Liquid-like clusters model

In addition to SMx and the cluster-continuum model, other continuum models have also been used to study reactions in liquids, including the polarized continuum model [133-135] (PCM), the conductor-like screening model (COSMO [136] and COSMO-RS [137,138]), the generalized COSMO [139] (GCOSMO) model, conductorlike PCM [140] (CPCM), and isodensity PCM [141] (IPCM). 

The first indications that certain systems might violate the phase rule came from computer simulations of small clusters of atoms. A number of studies revealed clearly defined solid-like and liquid-like forms [5-14]. These embraced both molecular dynamics and Monte Carlo simulations, and explored a variety of clusters. These included several based on atomic models with interparticle Lennard-Jones forces, which mimic rare gas clusters rather well. There were also models of alkali halide clusters. Hence, the existence of solid and liquid forms for such small systems seemed not only plausible but general, not restricted to any one kind of system. Shortly after these studies appeared, another, of a 55-atom cluster with Lennard-Jones interparticle forces, showed not only solid and liquid forms but also a form in which the surface of the cluster (with icosahedral structure) is liquid... 

Vibrational relaxation is a sensitive probe of local stmcture and dynamics [5]. Vibrational lifetimes and absorption spectra of the asymmetric CO stretching mode (-1990 cm ) of W(CO)6 in siq)ercritical CO2 are reported as functions of solvent density and temperature [6]. Close to the critical temperature, the observables are density-independent over a 2-fold range of density. A cluster model can explain the data if small fixed-size solute-solvent clusters are formed in the range of densities around the critical density. If the size, and therefore the properties, e.g., local density and spectrum of fluctuations, are density-independent then the observables also become density-independent. Such a stmcture may form if there is a liquid-like... 

The model is a simplified version of a model first developed by Lovett and Ben-Naim (1969). The idea is to define a sequence of n HBed particles as an n-cluster (mimicking the ice-like clusters of HBed molecules in liquid water). In doing so, we include all the HBing in the cluster as part of the internal partition function of the -cluster. The interaction potential between any pair of clusters (including the 1-cluster, i.e. the monomers, or non-HBed particle), is now the hard-rod HR) potential. (In the original model, this part was chosen to be a square-well potential. As with the primitive model discussed in Sec. 2.5.2, the... 

Recently, it was found that the surface layers of ice might be disordered by hydrocarbon contamination or HCl traces. " Molina first postulated the liquid-like ice surface of type II PSCs at stratospheric temperatures to explain HCl adsorption, which was supported experimentally by a zeroth-order rate dependence on HCl partial pressure. Further support comes from molecular dynamics simulations showing that the water molecules on the ice surface are substantially disordered even at 200 and the dynamics of water molecules in the uppermost bilayer of hexagonal ice is substantially faster than in the bulk leading to partial disorder. Therefore, small water clusters can be used to model the reactions occurring on type II PSCs. Both the nature of the type II PSCs and the detailed mechanisms of reactions within them have been the subjects of many studies. Whereas the reactions are reasonably well understood from a phenomenological point of view, there is still a vast amount of uncertainty in the microscopic view of the reaction mechanism. 

Figure 3 schematically illustrates the interference model where it is assumed that the ionomer peak arises from a preferred interparticle distance. Recently, Yarusso and Cooper have proposed an interpretation of the ionomer peak which is based on the liquid-like scattering from hard spheres originally described by Fournet. The Fournet model is quantitatively capable of fitting the X-ray peak from sulfonated polystyrene ionomers. In the case of zinc-neutralized material, about half of the ionic groups were found to aggregate into well ordered domains ( clusters ) with the remainder... 

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. 

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