THE APPLICATIONS OF NUCLEAR SPIN RELAXATION TO POLYMER DYNAMICS AND INTERMOLECULAR INTERACTIONS

The correlation functions of the side - groups and side ?chains of polymers are obtained for nuclear spin relaxation if the segmental motion of the polymers is described by VJGM model, these functions are derived from unequal two ?side and three -site jump internal rotation, diffusion internal rotation, restricted internal rotation and multiple internal rotation. The corresponding spectral density functions are also given, and these functions are used to interpret the nuclear spin relaxation data of the side-groups of some polymers. The average spectral density functions of side-groups are derived under the magic angle spinning, the correlation times and diffusion coefficients of the side-groups of crosslinked poly (methyl methacry-latcs) and solid poly(vinylbutyral) are obtained by using these average spectral density functions. The multiphase structures of nylon 6, poly (ethylenc glycol) and its complexes are investigated with cross ?polarization and magic angle spinning techniques.Three methods using     

Understanding Overhauser Dynamic Nuclear Polarisation through NMR relaxometry

Overhauser dynamic nuclear polarisation (DNP) represents a potentially outstanding tool to increase the sensitivity of solution and solid state NMR experiments, as well as of magnetic resonance imaging. DNP signal enhancements are strongly linked to the spin relaxation properties of the system under investigation, which must contain a paramagnetic molecule used as DNP polariser. In turn, nuclear spin relaxation can be monitored through NMR relaxometry, which reports on the field dependence of the nuclear relaxation rates, opening a route to understand the physical processes at the origin of the Overhauser DNP in solution. The contributions of dipole–dipole and Fermi-contact interactions to paramagnetic relaxation are here described and shown to be responsible to both the relaxometry profiles and the DNP enhancements, so that the experimental access to the former can allow for predictions of the latter.     

Relaxation of protons by radicals in rotationally immobilized proteins

Proton spin-lattice relaxation by paramagnetic centers may be dramatically enhanced if the paramagnetic center is rotationally immobilized in the magnetic field. The details of the relaxation mechanism are different from those appropriate to solutions of paramagnetic relaxation agents. We report here large enhancements in the proton spin-lattice relaxation rate constants associated with organic radicals when the radical system is rigidly connected with a rotationally immobilized macromolecular matrix such as a dry protein or a cross-linked protein gel. The paramagnetic contribution to the protein-proton population is direct and distributed internally among the protein protons by efficient spin diffusion. In the case of a cross-linked-protein gel, the paramagnetic effects are carried to the water spins indirectly by chemical exchange mechanisms involving water molecule exchange with rare long-lived water molecule binding sites on the immobilized protein and proton exchange. The dramatic increase in the efficiency of spin relaxation by organic radicals compared with metal systems at low magnetic field strengths results because the electron relaxation time of the radical is orders of magnitude larger than that for metal systems. This gain in relaxation efficiency provides completely new opportunities for the design of spin-lattice relaxation based contrast agents in magnetic imaging and also provides new ways to examine intramolecular protein dynamics.     

Nuclear magnetic resonance and spin relaxation in biological systems

Proton nuclear spin-lattice relaxation in biological systems is generally distinguished from that in inorganic systems such as rocks by the presence of locally disordered macromolecular environments. Rapid exchange of readily observed labile small molecules among differently oriented macromolecular sites generally nearly averages the spectral anisotropies in the small molecule resonances. The biological tissue is generally distinguished from the inorganic matrix by the presence of a significant population of protons in the solid components that are well connected by dipolar spin couplings. Magnetic coupling between the solid and the liquid components generally dominates the magnetic field dependence of the spin-lattice relaxation rates observed in the small molecule components which is generally described by a power law in the Larmor frequency. Recent theory involving a modification of the spin-phonon class of relaxation mechanism provides a quantitative understanding of these data in terms of the dynamics of the chain molecules generally present in the solid spin systems, folded proteins for example.     

Effect of Coupling between Rotational and Translational Brownian Motions on NMR Spin Relaxation: Consideration Using Green Function of Rigid Body Diffusion

Using the Green function of arbitrary rigid Brownian diffusion (Goldstein,Biopolymers33, 409–436, 1993), it was analytically shown that coupling between translation and rotation diffusion degrees of freedom does not affect the correlation functions relevant to the NMR intramolecular relaxation. It follows that spectral densities usually used for the anisotropic rotation diffusion (Woessner,J. Chem. Phys.37, 647–654, 1962) can be regarded as exact in respect to the rotation–translation coupling for the spin system connected with a rigid body.     

On the magnetic field dependence of nuclear spin relaxation (NRS) on surfaces

We have measured the nuclear spin relaxation rate for nuclear spin polarized⁷Li atoms adsorbed on a hot O-W(110) surface and found that it increases as the magnetic field strength approaches zero. The trend of the nuclear spin relaxation rate generally agrees with a logarithmic divergence, a consequence of the correlation function for two-dimensional diffusion. In principle, such experiments yield information on absolute values of diffusion rates for the adsorbed atoms. Supported in part by a Travel Grant from the North Atlantic Treaty Organization.     

Suppression of spin relaxation in an InAs nanowire double quantum dot

We investigate the triplet-singlet relaxation in a double quantum dot defined by top gates in an InAs nanowire. In the Pauli spin blockade regime, the leakage current can be mainly attributed to spin relaxation. While at weak and strong interdot coupling relaxation is dominated by two individual mechanisms, the relaxation is strongly reduced at intermediate coupling and finite magnetic field. In addition we observe a characteristic bistability of the spin-nonconserving current as a function of magnetic field. We propose a model where these features are explained by the polarization of nuclear spins enabled by the interplay between hyperfine and spin-orbit mediated relaxation.     

High-resolution study of nuclear magnetic relaxation dispersion of purine nucleotides: effects of spin-spin coupling

By combining magnetic field cycling in the range from 0.1mT to 7T with high-resolution NMR detection the T(1) relaxation dispersion (nuclear magnetic relaxation dispersion (NMRD)) of protons in the nucleotides adenosine mono-phosphate and guanosine mono-phosphate was measured in a site-specific way. While at high field the individual spins have distinctly different T(1) times, their scalar spin-spin interaction fulfills at low field the condition of strong coupling and leads to convergence of their T(1) dispersion curves. In addition, the spin-spin coupling can lead to oscillatory components in the relaxation kinetics traceable to a coupling between spin polarization and coherence in the relaxation process. As a consequence the NMRD curves do not directly reflect the spectral density function of the motional processes, but the effects of motion and spin coupling must be separated for a reliable evaluation. A theoretical approach is described allowing such an analysis.     

用NMR方法测定聚合物长侧基转动扩散系数

提出了一种测定聚合物溶液及有机固体中长侧基转动扩散系数的NMR方法,即先导出聚合物溶液和固体中长侧基运动的相关函数和谱密度函数,然后利用¹³C自旋弛豫数据求出扩散系数.同时还讨论了两个应用实例.     

Intrinsic electron spin relaxation due to the D'yakonov–Perel' mechanism in monolayer MoS2

Intrinsic electron spin relaxation due to the D'yakonov–Perel' mechanism is studied in monolayer Molybdenum Disulphide. An intervalley in-plane spin relaxation channel is revealed due to the opposite effective magnetic fields perpendicular to the monolayer Molybdenum Disulphide plane in the two valleys together with the intervalley electron–phonon scattering. The intervalley electron–phonon scattering is always in the weak scattering limit, which leads to a rapid decrease of the in-plane spin relaxation time with increasing temperature. A decrease of the in-plane spin relaxation time with the increase of the electron density is also shown.     

Relaxation times and line widths of isotopically-substituted nitroxides in aqueous solution at X-band

Optimization of nitroxides as probes for EPR imaging requires detailed understanding of spectral properties. Spin lattice relaxation times, spin packet line widths, nuclear hyperfine splitting, and overall lineshapes were characterized for six low molecular weight nitroxides in dilute deoxygenated aqueous solution at X-band. The nitroxides included 6-member, unsaturated 5-member, or saturated 5-member rings, most of which were isotopically labeled. The spectra are near the fast tumbling limit with T₁∼T₂ in the range of 0.50–1.1 μs at ambient temperature. Both spin–lattice relaxation T₁ and spin–spin relaxation T₂ are longer for ¹⁵N- than for ¹⁴N-nitroxides. The dominant contributions to T₁ are modulation of nitrogen hyperfine anisotropy and spin rotation. Dependence of T₁ on nitrogen nuclear spin state m_I was observed for both ¹⁴N and ¹⁵N. Unresolved hydrogen/deuterium hyperfine couplings dominate overall line widths. Lineshapes were simulated by including all nuclear hyperfine couplings and spin packet line widths that agreed with values obtained by electron spin echo. Line widths and relaxation times are predicted to be about the same at 250 MHz as at X-band.     

Direct spin–phonon coupling of spin-flip relaxation in quantum dots

Within the frame of the Pavlov–Firsov spin–phonon coupling model, we study the spin-flip assisted by the acoustical phonon scattering between the first-excited state and the ground state in quantum dots. We analyze the behaviors of the spin relaxation rates as a function of an external magnetic field and lateral radius of quantum dot. The different trends of the relaxation rates depending on the magnetic field and lateral radius are obtained, which may serve as a channel to distinguish the relaxation processes and thus control the spin state effectively.     

Electron spin polarization in a rotating triplet

The triplet model of electron spin polarization in fluid media is evaluated. The model consists of an initial singlet molecule rotating in a static, externally applied magnetic field. Intersystem crossing into different zero-field states is represented by a rate matrix diagonal in the molecular frame, and this matrix is expressed as an effective spin operator. The triplet rotates, and the motion affects the polarization in the laboratory frame, and also causes spin relaxation in the triplet manifold. The triplet is chemically quenched, and the polarization appears in the doublet fragments. The model is treated in a density matrix formalism and on the basis of anisotropic rotational diffusion of the triplet molecule. Explicit expressions are obtained in terms of the molecular parameters, the various rate constants, and the rotational correlation time.     

19F nuclear spin relaxation and spin diffusion effects in the single-ion magnet LiYF4:Ho3+

Temperature and magnetic field dependences of the ¹⁹F nuclear spin-lattice relaxation in a single crystal of LiYF₄ doped with holmium are described by an approach based on a detailed consideration of the magnetic dipole-dipole interactions between nuclei and impurity paramagnetic ions and nuclear spin diffusion processes. The observed non-exponential long time recovery of the nuclear magnetization after saturation at intermediate temperatures is in agreement with predictions of the spin-diffusion theory in a case of the diffusion limited relaxation. At avoided level crossings in the spectrum of electron-nuclear states of Ho^{3 +} ions, rates of nuclear spin-lattice relaxation increase due to quasi-resonant energy exchange between nuclei and paramagnetic ions in contrast to the predominant role played by electronic cross-relaxation processes in the low-frequency ac-susceptibility.     

Nuclear magnetic relaxation by regularly distributed dense paramagnetic ions in a quasi-two-dimensional system

The ¹H nuclear spin-lattice relaxation behavior was characterized in the perovskite-type layered structure quasi-two-dimensional Heisenberg paramagnets, (C_nH_2n+1NH₃)₂MnCl₄,with different chain lengths (n=8, 10, and 12). In contrast to the case of the short-chain compound with n=8, the nuclear spin diffusion to the electron spin system alone is not able to fully account for the spin-lattice relaxation in the compounds with longer chain lengths. Our results are discussed in light of the nuclear magnetic relaxation by the regularly distributed dense paramagnetic ions.     

Field-dependent nuclear relaxation of spins 1/2 induced by dipole-dipole couplings to quadrupole spins: LaF3 crystals as an example

A general theory of spin-lattice nuclear relaxation of spins I=1/2 caused by dipole-dipole couplings to quadrupole spins S1, characterized by a non-zero averaged (static) quadrupole coupling, is presented. In multispin systems containing quadrupolar and dipolar nuclei, transitions of spins 1/2 leading to their relaxation are associated through dipole-dipole couplings with certain transitions of quadrupole spins. The averaged quadrupole coupling attributes to the energy level structure of the quadrupole spin and influences in this manner relaxation processes of the spin 1/2. Typically, quadrupole spins exhibit also a complex multiexponential relaxation sensed by the dipolar spin as an additional modulation of the mutual dipole-dipole coupling. The proposed model includes both effects and is valid for an arbitrary magnetic field and an arbitrary quadrupole spin quantum number. The theory is applied to interpret fluorine relaxation profiles in LaF3 ionic crystals. The obtained results are compared with predictions of the 'classical' Solomon relaxation theory.     

Simulation of slow-motion CW EPR spectrum using stochastic Liouville equation for an electron spin coupled to two nuclei with arbitrary spins: matrix elements of the Liouville superoperator

An algorithm is developed that extends the well known nitroxide slow-motional continuous wave electron paramagnetic resonance (EPR) simulation technique developed originally by Meirovitch et al. [E. Meirovitch, D. Inger, E. Inger, G. Moro, J.H. Freed, J. Chem. Phys. 77 (1982) 3915-3938], and implemented by Schneider and Freed [D.J. Schneider, J.H. Freed, Calculating slow motional magnetic resonance spectra: a user's guide, in: Biological Magnetic Resonance, vol. 6, Plenum Publishing Corporation, 1989]. This paper deals with the more general case of coupling of one electron spin to two nuclear spins. A complete listing of the matrix elements of the Liouville superoperator for this extension has been included. This advance has been successfully tested by reproducing the observed spectral lineshapes of a solution of the novel radical Mes(*)(CH(3))P-PMes(*) [Mes(*)=2,4,6 (tBu)(3)C(2)H(2)] in tetrahydrofuran (THF), in which the radical is undergoing slow tumbling, with the coupling of one electron spin to two physically and magnetically inequivalent phosphorus ((31)P) nuclei.     

Flux relaxation phenomena in the presence of weak AC magnetic fields

Relaxation of flux profile and magnetisation due to non-linear vortex diffusion in a superconducting slab settled in a parallel-to-the-surface DC and superimposed weak AC magnetic fields is studied for several kinds of the non-linear vortex diffusivity, corresponding to different possible shapes of the current–voltage characteristics of the superconductor. The evolution of the dynamic vortex response on applied weak AC field due to flux relaxation process is studied and relaxation characteristics of the AC magnetic susceptibility are calculated. The flux creep rate and magnetisation decay are shown to be enhanced significantly in the case of strongly non-linear regime of vortex diffusion if even rather weak AC magnetic field is applied. The possibility of ‘dynamical melting' of the vortex lattice occurring at rather high levels of the induced current density (j>j_c) is also demonstrated.     

Anisotropic spin fluctuations and superconductivity in "115" heavy fermion compounds: ⁵⁹Co NMR study in PuCoGa₅

We report results of ??Co nuclear magnetic resonance measurements on a single crystal of superconducting PuCoGa? in its normal state. The nuclear spin-lattice relaxation rates and the Knight shifts as a function of temperature reveal an anisotropy of spin fluctuations with finite wave vector q. By comparison with the isostructural members, we conclude that antiferromagnetic XY-type anisotropy of spin fluctuations plays an important role in mediating superconductivity in these heavy fermion materials.