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Abstract:
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.
