CCL: Help with halogen bonding

Dear Mariya
The distance between the two bonded atoms should equal to or less than the sum of the VDW radii of the atoms. NBO and AIM could be used as a further analysis to prove if there is a bond or not.
Halogen bonds are weak bonds, you should use MP2 (better) or B3LYP (not recommended because of lacking of dispersion in DFT; but one paper shows a comparison between performance of these two methods regarding to halogen bonding, and proved that both of them are giving good results) with big basis set with diffuse functions, most of the papers done on halogen bonds used B3LYP/aug-cc-pVDZ.
Your calculation shaw that the distance between the halogen atom and the Y atom is around VDW + 0.1, I think you didn't use a big basis set. If you are studying Flourine or Chlorine atom (X), you should use 6-311+G** at least as a basis set. For Bromine or Iodine, you should use aug-cc-pVDZ basis set to include relativistic effect.

There are many papers/reviews on this subject, you may like to have a look on the following:

* Fourmigué, M. "Halogen bonding: Recent advances." Current Opinion in Solid State and Materials Science 13(3-4): 36-45.
* Auffinger, P., F. A. Hays, et al. (2004). "Halogen bonds in biological molecules." Proceedings of the National Academy of Sciences of the United States of America 101(48): 16789-16794.
Kovács, A. and Z. Varga (2006). "Halogen acceptors in hydrogen bonding." Coordination Chemistry Reviews 250(5-6): 710-727.
* Politzer, P., P. Lane, et al. (2007). "An overview of halogen bonding." Journal of Molecular Modeling 13(2): 305-311.
 
Sincerely;
M. Ibrahim 

On Fri, May 14, 2010 at 8:28 AM, Mariya al-Rashida maria_al_rashida(!)http://hotmail.com" target="_blank">hotmail.com <owner-chemistry*o*ccl.net> wrote:

Sent to CCL by: "Mariya  al-Rashida" [maria_al_rashida~!~http://hotmail.com" target="_blank">hotmail.com]
Hello everyone

Im currently investigating crystal structures of some organic halogen compounds. I'm pretty sure i've come across some "Halogen Bonds" [haolgen bonds are non covalent interactions C-X...Y where X = halogen acting as lewis acceptor and Y = N or O acting lewis base]. What i know > from literature, both exp and theoretical is that these interactions are found at distances equal to or slightly shorter than sum of van der wall radii. Whereas i'm getting these interactions at distance which is equal to sum of van der wall radii + 0.1. Does this still count as a halogen bond ??? Or should it be rather a weak type of haolgen bonding?

Also does anyone know of any relavent theoretical literature for halogen bonding

Your comments will be highly apreciated
Mariiya



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