It is shown that aqueous solvation free energies, ∆G sol , can be expressed quantitatively in terms of properties of the molecular surface electrostatic potentials of the solutes. The latter are obtained computationally by the B3P86/6-31+G** density functional procedure. Regression analyses and an experimental database encompassing 50 solutes of various types are used to obtain an analytical representation of ∆G sol which reproduces the experimental values with a standard deviation of 1.57 kJ/mol and an average absolute deviation of 1.14 kJ/ mol.

We have investigated the consequences of varying the three parameters in Becke's hybrid exchange-correlation functional, which includes five contributions: Hartree-Fock exchange, local exchange, Becke's gradient exchange correction, local correlation, and some form of gradient correlation correction. Our primary focus was upon obtaining orbital energies with magnitudes that are reasonable approximations to the electronic ionization potentials; however, we also looked at the effects on molecular geometries and atomization enthalpies. A total of 12 parameter combinations was considered for each of three different gradient correlation corrections: the Lee-Yang-Parr, the Perdew-86, and the Perdew-Wang 91. Five molecules were included in the study: HCN, N 2 , N 2 O, F 2 O, and H 2 O. For comparison, a Hartree-Fock calculation was also carried out for each of these. The 6-31+G * * basis set was used throughout this work. We found that the ionization potential estimates can be greatly improved (to much better than Hartree-Fock levels) by increasing the Hartree-Fock exchange contribution at the expense of local exchange. In itself, this also introduces major errors in the atomization enthalpies. However, this can be largely or even completely counteracted by reducing or eliminating the role of the gradient exchange correction.

No abstract

No abstract

Relative tautomerisation energies, enthalpies, entropies, Gibbs free energies, and dipole moments for 5-methyl-2,4-dioxo-imidazolidine and its thio analogous have been studied in the gas phase using hybrid density functional at the B3LYP level of theory using 6-31(d) and 6-311+(2df,2p) basis sets with full geometry optimization. A comparative investigation of their energies revealed that the keto form of the compound is the predominant tautomer with a very high tautomeric energy barrier. The findings conclude that the intramolecular prototropic tautomerisation process is thermodynamically unfavored.