Studies of unoccupied orbitals of BF3 and BCL3 by electron transmission spectroscopy and multiple scattering Xα calculations

Electron transmission spectroscopy (ETS ) and bound-state and continuum multiple-scattering Xα (MS -Xα) calculations are employed to characterize the unoccupied a′₂, a′₁ and e′ orbitals of BF₃ and BCl₃. The a′₂ orbital of BF₃, which produces a peak about 7 eV below threshold in the x-ray absorption spectrum (XAS ), generates a scattering resonance at 3.5 eV in ETS. Similarly, the e′ orbital that lies about 2 eV above threshold in XAS occurs about 13?16 eV above threshold in ETS . Dissociation of F^? from BF₃ due to electron attachment is attributed to a core-excited shape resonance involving an e″ → a″₂ excitation and electron capture into the a′₂ orbital. In BCl₃ all the unoccupied orbitals lie at lower energy than in BF₃ and are closely spaced, making definitive spectral assignments difficult. Both Hartree-Fock (HF ) and MS -Xα methods apparently underestimate the stability of the unoccupied e′ orbital of BCl₃. Vibronic coupling due to out-of-plane bending may significantly affect the spectral intensities. Feshbach resonances are observed for BCl₃ at energies close to those observed in the vacum-UV absorption spectrum. The lower energies of the unoccupied orbitals of BCl₃ are consistent with their stronger bonding to nucleophiles.