Tetrahedral bonding with free rotation

For the case of tetrahedral bonding with free rotation (equation 2.40)... 

Calculation of the total energy of 8 and 9 shows that 8 is about 8 kcal/mol more stable than 9. Bending the hydrogens up in 9, to make the carbons tetrahedral leads to an even more unfavourable system. With this favouring of the assymmetric intermediate, free rotation around the C-C bond becomes possible. This can cause the change in stereochemistry observed for the epoxidation of deuterio substituted ethylene (ref. 8). 

The thermophysical properties necessary for the growth of tetrahedral bonded films could be estimated with a thermal statistical model. These properties include the thermodynamic sensible properties, such as chemical potential /t, Gibbs free energy G, enthalpy H, heat capacity Cp, and entropy S. Such a model could use statistical thermodynamic expressions allowing for translational, rotational, and vibrational motions of the atom. 

The unperturbed dimensions of various condensation polymers obtained by the present method are listed in Table 10. A polyelectrolyte chain, sodium polyphosphate, has been included because theta-solvent results are available. The freely-rotating chain dimension (Lzyof of poly(dimethylsiloxane) in the table is due to Flory and his coworkers (705), that for the polyphosphate chains is taken directly from the paper of Strauss and Wineman 241 ), while most of the others have been calculated in the standard manner with the convenient and only negligibly incorrect assumption that all the aliphatic bond angles are tetrahedral. The free-rotation values for the maleate and fumarate polyesters are based on parameters consistent with those of Table 6 for diene polymers. 

Intramolecular forces must dominate the correlation, and this notion provides the basis for a theoretical discussion of g,--values. Intermolecular forces may be taken into account as a secondary perturbation. An important case concerns the arrangement of segmental dipoles, aligned perpendicularly with respect to the chain contour, along a polymeric molecule which has a backbone of carbon atoms. Part of the dipolar correlation is fixed, of course, by the tetrahedral nature of the carbon valence, but part depends on the possible rotation about the C-C bonds of the chain. For completely free rotation it may be shown that gT = 11/12. 

As we have seen in the previous chapter, the hydrocarbon skeleton is responsible for the shape and flexibility of organic molecules. In the case of alkane molecules, the molecular structure is based on tetrahedral units and the molecular dynamics is the consequence of relatively free rotations about the carbon-carbon single bonds. These rotations give rise to different conformations. However, with the exception of small-ring molecules, the alkanes, as compounds containing only carbon and hydrogen, are relatively weakly reactive substances. 

Note that it is important when working out possible isomers to remember the limitations of the two-dimensional representation of structures on paper. Thus the structures represented in Figures 10.21a and 10.21b are not isomers at all. In Figure 10.21a the chain appears to turn a corner on paper, but remember that, in reality, the structure around each carbon atom is tetrahedral and that there is free rotation around each bond. In Figure 10.21b, one structure is just the other turned over on the paper. It is crucial to remember that isomers are compounds with the same molecular formula but with different arrangements of atoms in the molecules. 

The presence of four fundamental frequencies in the Raman spectrum of Pb(C2H5)4 is consistent with a regular tetrahedral structure of the PbC4 central skeleton [1]. conformation was assumed for the interpretation of the IR data [6, 9]. Results of statistical-thermodynamic calculations were in accord with this model for Pb(C2H5)4 in the vapor state, although the existence of conformations of symmetry lower than V, had to be considered because of hindered rotations of the peripheral methyl groups [9]. Rotation about the Pb-C bond is free, or at best, restricted by a barrier of less than about 1 kcal/mol [7]. 

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