Effects of solvation

With respect to the linearity of uranyl, it is interesting to note that significantly bent 0=U=0 moieties (-160-170°) were predicted theoretically for the hydroxo complexes U02(0H)(H20) (n = 0,3-5), U02(0H)2, and dimeric (U02)2(0H)2(H20)n (n=0,6) [228-230], In the case of U02(0H)2(H20)3, bending of the uranyl unit was observed when two OH groups were not separated by an aqua ligand. This was rationalized [231] by a strong 7i-donation from OH to the uranium atom, which results in a competition between oxo and hydroxo ligands. 

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 

167-170 pm, in a Hartree-Fock SCF calculation, even when the first or second hydration spheres are explicitly considered in the model [237]. 

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Also use constant dielectric Tor MM+aiul OPLS ciilciilatimis. Use the (lislance-flepeiident dielecinc for AMBER and BlO+to mimic the screening effects of solvation when no explicit solvent molecules are present. The scale factor for the dielectric permittivity, n. can vary from 1 to H(l. IlyperChem sets tt to 1. .5 for MM-r. Use 1.0 for AMBER and OPLS. and 1.0-2..5 for BlO-r. 

In fee absence of fee solvation typical of protic solvents, fee relative nucleophilicity of anions changes. Hard nucleophiles increase in reactivity more than do soft nucleophiles. As a result, fee relative reactivity order changes. In methanol, for example, fee relative reactivity order is N3 > 1 > CN > Br > CP, whereas in DMSO fee order becomes CN > N3 > CP > Br > P. In mefeanol, fee reactivity order is dominated by solvent effects, and fee more weakly solvated N3 and P ions are fee most reactive nucleophiles. The iodide ion is large and very polarizable. The anionic charge on fee azide ion is dispersed by delocalization. When fee effect of solvation is diminished in DMSO, other factors become more important. These include fee strength of fee bond being formed, which would account for fee reversed order of fee halides in fee two series. There is also evidence fiiat S( 2 transition states are better solvated in protic dipolar solvents than in protic solvents. 

The effect of solvation of transition states has been discussed in relation to aromatic nucleophilic substitution. 

The former, which occurs in tetrahydrofuran, favors dimerization, while the latter, which takes place in hexamethylphoshoramide, is shifted far to the left. In spite of the complicating effects of solvation and association with counter ions, it appears that within a reaction series of conjugated radical ions, the following relation holds... 

As BH dissociates into H+ and the uncharged base B, the dielectric constant can exert only a minor effect on the mutual coulombic attraction, so that even in water (e = 78.5) the pKa values of aliphatic amines do not differ much from the above picture of the influence of solvent basicity. That piiTa(water) lies between ptfa(m.cresoi) and p a(acetlc acid) instead of between the latter and p.Ka(pyridine) may be ascribed to effects of solvation however, the p a(Water) values of the aromatic amines are low owing to effects of mesomerism. 

This coefficient has various names (medium effect, solvation activity coefficient, etc.) the name recommended by the responsible IUPAC commission is the transfer activity coefficient. In this book the effect of solvation in various solvents will be expressed exclusively in terms of standard Gibbs transfer energies. 

The effect of solvation on uracil and thymine photophysics has been studied by Gustavvson and coworkers, who have studied uracil with four explicit water molecules and PCM to study distorted geometries [92,93,149], The conical intersection connecting Si to the ground state that was found in the gas phase is also present in solution. The barrier connecting the Si minimum to the conical intersection is lower in solution, however, causing much shorter lifetimes. So the nanosecond lifetime which is observed in the gas phase is not observed in solution but a picosecond lifetime is observed. 

In selected cases, the effect of solvation on the crystalline structure formed is, however, considerably more pronounced. For example, the observed packing in the crystal of 2,4,6-tris( 1,3-propylenediamine-N,N -)cyclotriphosphazene (4) dihydrate (Fig. 6) is due to strong intermolecular hydrogen bonds between molecules of water and suitable couples of N-H groups on the host moiety M). The HzO species form also continuous H-bonded layers of solvation around the cyclophosphazene derivatives, thus stabilizing the crystal lattice. 

A computational study was concerned with the effect of solvation on the radical ion involved in CDP photolyase enzyme-catalysed reversion of thymine and uracil cyclobutane dimers stimulated by visible light <06T6490>. 

Tanaka et al75 proposed the mechanism described in Scheme 27 to explain the catalysis, which incorporates the effects of solvation. The system was characterized in the visible region. 

In the gas phase, ions may be isolated, and properties such as stability, metal-ligand bond energy, or reactivity determined, full structural characterization is not yet possible. There are no complications due to solvent or crystal packing forces and so the intrinsic properties of the ions may be investigated. The effects of solvation may be probed by studying ions such as [M(solvent) ]+. The spectroscopic investigation of ions has been limited to the photoelectron spectroscopy of anions but other methods such as infrared (IR) photodissociation spectroscopy are now available. 

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