CCL: Dipole Moments and Molecular Fragments

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,

University of Manitoba

Winnipeg, MB

R3T 2N2