Covalent hydration free energy calculations

For many chemical problems, it is crucial to consider solvent effects. This was demonstrated in our recent studies on the hydration free energy of U02 and the model reduction of uranyl by water [232,233]. The ParaGauss code [21,22] allows to carry out DKH DF calculations combined with a treatment of solvent effects via the self-consistent polarizable continuum method (PCM) COSMO [227]. If one aims at a realistic description of solvated species, it is not sufficient to represent an aqueous environment simply as a dielectric continuum because of the covalent nature of the bonding between an actinide and aqua ligands [232]. Ideally, one uses a combination model, in which one or more solvation shells (typically the first shell) are treated quantum-mechanically, while long-range electrostatic and other solvent effects are accounted for with a continuum model. Both contributions to the solvation free energy of U02 were... 

Little is also known about photoelectron spectra of pteridines. The unsubstituted nucleus and its 4-methyl- and 2,4,6,7-tetramethyl derivative have been recorded and analyzed in terms of both a simple Huckel-model and semiempirical calculations <86CB1275>. The assignment of the n- and n-type PE bands of pteridine indicates that the low energy band is associated with an ionization process involving the nitrogen n electrons and followed by rc-bands. Furthermore, the pKa of free pteridine, which is masked in aqueous solution by partial covalent hydration, is suggested from the PE data to be in the order of —2. 

An electrostatic hydration model, previously developed for ions of the noble gas structure, has been applied to the tervalent lanthanide and actinide ions. For lanthanides the application of a single primary hydration number resulted in a satisfactory fit of the model to the experimentally determined free energy and enthalpy data. The atomization enthalpies of lanthanide trihalide molecules have been calculated in terms of a covalent model of a polarized ion. Comparison with values obtained from a hard sphere modeP showed that a satisfactory description of the bonding in these molecules must ultimately be formulated from the covalent perspective. 

M. D. Erion and M. R. Reddy, J. Comput. Chem., 16,1513 (1995). Calculation of Relative Free Energy Differences for the Covalent Hydration of Organic Compounds A Combined Quantum Mechanical and Free Energy Perturbation Study. 

---