Bonding delocalized, molecule type

The model of the chain of hydrogen atoms with a completely delocalized (metallic) type of bonding is outlined in the preceding section. Intuitively, a chemist will find this model rather unreal, as he or she expects the atoms to combine in pairs to give H2 molecules. In other words, the chain of equidistant H atoms is expected to be unstable, so it undergoes a distortion in such a way that the atoms approach each other in pairs. This process is called Peierls distortion (or strong electron-phonon coupling) in solid-state physics ... 

Hybridization. The simplest way to think about bonding in [1.1.1 Jpropellane is to characterize the state of hybridization at the two types of carbon atoms and then combine the hybrids pairwise into localized bond orbitals. In ordinary molecules, these have electron occupancy close to two, and the corresponding antibonds are nearly unoccupied. Bond delocalization can then be introduced by considering interactions of the occupied localized bond orbitals with the unoccupied localized antibond orbitals. 

Isodesmic and homodesmotic reaction schemes use small molecules which retain bond types on both sides of the equation to model the relevant structural components of the cyclic conjugated species. o The homodesmotic reactions match bond and hybridization types for C—C and C—H on both sides of the equation. Calculation of the isodesmic stabilization energy (ISE) from the additivity of the enthalpies of formation of CH4, H3C—CH3, and H2C=CH2 is analogous to the RE derived above for benzene in that the reference system has noninteracting ethylenic double bonds and the calculated energy includes both the cyclic and noncyclic 7r-electron delocalization. Isodesmic reactions, like the RE calculation, tend to overestimate the stabilization because of the inclusion of the noncyclic delocalization energy. 

Boranes are typical species with electron-deficient bonds, where a chemical bond has more centers than electrons. The smallest molecule showing this property is diborane. Each of the two B-H-B bonds (shown in Figure 2-60a) contains only two electrons, while the molecular orbital extends over three atoms. A correct representation has to represent the delocalization of the two electrons over three atom centers as shown in Figure 2-60b. Figure 2-60c shows another type of electron-deficient bond. In boron cage compounds, boron-boron bonds share their electron pair with the unoccupied atom orbital of a third boron atom [86]. These types of bonds cannot be accommodated in a single VB model of two-electron/ two-centered bonds. 

Conjugated diene (Section 10 5) System of the type C=C—C=C in which two pairs of doubly bonded carbons are joined by a single bond The tt electrons are delocalized over the unit of four consecutive sp hybridized carbons Connectivity (Section 1 6) Order in which a molecule s atoms are connected Synonymous with constitution Constitution (Section 1 6) Order of atomic connections that defines a molecule... 

CC and CH bond orbitals but also for the CTL, ami CH3 group orbitals. If the local symmetry elements are preserved in the full molecule, the 7r (or a) local orbitals can combine to give v (or o) molecular orbitals. The reader should, therefore, not be surprised to find, for instance, tt type molecular orbitals in cyclopropane which are delocalized over the CH2 groups. 

All of the delocalization discussed so far involves n electrons. Another type, called hyperconjugation, involves <7 electrons. When a carbon attached to at least one hydrogen is attached to an unsaturated atom or one with an unshared orbital, canonical forms such as 110 can be drawn. In such canonical forms, there is no bond at all between the carbon and hydrogen. The effect of 110 on the actual molecule is that the electrons in the C—H bond are closer to the carbon than they would be if 110 did not contribute at all. 

The molecules (or atoms, for noble gases) of a molecular solid are held In place by the types of forces already discussed In this chapter dispersion forces, dipolar interactions, and/or hydrogen bonds. The atoms of a metallic solid are held in place by the delocalized bonding described in Section 10-. A network solid contains an array of covalent bonds linking every atom to its neighbors. An ionic solid contains cations and anions, attracted to one another by electrical forces as described in Section 8-. 

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