PI-ASTERISK SIGMA-ASTERISK ORBITAL MIXING - SYNTHESIS AND EXCITED-STATE STUDIES OF 7-CHLORO-2-(TRIMETHYLSILOXY)NORBORNENE AND 9-CHLORO-3-METHOXY-EXO-TRICYCLO[5.2.1.02,6]DEC-3-ENE

anti-7-Chloro-2-(trimethylsiloxy)norbornene (AntiCl) and syn-7-chloro-2-(trimethylsiloxy)norbornene (SynCl) as well as the epimeric exo-9- and endo-9-chloro-3-methoxy-exo-tricyclo[5.2.1.0(2,6)]dec-3-ene (GExoCl and GEndoCl, respectively) have been synthesized, their electronic absorption and electron transmission (ET) spectra have been determined and their photochemistry at 254 nm has been studied. MO calculations have been carried out to aid in the interpretation of the spectra. In contrast to exo-6-chloro-2-(trimethylsiloxy)norbornene (ExoCl), the calculations show that the LUMO's of AntiCl and SynCl contain relatively little C-Cl sigma* character, similar to endo-6-chloro-2-(trimethylsiloxy)norbornene (EndoCl). This is consistent with the relatively low (compared to ExoCl) quantum efficiencies of disappearance for these compounds (PHI(dis)(AntiCl) = 0.0072, PHI(dis)(SynCl) = 0.0027). The ET spectra reveal that the pi* levels in all of the chloro-substituted compounds are appreciably stabilized relative to that of 2-(trimethylsiloxy)-norbornene (TMSNB). However, a natural bond orbital (NBO) analysis shows that much of this stabilization derives from electric field (i.e., inductive) effects. Only in the case of ExoCl does the NBO analysis indicate sizable through-space (TS) and through-bond (TB) coupling between the pi* and C-Cl sigma* orbitals. Ab initio calculations on the extended chloro enol ethers GExoCl and GEndoCl (i.e., gamma-chlorides) show that the LUMO's of these molecules contain a small amount of C-Cl sigma* character. In addition, an NBO analysis shows that there are small TB interactions between the enol ether sigma* and C-Cl sigma* orbitals in both GExoCl and GEndoCl. However, photoinduced cleavage of the C-Cl linkage following pi --> pi* excitation at 254 nm is not observed in these compounds.