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 theatmospheric 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 ofa molecule, M, is defined as -ΔH 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 (-ΔG) of reaction. Adiabatic ionizationenergy (AIE) and adiabatic electron affinity (AEA) are the energy difference between neutralmolecule 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 moleculeand its anion or cation at the equilibrium geometry of the neutral molecule is called the verticalelectron affinity (VEA) and ionization energy (VIE) respectively ]1[.Barbituric acid (BA) is a well-known organic compound based on a pyrimidine heterocyclicskeleton. BA itself is not pharmacologically active but it is the parent compound of a large classof barbiturates that act as central nervous system depressants. They are used in medicine assedatives, 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 quantumchemistry package. There are three suitable protonation sites in BA molecular structure namedO6, O8, and N9. To find the topical PA and GB of BA, the neutral molecule was protonatedfrom nitrogen and two oxygen atoms. PA of this molecule was calculated as 781.31, 779.63 and694.9 kJ/mol when it was protonated from O8, O6 and N9 sites, respectively. Also the GB of BAwas 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 AIEand VIE of BA. The AEA and VEA of the BA were obtained as -0.42 and -0.18 eV, respectively.

کلیدواژه ها:

Barbituric Acid ، Proton Affinity ، Gas Phase Basicity ، Electron Affinity ، Iionization Energy