Molecular structures of fourteen xanthine tautomers were calculated by using the B3LYP/6-311G**method both in the gas and aqueous phases with full geometry optimization.The Onsager solvation model was employed for aqu...Molecular structures of fourteen xanthine tautomers were calculated by using the B3LYP/6-311G**method both in the gas and aqueous phases with full geometry optimization.The Onsager solvation model was employed for aqueous solution calculations.The structures,total energy,standard enthalpy,standard entropy and standard free energy were obtained.The calculations showed that xanthine existing as the diketo form is the predominant isomer in the gas and aqueous phases.The dioxo-N（7）（H）isomer is more stable than the dioxo-N（9）（H）isomer in the gas and aqueous phases.The results are in good agreement with available experimental results.The entropy effect on the Gibbs free energy of xanthine base is very small and there is little significance for the tautomeric equilibria of the base.The enthalpic term is dominant in the determinatio of tautomeric equilibria.The free energy of solvation is well correlated with the dipole moments of xanthine tautomers.Additionally,origins of the solvent effects were examined by analyzing the changes of the dipole moments due to the change of a solvent polarity.展开更多
In an attempt to assist the experimental findings, density functional theory (DFT) studies at the B3LYP/6-311+G(3df,2p) level on the structures, vibrational frequencies and relative stabilities of disulfides XSSY and ...In an attempt to assist the experimental findings, density functional theory (DFT) studies at the B3LYP/6-311+G(3df,2p) level on the structures, vibrational frequencies and relative stabilities of disulfides XSSY and their isomeric thiosulfoxide structures SSXY where X and Y are H, F, Cl, Br and I, have been reported in the present paper. The agreement with the available observed values experimentally is good. The calculated results suggest that the linear structures are more stable than their branched structures thermodynamically for all S2XY isomers. Thiosulfoxide difluoride SSF2, which is the only disulfide known to exist in two stable isomeric forms, is the most closest to its disulfide isomer energetically in all tautomers discussed in this paper. The activation energies for the unimolecular isomerization at room temperature were also predicted at the same level, and the calculated results indicated that the chain forms of XSSY are stable with respect to their corresponding branched forms kinetically. The intramolecular atom transfer rearrangement reaction are not likely processes because of the high potential barriers.展开更多
Molecular structures for tautomers of cytosine,uracil and thymine were calculated by the B3LYP/6-311Gmethod,both in the gas and aqueous phases,with full geometries optimization.The Onsager solvate theory model was...Molecular structures for tautomers of cytosine,uracil and thymine were calculated by the B3LYP/6-311Gmethod,both in the gas and aqueous phases,with full geometries optimization.The Onsager solvate theory model was employed for aqueous solution calculations.The structures,total energy,standard enthalpy,standard entropy and standard Gibbs free energy were obtained.The calculation results show that the oxo-amino or dioxo tautomers are the most stable isomer both in the gas and aqueous phasee.The results are in good agreement with available experimental data.The entropy effect on the Gibbs free energy of cytosine,uracil and thymine base is very small and there is little significance for the tautomeric equilibria of these base.The enthalpic term is dominant in the determination of tautomeric equilibria.The Gibbs free energy of solvation is well correlated with the dipole moments of cytosine,uracil and thymine tautomers.展开更多
文摘Molecular structures of fourteen xanthine tautomers were calculated by using the B3LYP/6-311G**method both in the gas and aqueous phases with full geometry optimization.The Onsager solvation model was employed for aqueous solution calculations.The structures,total energy,standard enthalpy,standard entropy and standard free energy were obtained.The calculations showed that xanthine existing as the diketo form is the predominant isomer in the gas and aqueous phases.The dioxo-N（7）（H）isomer is more stable than the dioxo-N（9）（H）isomer in the gas and aqueous phases.The results are in good agreement with available experimental results.The entropy effect on the Gibbs free energy of xanthine base is very small and there is little significance for the tautomeric equilibria of the base.The enthalpic term is dominant in the determinatio of tautomeric equilibria.The free energy of solvation is well correlated with the dipole moments of xanthine tautomers.Additionally,origins of the solvent effects were examined by analyzing the changes of the dipole moments due to the change of a solvent polarity.
文摘In an attempt to assist the experimental findings, density functional theory (DFT) studies at the B3LYP/6-311+G(3df,2p) level on the structures, vibrational frequencies and relative stabilities of disulfides XSSY and their isomeric thiosulfoxide structures SSXY where X and Y are H, F, Cl, Br and I, have been reported in the present paper. The agreement with the available observed values experimentally is good. The calculated results suggest that the linear structures are more stable than their branched structures thermodynamically for all S2XY isomers. Thiosulfoxide difluoride SSF2, which is the only disulfide known to exist in two stable isomeric forms, is the most closest to its disulfide isomer energetically in all tautomers discussed in this paper. The activation energies for the unimolecular isomerization at room temperature were also predicted at the same level, and the calculated results indicated that the chain forms of XSSY are stable with respect to their corresponding branched forms kinetically. The intramolecular atom transfer rearrangement reaction are not likely processes because of the high potential barriers.
基金The work were supported by National Natural Science Foundation of China(No.20702061)financial support from the Chinese Academy of Sciences(KSCX2-YW-R-172)
文摘Molecular structures for tautomers of cytosine,uracil and thymine were calculated by the B3LYP/6-311Gmethod,both in the gas and aqueous phases,with full geometries optimization.The Onsager solvate theory model was employed for aqueous solution calculations.The structures,total energy,standard enthalpy,standard entropy and standard Gibbs free energy were obtained.The calculation results show that the oxo-amino or dioxo tautomers are the most stable isomer both in the gas and aqueous phasee.The results are in good agreement with available experimental data.The entropy effect on the Gibbs free energy of cytosine,uracil and thymine base is very small and there is little significance for the tautomeric equilibria of these base.The enthalpic term is dominant in the determination of tautomeric equilibria.The Gibbs free energy of solvation is well correlated with the dipole moments of cytosine,uracil and thymine tautomers.
