Electronic, Raman and resonance Raman spectroscopy of [NBu4][RuBr4(MeCN)2]

Abstract

Raman spectra taken at resonance with the e_u(Br pπ) → b_2g(Ru dπ) charge-transfer (CT) transition of the trans-[RuBr₄(MeCN)₂]^– ion, as its [NBu₄]⁺ salt at ca. 80 K, consist of long overtone progressions in ν₁(a_1g), the symmetric RuBr stretch (at 188.5 cm^–1), together with combination band progressions in which ν₁ is the progression-forming mode and the enabling modes include ν₂(b_1g), ν_as(RuBr), and ν₄(b_2g), δ_s(BrRuBr). The excitation profile of the ν₁ band approximately follows the contour of the e_u → b_2g CT transition referred to above, consistent with the operation of the A-term scattering mechanism. The spectroscopic data allow the determination of the harmonic wavenumber (ω₁) and anharmonicity (x₁₁) of the ν₁(a_1g) mode to be 188.8 cm^–1 and –0.15 cm^–1, respectively. Some comparative data on the analogous chloride ion, trans-[RuCl₄(MeCN)₂]^–, are also given.