From owner-chemistry@ccl.net Fri Nov 4 11:48:00 2005 From: "Phil Hultin hultin.:.cc.umanitoba.ca" To: CCL Subject: CCL: Dipole Moments and Molecular Fragments Message-Id: <-29880-051104114208-26146-bCaPOzjGcSYG1m6pFbl7xg*_*server.ccl.net> X-Original-From: "Phil Hultin" Content-Type: multipart/alternative; boundary="----=_NextPart_000_000A_01C5E125.203F5C30" Date: Fri, 4 Nov 2005 09:49:53 -0600 MIME-Version: 1.0 Sent to CCL by: "Phil Hultin" [hultin|*|cc.umanitoba.ca] This is a multi-part message in MIME format. ------=_NextPart_000_000A_01C5E125.203F5C30 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit 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 hultin%a%cc.umanitoba.ca http://umanitoba.ca/chemistry/people/hultin ------=_NextPart_000_000A_01C5E125.203F5C30 Content-Type: text/html; charset="us-ascii" Content-Transfer-Encoding: quoted-printable

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

hultin%a%cc.umanitoba.ca

http://umanitoba.ca/= chemistry/people/hultin

 

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