Organic molecules in solution are hardly ever rigid. There are a variety of dynamic processes, with rotations being the ones having a profound effect on NMR spectra. The best known example is the free rotation of a methyl group, which leads to the three protons of the methyl group not being distinguishable in a 1H-NMR spectrum. Free rotation around each bond is not always possible, instead there are preferred orientations, called rotational or conformerical isomers. By rotation about one or more bonds a conformer can be changed to another conformer.
Whether a rotation takes place or not, and how quickly this rotation is, depends on two parameters: the energy required for the rotation (the rotational barrier), and the available energy (the temperature). From the rotation rate at a certain temperature, it is thus possible to determine the rotational barrier and thus the bond strength of a bond. We have done this in the projects Grubbs-II and Bis-NHC.
In flexible molecules, it is possible that several conformers are stable at room temperature and are interconvertible by (multiple) rotations. These conformers can all be populated equally, so they are equally likely. They also can, depending on their relative energies, be populated differently. The issue, which conformer is populated to which extend may be a question that can be answered by RDC (see Lactone). A current project that deals with such a flexible molecule is the Tetrapeptide.
Coworker on that area
|Dr. Lukas Kaltschneefirstname.lastname@example.org|
|Dr.-Ing. Andreas Kolmer|
|Dr. Volker Schmidts||L2|07 8||+49 6151 email@example.com|
|1||C. M. Thiele, A. Marx, R. Berger, J. Fischer, M. Biel, A. Giannis, Determination of the relative configuration of a five-membered ring compound using RDCs, Angew. Chem. 2006, 118, 4566-4571, Angew. Chem. Int. Ed. 2006, 45, 4455-4460.|
|2||S. Leuthäußer, V. Schmidts, C. M. Thiele, H. Plenio, π-Face Donor Properties of N-Heterocyclic Carbenes in Grubbs II Complexes, Chem. Eur. J. 2008, 14, 5465-5481.|
|3||C. M. Thiele, V. Schmidts, B. Böttcher, I. Louzao, R. Berger, A. Maliniak, B. Stevensson, On the Treatment of Conformational Flexibility when using Residual Dipolar Couplings for Structure Determination, Angew. Chem. 2009, 121, 6836-6840, Angew. Chem. Int. Ed. 2009, 48, 6708-6712.|