Crystal structure determination of thymoquinone by high-resolution X-ray powder diffraction

Abstract

The crystal structure of 2-isopropyl-5-methyl-1,4-benzoquinone (thymoquinone) and its thermal behavior—as necessary physical and chemical properties—were determined in order to enhance the current understanding of thymoquinone chemical action by using high resolution x-ray powder diffraction, Fourier transform infrared spectroscopy (FTIR), and 3 thermo-analytical techniques thermogravimetric analysis (TGA), differential thermal analysis (DTA), and differential scanning calorimetry (DSC). The findings obtained with high-resolution x-ray powder diffraction and molecular location methods based on a simulated annealing algorithm after Rietveld refinement showed that the triclinic unit cell was a=6.73728(8) Å, b=6.91560(8) Å, c=10.4988(2) Å, α=88.864(2)^o, β=82.449(1)^o, γ=77.0299(9)^o; cell volume=472.52(1) ų, Z=2, and space group $P\bar 1$ . In addition, FTIR spectrum revealed absorption bands corresponding to the carbonyl and C-H stretching of aliphatic and vinylic groups characteristically observed in such p-benzoquinones. Also, a chemical decomposition process starting at 65°C and ending at 213°C was noted when TGA was used. DSC allowed for the determination of onset at 43.55°C and a melting enthalpy value of ΔH_m=110.6 J/g. The low value obtained for the fusion point displayed a van der Waals pattern for molecular binding, and the thermograms performed evidence that thymoquinone can only be found in crystalline triclinic form, as determined by DRX methods.