Structure deformations, solute-solvent interaction

Two factors need to be accounted for in the calculation of the so-called r -struc-ture of a molecule from LCNMR data. The first is the effect of molecular harmonic vibrations on the observed dipolar splittings, which has been generally recognized since the late 1970s [22] most LCNMR stmctures published since 1980 contain these corrections. The second factor is molecular deformations caused by interaction of the solute with the medium (due to correlations between molecular vibrations and solute reorientations), which as might be expected, are more important for some liquid crystals than others. These effects have been widely recognized since the early 1980s, and have been studied in detail (see for example [15, 23-26]). Procedures to correct for these effects have been published [27], and solvent systems which produce minimal structural distortions have been identified [12, 28, 29]. The problem and its solution have recently been re-emphasized by Diehl and coworkers [30]. 

Ab initio calculations predict a structure with C2 symmetry for the free dini-tramide ion, N(N02)2 (Fig. 10.5), while in solution and in the solid state the local symmetry is essentially C3. This can be explained on the basis of weak cation-anion interactions or interactions with the solvent, since the dinitramide ion is very easy to deform because of the very small barrier to rotation of the NN02 moiety (< 13 kj mob1) [63]. 

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