PHOTOCHEMICAL ACTIVATION OF DISTAL FUNCTIONAL-GROUPS IN POLYFUNCTIONAL MOLECULES - PHOTOCHEMISTRY AND PHOTOPHYSICS OF THE SYN-7-CHLOROBENZONORBORNENES AND ANTI-7-CHLOROBENZONORBORNENES

The spectroscopic (electronic absorption, fluorescence, and electron t ransmission (ET)) and photochemical properties of anti-7-chlorobenzono rbornene (BAntiCl) and syn-7-chlorobenzonorbornene (BSynCl) are report ed. These are complemented by ab initio MO calculations. The BSynCl is omer, the LUMO of which contains relatively little C-Cl sigma charact er, is relatively unreactive upon photolysis (254 nm) in methanol (Phi (dis)(BSynCl) = 0.0014). The LUMO of BAntiCl is calculated to contain a significant amount of C-Cl sigma character, consistent with the muc h greater reactivity (Phi(dis)(BAntiCl) = 0.23) of this isomer. The ET spectra reveal that the pi levels of these compounds and of their ex o-2- and endo-2-chlorobenzonorbornene isomers (BExoCl and BEndoCl, res pectively) are appreciably stabilized relative to that of benzonorborn ene (BNB). A natural bond orbital (NBO) analysis shows that much of th is stabilization derives from inductive effects, with sizable through- space (TS) (0.07 and 0.10 eV) and through-bond (TB) (0.03 and 0.06 eV) coupling between the pi and C-CI sigma* orbitals for BAntiCl and BEx oCl, respectively.