Self-Consistent Heats of Formation for the Ethyl Cation, Ethyl Bromide, and Ethyl Iodide from Threshold Photoelectron Photoion Coincidence Spectroscopy

The dissociative photoionization onsets of Br and I loss reactions were measured for C2H5Br and C2H5I, respectively, by threshold photoelectron photoion coincidence (TREPICO) spectroscopy to establish the heats of formation of these two basic ethyl halides. The appearance energy of ethyl cation from ethyl bromide was found to be 1074.2 +/- 0.8 kJ mol(-1), and that from ethyl iodide was found to be 1016.4 +/- 0.8 kJ mol(-1). The heats of formation of ethyl bromide and ethyl iodide are interconnected through the ethyl cation. In establishing the thermochemistry of the ethyl halides, the ethyl cation heat of formation was concluded to be 915.5 +/- 1.3 kJ mol(-1) on the basis of a recent value for ethyl radical heat of formation and the well-established ionization energy and on the basis of ab initio isodesmic calculations using recent PEPICO data. Using this anchor, we obtained the following heats of formation: Delta H-f(0k)o[EtBr] = -40.8 +/- 1.5 kJ mol(-1) and Delta H-f(0k)o[EtI] = 6.3 +/- 1.5 kJ mol(-1). These results are more consistent with the higher Et Br heats of formation values in the literature, contrary to recent findings. For ethyl iodide, the latest calorimetric value does not agree within the claimed accuracy.