Date:
From: Gautam R. Desiraju <gautam_desiraju@yahoo.com>
To: Steve Scheiner <scheiner@cc.usu.edu>,
Subject: RE: IUPAC
Dear all,
I like Steve Scheiner's recommendations, which have the advantage of flexibility. Most people have little problems with most hydrogen bonds. The situations at the fringes excite comment, and the concept of the hydrogen bond advances now and then because of this excitement. C--H.....O bonds were in the fringe in the 1960s and blue shifted hydrogen bonds were in the fringe in 2000. Today's fringe becomes tomorrow's mainstream.
Still, we seem to be nearing the end and I suspect most of the fringe situations have already been encountered. Any "definition" we try and provide now may last for a while. So we need to be careful.
Arunan's original recommendation is general and flexible:
Hydrogen bonding occurs when an electron deficient hydrogen interacts with an electron rich region either within the same or another molecular entity.
(1) Do we need the word "molecular"?
(2) I like the words "electron deficient". If the H-atom in X--H......A is not positive with respect to X, then there is no hydrogen bond. If necessary we can explain what we mean by "electron deficient" but not mentioning it would be (in my opinion) a mistake. We should avoid a situation where any X--H group pointing at any atom A, is called a
hydrogen bond.
For the notes I have the following points to make:
(1) In the crystal engineering and crystallographic literature, hydrogen bonds are contrasted with van der Waals interactions. The latter are generally taken as the sum of the dispersive and repulsive interactions. Including hydrogen bonds within the subset of van der Waals interactions will surely cause confusion within this community. See also below.
(2) We need to say somewhere that the van der Waals separation criterion does not always work for weak hydrogen bonds. Indeed this criterion has no real scientific basis for any kind of hydrogen bond. Given this fact, if we were to include hydrogen bonds within the group of van der Waals interactions, there is bound to be further confusion.
(3) Can we avoid a mention therefore of van der Waals interactions?
(4) Chlorine and other halogen bonds are quite new. Perhaps we could wait before we start defining these interactions.
I look forward to further discussion with members.
Best wishes,
G R Desiraju