The Electronic Spectra of Conjugated Molecules

Even though a semiempirical method can never be used to prove or disprove an experimental fact, it has a high stimulating power and allows the experimenter to classify and understand such facts. 

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Venkataraman, K. (1952) The Chemistry of Synthetic Dyes, 2 volumes. NY Academic Press 32Platt, J. (1964) Systematics of the electronic spectra of conjugated molecules. NY John Wiley Sons, paper 20... 

Platt, J. R., Systematics of the Electronic Spectra of Conjugated Molecules A Source Book, John Wiley Sons, Chicago,... 

Simple molecular orbital theory provides a useful pictorial view of the electronic structure of molecules. It is particularly valuable in predicting the distribution of electronic charge over a molecule, and for planar w-electron systems predictions of molecular stability are often very accurate. By judicious choice of parameters the simple molecular orbital theory can also account for the gross features of the electronic spectra of conjugated molecules as, for example, in Huckel theory. 

Two models are particularly well suited for the calculation of the electronic spectra of conjugated ir-electronic systems. Both are based on the principle of differential overlap and involve no multicenter repulsion integrals. The Pariser-Parr-Pople (PPP) model (7), whose basis set contains only atomic orbitals of it symmetry, is particularly good for irir transitions in hydrocarbons and other molecules with a limited number of heteroatoms. Because of the unreliability of the PPP method in the presence of heteroatoms, we chose a second model which is available in two versions (CNDO/S (8) and INDO/S (9) for the calculations on 1 and 2. This model utilizes a basis set which contains all valence shell orbitals and leads to less uncertainty in parameterization even when a large number of heteroatoms are present. This model works well for irir transitions but often fails when applied to nir, air and ira transitions. This should not be a serious limitation in the present application since the only transitions of sufficient intensity to be of interest are of the wir type. 

Essential features of the electronic spectra of linear polyenes have already been mentioned in Chapter 1. The HOMO LUMO transition is shifted to longer wavelengths as the number of conjugated double bonds increases, and this is easy to rationalize in the FEMO model. However, the next transition that is to be expected from the simple MO model is only allowed if the molecule does not have a center of symmetry that is, if it is not in the all-trans configuration. (Cf. Example 1.7.) The absorption spectrum of /S-caro-... 

The SHM has been extensively used to correlate, rationalize, and predict many chemical phenomena, having been applied with surprising success to dipole moments, esr spectra, bond lengths, redox potentials, ionization potentials, UV and IR spectra, aromaticity, acidity/basicity, and reactivity, and specialized books on the SHM should be consulted for details [21], The method will probably give some insight into any phenomenon that involves predominantly the n electron systems of conjugated molecules. The SHM may have been underrated [50] and reports of its death are probably exaggerated. However, the SHM is not used very much in research nowadays, partly because more... 

We have shown [13,79, 80] that the electronic spectra of c -l,3-buta-diene [81], cyclopentadiene, aromatic five-membered heterocycles [48,79], and norbornadiene [80] can be understood on the basis of a model with two interacting double bonds. Cyclopentadiene (CP) is the prime example of a ring-shaped molecule with a conjugated rr-electron system, and its structure can be related to that of short polyenes such as cM-1,3-butadiene (CB) and the simplest heterocycles, such as pyrrole (PY), furan (FU), and thiophene (TP). In the series, ci5-l,3-butadiene, cyclopentadiene, norbornadiene (NB), the latter is the most complex system, with the two ethylenic units coupled through indirect conjugation and 77, a interaction. One more system will be added here to the set of molecules with two interacting double bonds methylenecyclopropene (MCP) [(1) in Fig 3] the simplest cross-conjugated 77-electron system. 

Conjugation of two chromophores not only results in a balhochromic shift but increases the intensity of the absorption. These two effects are of prime importance in the use and interpretation of electronic spectra of organic molecules because conjugation shifts the selective light absorption of isolated chromophores from a region of the spectrum that is not readily accessible to a region that... 

In the past a great effort was made to find a unified interpretation of the observed spectra and to rationalize trends of variations in the relevant spectral features presented by polyconjugated systems [41,42]. The important conclusion reached is that the vibrational spectra of such molecules can be explained only if one takes explicitly into account the fact that the structure (equilibrium nuclear geometry and its nuclear vibrational displacements) of the conjugated backbone is strongly coupled with the TT-electron distribution. This means that any modification of the 7r-electron distribution induced by various mechanisms such as... 

Burawoy A, Critchley JP (1959) Electronic spectra of mganic molecules and their interpretation -V effect of terminal groups containing multiple bonds on the K-bands of conjugated systems. Tetrahedron 5 340-351... 

The results of the above cited applications [18-28,45] have clearly shown that CS INDO method is fairly successful in combining equally satisfactory predictions of electronic spectra and potential surfaces (especially along internal rotation pathways) of conjugated molecules, a goal never reached by other NDO-type procedures. CS INDO shares, at least partly, the interpretative advantages of the CIPSI-PCILO-CNDO procedure [32,33,36,37], coming from using the same hybrid AO basis sets, but improves its predictive capabilities as far as spectroscopic and photochemical properties are concerned. 

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