Ernest Becker (September 27, 1924 – March 6, 1974) was a Jewish-American cultural anthropologist and writer. He is noted for his 1974 Pulitzer Prize-winning book, The Denial of Death. Dr Bernard Bekker. Stellenbosch University. Prof Alan Brent. Stellenbosch University. Clohessy,Warren Brettenny and E. Ernest van Dyk TEA BREAK (15h10 - 15h40).

Prezentaciyu na temu pajka. CF 3–Ph reductive elimination from [(Xantphos)Pd(Ph)(CF 3)] ( 1) and [( i-Pr-Xantphos)Pd(Ph)(CF 3)] ( 2) has been studied by experimental and computational methods. Complex 1 is cis in the solid state and predominantly cis in solution, undergoing degenerate cis–cis isomerization (Δ G ≠ exp = 13.4 kcal mol –1; Δ G ≠ calc = 12.8 kcal mol –1 in toluene) and slower cis–trans isomerization ( Δ G calc = +0.9 kcal mol –1; Δ G ≠ calc = 21.9 kcal mol –1). In contrast, 2 is only trans in both solution and the solid state with trans-2 computed to be 10.2 kcal mol –1 lower in energy than cis-2. Kinetic and computational studies of the previously communicated ( J. 2006, 128, 12644), remarkably facile CF 3–Ph reductive elimination from 1 suggest that the process does not require P–Pd bond dissociation but rather occurs directly from cis-1. The experimentally determined activation parameters (Δ H ≠ = 25.9 ± 2.6 kcal mol –1; Δ S ≠ = 6.4 ± 7.8 e.u.) are in excellent agreement with the computed data (Δ H ≠ calc = 24.8 kcal mol –1; Δ G ≠ calc = 25.0 kcal mol –1). Δ G ≠ calc for CF 3–Ph reductive elimination from cis-2 is only 24.0 kcal mol –1; however, the overall barrier relative to trans-2 is much higher (Δ G ≠ calc = 34.2 kcal mol –1) due to the need to include the energetic cost of trans–cis isomerization.

This is consistent with the higher thermal stability of 2 that decomposes to PhCF 3 only at 100 °C and even then only in a sluggish and less selective manner. The presence of excess Xantphos has a minor decelerating effect on the decomposition of 1. A steady slight decrease in k obs in the presence of 1 and 2 equiv of Xantphos then plateaus at [Xantphos]: 1 = 5, 10, and 20. Specific molecular interactions between 1 and Xantphos are not involved in this kinetic effect (NMR, T 1 measurements). A deduced kinetic scheme accounting for the influence of extra Xantphos involves the formation of cis-[(η 1-Xantphos) 2Pd(Ph)(CF 3)] that, by computation, is predicted to access reductive elimination of CF 3–Ph with Δ G ≠ calc = 22.8 kcal mol –1. Contoh program dengan visual foxpro 9 download.