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The question about dipole moments of charged species is
related to another issue, which is somewhat of a hobby-horse of
mine. In organic chemistry and biochemistry particularly,
structural and mechanistic rationales are often based on the idea of
“fragment
dipoles” and their interactions with one another. In small molecules
these are usually dipoles said to be associated with polar covalent bonds,
while in proteins the so-called helix dipole is another
example. For many years I accepted the idea that “intramolecular
dipole-dipole repulsions” were good explanations for all kinds of
phenomena, but I am in serious doubt about that idea now. When you start
to
dissect the overall dipole moment of a molecule, you enter into the same kind
of origin-dependence that you see in ions. What makes the arbitrary choice
of
a bond dipole or a helix dipole more significant than the infinitude of other
point-to-point dipoles that could be defined within the molecular charge
envelope? I think chemists have attached too much significance to the
undeniable separation of charge that exists between bonded atoms of different
electronegativities, mainly because there was no way to demonstrate that these
charge separations were not necessarily quantitatively or qualitatively
different from any others that might be defined for the
system. We have looked at charge distributions to seek evidence for
fragment dipoles and their interactions, and we haven’t seen anything
convincing. Do others have any opinions on
this? Dr. Philip G. Hultin Associate Professor of
Chemistry, R3T 2N2 http://umanitoba.ca/chemistry/people/hultin |