Ab initio Hartree–Fock and density functional theory investigations on the conformational stability, molecular structure and vibrational spectra of 3-(2-(4-methylpiperazin-1-yl)-2-oxoethyl)benzo[d]thiazol-2(3H)-one

In the present work, the conformational analysis of 3-(2-(4-methylpiperazin-1-yl)-2-oxoethyl)benzo[d]thiazol-2(3H)-one molecule (abbreviated as 3MOT) was performed by using ab initio Hartree–Fock (HF) method with 3-21G as the basis set to find the most stable form of 3MOT. Seven staggered stable conformers were observed on the torsional potential energy surfaces. It is shown that 3MOT_1 conformer (D1 = 68.1°, D2 = 73.4°, D3 = −177.4° and D4 = −79.9°) is the most stable form which has chair conformation with the methyl group linked to nitrogen in the axial position. The geometrical parameters, vibrational frequencies and IR intensities of 3MOT were calculated by DFT(B3LYP) method using 6-31G(d), 6-31+G(d), 6-31G(d,p), 6-311G(d), 6-311+G(d), 6-311G(d,p) and 6-311++G(d,p) 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. 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.