Clusters dissociation

The crystallinity levels in ethylene ionomers are generally low due to their high levels of branching and the clustering of the metal salts. At high temperatures, the clusters dissociate and the individual chains can move independently in the molten state, permitting them to be molded. When the ionomer is cooled dusters reform, crosslinking the chains... 

It is proposed that the rate dependence on concentration and pressure involves cluster dissociation and that the monomeric species, Ru(CO), is responsible for the high activity of this system. Dissociation is well known for ruthenium carbonyl clusters (25,27-31). Piacenti and co-workers (31) have demonstrated that at temperatures above 80° and CO pressures greater than 150 psi, monomeric ruthenium carbonyl is observed in significant quantities due to the equilibrium,... 

Thus, increases in CO pressure favor cluster dissociation and the formation of larger quantities of 2, High dilution should also favor the formation of 2, resulting in greater Ru(C0)3 to cluster ratios and greater WGSR activity. 

CLUSTER DISSOCIATION FAVORED BY DILUTION OR INCREASES IN CO PRESSURE... 

These effects can all be enhanced if the point defects interact to form defect clusters or similar structures, as in Fej xO above or U02, (Section 4.4). Such clusters can suppress phase changes at low temperatures. Under circumstances in which the clusters dissociate, such as those found in solid oxide fuel cells, the volume change can be considerable, leading to failure of the component. 

We will discuss here ZTRID bond strength determinations for the two cluster-dissociation reactions. 

To complete the RRKM calculations for the cluster dissociation rates and final bare 4EA molecule product distributions, the cluster binding energy E0 and the energy v of the chromophore vibrational state to be populated must be found. These can be estimated from selected fits to the experimental rates and intensities (Hineman et al. 1993a). The results of the rate and product distribution calculations are presented in Table 5-4. The predictions of the model are quite good—less than 30% error for all observations for the 4EA(N2)1 and 4EA(CH4), clusters. 

I trust that this book gives the flavor of the pace, excitement, and accomplishments of the last few years of cluster research. For me, the most surprising and important feature of this volume is the breadth that this new area of physical chemistry demonstrates. The various experimental chapters cover ionic chemistry, hot atom chemistry, photochemistry, neutral molecule chemistry, electron and proton transfer chemistry, chemistry of radicals and other transient species, and vibrational dynamics and cluster dissociation. Of at least equal importance is that theoretical potential energy surface studies are not accessible for cluster systems and are being pursued. All of us associated with this project have tried to convey the fresh insights and contributions that van der Waals cluster research has brought to physical chemistry. 

We have emphasized the importance of open d-orbitals and a proper atomic state if should dissociate with a low barrier on a transition metal surface. For clusters, however, the same type of dissociation puts up another requirement, which will turn out to be even more significant in the present context There must be at least one open shell valence orbital (of s-character) on the cluster, otherwise the sd-hybridization will not take place (8). For an infinite surface, this requirement can always be satisfied since states with open valence orbitals must at least be reachable by a low energy excitation. For clusters, the same type of excitation may be much more expensive. Since all nickel clusters dissociate it seems clear that a dissociative state is reachable in all cases for nickel. The question that has worried us for the past years is why the same type of states do not always seem reachable for iron and cobalt clusters. The answer to this question is discussed in section VI. 

Figure 31.40. The IRE-BP Is an Aconitase. (A) Aconitase contains a relatively unstable 4Fe-4S cluster at its center. (B) Under conditions of low iron, the 4Fe-4S cluster dissociates and appropriate RNA molecules can bind in its place.

Benzene...At2 is stable even at 64 K higher clusters dissociate at this temperature. Due to the limited set of successful MD runs we cannot say anything more than that clusters with 3-8 argons dissociate at about 40 K. 

DYNAMICS AND ENERGY RELEASE IN BENZENE/AR-CLUSTER DISSOCIATION... 

Bemshtein, V. and Oref, I. (2000) Dynamics and energy release in benzene/Ar cluster-dissociation, J. Chem. Phys.. 112. 686-697. 

Another limitation of TDSCF of a very different type is related to the time domain. The difference between the exact Hamiltonian and the TDSCF one can be viewed as a perturbation that is neglected in the mean-field approximation, and the integrated effect of which is bound on the whole to grow in time. The fact that TDSCF can be in serious error when pursued for very long time scales has been noted for several cluster dissociation processes.47,49,50 However, the long-tail time behavior was not of physical importance in the examples studied. Obviously this limitation of TDSCF is pertinent only to very slow processes (when in competition with much stronger channels). 

When clusters comprise several loosely bound molecules, the atoms within each molecule are held together by strong bonds while the molecules themselves are attracted to neighboring molecules by weak bonds. This discrepancy in forces translates into disparities in the respective vibrational frequencies. In the case of the HF dimer the two H—F vibrational frequencies are ca. 4000 cm while the four van der Waals frequencies are 480, 400, 210, and 150 cm (Quack and Suhm, 1991). The consequence of these different frequencies and forces is that energy cannot flow as freely among constituent units of the cluster as it can within each molecule. That is, there are bottlenecks in the phase space which means that IVR is inhibited. Thus, cluster dissociations, especially when energized by infrared radiation, provide some of the rare example of nonstatistical decay. 

It is useful to categorize the various classes of dimers and clusters before discussing the theoretical approaches and models for understanding the cluster dissociation rates and energy partitioning. Clusters fall into three broad classes ... 

Another interesting system is C1H- -Ar. Although it has not been studied experimentally, a close coupling calculation has shown that this cluster dissociates with a rate of 4 X 10 sec (Hutson, 1984). The HCl bond absorbs infrared radiation at 2886 cm f Since the binding energy is 116 cm, we have a total of 2770 cm" of energy to dispose. Suppose first that the decay converts all of this energy into translations. If... 

It is interesting to compare the Ewing and Miller-Leroy methods for correlating the cluster dissociation data. One is based on the energy gap, the other on the absorption frequency shift. The energy gap law is expected to correlate the lifetime data if the energy gap is the primary factor that determines the lifetime. It only treats excitations to oscillators that are connected to the van der Waals bond. As a result, it cannot account for the different lifetimes of the (HF)2 dimer when the different HF stretches are excited. Thus, numerous exceptions to the energy gap law are expected. On the other hand, the correlation as well as the exceptions provide considerable physical... 

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