Thermochemistry of gaseous ethylsilanes and their radical cations

The dissociation behavior of energy-selected tetraethylsilane, triethylsilane, and diethylsilane photocations is studied using the threshold photoelectron–photoion coincidence (TPEPICO) technique. In the 8–12.5 eV photon energy range, 0 K dissociation onsets have been measured from the TPEPICO data. The dissociation channels observed include loss of ethane, hydrogen molecule, ethyl radical and hydrogen atom, depending upon the molecular ion under investigation. The thermochemistry of the molecular ions and dissociation fragments is obtained by an analysis that takes into account the kinetics and internal energy distributions of the ions. The various dissociation onsets permit the reevaluation of both neutral and ionic silane thermochemistry. We observed 298-K ethyl group values of 60 ± 10 and 94 ± 10 kJ mol⁻¹ for neutral and ionic silanes, respectively. These values are considerably smaller than the previously reported values of 86 and 130 kJ mol⁻¹, respectively. Finally, a Δ_fH° (298 K) of −141.5 ± 21 kJ/mol for neutral diethyl silane is derived from the dissociative ionization onset of diethylsilane.