Spin-lattice relaxation time, tunneling

When the H- H dipole-dipole interaction can be measured for a specific pair of H nuclei, studies of the temperature dependence of both the H NMR line-shape and the H NMR relaxation provide a powerful way of probing the molecular dynamics, even in very low temperature regimes at which the dynamics often exhibit quantum tunnelling behaviour. In such cases, H NMR can be superior to quasielastic neutron scattering experiments in terms of both practicality and resolution. The experimental analysis can be made even more informative by carrying out H NMR measurements on single crystal samples. In principle, studies of both the H NMR lineshape and relaxation properties can be used to derive correlation times (rc) for the motion in practice, however, spin-lattice relaxation time (T measurements are more often used to measure rc as they are sensitive to the effects of motion over considerably wider temperature ranges. 

For the partially deuterated benzoic acid (C6D5COOH), the solid state H NMR spectrum is dominated by the intra-dimer H- H dipole-dipole interaction. In a single crystal, both tautomers A and B are characterised by a well-defined interproton vector with respect to the direction of the magnetic field (Fig. 1). Proton motion modulates the H- H dipole-dipole interactions, which in turn affects the H NMR lineshape and the spin-lattice relaxation time. It has been shown that spin-lattice relaxation times are sensitive to the proton dynamics over the temperature range from 10 K to 300 K, and at low temperatures incoherent quantum tunnelling characterises the proton dynamics. A dipolar splitting of about 16 kHz is observed at 20 K. From the orientation dependence of the dipolar splitting, the... 

Biomedical Applications. Spin-lattice relaxation times Ti and Tid as well as NMR second moment were employed to study the molecular dynamics of riboflavin (vitamin B-2) in the temperature range 55-350 K. The broad and flat T1 minimum observed at low temperatures is attributed to the motion of two non-equivalent methyl groups. The motion of the methyl groups is interpreted in terms of Haupt s theory, which takes into account the tunneling assisted relaxation. 

Tropolone is easily deuterated and the Sj-Sq fluorescence excitation spectrum of jet-cooled TRN(OD) was reported by Sekiya et al. [40]. The observed doublet separation, DS = 2 cm 1, is near the 2.2 cm value reported by Alves and Hollis [22], and about 10% of the value observed for TRN(OH). Presently the only available experimental estimate for the ZP tunneling splitting of Sq TRN(OD) is Ao <0.17 cm 1 [80] obtained using a chloroform solvent and NMR spectroscopy to measure the deuteron spin-lattice relaxation time. 

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