Dear Forum Members,

I am facing a problem with the Absorbance Ratio values between a Monomer and a Dimer of Pharmaceutical Drug. The Absorbance Ratio of the Dimer of that particular compound is almost half that of the Monomer. Is it same with all the Dimer compounds. If yes, why is it so, any theory behind it. I have taken well standardized Dimer compound and well standardized Monomer compound, prepared the same concentration solution for both and injected. I am getting almost half the response for Dimer when compared against its monomer. Even the same solutions I have checked in the UV-Vis Spectrophotometer, and the case is same. The spectrums of both the compounds are exactly same with same UV max values. I feel theoretically the Absorbance ratio of Monomer and Dimer shall be 1 when we take the molar concentrations and calculate.

The Monomer contains one conjugated double bond (two double bonds),and two -C=O groups where as,

the dimer consists of two conjugated double bonds (four double bonds), three -C=O groups and one
-COOH group.

These are the only chromophores available in the molecule. The compounds are showing a wavelength maxima of 238 nm, and was analyzed at the same wavelength.

I am waiting for your valuable responses.

Sorry, I intended to look up the rules on lambda max for C=C and C=O, but forgot. From those rules it may be deduced that your monomer has one C=O in conjugation with the two C=C (there are other possibilities) as chromophore. In any case, unless there is a strange coincidence, at least one of your chromophores must stay intact in dimerization, thus no wavelength change. Now the absorbance of a compound can be related to complicated quantum mechanical considerations which one would expect to be quite different between a dimer and monomer in a relatively small molecule. You should not be surprised to see such a difference.