Spectroscopic characterisation of the monoclinic and orthorhombic forms of paracetamol

The metastable orthorhombic form of paracetamol was prepared from the melt of the commercially available monoclinic form. Distinct differences were observed in the infrared spectra of both forms, especially in the region 1260-1225 cm^-1, in which is observed three strong absorptions of approximately equal intensity in spectra of the monoclinic form, and two absorptions, one strong and one medium, in spectra of the orthorhombic form. No diagnostically useful differences were observed in the Raman spectra of the two forms. A ¹³C CP/MAS solid-state NMR spectrum of the monoclinic form and a spectrum of a mixture of forms prepared from a melt were obtained. A spectrum of the orthorhombic form was obtained from these spectra by difference spectroscopy. The spectra show that the carbons in the paracetamol molecules are all in unique chemical environments in both crystalline forms, and that clear well-resolved differences in the chemical shifts of particular carbons in both forms can be observed.