An infrared spectroscopic study of p-n-alkoxybenzoic acids

The infrared spectra of various p-n-alkoxybenzoic acids are reported. The carbonyl stretching region of these materials is surprisingly sensitive to packing within the various crystal forms, a consequence of the sensitivity of these modes to hydrogen bonding and transition dipole coupling. The isotypic crystal form found in those acids with n>6 are characterized by a splitting in the carboxyl C=O stretching region, a consequence of interactions between the disordered arrangement of carboxyl groups from layer to layer. This splitting is not observed in those alkoxybenzoic acids where n<7. The infrared spectra also show that there is a solid state transformation from the crystalline state to conformationally disordered or "condis" crystals as intermediates between the crystalline and liquid crystalline forms.