Bond-angle and bond-length dependence of bound-state resonances in inner-shell excitation spectra of SF4

SF4 has a pseudo-trigonal bipyramidal (C2v) structure with a ligand missing at one of the equatorial sites (S-F bond lengths: R(ax) > R(eq)) and an approximate electronic structure of S4+[(3s)2(3p)0]·(F−)4 with unoccupied S 3pσ orbitals of a1, b1(ax) and b2(eq) symmetry. Contrary to an empirical relationship between bond lengths and σ resonance energies, ab initio Δ SCF calculations for the core-to-valence bound-state resonances predict the ordering b(eq) < a1 < b (ax), indicating that the bond angle effect is dominant in SF4. The bond-angle dependence is discussed qualitatively in terms of crystal field theory and Walsh diagrams. The bond-length and bond-angle dependence are discussed quantitatively via the ab initio calculations.