Date: Sun, 17 Sep 2006 11:53:10 +0100
From: AC Legon <A.C.Legon@bristol.ac.uk>
To: E. Arunan <arunan@ipc.iisc.ernet.in>, Task Group <A.C.Legon@exeter.ac.uk>,
bene@dcci.unipi.it, david.clary@mps.ox.ac.uk,
David Nesbitt <djn@jila.colorado.edu>, gautam_desiraju@yahoo.com,
henrik@alkali.otago.ac.nz, ibon@iqm.csic.es, jdannenberg@gc.cuny.edu,
kleinermanns@uni-duesseldorf.de, klein@institut.physiochem.uni-bonn.de,
pavel.hobza@uochb.cas.cz, robert.crabtree@yale.edu,
sadlej@maja.chem.uw.edu.pl, scheiner@cc.usu.edu
Subject: Re: IUPAC


Dear Arunan,

I think that Steve's short definition is good. I agree that there
should be the caveat that the list of properties of the hydrogen
bonds is not exhaustive and that there will be hydrogen bonds
some of whose individual properties fall outside the list.
(Since this is an IUPAC effort, we should use 4 or 5 kJ mol-1
rather than 1 kcal mol-1, even though kcal mol-1 is always used
in the USA!)

My own view is that the propensity for hydrogen bonds to be
non-linear is in fact a characteristic of the H bond. If there
is an H-bond interaction between two molecules, that is between
the electron- deficient region near H and and electron-rich region
of the 'proton' acceptor, there will always be secondary
interactions between the electron-rich region of the 'proton
donor' molecule and the electron-deficient region of the 'proton-
acceptor' molecule. The stronger the secondary interaction, the
larger will be the non-linearity of the hydrogen bond in general,
assuming that symmetry allows the non-linearity.

I think it would be a very good idea to include somewhere in the
final document definitions of:
1) a covalent interaction through the example of H2 (electron
 sharing between nuclei; order of magnitude for the stretching
force constant and the dissociation energy),
2) an ionic bond through the example of KCl (electron transfer
from K to Cl, as indicated by nuclear quadrupole coupling
constants; order of magnitude for stretching force constant
and dissociation energy),
3) a pure London dispersion interaction through the example of
Ar2 (as I recall the stretching force constant and dissociation
energy are conveniently 1 N m-1 and 1 kJ mol-1, respectively,
in SI).

Best wishes,

Tony.