I had cases where two 6-membered aromatic rings are slightly displaced with each other (I mean to say, they are not exactly face-to-face). The distance between their centroids is 4.2-4.4 Angstroms.
Would it be appropriate to say, "There are weak displaced pi-pi interactions are present in these cases."?
If possible, please suggest me some references.
Thanks in Advance.
Hava a look at this article:
Martinez, Chelsea R., e Brent L. Iverson. “Rethinking the Term ‘pi-stacking’”. Chemical Science 3, nº 7 (2012): 2191–2201. doi:10.1039/C2SC20045G.
Hava a look at this article:
Martinez, Chelsea R., e Brent L. Iverson. “Rethinking the Term ‘pi-stacking’”. Chemical Science 3, nº 7 (2012): 2191–2201. doi:10.1039/C2SC20045G.
Dear Nagapradeep
The limit to interaction pi-pi is the some of the van der Waals radius (VDW) between atoms involved. As we know, the some VDW to C-atoms is about 3,6 Angstroms. Thus, I think that to this case wich you described (with distance between their centroids 4.2 - 4.4 Angstroms) doesn't have interaction pi-pi.
If you're looking for an exact number Janiack (J. Chem. Soc., Dalton Trans., 2000, 3885–3896) suggests 3.3-3.8 A. This is supported by recent work by Alvarez (Dalton Trans., 2013, 42, 8617) which suggests an accurate VDW radii for C is 1.77 A. As mentioned by others 4.2-4.4 is too far for any true pi-pi interaction.
Dear Nagapradeep
The distance between two centroids of 4.2-4.4 Angstroms is possible for pi-pi interactions if other evidence is also present as it is much more than generally accepted limit of 4 A. You can go through one of my recent paper: Folded conformations due to arene interactions in dissymmetric and symmetric butylidene-linker models based on pyrazolo[3,4-d ]pyrimidine, purine and 7-deazapurine,
Acta Cryst. (2014). C70, 555–561.
and references therein.
@ Dr. K. Avasthi, Thanks for your straight forward answer along with valuable reference.
Surprisingly, the RING server used 6.5 A based on their statistical analysis on the PDB. They classified pi-pi interaction into 5 sub-types, among which "the orthogonal conformation is found at [5.36,5.4] A. Both the parallel and the tilted edge-to-face are mostly found at [5.94, 5.99](http://nar.oxfordjournals.org/content/44/W1/W367.full.pdf+html)
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