Conjugation, spectroscopic effects

We discussed the way in which conjugation affects reactivity in Chapter 10. and mentioned its spectroscopic effect there as well. 

The structural features and the spectroscopic characteristics of the thiirene dioxide system (22) are of special theoretical interest since, on the basis of analogy with cyclopropenone (23), it is a possible nonbenzenoid aromatic system with all the physical and chemical implications involved. Aromatic and/or conjugative effects, if any, require transmission through the d-orbitals of the sulfur atom. 

Effects of spacer groups on the formation and properties of the mixed-valence states of conjugated ferrocene dimers have been extensively studied by both electrochemical and spectroscopic methods. It should be noted that a characteristic feature in the electronic spectra of ferrocene dimers with conjugated spacer groups is the appearance of metal-to-ligand charge transfer (MLCT) bands in the neutral form as well as IT bands in the mixed-valence state. The dimer Fc — CH=CH — Fc... 

Based on the fundamental dipole moment concepts of mesomeric moment and interaction moment, models to explain the enhanced optical nonlinearities of polarized conjugated molecules have been devised. The equivalent internal field (EIF) model of Oudar and Chemla relates the j8 of a molecule to an equivalent electric field ER due to substituent R which biases the hyperpolarizabilities (28). In the case of donor-acceptor systems anomalously large nonlinearities result as a consequence of contributions from intramolecular charge-transfer interaction (related to /xjnt) and expressions to quantify this contribution have been obtained (29). Related treatments dealing with this problem have appeared one due to Levine and Bethea bearing directly on the EIF model (30), another due to Levine using spectroscopically derived substituent perturbations rather than dipole moment based data (31.) and yet another more empirical treatment by Dulcic and Sauteret involving reinforcement of substituent effects (32). 

Meanwhile there is overwhelming evidence that the basic assumptions of the SSH model are not applicable to 7i-bonded conjugated polymers. Coulombic and electron-electron correlation effects are large while electron-phonon coupling is moderately weak. As a consequence, the spectroscopic features in this class of materials are characteristic of molecular rather than of inorganic crystalline semiconductor systems. There are a number of key experimental and theoretical results that support this assignment ... 

In terms of the Dewar-Chatt model of bonding, for v metal complexation one double bond is effectively removed from the fullerene conjugation system due to extensive interaction between metal d orbitals and the fullerene HOMO and LUMO (7). The remaining 29 double bonds then behave almost identically to uncomplexed C60 with their IR, Raman, UV-vis, and 13C NMR spectra showing only slight perturbations, mainly as a result of diminution of symmetry effects. Nevertheless, it is important to state that the fullerene metal interaction is not confined purely to the former s HOMO and LUMO, and that other molecular orbitals are energetically suitable for interaction 89,90). The spectroscopic evidence cited for the preceding statement is as follows ... 

The properties of a compound with isolated double bonds, such as 1,4-pentadiene, generally are similar to those of simple alkenes because the double bonds are essentially isolated from one another by the intervening CH2 group. However, with a conjugated alkadiene, such as 1,3-pentadiene, or a cumulated alkadiene, such as 2,3-pentadiene, the properties are sufficiently different from those of simple alkenes (and from each other) to warrant separate discussion. Some aspects of the effects of conjugation already have been mentioned, such as the influence on spectroscopic properties (see Section 9-9B). The emphasis here will be on the effects of conjugation on chemical properties. The reactions of greatest interest are addition reactions, and this chapter will include various types of addition reactions electrophilic, radical, cycloaddition, and polymerization. 

---