Journal of Magnetic Resonance, Series A最新文献

The 1D-EXSY pulse sequence 90°—t₁—90°—t_m—90°—t₂(obs), with a minimum number of judiciously chosent₁values and a series of mixing timest_m, can provide NMR data for measurement of site-to-site rate constants in multisite systems undergoing chemical exchange. A condition can be specified for optimizing the choice oft₁values. The data are analyzed by an improved weighted linear-least-squares fitting procedure that uses matrix algebra to help evaluate the weighting factors. This method is capable of providing highly accurate rate constants and realistic error estimates, without an inordinate demand on instrument time. Three experimental applications, to E/Z methyl exchange inN,N-dimethylpropionamide, to H_E/H_Z/H_Solventproton exchange in acrylamide, and to methoxy exchange between stereoisomeric 4,6-dimethyl-2-methoxy-1,3-dioxanes and methanol, are demonstrated.

A simple and effective method for selectively imaging either one of two chemical species in a two-component system is presented and demonstrated experimentally. The pulse sequence employed,selective-echochemical-shiftimaging (SECSI), is a hybrid (frequency-selective/T₁-contrast) technique that is executed in a short period of time, utilizes the full Boltzmann magnetization of each chemical species to form the corresponding image, and requires only hard pulses of quadrature phase. This approach provides a direct and unambiguous representation of the spatial distribution of the two chemical species. In addition, the performance characteristics and the advantages of the SECSI sequence are compared on a common basis to those of other pulse sequences.

A 300 MHz high-resolution, high-pressure NMR probe which operates in the pressure range of 1 bar to 9 kbar at temperatures of −30 to 100°C is described. Specialized novel design features of the probe are discussed and test spectra showing resolution better than 1 Hz (<3.0 × 10⁻⁹) for 8 mm samples are presented. Potential biochemical applications of this probe are illustrated by experiments dealing with the pressure-induced unfolding of hen egg white lysozyme.

This study is concerned with the effects of applying selective pulses to systems with strong second-order scalar couplings in isotropic phase, where different transitions (rs) are associated with different transition matrix elementsF⁺_(rs). Two unusual features can be distinguished: the nutation angle (“flip angle”) depends on the matrix element of the irradiated transition (rs), and, in contrast to the behavior of an isolated spin-$\text{1}\text{2}$ system, the norm of the three single-transition operators [I^(rs)_x,I^(rs)_y,I^(rs)_z] associated with the fictitious spin-$\text{1}\text{2}$ space of the irradiated transition (rs) is generally not conserved. It is necessary to consider the single-transition operators [I^(rp)_x,I^(rp)_y,I^(rp)_z] and [I^(sq)_x,I^(sq)_y,I^(sq)_z] associated with all connected transitions (rp) and (sq) that share a common energy levelrorswith the irradiated transition (rs). If the pulse applied to the (rs) transition is sufficiently selective, the transverse componentsI^(rp)_x,I^(rp)_y,I^(sq)_x, andI^(sq)_y, can be neglected, since their expectation values remain equal to zero after application of a selective pulse to the (rs) transition, but the longitudinal componentsI^(rp)_zandI^(sq)_zacquire nonvanishing expectation values. When the selective pulse affects several transitions simultaneously, the response varies from one transition to another, depending on the matrix elements and the connectivities. These effects manifest themselves in unusual amplitudes and phases of signals excited by selective pulses, in particular in selective two-dimensional correlation spectra.

The spin–lattice relaxation rates of⁷Li,²³Na,²⁷Al,⁷¹Ga, and¹³⁹La nuclei in electrolyte solutions have been investigated as functions of the isotopic composition of the solvent. The results show that quadrupolar relaxation of the nuclei of monoatomic ions is extremely sensitive to the symmetry of the solvation shells. The data are compared to the results for the²H and¹⁷O nuclei of the solvent molecules and the¹⁴N nucleus of the nitrate anion. A semiquantitative description of the observed effects is given.

A novel approach to the calculation of powder lineshapes is presented. Lineshapes from asymmetric second-rank tensor interactions are produced by summing lineshapes from symmetric tensors. This approach significantly reduces the calculation time, greatly facilitating iterative nonlinear least-squares fitting of experimental spectra. Results are reported for anisotropic¹³C NMR lineshapes of glycine, benzoic acid, and octanoic acid in a urea inclusion compound. Lineshapes from nonuniform distributions of crystallite orientations can also be calculated by using distributions of the Euler angles that define the orientation of the principal-axis system of the chemical-shielding tensor with respect to the magnetic field.