Homolytic Se−H Bond Energy and Ionization Energy of Benzeneselenol and the Acidity of the Corresponding Radical Cation

The homolytic Se−H bond energy of benzeneselenol, a very efficient radical trap in solution and in the gas phase, was determined experimentally by using two independent approaches in a Fourier-transform ion cyclotron resonance mass spectrometer. The hydrogen−selenium bond strength was concluded to be 76−80 kcal mol -1 based on the measurement of the efficiency of hydrogen atom abstraction from benzeneselenol by several radical cations. The Se−H bond energy was also determined indirectly through the use of a thermochemical cycle. This approach required the measurement of the adiabatic ionization energy of benzeneselenol and the proton affinity of C 6 H 5 Se • . An adiabatic ionization energy of 8.3 ± 0.1 eV was obtained by measurement of the efficiencies of various electron transfer reactions in the forward and reverse directions. The proton affinity of C 6 H 5 Se • (acidity of the benzeneselenol radical cation) was found to be 200 ± 3 kcal mol -1 based on the measured efficiencies of several proton transfer reactions. These two values yield a homolytic Se−H bond enthalpy of 78 ± 4 kcal mol -1 (at 298 K) for benzeneselenol. This value is significantly different from the only estimate (67 kcal mol -1 ) currently available in the literature.