In what mode of operation are n-pi and pi-pi interactions most dominant? Reverse phase or normal phase? In my research I have tried to make estimates of a stationary phase’s ability to participate in these types of interactions. My findings are that, in the reversed phase mode, these types of interactions have a positive contribution to retention, and in the normal phase mode, they are negligible (no influence either way).

I know that there is a class of chiral stationary phases, Pirkel type, that are essentially pi complexation stationary phases. In light of the fact that they seem to work almost exclusively in the normal phase mode, I reason that n-pi and pi-pi interactions contribute to retention more in the normal phase mode than in the reverse phase mode. However my experimental results disagree.

I’d greatly appreciate any discussion on this topic, or suggestions of papers (theory or application), that deal with these types of interactions and their role in chromatography. I see that it is necessary to have a stationary phase (like a phenyl phase) and a solute that is capable of these types of interactions.

Thanks

There is a recent paper by Lloyd Snyder that talks about pi-pi...

How do YOU measure it? How do you know, if there is a specific pi-pi interaction?

Among true pi-pi, there used to be a nitrophenyl phase around by Macherey Nagel. Don't know if they are still making it.

If my memory does not fail me there also is something on this in infos about Hypercarb("graphitised carbon") columns (Thermo now?).

I've been using the method of Abraham, Kamlet, and Taft to assess the n-pi, pi-pi interactions.

Thread author

Thread author,
Would you please publish the reference for the Abraham, Kamlet, and Taft article. Thanks in advance.
Regards,
Mark

Hi, Mark!

References:

1. C.S. Sychov, M.M. Ilyin, V.A. Davankov, K.O. Sochilina. / J. Chromatography A, 1014 (2004)

2. V.A. Davankov, C.S. Sychov, M.M. Ilyin, K.O. Sochilina. / J. Chromatography A, 987 (2003) 67-75

You`re not quite right, pi-interactions mostly contribute to retention in non-polar eluents, but they contribute the retention in aqueous eluents, too, but less. Pi-interactions is the only interactions possible for PGC and hypercrosslinked polystyrene (HCL PS)in non-polar eluents (we called this mode "quasi-normal phase", and they greatly contribute the retention on Pircle phases and most of all on Chiralcel modified cellulose and amilose. In fact, all phases with aromatic fragments can participate in pi-interactions. The easiest way to estimate contribution of pi-interactions in quasi-normal phase mode is to measure the selectivity anthracene/phenol using for example 7% 2-propanol in hexane. The alpha is zero for silica-gel, 0.16 (Kromasil DMB), 0.72 (Pirkle ULMO RR), 3.41 (Chiralcel OJ-H)... "infinity" for HCL PS and I`m positive the same result you will get for any graphitic phase (2-propanol have for this mode actually negligable elution strength). In RP mode the best way is to measure the log selectivity for nitrobenzene and benzene, is it negative for conventional C18 packings, slightly positive for Hamilton PRP-1 or PLPR-S, and highly positive for HCLPS and PGC.
I used principal component method and correlations with Hammet-Taft constants to prove that the effect is pi-interactions. Unfortunately, published data is rather "old", I mean, now I have plenty of unpublished experimental material on this topic.
For example, now I know the eluotropic raw of solvents for quasi-normal phase mode, and the mechanism of adsorbtion and desorbtion in this mode is also almost clear.
This material is to some extent published, but in Russian. I think I'll write an article to J.Chrom., but I'm waiting for results on graphitic phases now.
If you want - I can send you my report "The role of pi-interactions in LC" for the last conference on LC in Moscow, it is in microsoft powerpoint format.

Constantine

There is an excellent review, by Abraham, in a recent Journal of Chromatography A, volume 1037, p29-47, "Determination of sets of solute descriptors from chromatographic measurements." It mainly focuses on efforts to generate descriptors for solutes which can then be used in the characterize a stationary phase (or any other applicable system). (This whole issue of J. Chrom. A is excellent, it is a review of techniques using chromatography to make physical/chemical measurements, binding constants, etc...)

Abraham, M. A. Chem. Soc. Rev. 1993, v22, p72-83.
This is a great reference as it outlines the application of the theory and has a huge table of probe solutes and their descriptors.

Tan, L. C.; Carr, P. W.; Abraham, M. A. J. Chromatogr. A 1996, 752, 1-18.
This is a good example of how the theory is applied to characterize stationary phases.


One theory I have come up with to account for my findings is this. n-pi and pi-pi interactions are a non-polar in nature, so they should occur in the non-polar domain of a system. In HPLC, in reversed phase, they should contriubte to retention because the stationary phase is non-polar, and in normal phase, they should detract from retention, because the mobile phase is non-polar. My findings are that in reversed phase, they do contribute to retention, and in normal phase, they are zero.

It may be the stationary phases I use are simply poor in the ability to participate in these types of interactions. Perhaps a true, dedicated, pi-complex stationary phase, would have a much more significant value for the pi-interaction. I'll try this and see.


Thread Author

It is zero on bare silica-gel. Have you tried Whelk-1,0 or Pirkle ULMO or Chiralcel OD, OJ, AJ ?
Retention of PAHs is great on hexane-based eluents, and it doesnt denend on propanol`s concentration (the maximum elution strenght has CHCl3 or CH2Cl2). What else proof do you want?