[CHEKAL]

how would i be able to distinguish between cis and trans diastereomers using H-NMR? i know its something to do with the coupling constant.

also, how would i be able to determine a rough ratio of cis:trans diasteroisomers using H-NMR?

thanks

[discodermolide]

What are cis and trans diastereoisomers? What exactly are you referring to here?

[CHEKAL]

i have a compound with two chiral centres, cis being both centres have the same configuration relative to one another, trans being different configuration to one another

[discodermolide]

i have a compound with two chiral centres, cis being both centres have the same configuration relative to one another, trans being different configuration to one another

You mean like this, for example?

[CHEKAL]

yes, ive run the nmr spectrum of the compound, i just need to distinguish between them and determine an approximate ratio of the two relative configurations

[discodermolide]

Well you can look at the ratio of the peaks next to the chiral centers.
The coupling constants for the H-H coupling should be different, look up the Karplus equation.
This of course if you can differentiate the peaks.

[CHEKAL]

which configuration would have a higher coupling constant?

[discodermolide]

Probably the anti configuration. As I suggested look up the Karplus equation.
This tells you the relationship between the J value and the angle between the two protons.

[CHEKAL]

how would i do it in the case of no peak of the adjacent carbons to the chiral centres? i have an aziridine and the predicted spectrum nor my actual spectrum does not show any singlets that havnt been accounted for

[discodermolide]

You mean you have a cis and trans aziridine? Like this?

[CHEKAL]

yes, i will get a structure uploaded, watch this space

[CHEKAL]

structures

[discodermolide]

I would expect that the trans coupling would be larger than the cis coupling.
You should easily see this in the NMR.

[CHEKAL]

how would i be able to see this?

[discodermolide]

The chemical shifts of the protons should be quite different, each will be a doublet the distance between the lines is the coupling constant, J in Hz.