Hydration shell geometry

Fig. 7. Hydration shell geometry of the aqueous chloride ion, as obtained from neutron scattering data.

Some recent papers permit an exciting outlook on the degree of sophistication of experimental techniques and on the kind of data which may be available soon. In the field of NMR spectroscopy, a publication by Hertz and Raedle 172> deals with the hydration shell of the fluoride ion. From nuclear magnetic relaxation rates of 19F in 1M aqueous solutions of KF at room temperature, the authors were able to show that the orientation of the water molecules in the vicinity of fluoride ions is such that the two protons are non-equivalent. A geometry is proposed for the water coordination in the inner solvent shell of F corresponding to an almost linear H-bond and to an OF distance of approximately 2.76 A, at least under the conditions chosen. 

The rates measured reflect directly the physical-chemical properties of the water molecule in the nearby hydration shell. These measurements can gauge the properties of the water in an active site where biochemical catalysis takes place. Insertion of proton emitter in a specific site can be useful to determine the internal geometry of a site in a soluble, functioning protein. The subnanosecond measurement is equivalent to a strobe light that freezes in time a transient conformation of a protein. 

Hydration shells from simulations of Sm " " are actually a mixture of equal amounts of eight- and nine-fold coordinated ions adopting the geometry of a square antisprim or a tricapped trigonal prism, respectively [67]. Figure 4.8 (bottom) gives an example of the trajectories of the water molecules in the first and... 

Ayocmplxs Glendening and Feller (1996) have calculated geometries of Mg, Ca, Sr, Ba and Ra aquo complexes with one to six water molecules at the Hartree-Fock/MP2 level. Alkali earth cations in aqueous solutions, however, have larger hydration shells Seward et al. (1999) measured EXAFS of Sr solutions up to 300 C. The hydration sphere for Sr appears to decrease from 8 to 6 water molecules with... 

Figure 13. Calculated infrared spectra for the HDO molecules (a) in the first hydration shell of Li (b) for the different types of water-ion geometries found in the second hydration shell of Li, and (c) in the first hydration shell of the formate anion. Figure (a) was reprinted with permission from Ref. 76. Copyright Elsevier. Figure (b and c) were reprinted with permission from Ref. 77. Copyright American Chemical Society.

The results for a 1.5 molal solution of NiCl in D O are shown in Fig. 6. The first peak corresponds to the Ni-0 distance and the second peak to the Ni-D distance the ratio of the areas under these peaks is of course 1 2, and integration of the peaks yields a hydration number of 5.8 0.2 which is invariant of concentration. This fact, taken together with the sharpness of the p aks reflects the stability of the first hydration shell of the Ni ion. From the position of the two peaks and a knowledge of the geometry of the D O molecule it becomes apparent that the D O molecules are tilted at an angle to the Ni-0 axis this angle increases with increasing concen-... 

Table 12.7 lists the structural properties of the hydration shell of UOjfaq) and UO (aq). For the purposes of comparison, past experimental and theoretical data for the first shell of AnO (An = U, Np, Pu) are also reported in Table 12.7. The hydration shell structure of UO (aq) has been described in detail elsewhere [150] so we will focus on the shell structure of UOjfaq). As shown in Figure 12.8, the AIMD simulations indicate that the first shell of UOjfaq) has five water molecules in the equatorial plane, in contrast to the QM/MM prediction of 4.51. The predicted U(V)=0< distance is very close to previous measurements of other actinyl(V) ions (Np(V) and Pu( V)) and are greater than the previous predicted value by 0.07A. Also, our average first-shell U-0 bond distance is slightly longer than the previous simulated value, which is expected since the first shell of the AIMD simulations contains more water ligands. Previous gas-phase structures exhibit slightly longer bonds as expected. Relative to UO (aq), UO Caq) shows a lengthening of 0.08A and 0.1 A for the U=Oai and bonds, respectively, because of reduced electrostatic attraction. Other first-shell properties of UOjfaq) and UO faq), such as the intramolecular water geometry and tilt angles, compare closely.

The dioxo form for both molecules is more stable by at least about 10 kcal mol than any of the corresponding 12 other species. Optimized geometries for monohydrates and dehydrates, as well as their relative energies help explore the tau-tomerization pathways if they emerge in aqueous solution. Schneider and Berman [67] determined, for example, ordered hydration sites for the nucleotide bases in B-type conformations using crystal structure data on 14 B-DNA decamers. The number of the water molecules, W, within 3.4 A of the atoms of the nucleotide bases were found as 101W/42G, 84W/43C, 92W/43A, and 95W/45T (G, C, A, T the standard code for nucleobases). The two to three water molecules that were identified per base on average in their first hydration shell confirm the importance of such studies. 

For the Tar—Tar kissing loops, the P—B calculations are unable to discern their propensity to accumulate counterions accumulation at the loop—loop interface (data not shown). This is because the fully hydrated ions as defined by the Stem layer cannot penetrate into the central cation binding pocket (data not shown). Similarly, the axial spine of counterion density observed in the A-RNA helix (Fig. 20.5) is not captured by the P—B calculation (Fig. 20.7). No noticeable sequence specificity is observed in the counterion accumulation patterns in the P—B calculations, even though the sequence effects are explicitly represented in the P—B calculation through the appropriate geometry and assignment of point-charges. This is because the sequence specificity observed in the molecular dynamics simulations usually involves first shell interactions of base moieties with partially dehydrated ions, which cannot be accurately represented in the P—B framework. 

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