The out-of-phase (C=O)2 stretching mode for maleic anhydrides,vout-of-phase (C=O)2, is in Fermi resonance with a B1 combination tone, and the extent of Fermi resonance interaction can be altered drastically depending upon the particular solvent system used in the study. The vout-of-phase (C=O)2 and vin-phase (C=O)2 modes can be differentiated by solvent system studies, since the out-of-phase mode shifts more in frequency than the in-phase mode. Both v(C=O)2 modes decrease in frequency as the polarity of the solvent system increases or the amount of intermolecular hydrogen bonding increases, such as in the case of an increasing mole % concentration of chloroform.
