THE INTERACTION VECTOR MODEL AND THE INTENSITY OF THE SECONDARY TRANSITION OF THE BENZENE CHROMOPHORE IN STRAINED NATURAL MOLECULES - SESAMOL, MELACACIDIN, TETRAHYDROPAPAVEROLIN, RENIFOLIN, PTEROCARPIN, PELTOGYNOL

A method (the New Vector Model : NVM) designed to approach the intensi ty of the secondary transition of the benzene chromophore with -CH3 an d -OR substituents, has been recently published. 1 This method is base d on MNDO calculations. It has been applied to strained chromophores w ith fused rings, 2 assuming that it is possible to take into account t he incidence of strain on intensity in introducing a strain vector in the vector scheme used to calculate the transition moment vector. In a nother step it has been derived a much simpler method which avoids the quantum calculations. It can be brought into play using graphically s imple vector addition rules. 3 This method is based on a new concept : the interaction vector (underneath, the method is called the Interact ion Vector Model : IVM). It has been designed to study alkyl and -OR s ubstituents, since we are mainly involved in the study of natural prod ucts and many natural products display such substituents. This IVM has not been designed to take into account the perturbations induced by t he strain on intensity. The present work is devoted to the adaptation of the IVM to strained benzene chromophores. It is given several examp les on the way it can be used on strained natural molecules. Experimen tal intensity is given in the present work as epsilon(sm), the maximum of the smoothed absorption curve, as it has been defined by BALLESTER and RIERA 4 (the calculated value is : epsilon(sm,c)).