Dipole moments in solution

On the other hand, electric dipolar moments of the solute molecules can be obtained with standard methods in ab initio molecular orbital calculations, whereas the induced dipole moments in solution are determined from differences between the values obtained in solution and in the gas phase. 

It remains to mention derivatives with just one substituent on the NH2 group. Concerning the configuration, they should take an intermediate position in addition, one has to deal with the conformation on C—N. These compounds have been less studied. Compared to structurally very similar amidines, a remarkable difference was found101. While AW -dimethyl-d-nitrobenzamidine in the crystal is in the form E,sp (41), A -methyl-4-nitrobenzamidoxime is Z.ap (42). Dipole moments in solution are compatible with a conformational equilibrium, but analysis is made imprecise by the mesomeric moments which are not known exactly102. In a previous analysis129 of /z of derivatives similar to 42 (H and Cl in place of NO2) the form Z.ap was also preferred, although the presence of a minor conformer could not be completely excluded. 

Usually, though, the solvent molecules do organise themselves in a special way around the polar species for the reason cited above. The effect depends principally on the anisotropy of the solute molecules, and many semi-empirical equations have been devised on this basis to explain the apparent change of dipole moments in solutions. An alternative approach has been to quote gas-and solution-determined dipole moments separately and to use the appropriate value in any particular situation. 

Beryllocene, Cp2Be, prepared in 1956 by E. O. Fischer, was intensively examined to determine its structural features. This work quickly abandoned a structural model based on ferrocene, and different bonding modes for the two Cp rings were demonstrated by Cp2Be s high dipole moment in solution. The slipped sandwich structure, with... 

In the second theory the formation of intermolecular complexes is not assumed and the variation of the dipole moment with concentration is considered to be caused by the limited orientation of the dipole molecules in the external field, due to the field of the surrounding polar molecules. The dioxan effect — For the measurement of dipole moments in solution... 

Substance Dipole moment in solution D Boiling point °C Substance Dipole moment in solution D lioiltng point °C Differ erne in boiling point °C... 

A curious feature of the solid-state structural results is that the MN3 unit is not coplanar as found in the first-row transition or Group IIIA series ( 2) but pyramidal, the metal atom being out of a plane defined by the three nitrogen atoms by ca. 0.4 A. Since the molecules have no dipole moment in solution (2 ) the geometry could well be due to packing forces in the solid state. 

Exner has compared and statistically tested the various equations used to determine dipole moments in solution [42]. 

The material presented in the chapter illustrates the difficulties connected with experimental determination and theoretical computation of dipole moments in general and heterocyclic compounds in particular. It is obvious that more work is needed on solute-solvent interactions, role of the polarity of the solvent, dipole moments in solution v. dipole moments in the gas phase, dielectric constant measurements V5. microwave spectroscopic data, improvements in calculations, better solvatochromic equations, etc. 

After beryllocene was first prepared in 1959, early work showed its IR spectmm to be more complex than those of the symmetrical sandwich molecules such as ferrocene, and it was also found to have a large dipole moment in solution, 2.24 D in cyclohexane. It was thus clear from the start that the molecular symmetry was lower than that of ferrocene, though it was not clear whether this involved two dissimilar rings or two identical but asymmetric rings. An early X-ray diffraction study appeared to show a unit-cell structure formally identical to that of ferrocene, implying a centrosymmetric molecule (12.IVa), which is incompatible with the observation of a dipole moment in solution. Thus there was a possibility that the molecular structure in the crystalline phase was not the same as that in gaseous and solution phases, but it was also possible that the crystallographic results could be attributed to disorder in the crystal. 

Dipole moments in solution See corresponding chapter of this Hwdbook ... 

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