Linearly-conjugated molecules

Occur at shorter wavelengths than do absorptions due to n — n transitions 

Relatively strong absorptions with max values of 103 to 1051 moDcnr1 

The absorption band occurs at longer wavelength in a polar solvent than in a nonpolar solvent (the absorption shows a red shift or bathochromic shift) 

Relatively weak absorptions with emax values of 1 to 1021 moL cnr1 

---

The wavefunctions of delocalized 7T orbitals of linear conjugated molecules can be drawn following the number of nodes (open circles)... 

There is another class of isomerization reaction which proceed through a cyclic transition state, known as a sigmatropic reaction. This is formally the shifting of a single (a) bond between the terminal carbon atoms of a linear conjugated molecule, according to the example drawn in Figure 4.40. 

In a dark reaction the relevant frontier orbital is the HOMO, while it will be the LUMO for a photochemical reaction. In the case of linear conjugated molecules there are then two simple rules ... 

To explain pericyclic reactions, the energies and the location of the nodes of the pi MOs of noncyclic, linear, conjugated molecules are needed. These can be obtained by the application of the following rules ... 

Most linear conjugated molecules and polymers possess spatial symmetries, while cyclic polymers possess axial symmetry. Conjugated systems also possess an approximate particle-hole symmetry. These symmetries characterize the electronic... 

Work on smaller systems increasingly includes treatment of both electronie and vibrational responses. Some papers which include work on the development of the vibrational theory, or are primarily concerned with large vibrational effects, are grouped together in a separate section, but many of the papers discussed in the other sections report results for both types of response for particular molecules. There is a substantial amount of work on linear conjugated molecules where papers, or series of papers, begin with calculations on small oligomers and eventually extrapolate results to make predictions about infinite polymeric systems. Readers should bear in mind that the decision as to whether to include such papers in one of the small molecule sections or in the Polymer section (4.9) is often rather arbitrary. 

---