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