Hydrogen bonding involving the hexacyanocobaltate(III) anion. 1. Cobalt-59 nuclear magnetic resonance studies

Donald R. Eaton, Carol V. Rogerson, Alan C. Sandercock

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24 Citations (Scopus)


59Co chemical shifts, line widths, viscosities, and electronic spectral data are reported for solutions of potassium hexacyanocobaltate(III) in mixed solvents involving dimethyl sulfoxide, water, formic acid, trifluoroacetic acid, and propionic acid. The chemical shifts cover a range of some 350 ppm, and it is concluded that hydrogen-bonding effects are responsible for the differences. The larger high-field shifts and by inference the stronger hydrogen bonds are associated with the stronger acids. There is evidence for preferential solvation of the hexacyanocobaltate anion in mixtures of dimethyl sulfoxide and acids, but not in water/acid mixtures. In the dimethyl sulfoxide mixtures the hydrogen bonds become progressively weaker as more acid molecules are incorporated into the second coordination sphere. Interactions with potassium or hydrogen cations do not contribute significantly to the shifts or line widths. The energy of the first d-d electronic transition shows parallel changes. The NMR line widths show only modest changes in mixed aqueous solutions, but larger changes (factors of up to several hundred) in dimethyl sulfoxide solutions. Since T1 is reduced by a similar factor, "chemical exchange" is not responsible for the broadening. The larger changes in relaxation rate are associated with the formation of the stronger hydrogen bonds. Relaxation rates are not proportional to viscosity, showing that changes in relaxation mechanism rather than simple variations of the correlation time are involved. These results are interpreted in terms of changes in the lifetimes of the hydrogen bonds. The stronger hydrogen bonds (with trifluoroacetic acid) have lifetimes greater than the rotational correlation time of the complex leading to an efficient quadrupolar relaxation mechanism for complexes with unsymmetrically substituted second coordination spheres. Estimates of the quadrupole coupling constants and the rotational correlation times show that this mechanism suffices to account for the observed line widths. The weaker hydrogen bonds (with water) have lifetimes shorter than the rotational correlation times, and other relaxation mechanisms dominate. Formic acid shows intermediate behavior.

Original languageEnglish
Pages (from-to)1365-1371
Number of pages7
JournalJournal of Physical Chemistry
Issue number8
Publication statusPublished - 1982
Externally publishedYes

ASJC Scopus subject areas

  • General Engineering
  • Physical and Theoretical Chemistry


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