Multiple exchange

The experiment starts at equilibrium. In the high-temperature approximation, the equilibrium density operator is proportional to the sum of the operators, which will be called F. If there are multiple exchanging sites with unequal populations, p-, the sum is a weighted one, as in equation (B2.4.31). 

We have done our experiments with hectorite, which is a 2 1 smectite that develops negative layer charge by substitution of Li for Mg in the octahedral sheet.Samples were prepared by multiple exchange in 1.0 and 0.1 M CsCl solutions until essentially complete Cs-exchange was reached (97% of the interlayer cations). Temperature dependent data are essential to interpret the results, because there is rapid exchange of Cs among different interlayer sites at room temperature (RT). 

The Use of deuterium has revealed interesting mechanistic details. Over Rh/C, Pt/C, and Pd/C, the butanone resulting from the deuteriumation of methyl vinyl ketone contains substantially more deuterium at position 4 than at 3 (Table 2.7). This can generally be accounted for by the classic mechanism assuming the a-carbon (position 3) remains preferentially attached to the surface while the P-carbon (position 4) undergoes multiple exchange by repeated second-point attachment (Fig. 2.27). However, over Pd/C, even less deuterium... 

FIGURE 2.27 Two possible mechanisms to explain deuterium distributions resulting from deuteriumation and exchange of methyl vinyl ketone. Deuterium is assumed to move subsurface and to attack adsorbed carbons from threefold hollows. Path A multiple exchange by repeated addition-rotation-abstraction at the [3-carbon (classic mechanism) path B I -A hydrogen shift followed by desorption as enol. 

FIGURE 9.4. Distribution patterns of deuteron cyclopentanes Multiple exchange after H/D exchange over conventional R catalysts with Ptn particles. Abscissa Value of x inC5H(io-x)D c fraction ordinate rel. abundance. 

The last step is analogous to the reaction which accounts, in part, for multiple exchange in methane (59), e.g.. 

This mechanism employed the postulated multiple bonds. It might be that with some metals and at higher temperatures the dehydrogenation is deeper and the multiplicity of bonds is even higher (e.g., that HC=M is also formed). In spite of this uncertainty, the multiple exchange of CH4 became a very good diagnostic tool for the multiple metal-carbon bonds. 

The ease of dissociative adsorption of multiple exchange and of fl-H elimination suggest that the transition... 

F. A Theory for Calculating the Initial Distributions of Products in Multiple Exchange Reactions. 238... 

Multiple Exchange Reactions, An exchange reaction in which more than one deuterium atom is introduced into the hydrocarbon molecule on each interaction of the molecule with the catalyst is classified as a multiple exchange reaction. The existence of such a process may be recognized in three ways, exactly analogous to those described above for simple exchange reactions ... 

In the study of multiple exchange reactions, it is desirable to work with a high ratio of deuterium to hydrocarbon. This minimizes the influence of isotopic dilution of the deuterium on the rate of production of the more highly deuterated species during the early part of the reaction. The use of deuterium containing as small a percentage of hydrogen as possible is also important if true initial distributions of products are to be obtained. 

Multiple Exchange Reactions. The possible mechanisms which may lead to multiple exchange are necessarily more complicated than those responsible for simple exchange. Four alternatives may be considered. 

The determination of whether the multiple exchange occurs by an a-a, a-/3, or a-y process is obviously an important step in the elucidation of the mechanism. Careful examination of the initial distributions of products can be most useful both in this connection and in providing evidence about the types of adsorbed species responsible for the exchange reaction. This examination may be carried out in two ways, either (a) the comparison... 

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