Overtone spectroscopy of chlorine substituted benzene derivatives.

The absorption spectra of di-, tri- and tetra-derivatives of chlorobenzene have been studied in their pure form in the spectral range 400-20,000 cm(-1). A large number of bands associated with the fundamental, the overtones and the combination frequencies of C-H stretching mode have been observed. Vibrational frequencies, anharmonicity constants and dissociation energies, for the C-H stretch vibrations for the six molecules have been determined using local mode model. The C-H stretch frequencies obtained from experiments are compared with the corresponding frequency determined theoretically using RHF and DFT methods with same 6-31+G* basis set. This information has been used for the assignment of several combination bands as well as some weak overtone bands. Effect of hydrogen atom substitution by chlorine atom has been studied by measuring changes in the vibrational frequency of the C-H stretching mode and the C-H bond length. Frequency changes have been well correlated with the change in charge density on carbon as well as chlorine atoms.