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

Vibrational spectral measurements, namely, infrared (4000–400 cm−1) and Raman (3500–50 cm−1) spectra have been made for 2-Bromohydroquinone. Optimized geometrical structures, harmonic vibrational frequencies and intensities have been computed by the ab initio (RHF), B-based (BLYP, BP86) and B3-based (B3P86, B3LYP, B3PW91) density functional methods using 6-31G(d) basis set. A complete assignment of the observed spectra has been proposed. Coupling of vibrations has been determined by calculating potential energy distributions (PEDs) at BP86/6-31G(d) level of theory. In the computed equilibrium geometries by all the levels, the bond lengths and bond angles show changes in the neighborhood of Bromine. Similarly, the vibrational spectra exhibit some marked spectral features unlike in hydroquinone and phenol. On the other hand, the infrared spectrum shows a clear evidence of OH⋯O bonding near 3200 cm−1 as in hydroquinone. Evaluation of the theoretical methods demonstrates that all the levels but the RHF have reproduced frequencies fairly accurately in the 2000–500 cm−1; below 500 cm−1 the RHF has performed reasonably well.