Context: Identification of optimal solid form of an active pharmaceutical ingredient and form control are very important in drug development. Thus, the structural information of these forms and in-depth insight on the modes of molecular interactions are necessary, and vibrational spectroscopic methods are well suited for this purpose.
Objective: In-depth structural analysis of different solid forms of indomethacin (IND) using Raman and infrared (IR) spectroscopy is the objective. We have investigated the modes of molecular interactions in polymorphs ( and ), amorphous and discovered cocrystals of IND with nicotinamide (NIC) and trans-cinnamic acid (CIN) coformers.
Materials and methods: The solid forms of IND have been prepared; their purity has been verified by differential scanning calorimetry and powder X-ray diffractometry and then studied in the solid-state by Raman and IR spectroscopy. The modes of the interactions were closely investigated from the vibrational data.
Results: The key vibrational features of IND solid forms have been specified. The IR (C=O) band at 1713 cm-1 attributed to cyclic acid dimer of  IND has disappeared in IND–NIC/CIN whilst retained in IND–SAC cocrystal.
Discussion: IND cocrystallizes in different conformations and crystal lattices with different coformers. The cyclic acid dimer of IND has been kept on its cocrystallization with saccharin and it could have been broken with NIC and CIN.
Conclusions: The complementary nature of Raman and IR spectroscopy allowed unambiguous investigation of the chemical composition of pharmaceutical materials which is of particular importance in the absence of detailed structural information, as in the case of IND–NIC and IND–CIN.