1,4-Dihydroquinone and its Derivatives: Vibrational Spectroscopic (FT-IR and Raman), Molecular Structure and Quantum Chemical Investigations

Hydroquinones constitute a family of antioxidants, but are also known as electron donor molecules in charge transfer complexes. Raman and IR spectra of 1,4-dihydroquinone (H₂Q) and its derivatives, 2,5-dichloro-1,4-dihydroquinone (H₂QCl₂) and 2-methoxy-1,4-dihydroquinone (H₂QOCH₃) in solid form have been recorded. Theoretical calculations using DFT (B3LYP) methods have been performed giving optimized structures, energies and harmonic vibrational frequencies. The comparison between the vibrational frequencies calculated and scaled with Raman and FT-IR experimental values shows good agreement. The influence of the substitutions on the electron donor capacity, geometrical parameters and normal modes of vibrations have been discussed. The presence of –Cl group increases the O–H bond length and decreases the electron donor capacity and inversely for the presence of –OCH₃. The O–H stretching vibrations shift to higher wave numbers in H₂QCl₂ and H₂QOCH₃. The aromatic C–H stretching vibrations are not much affected by the presence of –Cl and –OCH₃. The C–C and C=C stretching vibrations shift to higher wave numbers in H₂QCl₂ and shift to lower wave numbers in H₂QOCH₃. These studies show the influences of substitutions on molecular properties of hydroquinones.