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, OH₄ ²⁺, SH₄ ²⁺, and SeH₄ ²⁺. The tetrahydridoselenonium dication SeH₄ ²⁺ is predicted to have a tetrahedral (T _d )structure, similar to OH₄ ²⁺ and SH₄ ²⁺, with short bonds to hydrogen (1.483 Å). Although deprotonation of SeH₄ ²⁺ is thermodynamically favored Cby 104 kJ mol^–1), such a reaction is inhibited by a large barrier (240kJmol^–1]. Thus, SeH₄ ²⁺ 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 SeH₄ ²⁺ is very high (2483 kJ mol^–1], as is the case for OH₄ ²⁺ and SH₄ ²⁺. Of the group IV onium dications (OH₄ ²⁺, SH₄ ²⁺, and SeH₄ ²⁺), SeH₄ ²⁺ displays the greatest kinetic and thermodynamic stability toward proton loss. Substantial solvent stabilization is required in order to generate SeH₄ ²⁺ in solution.