ABSTRACT Quantum chemical calculations have been carried out to investigate the vibrational frequencies of clonazepam. The FTIR spectrum of clonazepam was recorded in the region 4000 to 400 cm–1 in the solid phase. The optimized geometry, wave number and intensity of the vibrational bands of clonazepam were obtained by ab initio Restricted Hartree Fock (RHF) and Density Functional Theory (DFT) methods with complete relaxation in the potential energy surface using the 6-31G (d,p) basis set. A complete vibrational assignment aided by the theoretical harmonic frequency analysis is proposed. The observed and the calculated wavenumbers are found to be in good agreement. The experimental spectra also coincide satisfactorily with those of theoretical spectra. Theoretical spectrograms for the IR spectrum were also constructed in RHF and B3LYP levels. In addition to this the RHF and DFT based NMR calculation procedure was used to assign the 1H NMR chemical shift of clonazepam. Theoretical values are compared with the experimental data.
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