Anisotropy of thermal motion

A determination of the structure of l,4-dibromocycl[3,2,2]azine (lp) was complicated by the considerable anisotropy of thermal motion of the bromine atoms. It was concluded that the nitrogen configuration is probably planar, but, if not, the nitrogen atom is less than 0.07 A out of the mean plane of the molecule.55... 

The next level of approximation accounts for the anisotropy of thermal motions in a harmonic approximation and describes atoms as ellipsoids in one of the three following forms, which are, in fact, equivalent to one another ... 

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