Labile cluster mechanism

FIGURE 3.10 Hydrate labile cluster growth mechanism imposed on a pressure-temperature trace. (Reproduced from Christiansen, R.L., Sloan, E.D., in Proc. First International Conference on Natural Gas Hydrates (1994) New York Academy of Sciences. With permission.)... 

Labile clusters join and grow on the vapor side of the surface until a critical size is achieved. This can occur either by the addition of water and gas molecules to existing cavities, via the joining of cavities along the interface (as indicated in the cluster aggregation mechanism) or both. 

Molecular simulation methods have been applied to investigate the nucleation mechanism of gas hydrates in the bulk water phase (Baez and Clancy, 1994), and more recently at the water-hydrocarbon interface (Radhakrishnan and Trout, 2002 Moon et al., 2003). The recent simulations performed at the water-hydrocarbon interface provide support for a local structuring nucleation hypothesis, rather than the previously described labile cluster model. 

In order to verify which of the above nucleation mechanisms accurately represents hydrate nucleation, it is clear that experimental validation is required. This can then lead to such qualitative models being quantified. However, to date, there is very limited experimental verification of the above hypotheses (labile cluster or local structuring model, or some combination of both models), due to both their stochastic and microscopic nature, and the timescale resolution of most experimental techniques. Without experimental validation, these hypotheses should be considered as only conceptual aids. While the resolution of a nucleation theory is uncertain, the next step of hydrate growth has proved more tenable for experimental evidence, as discussed in Section 3.2. 

We find that the nucleation proceeds via "the local structuring mechanism/ " i.e., a thermal fluctuation causing the local ordering of CO2 molecules leads to the nucleation of the clathrate, and not by the labile cluster hypothesis, one current conceptual picture. The local ordering of the guest molecules induces ordering of the host molecules at the nearest- and next-to-nearest-neighbor shells, which are captured by a three-body host-host order parameter, f these thermodynamic fluctuations lead to the formation of the critical nucleus. 

The mechanism of addition of thiols to the labilized cluster [Os3(CO)n(MeCN)] to form [Os3(CO)io( -H)(//-SR)] (9) clusters has been elucidated. The kinetics of reactions of the hindered thiols ortho-, meta- and para-thiocresol, thiophenol, benzyl mercaptan, 2-naphthalenethiol, cyclopentanethiol, and cyclohexanethiol with [Os3(CO)ii(MeCN)j have been studied by UV-visible spectrophotometry and... 

Questions concerning the "ferrous-wheel" mechanism came soon after the Villafranca and Mildvan studies with the discovery in Gawron s laboratory that aconitase also contains labile sulfur (23), which, with iron, is indicative of the presence of Fe-S clusters. 

Organometallic osmium-carbonyl clusters such as [Os3(CO)10(NCCH3)2] are apoptosis-inducing agents with anticancer therapeutic potential. The mechanism appears to involve the loss of the labile acetonitrile ligand [85]. 

Iron regulatory proteins (IRPs) regulate the cellular iron level in mammalian cells. IRPs are known as cytosol mRNA binding proteins which control the stability or the translation rate of mRNAs of iron metabolism-related proteins such as TfR, ferritin, and 5-aminolevulinic acid synthetase in response to the availability of cellular iron [19-21] after uptake [5]. The regulatory mechanism involves the interaction between the iron-responsive element (IRE) in the 3 or 5 untranslated regions of the transcripts and cytosolic IRPs (IRP-1 and -2). IRP-1 is an iron-sulfur (Fe-S) protein with aconitase activity containing a cubane 4Fe-4S cluster. When Fe is replete, IRP-1 prevails in a 4Fe-4S form as a holo-form and is an active cytoplasmic aconitase. As shown in Fig. 3, when Fe is deplete, it readily loses one Fe from the fourth labile Fe in the Fe-S cluster to become a 3Fe-4S cluster and in this state has little enzymatic activity [22, 23]. 

More coimnonly, it is to be expected that the reactivity of a mixed-metal cluster will be comparable to that of the most labile metal. For example, the rates of CO dissociation from (/u,-H)Ru3 Os (/u.-COMe)(CO)io fall in the series as Rus (4500) > RU2OS (1100) > RuOs2(220) OS3 (1). If this is the case, the relative rates for a series can provide information concerning the number of metal atoms which participate in such a transformation (see also Mechanisms of Reaction of Organometallic Complexes and Ligand Substitution). 

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