Using IR

[labrat00]

Can anyone tell me how to use IR to differentiate two structurally similar compounds that have the same substituents but placed at different locations on the compound like o-nitrophenol and p-nitrophenol.

[Klaynos]

I'm no expert on this but, possibly bragg diffraction might be what you're looking for?

[DrDNA]

IR spectroscopy resolves stretching, wagging and bending frequencies of molecular bonds. Diffferent substituents near key bonds (called fingerprint regions) can shift these frequencies.

The NO2 group is electron withdrawing. OH is electron donating. A phenyl ring is a conjugated system.

The position of the nitro and hydryoxyl groups (ortho vs para vs meta) on the phenyl ring affects the conjugated system and effects the bond structures of each other (and therefore the frequency shifts). The bonds that connect the nitro group to the phenyl ring (N-C=) and the hydroxy group to the phenyl ring (O-C=) change the pheny ring and change each other.

In essence the fingerprint is changed by these electron withdrawing or electron donating substituents being o, p, or m on the phenyl ring.

I hope I explained that clearly, if not, let me know and I'll try to clarify. You also should look up the bond shifts.