Effective conjugation theory

Gelan and coworkers [32] also examined the Raman spectra of the model compounds, both quinonoid and aromatic, which include 6, 7, and 9, and compared these to the spectrum of polyisothianaphthene (1). Here, the vibrational bands of 1 were shown to match more closely with those of the quinonoid structures 6 and 7 than those of the aromatic structures such as 9 that were studied. In another recent paper [38], the Raman spectra of the model compounds and that of the pristine polyi.sothia-naphthene (1) are again compared. Using the effective conjugation coordinate theory to interpret the spectra, it is again concluded that the structure is quinonoid. 

Following the classical approach of vibrational dynamics (as in the case of o--bonded polymers) [67], the experimental data from the oligomers could in principle be used to obtain the phonon dispersion curves of the ideally infinite polymer. In the case of 7r-bonded systems, complications arise because the skeletal modes are strongly CL-dependent and cannot be treated in the standard way. The rationalization of the behavior of the totally symmetric Raman-active normal modes leads to the definition of a CL-dependent intramolecular potential on which the effective conjugation coordinate (ECO theory is based (see Section IV). 

Equations (15) and (16) define the so-called effective conjugation coordinate of ECC theory. The important conclusion that can be derived from Eqs. (11) and (17) is that only normal modes that contain an oscillation of the dimerization amplitude ( R vibration) can have relevant Raman cross sections. Then the two relevant lines of the Raman spectrum of polyenes are necessarily assigned to normal modes that involve a large contribution by the fl oscillation (FI modes in the ECC theory). More precisely, the treatment of the dynamical problem of polyenes in terms of ECC theory assigns the two strong Raman lines to two different combinations (in-phase and out-of-phase) of the R oscillation with C—H wagging vibration. 

In conclusion, a critical reading of the data reported in Table 28.1 and 28.2 and Figs. 28.6, 28.8, and 28.18 shows that the physics of the polyene systems can be interpreted in a unified way by the introduction of the concept of effective conjugation coordinate" strongly coupled with tt electrons. Moreover, with the help of ECC theory, it is possible to extract relevant information on the electronic structure and molecular geometry from... 

According to ECC theory of the TS modes those which contain the mode are selectively enhanced, their intensity being - - Lqp. Moreover th se modes may show frequency and intensity dispersion with effective conjugation length. The Raman spectra of the oligomers (fig.6) at fixed / xc 9 ve immediately, and without any calculation, such dispersions. The observed weak line [121,122] near 1550 cm shows dispersion (Th2 1556, Th3 1530, Th4 1519 and Thg 1507 cm. The frequencies calculated [83, 84] from pure MNDO, without refin-ment, give the following dispersion Th2 1571, Th3 1560, Th4 1554 and Thg 1549 cm . ... 

Because of the mentioned leveling effect of the solvent (or excess acid itself acting as such) the acidity cannot exceed that of its conjugate acid. In the case of water the limiting acidity is that of HsO. Proton-ated water, H30 (hydronium ion), was first postulated in 1907, and its preeminent role in acid-catalyzed reactions in aqueous media was first realized in the acid-base theory of Bronsted and Lowry. Direct experimental evidence for the hydronium ion in solution and in the... 

A familiar feature of the electronic theory is the classification of substituents, in terms of the inductive and conjugative or resonance effects, which it provides. Examples from substituents discussed in this book are given in table 7.2. The effects upon orientation and reactivity indicated are only the dominant ones, and one of our tasks is to examine in closer detail how descriptions of substituent effects of this kind meet the facts of nitration. In general, such descriptions find wide acceptance, the more so since they are now known to correspond to parallel descriptions in terms of molecular orbital theory ( 7.2.2, 7.2.3). Only in respect of the interpretation to be placed upon the inductive effect is there still serious disagreement. It will be seen that recent results of nitration studies have produced evidence on this point ( 9.1.1). 

Bodi MO theory and experimental measurements provide a basis for evaluation of the energetic effects of conjugation between a double bond and adjacent substituents. Table 1.16 gives some representative values. The theoretical values AE are for the isodesmic reaction... 

The examples that have been presented in this section illustrate the approach that is used to describe structure and reactivity effects within the framework of MO description of structure. In the chapters that follow, both valence bond theory and MO theory will be used in the discussion of structure and reactivity. Qualitative valence bond terminology is normally most straightforward for saturated systems. MO theory provides useful insights into conjugated systems and into effects that depend upon the symmetry of the molecules under discussion. 

Despite the success of these simple rules in the Jt-conjugated diradicals, most of them cannot be directly applied to the localized diradicals within the o-framework. In Sect. 5, we will demonstrate that the orbital phase theory works effectively for the localized 1,3- and 1,4-diradicals as well. 

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