Subject: Nature of the hydrogen bond

From: "Donald E. Williams" <[email-masked]>
Date: Fri, 25 Jul 1997 15:45:09 -0400

>
>
>[email-masked] writes:

>I am interesting in sharing any experience people have with mimicing hydrogen
>bonds in forcefields. The main contribution to an O...H bond is obviously the
>coulombic interaction. The other thing is dispersion.
>The coulombic interaction is usually represented by two point charges and
>dispersion by a -C/R**6 term in a forcefield - I have also seen the term
>R/R**10
>used.
> However, the hydrogen bonds also have a directional character which is not
>described by this simple model.
>Are really forcefields of the form described above generally able to describe
>structures with hydrogen bonds?

>I would very much like to know the view of my colleagues on what a hydrogen
>bond really is made of and what it takes to describe it.

>Best wishes

>Merethe

     Perhaps the most commonly used force field model for the hydrogen bond,
especially O-H...O, is the "disappearing hydrogen" model.  This intermolecular
force field is based on (12-6-1) functional form, representing repulsion,
dispersion, and coulombic interaction.  This model is non-directional, and
assigns *zero* dispersion and *zero* repulsion.
     Now many say that the main component of h-bond energy is electrostatic.
However, in the disappearing hydrogen model, by far the main component is
the reduction of hydrogen repulsion.  You can test this against crystal
structure data.  If you restore normal (i.e., hydrocarbon H) repulsion,
keeping the electrostatic part, the model fails completely.  On the other
hand, is you delete the electrostatic part, keeping the zero repulsion, the
fit to crystal data become only a little worse.
     What prevents the h-bond model from collapsing?  Of course, it is the
repulsion between the oxygens (in an O-H...O bond).
     What is the significance of the reduced H repulsion in the h-bond?
Well this is what always happens when a bond forms.  However, there
normally is some residual repulsion, say K shell electrons, which prevents
the bond from collapsing. Of course, H doesn't have an inner shell to
prevent collapse, so maybe the zero repulsion can be rationalized.
But there still is a proton there which needs to be considered.
     I believe that many elementary textbooks give a misleading picture
when they describe the h-bond as primarily electrostatic, something akin
to an ionic bond.  Really the h-bond is more closely related to covalency,
with reduced repulsion normally associated with the covalent bond.

-Donald Williams

-- 
Dr. Donald E. Williams		email:[email-masked]
Department of Chemistry
University of Louisville	phone:502-852-5975
Louisville, KY 40292		fax:  502-852-8149

---

From: David Case <[email-masked]>
Date: Mon, 28 Jul 1997 17:00:33 -0700 (PDT)

Donald Williams wrote:

> the reduction of hydrogen repulsion. 

I don't know what is the "most commonly used force field", but
functions like OPLS and AMBER (or the TIP3 and SPC models of water,
etc.) "disappear" the 12-6 (Lennard-Jones) part of the hdyrogen, but
*keep* the electrostatic part of the hydrogen in computing the
interaction.  These force fields do have directionality, since
nonlinear geometries have less favorable Coulombic interactions than do
linear geometries.  This angular dependence is by no means perfect (and
is especially questionable for second row acceptors, like sulfur), but
does capture much of the flavor of the angular dependence seen in
quantum chemistry calculations.

So, in this model, one can say (as in Donald Williams' comment) that the
main energetic component of the hydrogen bond is the reduction of the
hydrogen repulsion that would "otherwise" be there, and that the principal
determinant of the angular dependence is electrostatics.

 ....dave case