Conformational landscape, stability, potential energy curves and vibrations of 1,2,3,4 tetrahydroquinoline

Abstract

The twisted conformer with the equatorial hydrogen of the NH group is computed to be the most stable conformer of tetrahydroquinoline in S0 and S1 states at various levels of quantum chemical computations. The planar structure for equatorial orientation of H atom is found to be higher by 4000 ± 800 cm−1 in S0 and ∼3000 cm−1 in S1. For the axial orientation of H atom the barrier is 9500 ± 300 cm−1 in S0 and ∼8000 cm−1 in S1. The twisting angle for the optimized conformer is 29° ± 2° in S0. Observed IR spectra corroborate well with the theoretical calculations. The characteristic low-frequency vibrations appear at 97 cm−1 and 154 cm−1 in S0. The excited state structure differs from S0 and is termed “half-twist”. Potential energy curves corresponding to twisting and bending motions, as well as the one connecting the two motions are found out. Frontier molecular orbital calculations associated with the electron density plots clearly identify the changes on the density on excitation. Molecular electrostatic potential is calculated and the sites of electrophilic interactions are noted. Hardnesses' and electrophilicities are calculated for all the conformers to check their relative stability. The maximum hardness principle and minimum electrophilicity principles are not valid for most of the conformers. Transition state is found to obey the minimum electrophilicity principle.