Reduction CCSD study

Reductive elimination and oxidative addition are ubiquitous reaction steps in many TM-catalyzed processes. A recent study by Beste and Frenking (82) may serve as example for the general finding that relative energies of TM complexes with different coordination numbers may be subject to systematic errors at the DFT level of theory. Table 16 shows calculated energies at the CCSD(T)/n level and at B3LYP using three different basis sets, II-IV, for platinum complexes... 

The theoretical study of [RhCl(PH3)3]-catalyzed hydrosilylation of ethylene by the DFT, MP4 (SDQ), and CCSD(T) methods shows that the rate-determining step in the Chalk-Harrod mechanism is the Si—C reductive elimination (43). 

For small molecules it is possible to perform accurate computations with post-Hartre—Fock approaches, and in this respect harmonic frequencies computed at the CCSD(T) (coupled clusters with single, double, and perturbative inclusion of triple excitation [27]) level, with basis sets of at least triple- quality reach an overall accuracy of 15—20cm for closed-shell systems [e.g., 28, 29]. For radicals, the situation is not so well assessed, but some recent investigations confirm that analogous accuracy can be reached [e.g., 30-34]. However, the unfavorable scaling of the CCSD(T) model with the number of active electrons limits its applicability to very small systems only. In addition, a simple reduction of the computational cost by combining correlated QM methods with small basis sets is not to be recommended due to the quite unpredictable accuracy of the results. Thus, the extension of computational studies to large systems requires cheaper and at the same time reliable electronic structure models. 

The calculations presented in this section have shown that, whereas improvements in the correlation treatment in general increase the bond length, improvements in the one-electron treatment reduce it. The coupled-cluster hierarchy converges smoothly, with a reduction in the error by a factor of 3-5 at each level. Thus, whereas the intrinsic error (based on A and Aabs in Figure 15.3) at the Hartree-Fock level is about —3 pm, it is reduced to about —0.8 pm at the CCSD level and to about —0.2 pm at the CCSD(T) level. In contrast, the Mpller-Plesset hierarchy oscillates, with typical errors of about 0.5 pm at the MP2 level, 1.2 pm at the MP3 level and 0.4 pm at the MP4 level. Even for the small molecules in this study, the CISD model is inadequate, with an intrinsic error of —1.7 pm. Based on these observations, we may state that, in terms of computational accuracy and cost, the only successful models are Hartree-Fock, MP2, CCSD and CCSD(T). 

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