Inner-shell excitations in weak-bond molecules

It is proposed that Rydberg and valence σ* conjugate orbitals have separate existences and can be seen in the same spectrum if the σ* MO can be disentangled from the Rydberg manifold. Because the energy of the σ* MO is a consequence of the σ–σ* split resulting from bond formation, the spectra of molecules having weak bonds should show low-lying transitions to σ* in addition to the conjugate Rydberg bands. Inelastic electron scattering spectra in the x-ray region (270–730 eV) of molecules having bond strengths in the 20–50 kcal/mol regime clearly show well-isolated transitions to low-lying σ* MOs, and in some cases the simultaneous presence of virtual σ* and Rydberg conjugate orbitals. The general characteristics of excitations from C 1s, O 1s, and F 1s inner orbitals to σ* MOs are listed and illustrated by the x-ray spectra of several compounds in which the weak bond involves the O–O or O–F linkage. Quantitative inner-shell optical oscillator strengths derived from the energy loss spectra are reported for H2O, F2O, CF3OF, CF3O2CF3, (CH3)3COH, and (CH3)3CO2C(CH3)3. The valence-shell spectrum of F2O also has been determined. Low-lying inner-shell excitations to σ* valence MOs are identified by their relatively large term values and oscillator strengths. The term values of transitions to σ* MOs in weak-bond molecules correlate with the relevant bond lengths when considered together with the sum of the atomic numbers of the atoms forming the weak bonds.