Monomeric or dimeric? A theoretical and vibrational spectroscopic approach to the structural stability of 5-(4-metoxy benzoyl)-6-(4-metoxyphenyl)-3-methyl-2-thioxo-2,3-dihydropyrimidine-4(1H)-on

The structural and the spectral characteristics of a pyrimidine derivative, 5-(4-metoxybenzoyl)-6-(4-metoxyphenyl)-3-methyl-2-thioxo-2,3-dihydropyrimidine-4(1H)-on (BPOP) was studied using density functional theory methods (DFT/B3LYP) employing 6-31G(d), 6-31G(d,p), 6-311G(d) and 6-311G(d,p) basis sets. Two BPOP molecules form a dimeric structure linked over two mutual N-H center dot center dot center dot S=C hydrogen bonds on each of the pyrimidine rings. The quantum chemical calculations of the optimized molecular structures, the energies and IR & Raman spectra of the monomeric and the dimeric forms of BPOP have been performed. The HOMO-LUMO and the molecular electrostatic potential surface (MEP) have also been plotted. The thermodynamic functions including enthalpy, entropy and heat capacity values and Mulliken charges of both structures have been calculated. The detailed assignments of vibrational modes have been made on the basis of potential energy distribution (PED). More accurate new scaling factors were obtained for the four basis sets. It was concluded that the experimental wavenumbers are in better agreement with the results of the dimeric structures calculated by all the methods. B3LYP/6-311G(d) method regenerated the best calculated values among the others. (C) 2020 Elsevier B.V. All rights reserved.