NMR study of molecular reorientations in NH4H2AsO4

Zeeman (T1Z) and dipolar (T1D) spin-lattice relaxation times of protons in NH4H2AsO4 were measured as a function of temperature. The existence of a slow motion (τ ≈ 10−3 see) is established, which is most probably a low frequency hindered reorientation of H2AsO4 groups. This motion is slowed down below the Curie point Tc. A sharp increase of the dipolar relaxation rate above T = 314°K indicates the possibility of a new high temperature phase transition in this compound

Molecular motion studied by NMR powder spectra. II. Experimental results for solid P4 and solid Fe(CO)5

Experimental examples of the effect molecular motion on NMR powder spectra governed by axially symmetric shielding tensors are presented. In solid white phosphorus, P4 the 31P resonance was studied at 92 MHz and temperatures down to 4.2 K. At low temperatures the usual powder pattern for a rigid solid is observed, from which we obtain the shielding anisotrophy Δσ = σ‖ - σ⊥ = −405 ± 10 ppm. At room temperature on the other hand the 31P spectrum of solid P4 consists of a sharp symmetric line only, the frequency shifts due to the anisotropy of σ being averaged out by rapid molecular motion. In the β-phase of this solid at temparatures between 80 K and 120 K intermediate values for the jump frequency τ−1 were found, such that Δω and τ−1 differ by not more than about one order of magnitude. The spectra observed show the characteristic features calculated in the preceding paper. Values for the jump frequencies were obtained independently from the spin—lattice relaxation time T1 studied also at 92 MHz. At this frequency the relaxation at low temperatures is strongly dominated by relaxation through anisotropic sheilding, so that the relationship between τ−1 T1 is very simple and unambigous. The agreement of τ −1 obtained by analysis of the spectra of T1, respectively, provides a quantitative check of the lineshape calculation. As a first example, where the analysis of such NMR spectra is applied to obtained information about molecular dynamics in solid, 13C spectra in solid Fe(CO)5 were studied at 61 MHz. In contrast to Ni(CO)4, where we observe the usual powder spectrum due to an axially symmetric shielding tensor, strong deviations from such a pattern are observed in Fe(CO)5. The values found for the shielding anisotropy in these metal carbonyls are quite close to the one in free 13CO: Δσ = 401,395 and 425±15ppm, for 13CO,Ni(13CO)5, respectively. From the analysis of the spectra in solid Fe(CO)5 it follows that the intramolecular exchange between axial and equatorial carbonyls in the trigonal bipyramid Fe(CO)5 postulated for the molecule takes place even in solid with exchange frequencies up to 25 kHz. A comparison is given with values for τ−1 in the liquid, where they are found to be higher by about six orders of magnitude

Multiple pulse study of the proton shielding in single crystals of maleic acid

The proton NMR in single crystals of maleic acid has been studied by means of multiple pulse line-narrowing techniques. The magnetic shielding tensors of the four magnetically inequivalent protons forming hydrogen bonds could be determined independently. The transferability of the magnetic shielding tensor is put forward as a physical index of the hydrogen bond. The connection between the principal directions of the shielding tensors and the bond directions is demonstrated and used as a means to obtain refined hydrogen positions in the crystal structure. Only one composite line of the olefinic protons could be observed. Its analysis in terms of the individual tensors of the olefinic protons suggests that the double bond principal directions are reflected in those of the shielding tensors in contradistinction to the single bond systems. The origin of this effect is discussed

1H and 19F shielding and deuteron EFG tensors in tetrafluoroterephthalic acid. NMR and neutron-scattering determination of crystal structure

Results of measurements are presented of the 1H and 19F nuclear magnetic shielding, deuteron electric field gradient (EFG) and H, H′ dipole-dipole coupling tensors in tetrafluoroterephthalic acid (MA). The shielding tensors are measured using solid state homo/heteronuclear decoupling techniques. The H, H′ coupling tensor is measured under pulsed 19F decoupling conditions. The principal directions of the various tensor quantities are used to determine the crystal packing of TFTA. Results of a full structure determination by neutron scattering are also presented. They show, as do the NMR results, that the carboxyl protons occupy two positions on the hydrogen bond with essentially equal occupation probabilities. The structural conclusions derived from the NMR data are compared with the results of the final structure determination