Structural investigation of anhydrous nitrofurantion and its monohydrate based on terahertz/Raman vibrational spectroscopy and density functional theory

Abstract

Anhydrous nitrofurantion and its monohydrated form were characterized and analyzed by using vibrational spectroscopic techniques including terahertz time-domain spectroscopy and Raman spectroscopy at room temperature. The experimental results indicate that both spectroscopic techniques could recognize the above two forms clearly. In terahertz spectra, anhydrous nitrofurantion has absorption peaks at 1.26 and 1.60 THz, while the monohydrated form has three peaks at 0.67, 1.05 and 1.50 THz respectively. Raman spectra also show some differences between the anhydrous nitrofurantion and monohydrated form, but not significant as that of terahertz spectra since Raman-active vibrational modes are mostly from intra-molecular interaction of various functional groups within molecules while terahertz spectra is more sensitive to inter-molecular interaction within various solid-state crystalline forms. Density functional theory was performed to simulate the optimized structures and vibrational modes of these two forms. In addition, the characteristic bands of anhydrous nitrofurantion and its monohydrate are also assigned based on the simulation results from the density functional theory calculation.