Ab initio Hartree–Fock and density functional theory investigations on the conformational stability, molecular structure and vibrational spectra of 7-acetoxy-6-(2,3-dibromopropyl)-4,8-dimethylcoumarin molecule

In this work, conformational analysis of 7-acetoxy-6-(2,3-dibromopropyl)-4,8-dimethylcoumarin molecule (abbreviated as 7ADDC) was performed by using ab initio Hartree–Fock (HF) method with 3-21G as the basis set to find the most stable form of 7ADDC. Fifteen staggered stable conformers were observed on the torsional potential energy surfaces. It is shown that 7ADDC_1 conformer (D1 = 89.9°, D2 = −170.4°, D3 = −91.8°, D4 = −171.3° and D5 = 67.3°) is the most stable form. The geometrical parameters, vibrational frequencies and IR intensities of 7ADDC were calculated by HF and DFT (B3LYP) methods using 6-311++G(d,p), 6-31G(d,p) and 3-21G basis sets for the lowest energy conformer. The solid-state FT-IR spectra was recorded and compared with scaled frequency values. Many normal vibrational bands have been observed to be mixed because of the lacking of symmetry. The optimal scaling factors calculated for 7ADDC were found to be 0.9731 and 0.9067 for B3LYP/6-311++G(d,p) and HF/6-311++G(d,p), respectively. Determination of the relative weight contribution of each vibrational mode of the observed frequencies indicates the reliability and precision of the spectral analysis. The observed and calculated vibrational frequencies are found to be in good agreement.