Investigating the conformers of 1, 2, 3, 4-tetrahydroquinoxaline: A combined theoretical and experimental investigation through potential energy surface studies, FT-IR and UV-Vis absorption measurements

摘要

1, 2, 3, 4-tetrahydroquinoxaline (THQX) show two inequivalent conformers at various levels of quantum chemical computations along with the variation of basis sets in S-0 and S-1 states. They are separated by 450 +/- 100 cm(-1) and 55 cm(-1) in S-0 and S-1 states respectively. The twisted conformer with the equatorial orientation of hydrogen atom of both the NH groups and bearing C-2 symmetry is computed to be the most stable conformer (C1) in S-0. The global minima in S-1 state do not differ considerably although C-2 symmetry is lost. Two potential energy surfaces (PES) are plotted with respect to i) N-C-C-N dihedral angle and the NH orientation and ii) two different orientations of the H atoms of the NH groups. Four transitions states are found in S-0. Three of them are bent structures and out of them two possesses C-2 symmetry. Corresponding surfaces in S-1 yield two transition states. The twisting angle for the optimised conformer is 31 degrees +/- 1.5 degrees in So and 28 degrees +/- 1 degrees in S-1. The characteristic low frequency vibrations appear at 107 cm(-1 )and 160 cm(-1) in S-0. Apart from some weak lines most of the observed vibrational frequencies in FT-IR spectra corroborate well with the computed ones. Calculation of molecular electrostatic potential indicates the sights of electrophilic interactions. Frontier molecular orbitals are calculated. The peaks in the observed UV-Vis absorption spectra are explained corresponding to the associated molecular orbitals. These spectra corroborate well with the calculated ones. The calculations predict that the dominant character of these transitions would be pi-pi* in nature. (C) 2020 Elsevier B.V. All rights reserved.