Coincidences solid-angle effects

The quadrupolar quasi-static coupling in NF3 — 7.068 MHz— is very close to the value obtained from microwave measurements — 7.07 MHz — this feature may be interpreted as a clue that intermolecular effects are negligible in this compound unless, by a fortuitous coincidence, thes intermolecular effects are cancelled by some other mechanism, such as a molecular deformation in the solid (the coupling is very sensitive to slight variations of the pyramidal angle, as discussed below). 

Characterization of the samples by TGA and CHN analysis shows that the template was effectively removed (C < 0.2 wt%). Small-angle X-ray scattering data of the calcined solid shows a reduction in the unit cell due to thermal shrinkage, while the values for the Fenton samples coincide with the starting precursor. Our approach therefore completely preserves the unit cell corresponding to the diameter of the micelles contained in the mesophase. 

The spin part of the Hamiltonian in the solid is identical to that of the AX system, and the solution is the same—a pair of lines for A and a pair for X, as in Fig. 6.2. However, the separation is dependent on orientation of the vector r joining the spins relative to B0, because the effective coupling is D(3 cos2 0—1), rather than D alone. As indicated in Fig. 7.2, the lines cross as 0 is varied, becoming coincident when 0 = 54.7°, the angle for which the term (3 cos2 0 — 1) = 0. This angle that will appear frequently in our later discussion. 

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