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Dipole Moments of ff-Bonded Complexes

Pierre L. Huyskens ; University of Leuven, Celestijnenlaan 200F, B-3030 Heverlee, Belgium
Georges G. Siegel ; University of Puerto Rico, 00708 Mayaguez, Puerto Rico

Puni tekst: engleski pdf 10.874 Kb

str. 55-73

preuzimanja: 162



This paper deals with the information which can be obtained
from dielectric measurements about the structure of H-bonded
complexes in the liquid phase.
In the first part the basic equations used in the determination
of the dipole moments in the liquid phase are discussed. For pure
polar liquid Onsager's equations lead to values of the moments
which may differ from those of the gas phase. According to Kirkwood
these deviations are due to preferential orientation effects
between the molecules. In pure liquids these deviations are only
very important in the case of the formation of H-bonds.
The interpretation of experimental dipole data for self associated
compounds such as alcohols, carboxylic acids, amides, amines,
anilines and pyridines is presented.
A method used for the experimental determination of dipole
moments for one-one hydrogen bonded complexes is discussed.
μab depends not only on the moments of the separate partners μa
and μb but also on the angles -&a and Db which these moments
form with the direction of the hydrogen bond. Furthermore, flab ....
also depends on the dipole increment, /),.μ, originated by the displacements
of electrons and nuclei brought about by the formation
of the. ...b ond.
/),.μ in turn, will depend on the f),.pKa, the difference between
the pK. of the conjugated acid of the proton acceptor and that of
the acid. Sigmoidal curves are obtained which can be interpreted
as resulting from a tautomerism between »normal« and »proton
transfer« hydrogen bonds.
The dependence of /),.μ on the enthalpy of bond formation,
- /),.Hh, also gives a sigmoidal curve which is approximately the
same for all H-bonds of a given kind (0-H ... 0, 0-H ... N etc.)
in a given solvent.
This dependence can be used for the calculation of /),.μ. Using
this value with the experimental moments μab• μa and /lb it is then
possible to deduce angular parameters for a given H-bonded complex.
A few examples are discussed.

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