From: Richard
Henchman <henchman@manchester.ac.uk>
Date: Wed, Dec 22, 2010 at 9:53 PM
Subject: RE: IUPAC Hydrogen Bond Definition
To: Arunan <arunan@ipc.iisc.ernet.in>
Dear Prof. Arunan,
Likewise I can say the same thing about your review of the hydrogen bond. I'd
found reading about the meetings and reports very informative. Regarding your
paper, I think the key point one needs to decide is whether a hydrogen bond has
to be stable for the duration of a libration or whole-molecule vibration, or
whether it is only transient e.g. free motion. Also, is the use of the word
"bond" reasonable for such transient interactions?
An extreme case I've found helpful is two water molecules at various distances.
The angular HB potential would get smoother as the separating distance
increases. If the thermal energy is fixed, there would eventually be a point at
which the HB would not be stable i.e. the water molecules would be largely free
rotors. So again, the choice one has to make is whether any HB strength is
allowable or must it satisfy certain requirements e.g. be greater than an
arbitrary quantity such as k_BT or zero-point energy. There is a kind of
"Cheshire cat" aspect to the use of the word bond as it fades to an
interaction (which is used for electrostatics and van der Waals interactions)
or strengthens to a covalent bond. A similar case to consider is: at a few
thousand degrees C, is a chemical bond formed or not, even if the atoms are
only transiently in bonding range? Finally, I note that the current definition
the IUPAC group does not refer to time scale, implying that short-lived HBs are
valid.
One other point that occurred to me, the IUPAC current definition includes the
use of the word "attractive". When two water molecules are as a HB
dimer, half the time the interaction between them is repulsive (they oscillate
back and forwards). Wouldn't the definition thus imply that these molecules
have an HB between them only half the time? Or is that being too picky? Saying
it's electrostatic gets around this somewhat.
I'm not entirely clear where I will be in mid July - possibly at some Telluride
meetings in the USA. If am around in Manchester during your visit, I would be
very happy to meet you.
Best wishes,
Richard