Spectroscopic and electronic structure characterization of hydrogen bonding in 2-Bromohydroquinone

Abstract

2-Bromohydroquinone species form both intra- and inter-molecular H-bonds. The evidences for the H-bonds have been observed in the solid-phase Mid IR (4000−400 cm−1) and Near IR (8000−5000 cm−1) absorption spectra with two intensely broad bands near 3263 and 3222 cm−1 and a broad absorption in 7000−5200 cm−1 with a band maximum at 6335 cm−1. Dimer and trimer models with DFT's B3LYP and BP86 functionals using 6-31G(d) and 6-311++G(d,p) basis sets have been proposed for the OH⋯O bonding in 2-Bromohydroquinone. Out of the four predicted conformers, one trans and one cis conformers with nearly 1:1 Boltzmann populations, are separated by very small energy difference of 0.03 kcal/mol and this is confirmed by two chemical shifts at 9.0 and 9.3 ppm from an 1H NMR spectrum. On the basis of Quantum Theory of Atoms in Molecules (AIM) and reduced density gradient plots of electron densities and its isosurfaces by non-covalent interactions (NCI) method, the influence of the OH⋯Br, weak van der Waals and OH⋯O bond interactions on the stability of dimer and trimer structures have been analysed. From the natural bond orbital (NBO) analysis, it is explained that the formation of H-bonding may be attributed to charge transfer from the lone pair orbital n(O) of the base Oxygen into the vacant antibonding orbital σ*(OH) of the acid OH in the OH⋯O bonding. The Near IR concentration-dependent spectra demonstrate that the sample of 2-Bromohydroquinone is composed of OH⋯O bonded species and dissociate into non-bonded species as the mole fraction of the solvent (acetonitrile/1,4-dioxane) is increased. The band observed at 7095 cm−1 at 0.09 M is identified as the first overtone of the OH stretching mode of the weak complex of 2-Bromohydroquinone and acetonitrile. The solvent effects on all the observed solution-phase NIR spectra are not ruled out.