A combined theoretical and experimental study of the valence and Rydberg states of iodopentafluorobenzene

A new ultraviolet (UV) and vacuum ultraviolet (VUV) spectrum for iodopentafluorobenzene (C₆F₅I) using synchrotron radiation is reported. The measurements have been combined with those from a recent high-resolution photoelectron spectroscopic study. A major theoretical study, which includes both Franck-Condon (FC) and Herzberg-Teller (HT) analyses, leads to conclusions, which are compatible with both experimental studies. Our observation that the VUV multiplet at 7.926 eV in the VUV spectrum is a Rydberg state rather than a valence state leads to a fundamental reassignment of the VUV Rydberg spectrum over previous studies and removes an anomaly where some previously assigned Rydberg states were to optically forbidden states. Adiabatic excitation energies (AEEs) were determined from equations-of-motion coupled cluster with singles and doubles excitation; these were combined with time dependent density functional theoretical methods. Frequencies from these two methods are very similar, and this enabled the evaluation of both FC and HT contributions in the lower valence states. Multi-reference multi-root configuration interaction gave a satisfactory account of the principal UV+VUV spectral profile of C₆F₅I, with vertical band positions and intensities. The UV spectral onset consists of two very weak transitions assigned to 1¹B₁ (πσ*) and 1¹B₂ (σσ*) symmetries. The lowest unoccupied molecular orbital of a σ*(a₁) symmetry has a significant C-I* antibonding character. This results in considerable lengthening of the C-I bond for both these excited states. The vibrational intensity of the lowest 1¹B₁ state is dominated by HT contributions; the 1¹B₂ state contains both HT and FC contributions; the third band, which contains three states, two ππ*(1¹A₁, 2¹B₂) and one πσ*(2¹B₁), is dominated by FC contributions in the ¹A₁ state. In this ¹A₁ state, and the spectrally dominant bands near 6.7 (¹A₁) and 7.3 eV (¹A₁ + ¹B₂), the C-I bond length is in the normal range, and FC components dominate.