Arenes spectral properties

The VSEPR model we discussed in Chapter 10 accounts for molecular shapes by assuming that electron groups minimize their repulsions, and thus occupy as much space as possible around a central atom. But it does not explain how the shapes arise from interactions of atomic orbitals. After all, the orbitals we examined in Chapter 7 aren t oriented toward the comers of a tetrahedron or a trigonal bipyramid, to mention just two of the common molecular shapes. Moreover, knowing the shape doesn t help us explain the magnetic and spectral properties of molecules only an understanding of their orbitals and energy levels can do that. 

Selenenic acid 16b fixed in the 1,2,3-altemate conformation was also synthesized by a similar procedure (Scheme 11.7). Intriguingly, it was found that the difference in the conformation of the calix[6]arene macrocycle affects the properties and reactivity of the SeOH functionality. While the 1,2,3-altemate isomer 16b showed comparable stability to the cone isomer 16a, they showed different spectral properties (Table 11.1). 

Arenes spontaneously form intermolecular 1 1 complexes with a wide variety of electrophiles, cations, acids, and oxidants that are all sufficiently electron-poor to be classified as electron acceptors. Spectral, structural, and thermodynamic properties of these donor/ acceptor associates are described within the context of the Mulliken charge-transfer (CT) formulation. The quantitative analyses of such CT complexes provide the mechanistic basis for understanding arene reactivity in different thermal and photochemical processes. 

Consideration of the spectral and thermodynamic properties of arene CT complexes thus indicates that they are reasonably described within the framework of recent developments of the Mulliken formalism, in the case of both weak and strong complexes in the highly en-dergonic and nearly isergonic regions. Accordingly, let us now consider the structural consequences attendant upon charge transfer from the donor to the acceptor in such complexes. 

The donor/acceptor properties and the electronic coupling interactions determine the redistribution of electron density between the aromatic donor and the electron acceptor upon complexation. Significant changes in structure and reactivity of the coordinated arene can be rationalized in terms of spectral and thermodynamic properties within the framework of the CT formalism. This section is devoted to a consideration of the structural effects of arene coordination (in terms of donor/acceptor bond distance and type of bonding, distortion of arene planarity, expansion of the aromatic ring, and re-bond localization). 

Charge transfer as depicted by Mullilcen provides a single unifying basis for predicting arene reactivity based on the spectral, structural (both molecular and electronic), and thermodynamic properties of their intermolecular complexes, from stable organometallic derivatives to non-bonded collision complexes with very short lifetimes. 

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