Dielectric Relaxation Study of the Ion Solvation and Association of NaCF3SO3 ,
Mg(CF3SO3)2 ,
and Ba(ClO4)2 in N,N-Dimethylformamide,
A. Placzek, G. Hefter, H. M. A. Rahman and R. Buchner,
J. Phys. Chem. B115 (2011) 2234-2242. DOI: 10.1021/jp1116307
Abstract
Solutions of sodium trifluoromethanesulfonate, magnesium trifluoromethanesulfonate and barium perchlorate in
N,N-dimethylformamide (DMF) have been investigated using broadband dielectric relaxation spectroscopy at
25 °C. All spectra were dominated by a solvent relaxation process centred at ~15 GHz but also exhibited
one (for NaCF3SO3) or two (for the 2:1 salts) low amplitude processes, centred at frequencies below 2 GHz,
that could be attributed to the presence of ion pairs. Effective solvation numbers calculated from the
solvent relaxation amplitudes indicated strong solvation of all three cations, with evidence for the formation of a second
solvation sheath for Mg2+ and possibly Ba2+. Detailed analysis of the solute-related
processes showed that solvent-shared ion pairs (SIPs) were formed in NaCF3SO3 solutions
in DMF. The data for Mg(CF3SO3)2 and Ba(ClO4)2
solutions were not definitive but, consistent with the solvation evidence, favoured the presence of double
solvent-separated ion pairs and SIPs. Overall association constants, KA, were small for all three salts
in DMF and increased in the order: NaCF3SO3 < Ba(ClO4)2
< Mg(CF3SO3)2.
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