Siberian Federal University

Protonation constant (K_SeuH) of selenourea (Seu) and TlSeu⁺ stability constant (β₁) were determined by spectrophotometry in aqueous solution (298 K). In general, selenourea reveals the same protonation affinity as thiourea. Quantum-chemical calculations prove that increase of solvent dielectric constant leads to: a) stabilization of selenourea's selenone species, b) rise of selenourea's proton affinity, c) while S-protonated species dominates it becomes less stable than the N-protonated one. The TlSeu⁺ stability constant, determined under ionic strength I = 1 (0,5 M HClO₄ + NaClO₄), made up lgβ₁ = 0,99. Increased TlSeu⁺ stability in comparison with TlTu⁺ (Tu - thiourea) may be explained in terms of higher Se atom polarizability.