Vibrational spectra, normal modes, ab initio and DFT calculations for 6-Chloro- and 7-Chloro-4-bromomethylcoumarins

Vibration spectral measurements – Infrared (4000–400 cm−1) and Raman (3500–50 cm−1) spectra – have been made for the solid samples of 6-Chloro- and 7-Chloro-4-bromomethylcoumarins. Ground electronic state energies, equilibrium geometries, harmonic vibrational spectra and normal modes have been computed using ab initio – RHF/6-31G* – and DFT – B3LYP/6-31G* levels of theory. The optimization yielded three structures for each molecule, with one being a transition state structure. Of the remaining two conformers, one belongs to Cs symmetry and the other belongs to C1, the latter being the most stable one. The optimized dihedral angle for –CH2Br group is 111° in agreement with X-ray diffraction results reported for the similar molecular systems. Assignment of all the observed spectral bands has been proposed. The absorptions show band pattern revealing isomer characteristics and vibrational coupling in varying degrees; the Raman spectra show structural changes associated with the rings as well as lattice modes.