FT-IR and FT-Raman spectra of five polymorphs of chlorpropamide. Experimental study and ab initio calculations

IR- and Raman spectra were studied for five polymorphs of chlorpropamide (α, β, γ, δ and ε) (phase purity proved by DSC, X-ray single-crystal and powder diffraction). The equilibrium geometry and the vibrational spectra for different molecular conformations were calculated ab initio (DFT, B3LYP approximation). The assignment of the vibrational bands was made on the basis of the theoretical calculations and a comparison of the experimentally measured spectra for the crystalline polymorphs and the melt. The differences in the experimental vibrational spectra of the five chlorpropamide polymorphs were correlated with the X-ray structural data, and shown to be mainly due to the intermolecular interactions effect, and only to a smaller extent – to the conformational changes.