Electronic Structure of Conjugated Molecules

The structure polycyclic hydrocarbons (eg. Fig. 2) is taken with benzene bond lengths Rq = 1.397A and bond angles 27r/3. The Ohno formula[7] for V(Rpq) is 

The naphthalene results[35] in Table 1 illustrate the scope and success of standard PPP parameters for a classic series of experiments[49, 50, 51]. N = Ne = 10 

Similarly good agreement with experiment is found for biphenyl [37], ith 12 pi electrons, anthracene[54] or trans-stilbene[55], with 14 7r-electrons. The even-parity singlets of anthracene are systematically reassigned to lower energy and show strong correlation effects. The bond order pnm of sites n and m in the model-exact eigenstate X is, 

Large pnm (lMs) in the ground state indicate increased double-bond character and reduced bond lengths in the actual structure. Thus equal bond lengths and transfer integrals suffice for refinements that hardly change when t(R) is modulated according 

The linear correlation with slope AR/Ap = —0.36 shows that PPP bond orders can be used for excited-state relaxation. The comparable graph of AR/Ap for trans-stilbene has a linear correlation with slope —0.25. Similarly, it is seen in biphenyl[37] that the lowest triplet state has a bond order pattern corresponding to the quinonoidal structure, implying a planar geometry in the triplet state in conformity with the prediction of Lewis and Kasha[56]. 

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The Coulson Rushbrooke theorems have many important consequences that will lead us a long way towards a qualitative understanding of the electronic structure of conjugated molecules, particularly of their excited states. Dewar developed a simple perturbation method PMO theory) to evaluate the HOMO LUMO gap and the associated excitation energy for 7t-systems with an even number of conjugated atoms.284,285,288 Because the NBMO of odd AHs can be determined so easily, the system of interest is dissected into two odd AHs. Some examples are shown in Scheme 4.1. 

A 7i-conjugated systems is a molecule along the backbone of which occurs a continuous path of carbon atoms or heteroatoms, each carrying a p atomic orbital. The determination of the electronic structure of conjugated systems and their properties in terms of energy, electron and hole transport is very difficult. Electron correlation effects must be taken into account and the strong connection between, and mutual influence of, the electronic and geometric structures should be evaluated [91]. 

The electronic structures of A -phosphorins have been investigated by photoelectron spectroscopy and by CNDO/S calculations. It has been concluded that the n-electronic structure of these molecules is best described as a superimposition of the internal ylide structure (181), involving a delocalized 6 i-pentadienyl anion, and the HUckel 6 r-aromatic system (182), in which the phosphorus 2>dyt orbital is involved in conjugation. Which form predominates seems to depend largely on the nature of... 

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. 

Experimental studies of the electronic structure in conjugated molecules during metallization with Al and Ca has recently been performed by means of photoelectron spectroscopy chemically pure (oxygen free) thin films of a model molecule for t ra Ji s-polyacetylene shown in Figure 6, a,ai-diphenyltetradecaheptaene (hereafter denoted DPT), were prepared under ultra-high vacuum conditions. The evolution of the valence leotronlo levels in this molecule could than be monitored in situ by ultraviolet photoelectron spectroscopy (OPS) upon vapour deposition, basically in monolayer steps of Al or Ca. 

The questions of the effect that the Si atom has on the electronic structure of aromatic molecules can be expanded to include the effect of other substitutents on the Si, including different 7i-conjugated substituents. A second part of this question regards the presence of the Si-0- group or, generally, the effect of linear or cyclic... 

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