Quantum mechanical investigation of vibrational and electronic spectra of some 5-substituted isoquinolines

Abstract

Fourier Transform infrared (FTIR) and FT-Raman spectra of 5-bromoisoquinoline (5-BIQ), 5-aminoisoquinoline (5-AIQ) and 5-nitroisoquinoline (5-NIQ) have been recorded and analyzed. The geometric bond lengths and bond angles have been optimized. The equilibrium geometry, bonding features and harmonic vibrational frequencies have been investigated by ab-initio and density functional theory (DFT) methods. The assignments of the vibrational spectra have been carried out. The calculated HOMO and LUMO energies indicate that charge transfer occurs within the molecule. Stability of the molecule due to conjugative interactions arising from charge delocalization has been analyzed using extensive natural bond orbital (NBO) analysis. Electronic spectral properties have been computed in benzene medium for the three compounds and compared with the experimental values. Thermodynamic parameters such as zero point vibrational energy (ZPVE), thermal energy and specific heat capacity, entropy of 5-BIQ, 5-AIQ and 5-NIQ at 298 K in ground state have been calculated by HF and DFT methods.