Hydrated clusters

Pulse radiolysis studies of silver solutions with a low radiation dose per pulse have shown that the Ag2 species, instead of dimerizing into Ag4 + (Eq. 16), reacts with another cation to yield Ag3 + (Eq. 17).  

The transient Ag3 + ion have an intense absorption spectrum with two maxima, at 310 and at 265 nm. Its second-order decay leads to the cluster Ag4 +. Under total reduction conditions the neutral dimer Ag2 is observed at 275 and 310 nm. The optical transitions of low-nuclearity silver oligomers, the rate constants, and the extinction coefficients are derived from adjustment between experimental (Fig. 2, bottom) and calculated absorption spectrum evolution. An even number of atoms favors the high stability of the magic hydrated clusters Ag4 + (275 nm), Agg + and possibly Agi4 + (Fig. 2, top). After a longer time, the plasmon 

Because the total concentration of atoms is constant during the growth, the observed change with time of the total absorbance, l,E xnx x , results from the variation of the extinction coefficient per atom as a function of n and of the above time evolution of the size distribution. It has been shown for silver that the shape of the absorption band and the extinction coefficient value do not change further beyond n 13.  

The growth of thallium clusters has been also observed by time-resolved optical spectroscopy. The coalescence steps are comparable with those of silver and the final plasmon band of Tl is located at 300 nm.  

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When water undergoes self-ionization, a range of cationic species are formed, the simplest of which is the hydronium ion, HjO (Clever, 1963). This ion has been detected experimentally by a range of techniques including mass spectrometry (Cunningham, Payzant Kebarle, 1972), as have ions of the type H+ (HaO) with values of n up to 8. Monte-Carlo calculations show that HjO ions exist in hydrated clusters surrounded by three or four water molecules in the hydration shell (Kochanski, 1985). These ions have only a short lifetime, since the proton is highly mobile and may be readily transferred from one water molecule to another. The time taken for such a transfer is typically of the order of 10 s provided that the receiving molecule of water is correctly oriented. 

The reaction enthalpy switches from being exothermic to being endothermic between n = 3 and n - 4. In hydrated clusters, only reactions leading to partial replacement of the water molecules maintain thermodynamic exoergicity ... 

Mauritz et al., motivated by these experimental results, developed a statistical mechanical, water content and cation-dependent model for the counterion dissociation equilibrium as pictured in Figure 12. This model was then utilized in a molecular based theory of thermodynamic water activity, aw, for the hydrated clusters, which were treated as microsolutions. determines osmotic pressure, which, in turn, controls membrane swelling subject to the counteractive forces posed by the deformed polymer chains. The reader is directed to the original paper for the concepts and theoretical ingredients. 

Hsu, Barkley, and Meakin addressed the percolation aspect of hydrated clusters in relation to insula-tor-to-conductor transitions. As the concentration of clusters on a hypothetical grid, that is, a three-dimensional lattice, increases, islands of clusters will grow in size and become interconnected. Eventu-... 

Thus, the work, W(J), to form a hydrate cluster of n building units can be determined using the classical theory of nucleation. 

The profiles of the unleached glass surface showed essentially no change in intensity of the sodium and potassium ions as a function of depth. However, Si and B ion intensities were found to be consistently lower at the outer surface than within the bulk of the glass. Depth profiles for both ions have a similar shape. This effect is believed to result from surface hydration which alters the yield of ions from the borosilicate network. Unhydrated glass surfaces, introduced into the spectrometer immediately fter fracturing, showed little or no depression of either Si or B signals, and hydrate clusters, such as Si(0H)+ were much reduced in intensity. 

We present experimental results on photophysical deactivation pathways of uracil and thymine bases in the gas phase and in solvent/solute complexes. After photoexcitation to the S2 state, a bare molecule is tunneled into and trapped in a dark state with a lifetime of tens to hundreds of nanoseconds. The nature of this dark state is most likely a low lying nn state. Solvent molecules affect the decay pathways by increasing IC from the S2 to the dark state and then further to the ground state, or directly from S2 to S0. The lifetimes of the S2 state and the dark state are both decreased with the addition of only one or two water molecules. When more than four water molecules are attached, the photophysics of these hydrated clusters rapidly approaches that in the condensed phase. This model is now confirmed from other gas phase and liquid phase experiments, as well as from theoretical calculations. This result offers a new interpretation on the origin of the photostability of nucleic acid bases. Although we believe photochemical stability is a major natural selective force, the reason that the nucleic acid bases have been chosen is not because of their intrinsic stability. Rather, it is the stability of the overall system, with a significant contribution from the environment, that has allowed the carriers of the genetic code to survive, accumulate, and eventually evolve into life s complicated form. 

Our work on hydrated clusters manifests the value of gas phase experiments. Condensed phase studies reveal the properties of the bulk system. However, it is difficult to distinguish intrinsic vs. collective properties of a system. Gas phase studies, on the other hand, directly provide information on bare molecules. Moreover, the investigation of size selected water complexes can mimic the transition from an isolated molecule to the bulk. The comparison of gas phase experimental results with theoretical calculations can also provide a direct test of theoretical models. This test is in urgent need if theoretical modeling is to evolve into calculations of solvated systems with credibility. 

The microsolvation computations °° ° are excellent models of, for example, small hydrated clusters that can be observed in the gas phase. ° ° Still under intense computational study is how informative microsolvation computations are for understanding true solution phase chemistry. 

The synthesis of ordered mesoporous materials in the past decade greatly expanded the range of ordered porous materials and opened up many new opportunities in the design and applications of materials. Of particular importance is the synthetic methodology that is used for the preparation of these materials. Prior to this, the synthesis of porous materials generally involved the use of individual molecules or hydrated clusters of simple ions. In the synthesis of ordered mesoporous materials, however, it is the assembly of surfactant molecules that directs the condensation of inorganic precursors. 

The protein-solvent interface was studied in an explicit solvent environment of 3182 water molecules by MD simulations performed on metmyoglobin [31].Both the structure and dynamics of the hydrated surface of myoglobin are similar to that obtained by experimental methods calculating three-dimensional density distributions, temperature factors and occupancy weights of the solvent molecules. On the basis of trajectories they identified multiple solvation layers around the protein surface including more than 500 hydration sites. Properties of theoretically calculated hydration clusters were compared to that obtained from neutron and X-ray data. This study indicates that the simulation unified the hydration picture provided by X-ray and neutron diffraction experiments. 

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