Raman and solid state C-13-NMR investigation of the structure of the 1 : 1amorphous piroxicam :beta-cyclodextrin inclusion compound

The results of a Raman and solid state C-13-NMR spectroscopic investigation aimed at studying the conformation of piroxicam (P) and its interaction wi th beta-cyclodextrin (beta CD) in 1 : 1 amorphous P beta CD inclusion compo und are reported. The 1700-1200 cm(-1) FT-Raman and the C-13 CP/MAS NMR spe ctra of 1 : 1 P beta CD inclusion compound are discussed and assigned in co mparison with those of the three main modifications of piroxicam (alpha, be ta, and monohydrate). The FT-Raman and C-13-NMR results show that in 1 : 1 PPCD inclusion compound piroxicam mainly assumes the zwitterionic structure typical of monohydrate, even if the presence of a different structure, tha t is, beta form, is not excluded. Piroxicam monohydrate, differently from a lpha and beta forms, is characterized by a zwitterionic structure with an i nternal proton transfer and an increased charge delocalization, as shown by our spectroscopic results. The charge delocalization characteristic of thi s zwitterionic structure gives rise to the interaction with beta CD via ele ctrostatic and hydrogen bonds. The possibility of a host-guest interaction between piroxicam and beta CD is not excluded; the guest molecule can be ac commodated in beta CD cavity. by interaction via hydrophobic bonds. (C) 199 9 John Wiley & Sons, Inc.