Solvation structure

This chapter has given an overview of the structure and dynamics of lipid and water molecules in membrane systems, viewed with atomic resolution by molecular dynamics simulations of fully hydrated phospholipid bilayers. The calculations have permitted a detailed picture of the solvation of the lipid polar groups to be developed, and this picture has been used to elucidate the molecular origins of the dipole potential. The solvation structure has been discussed in terms of a somewhat arbitrary, but useful, definition of bound and bulk water molecules. 

An interesting phenomenon was observed when the CD of chiral molecules was measured in achiral solvents. The chiral solvent contributed as much as 10-20% to the CD intensity in some cases. Apparently, the chiral compound can induce a solvation structure that is chiral, even when the solvent molecules themselves are achiral. ... 

Warren GL, Patel S (2008) Comparison of the solvation structure of polarizable and nonpolarizable ions in bulk water and near the aqueous liquid-vapor interface. J Phys Chem C 112(19) 7455-7467... 

One should expect the activation entropy (AS ) to C=C rotation in Case 1 push-pull ethylenes to be negative, since the increase in polarity in the transition state should increase the order in the solvated structure. The effect should increase with increasing difference in polarity between ground and transition states, and also with increasing solvent polarity. These expectations have been completely borne out by experiments (78,140,143), as Table 22 shows. Contrary to what is generally found for conformational processes (144), AS values -20 e.u. are frequently found for C=C rotation in push-pull systems. 

This type of interaction is not restricted to one dimension, as indicated in the formulation. The bonds between AsCls and SeOa would be much weaker if the amphoteric properties of the molecules were less well developed. This is analogous to bulk effects in solutions and solvate structures involving amphoteric molecules. 

Some of the homoatomic polyanions formulated for solid state compounds are also stable in solution, and even new polyanions are accessible by crystallization from these approaches. The final chapter summarizes homoatomic polyanions of group 14 and group 15, which have been reported in solvate structures. 

Table 9 Reported neat solvate structures of polyanions of group 14 elements, which are existent in solid state as well as in solution...

Fig. 13 Homoatomic polyanions of group 14 elements, which are present in solvate structures (a) tetrahedral (E = Sn, Pb) (b) nine-atom cages Eg" (n = 2-4 E = Si-Pb) (c) trigonal bipyramidal Es (E = Si-Pb)...

Having a closer look at the reported solvate structures of homoatomic polyanions of group 14 elements, it becomes evident that only clusters and no rings or chains are stable in solid state as well as in solution (Fig. 13). Higher reduced polyanions with > 5, equivalent with a charge higher than —1 per atom, are not reported in solution so far. The nine-atom species is quite well established in solutions of ammonia and ethylenediamine. Their interesting reactivity is intensely studied and is discussed elsewhere [5, 125, 126]. The less-understood cluster... 

Table 10 Polyanions of group 15 in solvate structures, which are also known in solid state...

Polyanions of Group 14 and Group 15 Elements in Alkali and Alkaline Earth Metal Solid State Compounds and Solvate Structures... 

Another material that has a strong propensity to form solvate structures in crystallization experiments is trithiocyanuric acid (TTCA). Consequently, the pure crystalline phase of TTCA is difficult to obtain by crystallization from solution, but can be obtained instead by desolvation of these solvate phases. In this... 

Figure 11. The time-evolution of the solvation structure around the benzene-like solute site whose charge increases by e/2. The left panel depicts the pair correlation of this site with the N site of acetonitrile and the right panel the pair correlation with the C site of benzene. The equilibrium pair correlations for the ground (t = 0) arid excited (t = °°) solute states are also shown. The curves depicting the pair correlations for t > 0 are vertically offset from each other by 0.5.

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