Computational Studies on the Rotamers of 4-Aryl-2-Oxo-1,2,3,4-Tetrahydropyrimidines and 4-Aryl-2-Oxo-1,2-Dihydropyrimidines

Abstract

The potential energy surface of 2-oxo-1,2,3,4-tetrahydropyrimidines (THPMs) is explored by density functional theory at the B3LYP level with a 6-31++G** basis set. All minimum and maximum points display a flat boat conformation of the heterocyclic ring in which the C₄-aryl ring occupies a pseudo-axial position. In order to elucidate the comparative effect of the space orientation of the C₄-aryl ring or the 5-CO group towards the heterocyclic ring on the total energy content of THPM and also the changes in some characteristic bond lengths and angles, these groups are rotated around the C₄-Cn and C₅-C₇ single bonds, respectively. The results indicate an effect of the orientation of the 5-CO group on the investigated parameters compared to that of the C₄-aryl ring. Natural bond orbital (NBO) analysis is carried out to investigate the effect of various intramolecular interactions and also intermolecular interactions with a medium in the calculation on the stabilization energy compared to those in the gas phase calculation. The results indicate that the dipole moments and also the charge densities on the specific atoms are effectively changed by the orientation of the C4-aryl ring and the 5-CO group towards the heterocyclic ring. In addition, bond rotations around the C₄-Cn and C₅-C₇ single bonds in the oxidized compounds 2-oxo-1,2-dihydropyrimidines (DHPMs) are also carried out to compare the effects of these changes with those in THPMs.