2-Bromohydroquinone: structures, vibrational assignments and RHF, B- and B3-based density functional calculations

Area of Science:

• Molecular Spectroscopy
• Computational Chemistry
• Quantum Chemistry

Background:

• Vibrational spectroscopy provides insights into molecular structure and bonding.
• Understanding the effects of substituents on molecular vibrations is crucial for chemical analysis.
• 2-Bromohydroquinone's spectral properties are compared to related compounds like hydroquinone and phenol.

Purpose of the Study:

• To experimentally measure and theoretically compute the vibrational spectra of 2-Bromohydroquinone.
• To assign observed infrared and Raman spectral bands.
• To investigate the impact of bromine substitution on molecular geometry and vibrational frequencies.

Main Methods:

• Infrared (IR) and Raman spectroscopy were used for experimental measurements.
• Ab initio and density functional theory (DFT) methods (RHF, BLYP, BP86, B3P86, B3LYP, B3PW91) with a 6-31G(d) basis set were employed for computations.
• Potential Energy Distribution (PED) analysis was performed to determine vibrational coupling.

Main Results:

• Optimized geometries showed changes in bond lengths and angles near the bromine atom.
• Vibrational spectra exhibited distinct features compared to hydroquinone and phenol.
• Evidence of O-H...O hydrogen bonding was observed in the infrared spectrum around 3200 cm(-1).
• DFT methods accurately reproduced frequencies between 2000-500 cm(-1), while RHF performed well below 500 cm(-1).

Conclusions:

• The study successfully assigned the vibrational spectra of 2-Bromohydroquinone.
• Bromine substitution significantly influences the molecule's geometry and vibrational characteristics.
• Computational methods provide reliable predictions for vibrational spectra, aiding in molecular characterization.