We have studied the Uracil system with a set of 4,12b-Dihydro-3-thioxo-1H,7H-chromeno[4/,3/:4,5]pyrano[2,3- d]pyrimidine-1(2H)-one of different electronic features using several methods. Our aim is to find out a calculation method for the analysis of these molecules in biological systems. The combined IR and NBO study permitted us to justify the observed tautomeric preferences. The absolute predominance of the 2H-tautomer forms is greatly changed when the substituent group possesses anionic character; therefore the pH of the medium is relevant. When the calculations were carried out in solution, noteworthy changes in the behavior of charged substituents were observed. These facts may be relevant when studying the interactions of these molecules with biological receptors