Silenes ionization potentials

The first ionization potentials of several silenes have been predicted using MINDO/3 calculations (177). The ionization potentials found for the parent silene, 8.95 0.1 eV, (174) and 1,1-dimethylsilene, 7.7 eV (184), 7.5 0.3 eV (191), and 8.14 eV (176), have also been estimated from ab initio (174,176) or thermochemical (184,191) calculations. 

Only a few photoelectron spectra of silenes have been measured. Silene 25 has a first vertical ionization energy of ca. 8.85 eV, in agreement with the theoretical value of 8.95 eV30. The assignment is further supported by the observed fine structure of the first band which is similar to that found for ethene. The first ionization potential of the stable Brook silene 150 is 7.7 eV3, similar to what is found for Me2Si=CH2 2 (7.98 eV)276. 

Scheme 2. Ionization potentials of silene Sa and some methyl substituted derivatives. a Ref 21 - fb Estimated by AMI calculations scaled with the experimental values of silene Sa and l.l-dimethylsilene Sc - c Ref 22.

The photochemical reactivity goes up with increasing number of methyl groups at the double bond and decreasing ionization potential . Key intermediates in both the photochemical and the thermal oxidation of silenes 12, 7 and 13 are the siladioxetanes 14. These species are labile even in low-temperature matrices and could not be identified spectroscopically. Evidence for their formation comes from the observed oxidation products such as complexes 15 between silanones and formaldehyde and formylsilanols 16. 

---