ISSN 1017-2548(Print)
ISSN 2234-8530(Online)
Volume 17, Number 1
JKCSEZ 17(1)
February 20, 1973 

The purpose of this paper is to investigate the mechanism of Diels-Alder reaction by assuming pseudo molecular complex (PMC) which has characters both of ionic and radical bonds. We treated this complex quantum-chemically as an intermediate between the configuration without charge transfer (radical bond character) and the configuration corresponding to the charge transfer from Diene (R) to Dienophile (S) (ionic bond character). The wave function for the complex could be expressed as: Ψcomplex = Ψ (R, S) + ρΨ (R+, S-)where ρ is the extent of charge transfer which is a constant to measure the ionic character of PMC. It has been noticed that ρ is related to the difference between Fr + Fr' and Fs + Fs' in free valence (F) when R is united to S through atom r in R to atom s in S and atom r' in R to atom s' in S, That is, ρα ΔF = (Fr + Fr') - (Fs + Fs'). We have calculated ΔF values for more than forty Diels-Alder reactions. The calculated values of ΔF is reversely proportional to the values of Brown's paralocalization energy (Lp) as well as Dewar's differences of delocalization energy (ΔEdeloc.) with good linearity. This approach also presents a way of predicting the possibility and the easiness of diene synthesis between any two conjugate compounds. According to the considerations, it could be concluded that Diels-Alder reaction takes place through the united ionic-radical mechanism rather than the separated ionic or radical mechanism.