Monday, March 7, 2011

Seeking a universal description of hydrogen bonds

This morning I read a paper, Estimating the hydrogen bond energy, which is one of the most downloaded papers from the Journal of Physical Chemistry. It considers a relatively simple criteria (going back to Davidson in 1967) for estimating a bond energy in terms of the two-center shared electron number, sigma.

It also connects to the natural bond orbital approach of Weinhold where a hydrogen bond D-H...A is viewed as an interaction between the unoccupied anti-bonding orbital of the DH bond and the the occupied nonbonded natural orbital (e.g. lone pair) of the acceptor atom A.

The authors perform quantum chemistry [most DFT with B2LYP-D] calculations for hundreds of H bonds. They find correlations between sigma and the bond energy, the H...A length, the shift in the D-H stretch frequency.
There were a couple of things I found strange about the paper.

1. Many quantities are calculated with quantum chemistry at different levels of theory and compared. But, I could never find a case where a calculated quantity was actually compared to an experimental value.

2. It makes the following claim:
Intramolecular hydrogen bonds are very interesting, but they are not accessible directly from experiment. Hence, theoretical results are the only reference data. 
Surely this is an overstatement. Consider for example, the work discussed in this earlier post I made. 

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