What kind of NMR technique is used to determine the stereo chemistry of a new organic compound?
Dear Devulapally Srikrishna,
The NMR technique used to determine the stereo chemistry of a new organic compound is 1D1H-1H nOe difference:
The nOe (nuclear Overhauser effect) difference experiment is used, primarily, to define the stereochemistry within a molecule. Unlike all the above techniques, it's mode of operation relies on the direct, through-space interaction between nuclei, and is independent of the presence of through-bond couplings. This through-space
effect is akin to the forces experienced between two bar magnets when brought towards each other. The experiment involves the application of a radio-frequency field to a single resonance in the spectrum, such that the corresponding protons become saturated (that is, the population differences between their high and low energy levels are forced to zero). The system no longer has the initial population distribution between its energy levels, as defined by the Boltzmann distribution, and like any chemical system that has been perturbed from its equilibrium conditions, it will attempt to adjust itself so as to counteract the change. In the context of the nOe, this adjustment takes the form of changes in population differences for the energy levels of nuclei that are 'close in space' to the saturated proton. Recording of the proton spectrum after the period of saturation may, therefore, show changes in signal intensities for those protons in the vicinity of the saturated proton, due to these population changes. The observed intensity changes and termed steady-state NOEs and are usually quite small (often less that 10%), so are best seen in a difference spectrum in which a control spectrum without proton saturation is subtracted from the nOe spectrum, such that only differences between the two are
apparent.
In practice, the nOe only operates over a short range (< 4 Å) thus providing evidence of spatial proximity of two protons. However, distance is NOT the only parameter to influence the size of the nOe; if the nOe from proton A to B is 10 % and that from A to C is only 6 % it is not possible to say, from these data alone, that A is closer to B than to C. It is necessary to consider all the protons surrounding B and C to determine the relative influence of saturating A; it is recommended that you read the relevant section of a suitable textbook to appreciate this fully. It is also the case that nOe's in these experiments are seldom symmetrical; if saturating proton A gives a 10 % enhancement to B, then saturating B will not necessarily give a 10 % enhancement to A. Once again, the whole proton environment of protons A and B need be considered.
For more details on this technique, please use the following link:
http://www.chem.ox.ac.uk/spectroscopy/nmr/pdfs/organic%20nmr%201.pdf
Hoping this will be helpful,
Rafik
Dear Devulapally Srikrishna,
The NMR technique used to determine the stereo chemistry of a new organic compound is 1D1H-1H nOe difference:
The nOe (nuclear Overhauser effect) difference experiment is used, primarily, to define the stereochemistry within a molecule. Unlike all the above techniques, it's mode of operation relies on the direct, through-space interaction between nuclei, and is independent of the presence of through-bond couplings. This through-space
effect is akin to the forces experienced between two bar magnets when brought towards each other. The experiment involves the application of a radio-frequency field to a single resonance in the spectrum, such that the corresponding protons become saturated (that is, the population differences between their high and low energy levels are forced to zero). The system no longer has the initial population distribution between its energy levels, as defined by the Boltzmann distribution, and like any chemical system that has been perturbed from its equilibrium conditions, it will attempt to adjust itself so as to counteract the change. In the context of the nOe, this adjustment takes the form of changes in population differences for the energy levels of nuclei that are 'close in space' to the saturated proton. Recording of the proton spectrum after the period of saturation may, therefore, show changes in signal intensities for those protons in the vicinity of the saturated proton, due to these population changes. The observed intensity changes and termed steady-state NOEs and are usually quite small (often less that 10%), so are best seen in a difference spectrum in which a control spectrum without proton saturation is subtracted from the nOe spectrum, such that only differences between the two are
apparent.
In practice, the nOe only operates over a short range (< 4 Å) thus providing evidence of spatial proximity of two protons. However, distance is NOT the only parameter to influence the size of the nOe; if the nOe from proton A to B is 10 % and that from A to C is only 6 % it is not possible to say, from these data alone, that A is closer to B than to C. It is necessary to consider all the protons surrounding B and C to determine the relative influence of saturating A; it is recommended that you read the relevant section of a suitable textbook to appreciate this fully. It is also the case that nOe's in these experiments are seldom symmetrical; if saturating proton A gives a 10 % enhancement to B, then saturating B will not necessarily give a 10 % enhancement to A. Once again, the whole proton environment of protons A and B need be considered.
For more details on this technique, please use the following link:
http://www.chem.ox.ac.uk/spectroscopy/nmr/pdfs/organic%20nmr%201.pdf
Hoping this will be helpful,
Rafik
In addition to NOE experiments, which aren't always very clear-cut (seeing an NOE coupling is unambiguous, but if there is no signal, it either means that the nuclei are too far apart, or the scan wasn't sensitive enough, or there was oxygen dissolved in the solvent etc. etc.), one can look at coupling constants with other nearby protons, or use chiral shift reagents.
Is there only one stereocenter in the molecule of interest? If there are more than one, and one of the other stereocenters is already of known configuration, you might be able to distinguish one possible diastereomer from the other based on the degree of coupling (https://en.wikipedia.org/wiki/Karplus_equation)
There are also paramagnetic chiral compounds (chiral shift reagents: http://www.sigmaaldrich.com/chemistry/chemistry-products.html?TablePage=16265970) that will coordinate with the molecule of interest, and form a diastereomeric complex. The spectrum will change in some predictable ways depending on the absolute configuration of the stereocenter of interest.
There is also the Mosher ester protocol: http://www.nature.com/nprot/journal/v2/n10/full/nprot.2007.354.html
NOE experiments works when the molecule of interest has some rigidity. However, if it able to freely rotate you might be out of luck. I would also suggest you read up on chiral HPLC, which might be best was to resolve stereoisomers. It is used extensively in industry and your university might have one.
the realive stereochemistry can be derived from:
1. NOE (e.g. 1D DPFGNOE is very effective):
2. spin-spin couplings (particularly 3-bond J's);
3. chemical shifts
If you are not able to crystallize your molecule, than you should resort to traditional analysis based on:
1. Spin-spin couplings (from 1H or COSY experiments), doing the analysis on the base of Karplus equations
2. NOE, NOESY, ROESY, all of them based on Nuclear Overhauser effect.
If these two options don't give you an unambigous solution, then you have two more options:
3. Residual dipolar couplings (RDCs), for which you would need an alignment media (cross-linked polymer or lyotropic liquid crystal) that should be incorporated to your NMR sample. Therefore, you would need to resort to the broad bibliography on this topic and choose an alignment media compatible with a solvent you can use with that analyte to perform NMR experiments. Then, you would need to perform heteronuclear coupled experiments (for example a coupled HSQC, from which you can extract C-H couplings), and compare the couplings obtained from that experiment performed in isotropic conditions and then with the alignment media, and use computationally optimized models of your compound, along with a program specially developed to analyse this kind of data (normally they are free).
4. Also, you can check Goodman's DP4 method. In order to use this, you would need to generate computational models of all the diastereoisomers of your molecule, and compute their 1H and 13C chemical shifts with gaussian or an analogous program with the specifications that Goodman suggests.
In the end, the method compares the experimental chemical shifts with the computationally calculated ones and gives a probability to each diastereoisomer, of being the real one. It is quite reliable.
http://www-jmg.ch.cam.ac.uk/tools/nmr/DP4/
- In fact, all these experiments help you to determine stereochemistry, but you can also, in some cases, consider coupling constants. For example, in ring type cyclohexanes the trans-diaxial constant is higher than the axial-equatorial or equatorial-equatorial constants. All this information help you in stereochemistry determination
I agree with all the above techniques while crystallization is the best. You can perhaps use contact shift reagents to determine the stereochemistry of your compound as below
1. http://pubs.rsc.org/en/Content/ArticleLanding/1976/C3/C39760000647#!divAbstract
2. http://pubs.rsc.org/en/Content/ArticleLanding/1983/C3/c39830001392#!divAbstract
Additionally to NOE (mainly), a group from spain has developed a method with an auxiliary reagent.
http://onlinelibrary.wiley.com/doi/10.1002/chem.200500181/abstract
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