Effect of ion interactions on the IR spectrum of benzenesulfonate ion. Restoration of sulfonate ion symmetry in sodium benzenesulfonate dimer

•Structures and IR spectra of free benzenesulfonate ion and its salts have been calculated.•Bidentate coordination of counter-ions is energetically favorable for isolated ion pairs.•Symmetry of the sulfonate ion is restored for sodium benzenesulfonate dimer.•Experimental and theoretical IR spectra of sodium benzenesulphonate are compared.

Literature data concerning the assignment of IR spectra of benzenesulfonate salts that serve as model compounds for aromatic sulfonate-containing ionomers and polyelectrolytes have been analyzed. The structures and IR spectra of free benzenesulfonate ion and its potassium and sodium salts have been calculated in B3LYP/6-311G(d,p) approximation. The bidentate coordination of counter-ions is energetically favorable for isolated ion pairs. In this coordination, the symmetry of sulfonate ion changes noticeably, which manifests itself as strong splitting of calculated vibrational modes of asymmetric stretching vibrations of SO bonds, Δνas(SO3) = 154 cm−1 (K) and 180 cm−1 (Na). For sodium benzenesulfonate it is thermodynamically favorable to form a dimer (ΔG° = −37.6 kcal/mol) in which the joint effects of monodentate and bidentate coordinated Na cations result in equalization of SO bond lengths and thus a considerable restoration of C3V symmetry of the sulfonate ion. The IR spectrum of the dimer in which Δνas(SO3) splitting is considerably smaller much better matches the experimental spectrum than the spectrum of an isolated ion pair. The major absorption bands in the IR spectrum of sodium benzenesulfonate have been assigned to theoretical vibrational modes of the dimer and, based on visualization of modes, to vibrations of certain bonds in the anion. In particular, the bands at 1200 and 1186 cm−1 have been assigned to νas(SO3), that at 1049 cm−1 to νs(SO3), and those at 628 and 572 cm−1 to δ(oop)s(SO3), and δ(ip)as(SO3), respectively. The strong effect of sulfonate ion environment on the positions of the absorption bands of stretching vibrations of SO bonds makes it necessary to obtain data on exact structures of ion clusters for reliable assignment of absorption bands in experimental IR spectra of real sulfonate-containing systems.

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