I have a host-guest solution in an aprotic solvent medium. I suspect that there is a probability of existence of anion- π interaction during host-guest recognition process. What is the simple and best approach to investigate this phenomena if at all it is existing? (I have read several literature where they solve crystal structure, do some computational study and all. I didn't obtain any crystal of host-guest complex.) What may be the best way to approach towards solving this problem.
It sounds like you already know the answer. A crystal structure will let you know if the interaction exists. A computational study could help estimate the strength of the interaction. An alternative might be to adjust the pH of your solution so that the anion no longer exists, and see if that weakens or tightens your binding by whatever method you currently use to analyze it. You could also try replacing the pi-bearing group with a non-aromatic ring of the same size (or some other bit of molecule) to see what that does to the interaction.
But the 'best' approach isn't likely to also be 'simple'. Anion-pi interactions are still only marginally understood in the first place. Conducting a reasonable study of such an interaction isn't likely to be trivial.
@William K Patt I know a general idea how people have arrived at it's existence.But I didn't know how to develop an approach towards it.Your explainations really generated ideas in my mind.Thank you for your valuable inputs.
if it is possible, you can change the type of anions. In this case, maybe NMR will givie some hints.
@Weitao Gong
NMR chemical shift values fall around same value in two aromatic rings. Hence it was difficult to determine based on that.
Is there any cut off ppm value, above which anion- π interaction can exist?
Studying anion-pi interactions in liquid state is difficult however X-ray crystallography is straight forward for characterization of solid state assembly. UV-VIS spectroscopy might yield some information about anion-pi interactions in solution based on disturbance in the pi-pi* transition of your aromatic system. For anion-pi interactions to be effective the aromatic system needs to be electron deficient and the anion needs to be electron donating. Hence you might see a charge transfer effect which is reflected in UV-VIS spectroscopy.
@Ravikumar Iyyamperumal
Can you please tell me how disturbance in the pi-pi* transition of your aromatic system can be visualised in UV-Vis?
Is there any characteristic pattern?
If there is a strong charge transfer between your host and guest, you might be able to see red shift in your absorbance spectrum.
You do not have any chance to notice anion- π interactions in solution because they do not give a new absorption or emission band in any spectra unless your π-system is a strong electon-acceptor. NMR titration gives very ambiguous information on stability of such complexes because the same shift may be a result also of preferential solvation, which only formally complies with 1:1 complexation model. A real number of solvent molecules interacting with your anion is unknown but should be more than one. Hence, processing your titration data with 1:1 complexation model leads to a huge overestimation of calculated stability constant compared with a real value. More detailed discussion of this problem is in the chapter: http://www.sciencedirect.com/science/article/pii/B9780124095472124990
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