Ionization Energies of 1‐R‐2,2‐dimesitylethenols, 1,2‐Dimesityl‐2‐phenylethenol, and Some of their Keto Tautomers

The He(Iα) PE spectra of the two pairs of 1(Ph)‐3 and 2–4 have been recorded and assigned by comparison with spectra of related molecules, or by using simple MO arguments. This analysis shows that the PE spectra are in agreement with the assumption that the twist angles observed in the crystalline state are more or less conserved in the gas phase. The observed first adiabatic ionization energies I Ia allow the calculation of the molar reaction enthalpy Δ_rH mθ which accompanies the enolization of the radical cations of the title compounds. It is found that Δ_rH mθ ≈ –−70 kJ mol⁻¹ is roughly the same for both keto/enol pairs, in qualitative and quantitative agreement with expectation. These results complement and correct ionization energies determined previously by mass spectrometric methods and conclusions based on them.