the methyl groups and the chains have been isolated and compared with these parameters for similar motions in Fischer-Tropsch waxes. The T1 rho results revealed a minimum which is associated with the dangling motion of chain ends in the amorphous zone of the wax. The relaxation results strongly suggest that beeswax is branched to a much higher degree than Fischer-Tropsch waxes, including oxidised waxes. The high-resolution 13C spectrum of beeswax in the solid state shows that it resembles oxidised Fischer-Tropsch hard wax closely. The major difference is that a higher percentage of carbon atoms are involved in ester groups in beeswax. Oxidised hard wax contains a higher fraction of carbon atoms with double carbon-carbon bonds. The average chain length in beeswax, determined by ebullioscopic methods, is 40 carbon atoms.
The dynamic nuclear polarization of 13C nuclei in a suite of seven natural type Ia and Ib diamonds, using continuous wave S- and X-band microwave radiation, is described. The 13C signal enhancement and polarization time have been measured for one of the type Ib diamonds as a function of magnetic field in the vicinity of the resonance field. The total paramagnetic impurity concentration (P1 and other centers) in this diamond is 2×1018 cm−3 (23 atomic parts per million), while the concentration of P1 centers is 9.3 ppm. Since the central electron spin resonance (ESR) linewidth HL is comparable with H0γC/γe, flip–flip and flip–flop forbidden transitions take place simultaneously. Consequently thermal mixing plays an important role in the 13C signal enhancement. However, the 13C spin-lattice relaxation rate is determined to a large extent by the solid state effect (forbidden transitions). The 13C polarization rates have been measured for the suite of diamonds by executing dynamic nuclear polarization (DNP) experiments on both hyperfine and central ESR lines. It is shown that the polarization rate is proportional to the paramagnetic impurity concentration of the sample, in agreement with the existing theory. It has been found that in type Ib diamonds with relatively low nitrogen impurity concentrations the dynamic nuclear polarization of a single hyperfine line yields an equilibrium 13C polarization that is one-quarter of that obtained in the case of dynamic nuclear polarization of the central line. In samples containing P1 and P2 centers (type Ia), or in type Ib samples with relatively high concentrations of P1 centers, the same equilibrium 13C polarization is obtained for the DNP of hyperfine and central transitions. This phenomenon is explained in terms of a model in which thermal contact is established between the electron Zeeman reservoir and the nuclear spin reservoirs via the spin–spin interaction reservoir if HL⩾H0γC/γe.
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