Deformations solute-solvent interactions

The first semiempirical studies of the potential functions of silatranes revealed that their Si-<—N bond could be very easily deformed by crystal packing and solvation forces . Direct MINDO/3 and MNDO calculations demonstrate a significant shortening of the equilibrium, gas phase Si- —N distance due to the contribution of the solute-solvent interaction energy (within the Onsager reaction field model ) to the total enragy of 1-methyl- and 1-fluorosilatranes . 

Equation (23) predicts a dependence of xR on M2. Experimentally, it was found that the relaxation time for flexible polymer chains in dilute solutions obeys a different scaling law, i.e. t M3/2. The Rouse model does not consider excluded volume effects or polymer-solvent interactions, it assumes a Gaussian behavior for the chain conformation even when distorted by the flow. Its domain of validity is therefore limited to modest deformations under 0-conditions. The weakest point, however, was neglecting hydrodynamic interaction which will now be discussed. 

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]. 

Protic solvents always have more complex infrared spectra because of the presence of hydrogen bonding in the liquid state. In methanol, this involves interaction of the acidic proton on the OH group in one molecule with the oxygen atom in an adjacent molecule (fig. 5.15). The infrared spectrum shows a wide band centered at 3346 cm which is due to the -OH stretch. When methanol is dissolved as a dilute solute in carbon tetrachloride, this band is sharp and appears at 3644 cm . An -OH bending mode appears at 1449 cm. Another broad band due to -OH out-of-plane deformation is centered at 663 cm. The other features of the methanol spectrum are due to the vibrational modes of the CH3- group or to skeletal vibrations [27]. 

An analysis of the previously defined function B(T) for dilute solutions of polymers in polar solvents may be helpful for the understanding of the interaction between polymer and these solvents.33 Applying Frohlich s theory, in which deformation polarization is treated macroscopically, we consider the solution as a cpntinuous medium containing polar units. The dielectric constant of the continuous medium is taken as equal to the square of the refractive index of the solution n0. This value is very close to that of the solvent. Each polar unit is represented as a sphere of dielectric constant 0a, haying a point dipole located at its center. It must be stressed that polar units may be either whole... 

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