2,5-dibromonitrobenzene (monomer)
4,4’-dibromo-2,2’-dinitrobiphenyl (dimer)
The FTIR spectra for these two are basically the same. But the dimer spectrum shifted to the left. Isn't that when the mass increased, the peak shifted to higher wavenumber?
It follows from your Figure that you are trying to compare the spectra of NITRO-monomer with AMINO-dimer obtained by reduction reaction of NITRO-monomer.
Naturally the RED and the BLUE spectra are VERY different. But it should not be described as bands-shift.
The two spectra are for two different structures. Different bands for different bonds are present in the two structures. Generally shifting any bond to the left means that the bond becomes more stable and stronger. Resonance could be good interpretation for this shift.
The direct answer to your question is no. An increase in the mass of an atom usually lengthens the bond resulting in a shift to longer wavelength, lower wave number. In you text you refer to the mono and dimmer of a nitrobenzene but the spectra you show are of a nitro benzene and an amino benzene. The spectra show band changes consistent with the chemical group change
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