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-(benzyl)-2-chloropyridinium, and N-(2-chlorobenzyl)-2-chloropyridinium salts using a simple Huckel method in order to discuss their intramolecular photocyclization reaction in a qualitative method. Our calculation qualitatively predicts that photocyclization occurs through forming radicals as a reaction intermediate by breaking a C-CI bond after photoexcitation into a triplet state via intersystem crossing from an initially excited singlet state. We noticed that this C-Cl.bond breaking is aided by pi-complex formation between a chlorine atom and the pi-electrons of the neighboring ring in the triplet state and a stronger pi-complex bond makes C-CI bond breaking, ie., radical formation, much easier. A chlorine atom will form a stronger pi-complex bond to a benzyl ring of N-(benzyl)-2-chloropyridinium than a pyridinium ring of N-(2-chlorobenzyl)-pyridinium because the former can donate its pi-electron more easily than the latter. The chlorine at position 15 of N-(2-chlorobenzyl)-2-chloropyridinium salt in the excited state also provides its pi-electron to the benzyl ring. So this kappa-electron can increase the bond strength of the pi-complex. Therefore, the strength of pi-complex follows the order of N-(2-chlorobenzyl)-2-chloropyridinium, N-(benzyl)-2-chloropyridinium, and N-(2-chlorobenzy)-pyridinium salts and thus the radical formation rate. This provides us with an intramolecular photocyclization reaction rate of the same order as given above.