I synthesized a molecule which has two chiral centers. So, I got diastereomers which I am able to separate through column. How do I determine the configuration of these molecules?
One good way is to use single crystal X-ray diffraction analysis, if you can grow a single crystal.
In some cases, according to the structure, it is possible to determine by NMR spectroscopy.
Methods for studying chiral molecules. If there are chromophores, then apply Electronic Circular Dichroism Spectroscopy .For different functional groups - Vibrational Circular Dichroism Spectroscopy. For investigation at the phase interface - Sum Frequency Generation Vibrational spectroscopy, SFG .
Rajiv,
The most definitive way of determining relative stereochemistry of your diasteriomers would be as Volker said, X-ray crystallography. Other techniques are often dependent on the structure and type of your molecule. NMR techniques are useful, but are typically best applied in situations where your molecule is rigid and has a predictable configuration, i.e. the stereocenters are both attached to a ring. If you could provide the structure of your molecule and the reaction conditions used to derive it, you would get answers much more specific and useful to your current situation.
I wish you the best of luck in your research.
First you have to separate the diastereomers in a single pure isomer in a crystalline form , then determine the configuration best way by X-ray diffraction studies.
X Ray diffraction and NMR...after development in to a single isomer
I agree that X-ray diffraction of an isometrically pure crystal of your compound is the most unambiguous method to establish the absolute and relative configuration of your compound but I would suggest uploading a chemdraw of your compound or at least the chemdraw-generated name of your compound because knowing the particular structure you are trying to solve can help people suggests strategies (eg. Can your compound be condensed with a chiral auxiliary which would then allow, via NMR, the absolute configuration of your compound to be determined via its relative configuration with respect to the known auxiliary stereocenteres). Also if we know the physical characteristics of your compound (eg oil, low melting solid, etc) or what the structure is we can suggest methods to increase the crystallinity of your compound such as bromination of aryl rings or arylation of amines, etc.
I also found two diastereomers, I separated them by crystallization and I determined the configuration of each by x-rays diffraction
NOE is an efficient technique for confirming diastereomers if no single crystal is possible.
If your compond has two stereogenic centers it may have 4 (four) different configurations. Are you sure that the fractions you have separated are pure disatereomers or racemic mixtures (two enanteomers in each one)? The first thing is to measure the specific rotation of each sample,. If they are optically active you may proceed as suggested by the colleagues to determine the configuration (e.g. by using X ray difraction); if not, you have to resolve first the racemic mixtures (e.g. in a chiral collumn).
Dr. Vieira makes a very good point that you absolutely need to be sure you are working with enantiomerically pure diasteriomers (or at least reasonably enriched). In fact, mixtures of enantiomers often co-crystalize preferentially over enantiopure samples thus making X-ray diffraction an ineffective method to determine absolute stereochemistry (you will often obtain a disordered structure). I have submitted crystals from samples with >96% ee and obtained somewhat disordered crystals structures from crystals of
First of all you should check if the compound (or its enantiomer) has not been described before. If it has and an optical rotation value it is reported you should compare the value and sign of your molecule optical rotation and the literature data.
Secondly, if the compound is new or the optical rotation value is unknown, then you can turn to X-Ray diffraction or other, often less reliable or more cumbersome, options.
A third option is to chemically modify your molecule in order to transform it into a compound of known absolute stereochemistry. Typically a sugar, an amino acid or any well described compound.
Finally, you can try to make the molecule (or the other enantiomer) by a different route and check if you get a consistent optical rotation value.
Good luck in your research!
By X-ray diffraction if you have a cristal of one diastereomers.
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