Solvation of Uracil and Its Derivatives by DMSO: A DFT-Supported ¹H NMR and ¹³C NMR Study

¹H NMR and ¹³C NMR spectra of uracil, thymine, 5-hydroxymethyluracil, 5,6-dihydrouracil, and 5,6-dihydrothymine in DMSO-d₆ solutions have been measured. Additionally, molecular structures as well as NMR parameters of these compounds and their various solvates have been calculated using DFT B3LYP/6-311++G(2d,p) PCM(DMSO) method. The analysis of the chemical shift data for these compounds has shown that, indeed, in DMSO solutions they occur as equilibrium mixtures of free molecules and solvates in which solute and solvent molecules are joined by NH···O or OH···O hydrogen bonds. The populations of particular species present in the solutions have been estimated. Moreover, it has been found that 5,6-dihydrothymine exists in DMSO solution preferentially in conformation with the methyl group occupying the pseudoequatorial position. This finding is based on the molecular energy calculations and remains in full agreement with the interpretation of NMR data and theoretical calculations of NMR parameters.