Comparative studies of the vibrational spectra of 4-methyl 3-nitrobenzonitrile and 2-formyl benzonitrile have been made. The spectra are interpreted with the aid of normal mode analysis following full structure optimization based on the density functional theory and different basis set combination. While making complete assignment of vibrational wavenumbers, some interesting facts were noticed for example, how modes of vibrations, bond angles and bond lengths are affected by different groups attached to the benzene ring. The main aim of the present study is to investigate in detail the vibrational spectra and thermo dynamical properties of 4- methyl 3-nitrobenzonitrile and 2-formyl benzonitrile by B3LYP/6-311+G(2df,2p) method and basis set combination, which can presumably help in understanding its behavior i.e. effect on reactivity, stability, nature of state due to the different groups attached to the benzene ring. These effects are also correlated with the chemistry of attached groups.