Aj. Arduengo et al., PHOTOELECTRON-SPECTROSCOPY OF A CARBENE SILYLENE/GERMYLENE SERIES/, Journal of the American Chemical Society, 116(15), 1994, pp. 6641-6649
The photoelectron spectra [HeI and He II] are reported for a carbene,
a silylene, and a germylene (1,3-di-tert-butylimidazol-2-ylidene, 1,3-
di-tert-butyl-1,3,2-diazasilol-2-ylidene, and 1,3-di-tert-butyl-1,3,2-
diazagermol-2-ylidene). The experimental photoelectron spectra are ass
igned on the basis of predictions from density functional theory (DFT)
calculations and first-order time-dependent perturbation theory. The
predicted spectra agree well with the experimental ones both in ioniza
tion energy and band intensity. The carbene 1,3-di-tert-butylimidazol-
2-ylidene is found to have a highest occupied molecular orbital (HOMO)
that is essentially the in-plane lone pair of electrons at the carben
e center (C sigma-lp). The second ionization from the carbene occurs f
rom a a-molecular orbital (pi-3) that is largely the C=C double bond i
n the imidazole ring with some contributions from the nitrogens and th
e carbene center. The HOMOs of the silylene and germylene are derived
from the pi-3, orbital which changes character to become more concentr
ated on the two-coordinate main group IV center (Si or Ge) and less in
volved with the C=C double bond. The Si sigma-lp and Ge sigma-lp orbit
als are subjacent and responsible for the second ionization bands. The
molecular orbital structure and total electron distribution predicted
from the DFT calculations are used to illustrate the differences in s
tructure and chemistry in the carbene, silylene, and germylene compoun
ds. The X-ray structure of 1,3-di-tert-butylimidazol-2-ylidene is also
reported.