Exchange barriers

Comparison of Exchange Barriers. Among the most interesting results we have obtained are the intrinsic barriers for halide and alkoxide exchanges, shown in Table II. Note that... 

The activation parameters reported for the exchange process in 19 and 20 are definitively lower than the expected values for saturated systems (Table 2, compare to Me2Zn exchange barriers shown in Section II.C.l). Values in Table 2 are referred to the experimental rate of exchange kexp at the given concentration, i.e. which is neither concentration nor statistically corrected according to the molecularity of the reaction. The actual rate... 

TABLE 5. Comparison of ligand-exchange barriers with percent (TBP — SP) deviation in the crystal for spirosilicates 5135 ... 

TABLE 17. Bond lengths, 29Si chemical shifts and their temperature gradients [K = (<529SiTj — 29SiT2)/(Ti — T2)], and ligand exchange barriers in 120133... 

TABLE 20. 29Si NMR data and ligand exchange barriers (determined by 19F NMR) for hexacoordi-nated anionic silicates... 

FIGURE 26. Activation free energies for two exchange barriers in 189a- 189d as a function of the 29Si chemical shift211... 

In the xH-NMR-spectra of the dimethyl derivative 97b and the trimethyl derivative 97c, significant methyl signal broadening is only observable at —184 °C. From this a ligand exchange barrier between two identical trigonal bipyramids of about 5 (21) kcal (kJ)/mol was derived m). 

The rate constant for the exchange of lithiums within the trimer (Li-10)2/w-BuLi is reported to be 0.8 s-1 at — 33 °C, corresponding to an exchange barrier AG 24o of 14.7 kcalmol-1. The rate of lithium-lithium exchange is suggested to be faster within mixed complexes of the chiral lithium amides with n-BuLi than within the homoaggregates49. 

Fig. 4. Linear correlation between V-methyl exchange barriers and coordinative 29Si chemical shifts for 4a-4g.14 Reproduced with permission from the American Chemical Society.

The second process was too slow to permit measurement by the coalescence method, and the exchange barrier was determined using the SIR technique.72 The first order rate constant obtained by this method at 355 K was =1.02 s-1, corresponding to a free energy barrier AG = 20.9 0.1 kcal mol-1. This barrier was assigned to the inversion of configuration at silicon through a (0,0)-shift.73... 

Absorption barrier Any of the exchange barriers of the body that allow differential diffusion of various substances across a boundary. Examples of absorption barriers are the skin, lung tissue and gastrointestinal tract wall (USEPA, 1992a,b). 

Summary Introduction of a chiral ligand to a series of pentacoordinate silicon complexes led to the assignment of two intramolecular rate processes Si-N cleavage and pseudorotation. Linear correlations between and Si chemical shifts of the complexes, as well as between the latter and the N-methyl exchange barriers were attributed to variation in strength of N—>Si coordination. 

Table 2. Exchange barriers of the allyl anions 23-26 in THF-D8 23) a Ref. 24 (AH value)...

The evidence for Berry pseudorotation and turnstile rotation has been reviewed. Further evidence for the turnstile rotation for spiro-penta-oxyphosphoranes has been presented, based on the lowering of the exchange barrier as the distortion of the t.b.p. of the phosphorane increases. A... 

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