Normal Modes of [Cr(H2O)6]3+ (roughly octahedral - T symmetry)

    The Cr3+ cation forms a highly stable octahedral complex with six water molecules in acidic solutions and in certain salts.  The vibrational frequencies of this complex have been measured in at least one such salt, Cs[Cr(H2O)6](SO4)2.6H2O (Best et al. 1980, Inorg. Chem. 19:1958-1961; Best et al. 1984, J. Chem. Soc. Dalton Trans. 2611-2624).  In order to illustrate the vibrational modes, we have performed an ab initio calculation (HF/6-31G*) on Cr3+ in a cluster of 18 water molecules.  The water molecules arrange themselves into an inner Cr(H2O)6 complex surrounded by a tetrahedral arrangement of twelve outer water molecules.  Vibrations of the inner complex have been separated from the other vibrations, to make it easier to see what's going on.  Because the whole cluster has a lower symmetry (T) than a simple octahedral complex (Oh), all of the triply-degenerate bending vibrations are mixed up in a single symmetry class (F).  Measured frequencies are given for each mode, the ab initio frequencies are also given in parentheses.  For the bending modes, correlations between measured and predicted frequencies are tentative at best.

ν1 (A) - Symmetric Stretch
540 cm-1 (517 cm-1)

 ν3 (F) - Stretch
560 cm-1 w/ 50Cr (556 cm-1)
555 cm-1 w/ 53Cr (550 cm-1)

ν4? (F) - Bend
329 cm-1 (321 cm-1)

ν2 (E) - Stretch
502 cm-1 (464 cm-1)

 ν5? (F) - Bend
320 cm-1 (281 cm-1)

ν6? (F) - Bend
(225 cm-1)

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