Exchange reactions General Subject

McKee (21, 195) and McKee and Norton (219, 249, 250) have reported compensation effects in the exchange reactions of methane on several pairs of binary noble metal alloy catalysts. For each combination of elements kinetic measurements were made at a number of different compositions. Although the compensation behavior was generally very similar, there were perceptible differences in the values of B and e calculated for the various alloy combinations. The parameters found, by use of the formulas given in Appendix II, are summarized in Table IV, A-E, and are subject to the following comments. In consideration of data for the Pd-Rh alloys, the point for... 

At the present time the preparation of the trifluoromethylated derivatives of low valent transition metals by ligand-exchange reactions appears to be quite general. However, as exemplified by the nickel reaction above, the utility of the method is obviously subject to the inherent stability of the desired product. In many cases, such as the preparation of the trifluoromethyl derivatives of the cyclopentadienyl cobalt system, (CF3)Co(Cp)(CO)I and (CF3)2Co(Cp)(CO), the reaction of the dihalide with (CF3)2Cd glyme represents the simplest reaction... 

Two types of gas phase reactions are considered for the formation of diatomic molecules i) radiative association, ii) chemical exchange reactions. The rate constants6 are either taken from laboratory experiments or estimated by comparison with similar reactions and if possible they are corrected for low temperature conditions. Since rate constants are often not well known and subject to major revision as new experimental data become available they are a source of considerable uncertainty. For radiative association the reaction rate is equal to the product of the rate of collisions and the probability that a photon is emitted during the collision process (Bates, 1951). The general reaction scheme is of the form... 

The precursor of photosynthetic electron transfer is the lowest singlet excited state of the pigment complex P. It is generally accepted that upon population of this state, P disappears and the radical pair P Hl is formed in about 3 ps at room temperature (Martin et al.,1986, Woodbury et al.,1985, Holzapfel et al.,1990, Kirmaier and Holten, 1988) and somewhat faster, down to 700 fs, at low temperature (Fleming et al., 1988). However the precise nature of this reaction is subject to debate, especially the role of Bl, which is located between P and Hl (for a review see Martin and Vos, 1992). It has been proposed that P B is a virtual intermediate in a super-exchange mechanism (Michel-Beyerle et al., 1988, Warshel et al., 1988). Based on the observation of transients faster than P decay, Zinth and coworkers proposed that P l is a real intermediate, which decays with a faster rate than it is formed (Holzapfel et al., 1989, 1990). 

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