Polyenes and Aromaticity

Electrostatic potential map for 1,3-pentadiene shows negatively-charged regions (in red) and positively-charged regions (in blue). 

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In 1937, Coulson s mentor, Lennard-Jones, wrote a series of papers on molecular orbitals applied to polyenes and aromatic molecules, discussing the variations in bond lengths in conjugated and aromatic molecules from a theoretical point of view. 92 He involved his student in the work. Coulson defined... 

In order to extend the applicability of the same simple methods to olefinic, polyenic, and aromatic unsaturated hydrocarbons, additional parameters are required those for the bonds involving sp carbons. One must thus consider... 

G. W. Wheland, Proc. R. Soc. London, Ser. A 159, 397 (1938). The Electronic Structure of Some Polyenes and Aromatic Molecules. V—A Comparison of Molecular Orbital and Valence Bond Methods. 

Recently, the response theory formalism was applied for the calculation of singlet-triplet spectra in polyenes and aromatic hydrocarbons [83,133,134,132]. In this section we shall discuss mainly the vertical Tn <— So transitions probability in polyenes. The minimum level of accuracy includes CAS calculations of the ground state correlating all x electrons followed by linear and quadratic multi-configuration response calculations. Because the ethene molecule is the simplest representative of unsaturated hydrocarbons, different types of active spaces and AO basis sets were tested more thoroughly for this molecule in order to study the convergence of the S-T transition probability with respect to the level of accuracy. 

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