Structure and stability of the tetrahydridoselenonium dication,SeH4 2+

Ab initio molecular orbital theory with triple-zeta-valence plus polarization basis sets and with electron correlation incorporated at the fourth-order Møller-Plesset level has been used to study the tetrahydridoonium dications, OH₄²⁺, SH₄²⁺, and SeH₄²⁺. The tetrahydridoselenonium dication SeH₄²⁺ is predicted to have a tetrahedral (T_d)structure, similar to OH₄²⁺ and SH₄²⁺, with short bonds to hydrogen (1.483 Å). Although deprotonation of SeH₄²⁺ is thermodynamically favored Cby 104 kJ mol^–1), such a reaction is inhibited by a large barrier (240kJmol^–1]. Thus, SeH₄²⁺ lies in a deep potential well and as an isolated species should have a long lifetime in the gas phase. The estimated heat of formation, H°_f, for SeH₄²⁺ is very high (2483 kJ mol^–1], as is the case for OH₄²⁺ and SH₄²⁺. Of the group IV onium dications (OH₄²⁺, SH₄²⁺, and SeH₄²⁺), SeH₄²⁺ displays the greatest kinetic and thermodynamic stability toward proton loss. Substantial solvent stabilization is required in order to generate SeH₄²⁺ in solution.