Bent molecular shape, 263 table

In the last 10 years a very large number of different ring systems have been introduced into liquid crystal chemistry [35 a, 36]. Many of them are not very effective in terms of their mesogenity, and may be considered merely as curiosities. Table 14 presents some examples. Spiro ring systems such as compound numbers 1 and 9 in Table 14 are not planar rather, the rings are perpendicular one to another. This causes an unfavourable bent molecular shape. In seven-mem-bered rings (compound number 6) the directions of the substituent bonds are not parallel, which also leads to a bent molecular shape. Compound number 7 is derived from... 

In the electron-dot formula of water, H2O, there are also four electron groups, which have minimal repulsion when the electron-group geometry is tetrahedral. However, in H2O, two of the electron groups are lone pairs of electrons. Because the shape of H2O is determined by the two H atoms bonded to the central O atom, the H2O molecule has a bent shape with a bond angle of 109°. Table 10.3 gives the molecular shapes for molecules with two, three, and four bonded atoms. 

See Practice Problem Exercise 12.4) The four pairs of electrons around the sulfur require a tetrahedral arrangement. In this case two pairs are shared with hydrogen atoms, leaving two lone pairs. Thus the molecular structure is bent or V-shaped (case 5 in Table 12.4). 

Table 1.7, for example, in the difference between the volume change of anthracene and phenanthrene. The more regular shape of anthracene is clearly more favorable for tight molecular packing in the crystal than the bent shape of its isomer phenanthrene, whose crystal also has a lower packing coefficient and a lower heat of sublimation. These observations and conclusions are however quahtative and sporadic, and can hardly be used in systematic theories of molecular packing. 

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