[svalbard]

Hello all, thanks in advance for your help.

QUESTIONS ABOUT ROTAMERS:

I've had a difficult time getting a clear understanding of what rotamers are. I've consulted the IUPAC gold book and several over sources but am still somewhat confused. The clearest definition I've found is fromhttp://www.chemicool.com/definition/rotamer.html:

Rotamer: One of a set of conformers arising from restricted rotation about one single bond.

Ok, seems pretty clear but how is restricted rotation defined in this context? It seems like clarifying in a rotational barrier context would make more sense.

1. Is "free" rotation implied: The rotational barrier is so low that different conformations are not perceptible as different chemical species? (IUPAC definition)

2. or as hindered/restricted rotation: The inhibition of rotation of groups about a bond due to the presence of a sufficiently large rotational barrier to make the phenomenon observable? (IUPAC definition)

3. But free rotation is technically hindered or restricted in some sense, no matter how little, so why even differentiate?

4. Or both: either free and/or hindered?!

[MissPhosgene]

I'm not really sure what you are asking.

Rotamers are rotational isomers. Calling a species a rotamer is dependent on the experiment. Rotation has to be slow enough on the experiment time scale (high enough barrier) to "see" a single rotational isomer. Take the 1HNMR of DMF at RT. The two methyls appear to be indistinguishable when you look at the structure, but there are two methyl peaks in the spectrum. If you were to run a 1HNMR experiment on DMF at higher temp, there would be one methyl peak.  Lots of molecules are rotmers. Look up o-substituted biphenyls.

[svalbard]

I guess I'm more asking in the context of conformers.

"The existence of more than one conformation, usually with different energies, arises from hindered rotation about bonds. Butane has three rotamers: two gauche conformers, which are enantiomeric and an anti conformer, where the four carbon centres are coplanar. The three eclipsed conformations with dihedral angles of 0°,120° and 270° are not considered to be rotamers, but are instead transition state."

[MissPhosgene]

They are conformations which correspond to energy wells. Maybe you are over-thinking it.