[Rutherford]

I got confused while studying conformational analysis. I need to clarify what these terms mean: conformer, rotamer, atropisomer and axial chirality isomer. Here is what I think, if it's incorrect, please correct me:

Conformer - any conformational isomer that is produced by rotation around any single bond in a molecule.

Rotamer - any conformational isomer that is produced by rotation around one and the same single bond in a molecule.

Atropisomer - conformational isomers where the interconvert between them has a big activation energy, so they are regarded as different compounds.

Axial enantiomers - not sure. Where is the difference with the atropisomers?

What are the right definitions?

[AWK]

compare with:
conformational isomerism, atropisomer and axial chirality in Wikipedia

[Rutherford]

Okay, then:
Conformer - any conformational isomer that is produced by rotation around any single bond in a molecule.

Rotamer - conformational isomers where the interconvert between them has an activation energy.

Atropisomer - conformational isomers where the interconvert between them has a big activation energy, so they are regarded as different compounds. They exhibit axial chirality.


Which conformers are not rotamers?

[AWK]

rotamers are a special case of conformers (conformational isomers) with hindered rotation

[Rutherford]

I could use an example of a conformer that is not a rotamer. How can there be no hindered rotation at all?

[AWK]

http://goldbook.iupac.org/R05407.html

[Rutherford]

I need some real life examples, molecules to understand the difference.
For example, cyclohexane chair and boat conformations are produced from rotations around sigma bonds, but staggered and eclipse rotamers of e.g. propane are made in the same way. Where is the difference?

[Dan]

For example, cyclohexane chair and boat conformations are produced from rotations around sigma bonds, but staggered and eclipse rotamers of e.g. propane are made in the same way. Where is the difference?

Neither of these are not rotamers - the energy barriers to rotation about sigma bonds in cyclohexane and butane (note that propane does not have staggered/eclipsed conformers) have relatively low barriers to rotation - i.e. they interconvert rapidly at room temperature.

At the other end of the scale is compounds like BINAP, which has two conformers that can be isolated - the barrier to rotation is so high that one conformer would have to be be heated strongly for a long time in order to convert it to the other.

Rotamers interconvert slowly at room temperature, such that you might be able to see them as seperate species on the timescale of NMR for example, but they interconvert too rapidly at room temperature to be isolated. Amides and carbamates can quite often show this behaviour:

e.g. and

So: rotamers are conformers which interconvert slowly at room temperature, can usually be distinguished by e.g. NMR, but normally interconvert too rapidly for isolation. Atropisomers interconvert so slowly that they can (theoretically) be separated and will not undergo appreciable interconversion at room temperature. They are both special types of conformer.   

[Rutherford]

Thanks for clarifying this!

One more questions: How doesn't propane have staggered and eclipsed conformations? It doesn't have the gauche and anti types.

[Dan]

One more questions: How doesn't propane have staggered and eclipsed conformations? It doesn't have the gauche and anti types.

Sorry, that was a brainfart - propane does have staggered/eclipsed, but for propane gauche=eclipsed.