Experimental and theoretical studies on the thermodynamics and properties of 2-substituted 6(4)-methyl-1,4(1,6)-dihydropyrimidine-5-carboxylates were undertaken by 1 H NMR measurements and DFT (density functional theory) calculations. The ratios of tautomers a / b of dihydropyrimidines (DPs) 1 , 2 , and 3 were determined under various conditions to reveal the effects of temperature, solvent, and concentration on the thermodynamic data. The observed results, the free energy differences (Δ G ), enthalpy differences (Δ H ), and entropy differences (Δ S ), are discussed in terms of the molecular structures, dipole moments (DM), and the electrostatic potential maps calculated by the DFT to clarify the nature of the DPs. [ABSTRACT FROM AUTHOR]