Single-crystal NMR Data Research Articles

SCFit: Software for single-crystal NMR analysis. Free vs constrained fitting

The design and implementation of a software package for the analysis of single-crystal NMR data is presented. The SCFit software can treat spectra arising from various interactions: (i) chemical shift tensor only; (ii) chemical shift tensor and quadrupolar coupling tensor; (iii) dipolar and indirect nuclear spin-spin coupling tensors; (iv) all four interactions. The software is demonstrated on recently reported 17O and 31P single-crystal NMR data for triphenylphosphine oxide and for two of its halogen-bonded cocrystals. The 17O single-crystal NMR data represent a case where all four above-mentioned interactions simultaneously affect the spectra. SCFit can fit the chemical shift and quadrupolar coupling in two ways: (i) through an unconstrained fitting process where all tensor parameters are freely optimized or (ii) through a constrained fitting process where the principal components of the tensors may be fixed to values known previously with high precision via the analysis of powder samples. The second strategy is explored in an effort to reduce the number of unknowns in the fitting process; an improvement in the precision of the resulting tensor orientations is noted in some cases.

Determination of the 19F NMR chemical shielding tensor and crystal structure of 5-fluoro- dl-tryptophan

5-Fluoro- dl-tryptophan (5F-Trp) is a very sensitive probe used to investigate orientation and dynamics of biomacromolecules at the in situ level. In order to establish a 19F NMR strategy, the crystal structure and 19F chemical shielding tensor of 5F-Trp are reported. A novel approach was developed to use F–F homonuclear dipole–dipole coupling information to analyze single-crystal NMR data without determining crystal orientations. The measured values for the principal components of the shielding tensor are σ 11 = 0.9, σ 22 = −63.3, and σ 33 = −82.9 ppm relative to TFA in D 2O. The principal axes of the shielding tensors coincide with the indole ring symmetry, which makes it a straightforward and powerful tool to monitor protein alignment in oriented environments. Hartree–Fock (HF) and density functional theory (DFT) calculations of the chemical shielding tensors are also reported.

Origin of zero-field 27Al NMR spectra of ferromagnetically ordered GdAl 2

The NMR frequencies of 27Al in ferromagnetically ordered GdAl 2 were calculated for zero field ( T=4.2 K) on the basis of high-field single-crystal NMR data. Their variation with strength and direction of an external magnetic field was observed at a single crystalline sphere in the low-field range. The influence of the rf amplitude on the domain-wall spectrum was determined for a powder sample and by simulation.

Assignment of symmetry-related cadmium shielding tensors to lattice positions in cadmium acetate dihydrate

An oriented single-crystal NMR experiment was carried out for a cadmium acetate dihydrate crystal having 66% 13C enrichment of the acetate carboxylate groups. The resulting 113Cd- 13C direct dipole-dipole interactions give rise to an orientation-dependent modulation of 113Cd NMR linewidths for each of fdur magnetically distinguishable shielding tensors in the cadmium acetate crystal. Comparison of observed cadmium linewidths with those computed as a function of crystal orientation allows for unambiguous assignment of each of the four symmetry-related 113Cd shielding tensors to specific lattice positions in the cadmium acetate crystal. These assignments are in agreement with those made based upon empirical rules, and serve to confirm by means independent of previous assumptions the reliability of the assignment rules used previously. Confirmation of the validity and reliability of such rules is essential for the accurate interpretation of cadmium single-crystal NMR data in terms of shielding tensor-structure correlations to cadmium chemical shifts.

The determination of quadrupole coupling tensors from single-crystal NMR data

A brief description is given of a general procedure which we have used for a number of years for the analysis of NMR quadrupole splittings and chemical shift data from single crystals. Some of the advantages of the procedure are: (1) It includes, in a simple straightforward way, all possible restrictions imposed by crystal symmetry on the efg tensors and on relations between efgtensors of symmetry-related atoms. (2) When the crystal orientation is unknown or known with insufficient precision, the procedure allows a determination of the orientation fully or in part, depending on the crystal symmetry. (3) The procedure permits the analysis of data from rotation patterns obtained by rotating the crystal(s) about an arbitrary number of axes, not necessarily mutually orthogonal and not necessarily orthogonal to the magnetic field direction. The use of the method is illustrated by examples from determinations of deuteron quadrupole coupling tensors in some hydrates. In all cases the full efg tensors were determined from rotations about one, two, or three axes.