Resonance Raman spectroscopic studies on the Group 6B polyatomic cations S42+, Se42+, and Te42+

Abstract

The Raman and resonance Raman (r.R.) spectra of S₄²⁺ and Se₄²⁺ in oleum at room temperature, of Te₄²⁺ in H₂SO₄, and of Te₄[Al₂Cl₇]₂ at ca. 80 K, have been recorded with a variety of excitation lines. The r.R. spectra display long progressions, v₁ν₁, in the ν₁(a_1g) mode in each case, together with shorter progressions of the sort v₁ν₁+ν₂(b_1g). Analysis of the results leads to harmonic wavenumbers (ω₁) of 584.7, 321.8, and 219.5 cm^–1 for S₄²⁺, Se₄²⁺, and Te₄²⁺, respectively in solution, and anharmonicity constants (x₁₁) of –0.35, –0.55, and –0.30 cm^–1, respectively. Stretching force constants (valence force field) for S₄²⁺, Se₄²⁺, and Te₄²⁺ in solution are calculated to be 2.78, 2.10, and 1.45 mdyn Å^–1, respectively, the trans-bond stretch–stretch interaction constants being more than an order of magnitude greater than the cis-bond ones. The ν₁ band excitation profile of each ion maximises within the band contour of the lowest allowed electronic transition of each ion, i.e. that at 330 nm for S₄²⁺, 410 nm for Se₄²⁺, and 510 nm for Te₄²⁺. These data, coupled with detailed measurements on the Raman band polarizations at resonance, indicate that the assignment of the lowest allowed transition of each ion is π*(b_2u)â†�π(e_g),¹E_uâ†�¹A_1g.