Metallocenes molecular mechanics calculations

For decabenzylferrocene (Fig. 13) it was reasoned, based on qualitative model studies, that a 180° rotation about the (benzyl)CH2—C5 bond should not be possible (Section IV,B,6) (98). No stronger evidence from molecular mechanics calculations or dynamic NMR studies is, as yet, available. Ambient temperature NMR measurements of other bent-metallocene or half-sandwich pentabenzyl-Cp complexes (cf. Figs. 2, 10, and 11)... 

Although vibrational frequencies have been calculated and compared with experimental data since the early days of molecular mechanics refinements using full-matrix second-derivative procedures196,971, there are few reports on the application of this method in the field of transition metal chemistry. One of the few examples is a recent study on linear metallocenes [981. Here, the molecular mechanics force constants were obtained by adjusting initially assumed values by fitting the calculated vibrations to thoroughly analyzed experimental spectra. The average difference (rms) between experimental and calculated vibrations were of the order of ca. 30 cm-1.2 Table... 

This type of selectivity originates solely from steric interactions between the auxiliary ligands, polymer chain, and the incoming propene. It was first explained qualitatively by means of visualization of the structure of the catalyst precursors. A more quantitative approach led naturally to molecular mechanics models in order to explain and even predict the stereospecificity of catalysts with different ligand environments. Due to the limitations of MM models to describe metallocene complexes as well as bond breaking and bond formation processes (see Section 3.1.2.1), the models were initially based on some rigid core structures derived from the measured structures, e. g., of the dichloride precursors [25, 26]. In order to achieve more accurate results, core structures, calculated by ab initio methods, were employed later. A further step in this direction is the joint description of the... 

Corradini et al. examined in some detail by molecular mechanics15 and density functional studies100 the polymerization mechanism proposed by Cossee and the catalytic sites on TiC surfaces, including those proposed by Arl-man and Cossee13 and by Allegra.14 According to the calculations, for all these octahedral active sites a similar general mechanism of stereoselectivity occurs which is very similar to the one established several years later for stereospecific metallocenes (see previous section). The chirality of the site would determine a chiral orientation of the first C-C bond of the chain (for a A site,... 

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