I am comparing the IR spectra of methyl trans-cinnamate and trans-cinnamic acid. For the IR spectrum of methyl trans-cinnamate, aromatic overtones can be seen (2112 cm-1, 1995 cm-1, and 1923 cm-1), while they are not present in the IR spectrum of trans-cinnamic acid. Both of these molecules contain the same aromatic ring, so how come only one of them contains the aromatic overtones? I have attached the relevant spectra. Any help would be much appreciated, thank you!