A Summary of Electronic and Molecular Geometries

We have discussed several common types of polyatomic molecules and ions, and provided a reasonable explanation for the observed structures and polarities of these species. Table 8-4 provides a summation. 

Our discussion of covalent bonding illustrates some important points  

Molecules and polyatomic ions have definite shapes. 

The properties of molecules and polyatomic ions are determined to a great extent by their shapes. Incompletely filled electron shells and lone pairs of electrons on the central element are very important. 

There are oidy e electronic geometries. AU molecular geometries are simply special cases of these five basic geometries (Table 8-4). 

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TABLE 8-4 A Summary of Electronic and Molecular Geometries of Polyatomic Molecules and Ions ... 

In summary, it is noted that multiple bonding between the heavier Group 14 elements E (Ge, Sn, Pb) differs in nature in comparison with the conventional a and 7T covalent bonds in alkenes and alkynes. In an E=Ebond, both components are of the donor-acceptor type, and a formal E=E bond involves two donor-acceptor components plus a p-p n bond. There is also the complication that the bond order may be lowered when each E atom bears an unpaired electron or a lone pair. The simple bonding models provide a reasonable rationale for the marked difference in molecular geometries, as well as the gradation of bond properties in formally single, double and triple bonds, in compounds of carbon versus those of its heavier congeners. 

In summary the results observed in these studies [160] of poly(Sty-co-DVB) swelling in aromatic liquids serve to show that the method of measuring a is so sensitive that it can detect an effect caused by even the smallest modification in the molecular geometry of attached substituents, and that these differences correlate qualitatively with expectation based on the known principles of physico-organic chemistry of aromatic compounds. Since the observed a is the net effect of electronic attraction and steric hindrance between the sorbed molecule and the adsorption site, i.e. the monomer unit of the polymer, it would be impossible to separate quantitatively the electronic and steric contributions of a particular substituent. The ability to make such a differentiation, however, appears to be more promising with liquids that comprise homologous series of the type Z(CH2)nH (where Z is a phenyl, chloro, bromo or iodo substituent), since the added electronic contribution to Z by each additional methylene group is well known to be extremely small when n becomes >3 [165],... 

In summary, the rigged Born-Oppenheimer framework permits a general description of chemical reactions. By retaining the stationary geometry structures determined with modern electronic wave function methods the relationship between quantum electronic state and molecular species is established. The relaxation processes involve serial changes of quantum states. 

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