Distorted methyl-halide

After introducing the Non-Rigid Group Theory, and the main operations used for dealing with some simple molecules, let us compare this theory with those of Longuet-Higgins [5] and Altmann [10,11]. For this purpose, let us consider the hypothetical examples of a distorted methyl halide molecule, and the orthoformic acid, advanced by Watson in an earlier critical study [3Sj. 

Figure 8 The three conformations of the distorted methyl halide.

The Longuet-Higgins Molecular Symmetry Group of this molecule is the same as that of the distorted methyl halide (56), since the number of interchangeable identical particules, i.e., the number of feasible permutations, is the same in both systems. 

Table 9 Groups and group structures in the Altmann s, restricted and full NRG theories, and group structure in the Longuet-Higgins scheme for distorted methyl halide (DMH), symmetric orthoformic acid (SOA), and random orthoformic acid (ROA).

Summary Bulky monodentate ligands affect the structure and reactivity of siliconium ion complexes in several ways they enhance ionization, they promote a methyl halide elimination reaction, and they severely distort complex geometries. The latter effect enabled the assembly of a reaction coordinate model for the Berry pseudorotation, composed of crystal structures with varying NSiN and OSiO bond angles. In a competition between opposing effects of electron withdrawal by CF3 and steric bulk of a cyclohexyl ligand in the same molecule, a nonionic dissociation of the dative N->Si bond was observed. 

Bulky X-ligands, and in particular the /-butyl group, cause severe geomettical distortions of pentacoordinate silicon complexes. The most dramatic example is that mentioned above, in which the nitrogen ligands in 3 have moved from axial to equatorial positions upon elimination of methyl halide from 2. The crystallographic evidence for this geometry is depicted in Fig. 1. The extent of... 

As far as mean amplitudes are concerned, interplay between spectroscopy and electron diffraction may come about in two ways. Firstly, even for comparatively simple polyatomic molecules e.g. the methyl halides ) the general harmonic force field is not well determined from all the spectroscopic data available, i.e. vibration frequencies, isotopic frequency shifts, Coriolis zfita constants, and centrifugal distortion constants. In principle, experimental mean amplitudes from electron diffraction studies should provide valuable additional data. In practice, however, the experimental amplitudes have as yet rarely been of sufficient precision to be helpful. Secondly, for more complex molecules, mean amplitudes calculated from spectroscopic data (by way of what are inevitably very approximate force fields in many cases) are sometimes used as fixed parameters in the electron diffraction analysis in order to reduce the total number of parameters refined. 

These studies raised several fundamental questions about the effects of pincer substituents on the overall electronic structure of the resulting cobalt halide or alkyl compound. First, why are the electronic stractures of N-aryl and N-alkyl substituted bis(imino)pyridine cobalt chloride complexes different. Second, why for the N-alkyl derivatives is the chloride compound spin crossover and the methyl diamagnetic. What role does the distortion of the hgand play on the overall electronic structure ... 

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