Journal of Magnetic Resonance, Series A最新文献

The Dysonian line in the limitd< or ∼ δ, wheredis the thickness and δ the skin depth, was fitted to a combination of absorption and dispersion Lorentzian lines. This procedure allows one to determine not only microwave conductivity from the Dysonian line but also the truegvalue, linewidth, and paramagnetic susceptibility by the measurement of five parameters of the ESR absorption-derivative Dysonian line.

The role of time symmetry in composite-pulse design is examined by considering a phase-alternating composite pulse pair {π(I= $\text{1}\text{2}$), π/2(I= 1)}, where the spin-1 excitation pulse has been derived from its spin-$\text{1}\text{2}$ progenitor by halving the pulse durations. The quaternion calculus is used to define the quaternion elements (Euler–Rodrigues parameters) of each composite pulse. In this manner, it is shown how an Euler–Rodrigues (ER) parametrization of the consecutive rotations implicit in each composite pulse can be used to derive simple phase and amplitude relationships between the members of such a {π(I= $\text{1}\text{2}$), π/2(I= 1)} pulse pair. The simplicity and compactness of the ER parametrization is then used to identify optimal time-symmetric sequences for spin-1 excitation by using the Lagrange multiplier method.

An approximation technique for the calculation of pulsed-gradient NMR signals in confined spaces is introduced on the basis of a memory-function formalism and compared to the well-known cumulant expansion technique. The validity of the technique is investigated for the cases of a time-independent field gradient and a gradient consisting of two pulses of finite duration. It is found that the validity is governed by the ratio of two characteristic times: the time for the spins to traverse the dimensions of the confining space through diffusion and the reciprocal of the extreme difference between values of the precession frequency of the spin. Oscillations in the time evolution of the signal for the constant gradient, as well as oscillations in the (gradient) field dependence for the two-pulse gradient, which are both characteristic of the exact signals, are predicted by the new technique but not by the cumulant technique. The cumulant results are shown to arise as an approximate consequence of the memory results.

A generalization of the modified Solomon–Bloembergen–Morgan (MSBM) theory is presented for electron-spin quantum numberS= 7/2 taking multiexponential electron-spin relaxation into account. The theoretical approach closely follows P.-O. Westlund [Mol. Phys.85, 1165–1178 (1995)]. In the nonextreme-narrowing regime for the electron spin system, three correction terms to the traditional MSBM equations arise. They are all derived in closed analytical form to give the generalized equations in a simple and convenient form: MSBM × (1 + correction). The ESR lineshape function forS= 7/2 is also given in closed analytical form. A number of protonT₁NMRD profiles representing different Gd(III) complexes are investigated, using a spectral density function of the transient ZFS interaction which allows for two relaxation times. For Gd–macromolecular complexes the largest deviation from the traditional MSBM theory may be about 15.2%, but for the parameter sets investigated the corrections were about 2–3%. A computer program is developed which calculates the NMRD profile and the ESR lineshape (X band or Q band) for the set of model parameters. These parameters describe the electron spin system in terms of Δ_t(the root-mean-square value of the transient ZFS interaction) and τ_v1, τ_v2, andS₀(correlation times and an order parameter of the transient ZFS correlation function). For the paramagnetically enhanced nuclear (I= $\text{1}\text{2}$) spin relaxation, the parameters τ_R(the reorientational correlation time),q(the number of fast exchanging water molecules in the first hydration shell), andr_IS(the point spin–dipole distance) are also used.

Proton spin–lattice relaxation times were measured between 2.2 K and room temperature in [Zn(ptz)₆](BF₄)₂(ptz = 1-n-propyl-1H-tetrazole) and in the spin-crossover complex [Fe(ptz)₆](BF₄)₂. Three different types of intramolecular motion of the propyl group are suggested in [Zn(ptz)₆](BF₄)₂, namely tunneling and classical rotation of methyl groups and rotation of methylene groups. Correlation times and activation energies are calculated for tunneling rotation of the CH₃group and for the classical (hindered) rotations of –CH₃and –CH₂–CH₃as reorientations over a three-well asymmetrical potential. In [Fe(ptz)₆](BF₄)₂the mechanism for the paramagnetic relaxation is found to be of the rapid-diffusion type according to the theory of Lowe and Tse, and the intramolecular motions are suggested to be the same as for the zinc complex.