Does Tetrahydrofuran Ring Open upon Ionization and Dissociation? A TPES and TPEPICO Investigation

The threshold photoelectron spectrum (TPES) of tetrahydrofuran (THF) is compared to that of the unsaturated furan molecule. In general, there is a similarity in the orbital ionization profile for the two species, though unlike furan, THF exhibits (modest) vibrational detail only in the (9b)−1 X̃ 2B band. An adiabatic ionization energy of 9.445 ± 0.010 eV has been derived from the onset of the TPES spectrum. Threshold photoelectron photoion coincidence spectroscopy was used to explore the loss of a hydrogen atom from ionized THF over the photon energy range of 9.9−10.4 eV. RRKM fitting of the resulting breakdown curves yields an E 0 of 0.85 ± 0.03 eV (82 ± 3 kJ mol−1) (AE = 10.30 ± 0.04 eV). If the G3 IE of 9.48 eV is used to convert the experimental data from photon energy to THF ion internal energy, E 0 = 0.81 ± 0.01 eV (78 ± 1 kJ mol−1). The latter value is closer to the G3 E 0 of 72 kJ mol−1 for the formation of the cyclic ion 1. A variety of ring-opening reactions were also probed at the B3-LYP/6-31+G(d) and G3 levels of theory. The distonic isomer •CH2CH2CH2OCH2 + lies 70 kJ mol−1 higher than ionized THF, which places it within 1 kJ mol−1 of the threshold for the dissociation to 1. All of the probed H-loss products from the distonic isomer (which includes singlet and triplet species) lie significantly higher in energy than ion 1, eliminating the possibility that ionized THF dissociates to m/z 71 via a ring-opening reaction in the present experiment. The derived Δ‡ S value for the dissociation, 8 ± 5 J K−1 mol−1, is also consistent with the formation of 1. The experimentally derived E 0 values can be used to derive the Δf H o 0 for ion 1. Together with the Δf H o 0 values for the THF ion (752.0 ± 2 kJ mol−1, derived from the neutral Δf H o 0 of −154.9 ± 0.7 kJ mol−1 and experimental IE of 9.445 ± 0.010 eV) and H atom (218.5 kJ mol−1) our E 0 of 82 ± 3 kJ mol−1 yields a Δf H o 0 for ion 1 of 620 ± 4 kJ mol−1 (Δf H o 298 = 594 ± 4 kJ mol−1), in good agreement with the G3 Δf H o 0 of 621 kJ mol−1. Appearance energies for all fragment ions up to photon energies of 34 eV are also reported and discussed in comparison with the available literature.