Structural, vibrational and nuclear magnetic resonance investigations of 4-bromoisoquinoline by experimental and theoretical DFT methods

Quantum chemical calculations of energy, structural parameters and vibrational wavenumbers of 4-bromoisoquinoline (4BIQ) were carried out by using B3LYP method using 6-311++G**, cc-pVTZ and LANL2DZ basis sets. The optimised geometrical parameters obtained by DFT calculations are in good agreement with electron diffraction data. Interpretations of the experimental FTIR and FT-Raman spectra have been reported with the aid of the theoretical wavenumbers. The differences between the observed and scaled wavenumber values of most of the fundamentals are very small. The thermodynamic parameters have also been computed. Electronic properties of the molecule were discussed through the molecular electrostatic potential surface, HOMO–LUMO energy gap and NBO analysis. To provide precise assignments of 1H and 13C NMR spectra, isotropic shielding and chemical shifts were calculated with the Gauge-Invariant Atomic Orbital (GIAO) method.

Graphical abstractThe FTIR, FT-Raman, 1H and 13C NMR spectral measurements of 4-bromoisoquinoline and complete assignments of the observed bands have been reported. DFT calculations have been performed and the structural parameters of the compound were determined from the optimised geometry with MP2/6-31G** and B3LYP using 6-311++G** and cc-pVTZ basis sets. The energy, harmonic vibrational frequencies, depolarisation ratios, IR and Raman intensities were determined. The geometrical parameters, harmonic vibrational frequencies and chemical shifts were compared with the experimental data of the molecule. The influences of various factors affecting the stability of the compound have been discussed. A detailed natural bond orbital analysis was carried out..Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► FTIR, FT-Raman and NMR spectral studies of 4-bromoisoquinoline were carried out. ► Dihedral angles Br17C4C3C2 and Br17C4C9C10 (180°) shows the molecule is planar. ► The down field of C1 and C3 is due to the I effect of N2 and +M effect of Br17. ► The frontier orbital energy gap (EHOMO − ELUMO) is found to be 0.1682 a.u. ► The π → π* interaction between C1N2 bonding and C3C4 antibonding orbitals hss stabilisation 22.63 kJ mol−1.