Calculation of Proton and Electron Affinities Gas Phase Basicities and Ionization Energies of Barbituric acid

Proton transfer reactions and ionization are important processes that play a key role in the atmospheric chemistry and biochemistry. Proton affinity (PA) and gas phase basicity (GB) determine the capability of an atom or a molecule to accept a proton in the gas phase. The PA of a molecule, M, is defined as -∆H of its protonation in the gas phase, M(g) + H+(g) → MH+(g). The GB of M is defined as the Gibbs free energy change (-∆G) of reaction. Adiabatic ionization energy (AIE) and adiabatic electron affinity (AEA) are the energy difference between neutral molecule and its cation and anion when all species are in their ground electronic states, respectively. The electronic energy difference between the ground states of the neutral molecule and its anion or cation at the equilibrium geometry of the neutral molecule is called the vertical electron affinity (VEA) and ionization energy (VIE) respectively [1]. Barbituric acid (BA) is a well-known organic compound based on a pyrimidine heterocyclic skeleton. BA itself is not pharmacologically active but it is the parent compound of a large class of barbiturates that act as central nervous system depressants. They are used in medicine as sedatives, hypnotics, and soporifics. In this work we calculate the PA, GB, VIE, AIE, VEA and AEA of BA at the B3LYP/6-311++G** level of theory. All calculations were performed using Gaussian 09 quantum chemistry package. There are three suitable protonation sites in BA molecular structure named O6, O8, and N9. To find the topical PA and GB of BA, the neutral molecule was protonated from nitrogen and two oxygen atoms. PA of this molecule was calculated as 781.31, 779.63 and 694.9 kJ/mol when it was protonated from O8, O6 and N9 sites, respectively. Also the GB of BA was obtained as 745.29, 748.11 and 660.49 kJ/mol for protonation of O8, O6 and N9 sites, respectively. Theoretical calculations show that the protonated structure from the O8 site (M(O8)H+) was the most stable isomer. The values of 9.97 and 9.89 eV was calculated as AIE and VIE of BA. The AEA and VEA of the BA were obtained as -0.42 and -0.18 eV, respectively.