A SIMPLE HUCKEL APPROACH TO INTRAMOLECULAR PHOTOCYCLIZATION REACTION OF N-(2-CHLOROBENZYL)-PYRIDINIUM, N-(BENZYL)-2-CHLOROPYRIDINIUM, AND N-(2-CHLOROBENZYL)-2-CHLOROPYRIDINIUM SALTS

We have calculated the pi-electron density, atom self-polarizability, and free valence on each atom of N-(2-chlorobenzyl)pyridinium, N-(benz yl)-2-chloropyridinium, and N-(2-chlorobenzyl)-2-chloropyridinium salt s using a simple Huckel method in order to discuss their intramolecula r photocyclization reaction in a qualitative method. Our calculation q ualitatively predicts that photocyclization occurs through forming rad icals as a reaction intermediate by breaking a C-CI bond after photoex citation into a triplet state via intersystem crossing from an initial ly excited singlet state. We noticed that this C-Cl.bond breaking is a ided by pi-complex formation between a chlorine atom and the pi-electr ons of the neighboring ring in the triplet state and a stronger pi-com plex bond makes C-CI bond breaking, ie., radical formation, much easie r. A chlorine atom will form a stronger pi-complex bond to a benzyl ri ng of N-(benzyl)-2-chloropyridinium than a pyridinium ring of N-(2-chl orobenzyl)-pyridinium because the former can donate its pi-electron mo re easily than the latter. The chlorine at position 15 of N-(2-chlorob enzyl)-2-chloropyridinium salt in the excited state also provides its pi-electron to the benzyl ring. So this kappa-electron can increase th e bond strength of the pi-complex. Therefore, the strength of pi-compl ex follows the order of N-(2-chlorobenzyl)-2-chloropyridinium, N-(benz yl)-2-chloropyridinium, and N-(2-chlorobenzy)-pyridinium salts and thu s the radical formation rate. This provides us with an intramolecular photocyclization reaction rate of the same order as given above.