PHOTOELECTRON-SPECTROSCOPY OF A CARBENE/SILYLENE/GERMYLENE SERIES

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 assigned 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 ionization 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 carbene center (C sigma-lp). The second ionization from the carbene occurs from a a-molecular orbital (pi-3) that is largely the C=C double bond in the imidazole ring with some contributions from the nitrogens and the carbene center. The HOMOs of the silylene and germylene are derived from the pi-3, orbital which changes character to become more concentrated on the two-coordinate main group IV center (Si or Ge) and less involved with the C=C double bond. The Si sigma-lp and Ge sigma-lp orbitals 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 structure and chemistry in the carbene, silylene, and germylene compounds. The X-ray structure of 1,3-di-tert-butylimidazol-2-ylidene is also reported.