Towards understanding of different solid forms of formoterol fumarate: Combined computational and experimental approach

In this study, specimens of Formetrol fumerate were investigated using IR and Raman vibrational spectroscopy as well as quantum chemical calculations. The structure of formetrol fumerate was optimised using density functional theory calculations and the geometry optimization has been carried out on three different solvate crystal forms; di-hydrate, di-ethanolate, and di-isopropanolate in addition to the anhydrate form with and without intramolecular hydrogen bonding. Molecular assignments are proposed on the basis of ab initioB3LYP DFT calculations with a 6–31 G∗ basis set and vibrational wavenumbers. Crystallographic investigation has been carried out to formetrol anions and protonated anions arising from crystal structures of the studied conformers and it was evidenced that the di-hydrate form has the highest energy probably due to the greater possibility of intramolecular hydrogen bonding. Infrared and Raman spectra were calculated from the optimised structures. Many modes in the calculated spectra were matched with the experimental spectra and a description of the modes was given. By analysis of the theoretical vibrational modes, it was proved that formetrol fumerate specimens are likely to be dihydrate form with and without intramolecular hydrogen bonding. Additionally, several spectral features and band intensities in the stretching and bending regions were explained. Quantum mechanical calculations allowed improved understanding of formetrol fumerate and its vibrational spectra as an important β2 antagonistic compound in various pharmaceutical formulations. The obtained data could provide useful information about its interactions with excipients and other components.