Force-field calculations. See

Fig. 13.3. Metallocenes investigated by force field calculations (see Table 13.1).

PH3, PD3, PT3. The six normal modes of the PH3 (PD3) molecule form two totally symmetrical vibrations, Vi(Ai) and V2(Ai), and two doubly degenerate vibrations, V3(E) and V4(E). Force field calculations (see p. 172) showed that approximately the higher frequency mode in each of the two symmetry species is a bond stretch and the lower an angle deformation. 

For instance, the quantum chemical literature is full of comparisons between computed harmonic frequencies and observed fundamental ones. While this may be acceptable in situations where an anharmonic force field calculation (see Anharmonic Molecular Force Fields) is technically impossible, the anharmonicity a>i — v, of a fundamental may easily range from 2 to 200 cm- and therefore such a comparison is essentially meaningless for benchmark quality results. Yet for polyatomics, the experimentally derived harmonic frequencies are often associated with large uncertainties (cf. the C2H4 example at the end of this article) due to indeterminacies, and often the only meaningful comparison will be between computed and observed fundamentals, requiring the ab initio calculation of the anharmonic part of the force field. 

In order to represent 3D molecular models it is necessary to supply structure files with 3D information (e.g., pdb, xyz, df, mol, etc.. If structures from a structure editor are used directly, the files do not normally include 3D data. Indusion of such data can be achieved only via 3D structure generators, force-field calculations, etc. 3D structures can then be represented in various display modes, e.g., wire frame, balls and sticks, space-filling (see Section 2.11). Proteins are visualized by various representations of helices, / -strains, or tertiary structures. An additional feature is the ability to color the atoms according to subunits, temperature, or chain types. During all such operations the molecule can be interactively moved, rotated, or zoomed by the user. 

Incorporation of the (.S )-2-mcthyloctoxy group afforded optically active polymers with preferential helical screw sense (see Section 3.11.6.1). The observed helicity was corroborated by force field calculations, which indicated similar helical conformations for both dialkoxy- and dialkyl-substituted polymers. Based on their similar conformational properties, it was suggested that the origin of the spectral red shift was electronic, due to a a-n mixing interaction, as for polymers 76 above, rather than conformational. 

Thus, it is not surprising that, with few exceptions, force field calculations of organometallic systems start with a predefined bonding scheme. This is not unreasonable since the type of bonding may usually be determined from spectroscopic results, and it is often more or less constant within a class of similar compounds. Force field calculations can then be used to obtain a more detailed picture of the structural and dynamic properties of a molecule with a given connectivity. In spite of these restrictions on the modeling of organometallics, the results obtainable are potentially useful, especially for catalytic reactions (see also Chapter 7, Sections 7.2 and 7.4). 

The determination of the charge distribution in a molecule, needed here for the latter term, (Ges), has been a considerable problem in force field calculations, especially for transition metal compounds (see Sections 3.2.6 and 3.3.6). Most promising but not yet fully tested for transition metal complexes are semi-empiri-cal quantum-mechanical methods[ 103,1041. Future studies might show whether a combination of approximate methods for the computation of charge distributions and solvation will lead to a reliable approximation of solvation parameters of coordination compounds. 

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