For clustering reactions

COLLISIONAL AND STATISTICAL THEORIES FOR CLUSTER REACTION RATES... 

One of the motivations for studying Van der Waals complexes and clusters is that they are floppy systems with similarities to the transition states of chemical reactions. This can be taken one stage further by studying clusters that actually are precursors for chemical reactions, and can be broken up to make more than one set of products. A good example of this is H2-OH, which can in principle dissociate to fonn either H2 + OH or H2O + H. Indeed, dissociation to H2 O -t H is energetically favoured the reaction H2 + OH—> H2 O -t H is exothennic by about 5000... 

Silicon cluster reactions are an example of a newly emerging field of research which is very amenable to study with electronic structure methods. This exercise will examine the potential surface for silicon cation reacting with silane (SiH4). Such reactions are central to the growth of large silicon clusters, which occurs by sequential additions of -SiHj ... 

Metal-promoted alkyne-insertion reactions afford another good method (see structure 12 for cluster geometry and numbering)... 

With Sb even larger clusters can be obtained. For example reaction of Co(OAc)2-4H20 and... 

Illuminati and Marino reported an interesting example of the dependence of solvent effects on the position of the reacting center relative to the aza group. The rate constants for the reaction of 2- and 4-chloroquinoline with piperidine were compared in three different solvents, methanol, piperidine, and toluene. These data are reported in Table III. Three main points are apparent from these data (a) the different response of the two substrates to the action of the solvent, (b) the rates for 2-chloroquinoline in the three solvents tend to cluster around the highest reactivity level shown by 4-chloroquinoline in... 

The proton transfer in these clusters via the water bridge was found to be about three times as fast as a nonassisted transfer, underscoring the importance of the solvent for the reaction mechanism [98IJQ855]. In addition to the relative stabilities of the cytosine tautomers, the structures and properties of some cytosine derivatives have been investigated, mainly those of 5-hydroxycytosine 111 and 5,6-dihydroxycytosine 112 (Scheme 73) [99JST1, 99JST49]. 

Reactions between tantalum cluster ions Espenson and McCarley have estimated the rate coefficient for the reaction Ta6Clt +Ta6Cl 2 = 2 TagCl ... 

In this case water is effectively acting as a catalyst for the reaction by lowering the energy of activation. These catalytic water molecules are more likely to participate in the reaction under supercritical conditions because their high compressibility promotes the formation of solute-solvent clusters. 

A specific feature of reactions occurring in the autoclave is that the least soluble compounds are always precipitated from the homogeneous phase of the reaction. As a result, the equilibrium of the reaction is always shifted to the formation of these very insoluble compounds. Thus, it becomes clear that by varying the composition of the reaction mixture (mainly due to the introduction of new cations and anions) practically all types of the cluster forms being generated in the given system can be obtained in the solution. This is a clear advantage of the hydrothermal technique for cluster synthesis in the autoclave. 

In the present experiment, even when n = 59, the anionic clusters 0H (H20)B are still found to react very fast toward S02, approaching the gas collision limit of about 10 9 m3 s 1. Since S02 is a strong reductant in basic solution,125 no kinetic data are directly available for the reaction ... 

Figure 19. (a) Cluster size dependence of the rate constants for the reactions of CO2 with the large hydrated hydroxyl anions at T= 130 K O, experimental values for OH (H2O), —, calculated values for 0H (H20)n. (b) Dependence of rate constants on cluster size for the reactions of 0H (H20)n with SO2 at T = 135 K. Taken with permission from ref. 19. 

Recently, similar findings133 have been made in studies of the photoionization of NO containing water clusters134 which established that the cluster reaction analogous to the last step in reaction 36 occurs for n > 4. 

With respect to the thermodynamic stability of metal clusters, there is a plethora of results which support the spherical Jellium model for the alkalis as well as for other metals, like copper. This appears to be the case for cluster reactivity, at least for etching reactions, where electronic structure dominates reactivity and minor anomalies are attributable to geometric influence. These cases, however, illustrate a situation where significant addition or diminution of valence electron density occurs via loss or gain of metal atoms. A small molecule, like carbon monoxide,... 

---