Chromatography Forum

Can someone explain the difference between these two definitions?

No difference. Both terms mean selectivity based on the geometry differences of the test molecules, rather than strength of other interactions.

No difference. Both terms mean selectivity based on the geometry differences of the test molecules, rather than strength of other interactions.

That is also my idea. But in the article of L.C. Sander in anal chem 1999 is written:
These solutes (PAH) in each of these classes exhibit a fixed conformation structure constrained by fused ring systems, double bonds, and/or steric restrictions.

So if I interpret this sentence well it mean that steric selectivity is a part of shape selectivity.


Dolan also noted something concerning this topic.


Can someone give a clear statement?

No difference. Both terms mean selectivity based on the geometry differences of the test molecules, rather than strength of other interactions.

That is also my idea. But in the article of L.C. Sander in anal chem 1999 is written:
These solutes (PAH) in each of these classes exhibit a fixed conformation structure constrained by fused ring systems, double bonds, and/or steric restrictions.

So if I interpret this sentence well it mean that steric selectivity is a part of shape selectivity.


Dolan also noted something concerning this topic.


Can someone give a clear statement?

In my opinion if you only refer to alkyl (C18/C30) RP stationary phases.
steric selectivity (a term used by TANAKA) explains the behaviour for polymeric coated reversed phase systems against (poly)aromatic hydrocarbons compared to standard monomeric RP18 stationary phases. He compares only molecules with a SIMILAR SHAPE (this means: identical amount of atoms and approximately the same hydrophobicity but one is 3D and the other is planar! (see o-Terphenyl and Triphenylene!)

Sander also looked for condensed systems with same number of rings (and Pi electrons) but different shape (linear fused systems like anthracene behave different from phenanthrene (which also contains 3 fused rings but angular not linear!) Same for tetracene/chrysene!

The word "shape" distinguishes not only between the rigid and flexible structure of bridged (sigma bonding: Terphenyl) and condensed (Pi bonding: Triphenylene) molecules - interaction and retention is also controlled by the "shape" of the analytes even if they´re both planar. This is EXTREMELY important for separation of PAHs.

But steric selectivity can play a great and completely different role besides C18 - for example cyclodextrins show excellent separation of diastereomers (Vitamins D2/D3). The rigid cyclic structure leads to high steric selectivity and this is caused by the stereochemistry of the sugars.

In most cases people mean the same when using these explanations together with different C18 columns - so Mark is quite right.

I think this is a clear explanation. There is just a small difference between the two terms, but both can be used .
To keep it unambiguous the term Shape Selectivity contains both selectivity based on different structure (PAH like Sander) and diference of diastereomers (steric selectivity).

Correct me if I'm wrong

The above quote from Sanders seems to merely explain the different sources of rigidity of the molecules, that is the restriction toward rotation about a C-C bond. A fused ring or a double bond clearly are such restrictors, his last type is steric hindrance to rotation such as in biphenyls with bulky ortho groups. Some even exist as noninterchangeable chiral molecules. I don´t see any attempt here to distinguish between steric and shape in relation to chromatography. To me any attempt to distinguish would be to artificially complicate things even further.
Incidentally, a planar molecule is three dimensional!

I think some of the confusion stems from the fact that Sander characterized columns in terms of "steric selectivity". Snyder and Dolan characterized columns on the basis of 5 parameters, one of which was related to the shape of the molecule, but which did not correlate well with Sander's "steric selectivity". I believe they used the term "shape selectivity" or "steric resistance" (I'm out of the office this week, so doing this from memory; if I've got it wrong, please correct me!).

In common usage, "shape selectivity" and "steric selectivity" are used interchangeably (as has been pointed out). If you are referring to quantitative values, then you probably should specify "as defined by Sander" or "as defined by Snyder and Dolan" to avoid ambiguity.