E.C. Reynhardt

The dependences of ESR line widths and spin - spin relaxation times of single nitrogen defects on the concentration of nitrogen defects in diamond

Line widths and spin - spin relaxation times of P1 centres in synthetic Ib and natural Ia diamonds with concentrations of P1 and P2 centres covering the range 0.03 - 400 atomic parts per million have been measured. At concentrations higher than about ten atomic parts per million the line width is linearly dependent on the concentration. At lower concentrations the electron - dipolar contribution to the line width dominates and the width of the line remains constant. Since the pulse sequence employed for measurements eliminates the effects of inhomogeneous line broadening, of the line remains linearly dependent on the total paramagnetic impurity concentration, even at very low paramagnetic impurity concentrations.

13C relaxation in natural diamond

Abstract 13C spin-lattice relaxation times in the laboratory frame have been measured for one type Ia diamond and one type IIa diamond. Spin-lattice relaxation times for polarization in a field of 47 kG are shorter than spin-lattice relaxation times associated with the depolarization in a field of 47 kG after dynamic nuclear polarization in a field of 3.4 kG. If the 13C magnetization is inverted after dynamic nuclear polarization, the spin-lattice relaxation time is shortened, but it is still longer than the corresponding value for polarization in a field of 47 kG. It is suggested that spin-lattice relaxation via paramagnetic impurities is the main spin-lattice relaxation mechanism. A qualitative explanation is offered for the observed relaxation times.

Molecular motions and phase changes in the perovskite-type compound (C10H21NH3)2ZnCl4

A virgin sample of polycrystalline (C10H21NH3)2ZnCl4 shows two phase transitions, at 284 K and 363 K. In the low-temperature phase threefold reorientations of methyl and NH3 groups dominate the nuclear magnetic resonance relaxation results. The motion of the NH3 groups can be described as a three-fold jump among three potential minima of which one is shallower than the other two. In the intermediate temperature phase a defect motion of chain ends has been identified. In the high-temperature phase the spacing between adjacent layers of ZnCl4 tetrahedra is increased by ∼ 16% due to the non-intercalation of the chains. In this phase the chains experience a high degree of mobility, which is partly due to the diffusion of chain defects along the chain. On cooling the sample from the high-temperature phase, it transforms to the intermediate phase, but the transition to the low-temperature phase is absent.

Identification of defect chain motions in the low temperature orthorhombic phase of binary mixtures of n‐alkanes by means of nuclear magnetic resonance spin‐lattice relaxation time measurements

Powder x‐ray diffraction, differential scanning calorimetry, and nuclear magnetic resonance spin‐lattice relaxation time measurements on a series of n‐alkane binary mixtures are reported. The results have been interpreted in terms of a model in which the longer chains execute thermally activated end‐gauche defect motions, while the shorter chains are involved in twofold screw motions. The results are compared with infrared and x‐ray diffraction results.

Molecular motions in solid saccharides studied by NMR spectroscopy

Abstract Analyses of spin-lattice relaxation time and second moment measurements of the polycrystalline saccharides methyl α-D-mannopyranoside, methyl β-D-arabinopyranoside, methyl β-L-arabinopyranoside and β-D-allose have revealed, apart from methyl group reorientations in the methyl pyranosides, the jump motion of a hydroxyl proton in a hydrogen bond in β-D-allose and trans-gauche reorientations of CH 2 OH groups in methyl α-D-mannopyranoside and β-D-allose.

Temperature dependence of spin-spin and spin-lattice relaxation times of paramagnetic nitrogen defects in diamond

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.

Dynamic nuclear polarization of diamond. I. Solid state and thermal mixing effects

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.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) ex...

Dynamic nuclear polarization of diamond. II. Nuclear orientation via electron spin-locking

The polarization of 13C nuclei by means of nuclear orientation via electron spin-locking (Hartmann–Hahn cross-polarization between paramagnetic electrons and 13C nuclei) in a suite of natural diamonds has been investigated at 2.4 GHz and 9.6 GHz. The 13C polarization rate has been found to be independent of the microwave frequency, in agreement with theory. It is shown that since T1ρ(e)≪T1(e) for diamond, the effective polarization rate of 13C nuclei is relatively low. At low paramagnetic impurities (Ce<5 ppm) 13C nuclei in diamond are polarized faster by employing continuous wave microwave radiation to drive the forbidden transitions of the 13C-electron spin system, the so-called solid effect.

Nuclear spin-lattice relaxation and complex motions in polycrystalline solids

Spin–lattice relaxation rates in the laboratory and rotating frames have been derived for complex motions in polycrystalline solids consisting of combinations of two-site and three-site jumps in asymmetric and symmetric potentials. The theory is applied to experimental data for trimethylamineborane. A complex motion consisting of threefold reorientations of CH3 and BH3 groups and a threefold reorientation of the entire molecule has been analysed.

The structure and melting of paraffinic Fischer—Tropsch waxes

NMR results, which show that the mobile amorphous zone of a wax is formed only after voids in the amorphous zone have been filled by shorter chains, are presented. A model for the melting of a wax is proposed.