PI-ASTERISK SIGMA-ASTERISK ORBITAL MIXING - SYNTHESIS AND EXCITED-STATE STUDIES OF 7-CHLORO-2-(TRIMETHYLSILOXY)NORBORNENE AND CHLORO-3-METHOXY-EXO-TRICYCLO[5.2.1.02,6]DEC-3-ENE
Jj. Nash et al., PI-ASTERISK SIGMA-ASTERISK ORBITAL MIXING - SYNTHESIS AND EXCITED-STATE STUDIES OF 7-CHLORO-2-(TRIMETHYLSILOXY)NORBORNENE AND CHLORO-3-METHOXY-EXO-TRICYCLO[5.2.1.02,6]DEC-3-ENE, Journal of the American Chemical Society, 115(20), 1993, pp. 8969-8979
anti-7-Chloro-2-(trimethylsiloxy)norbornene (AntiCl) and syn-7-chloro-
2-(trimethylsiloxy)norbornene (SynCl) as well as the epimeric exo-9- a
nd loro-3-methoxy-exo-tricyclo[5.2.1.0(2,6)]dec-3-ene (GExoCl and GEnd
oCl, 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 c
arried 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-C
l sigma character, similar to endo-6-chloro-2-(trimethylsiloxy)norbor
nene (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 reve
al that the pi levels in all of the chloro-substituted compounds are
appreciably stabilized relative to that of 2-(trimethylsiloxy)-norborn
ene (TMSNB). However, a natural bond orbital (NBO) analysis shows that
much of this stabilization derives from electric field (i.e., inducti
ve) 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 c
hloro 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 c
haracter. In addition, an NBO analysis shows that there are small TB i
nteractions between the enol ether sigma and C-Cl sigma* orbitals in
both GExoCl and GEndoCl. However, photoinduced cleavage of the C-Cl li
nkage following pi --> pi excitation at 254 nm is not observed in the
se compounds.