Question

IR and Ramen spectra of crystalline barium rhodizonate have been reported. from an inspection of the data. it was concluded that the rhodizonate ion, C₆O₆^2-probably had D_6hsymmetry. the other possible structure can be deduced the rhodizonic acid molecule. In the D6h ion there is no C=C compared to the C_2hequivalent. Compare two anions and determine which anion fits the data shown below.

235 (IR) 1051 (IR) 280 (Ramen) 346 (Ramen) 436 (Ramen) 553 (Ramen) 1449 (IR) 386 (IR) 1546 (Ramen) 1252 (Ramen) 1669 (Ramen)

Answer 1

If the rhodizonate ion is planar and symmetrical (all C-C and C-0 bonds equivalent) it will belong to the D_6h point group. In this case, there will be eleven fundamental frequencies seven of which are Raman active and four i.r. active. No coincidences should be observed between i.r. and Raman frequencies. Any other less symmetrical structure would give rise to a much larger number of fundamentals and most alternative structures would show some i,r./Raman coincidences. In the observed i.r. spectrum, four strong bands were observed at 235, 386, 10511, and 1449 cm-1. The Raman spectrum contained six bands at 280, 346, 436, 553, 1252, 1546 and 1669 cm-l. No coincidences between i.r. and Raman bands were observed. The observed spectra therefore strongly favours D 6h symmetry for the anion. In support of this conclusion, no strong i.r. or Raman frequencies were found in the region normally associated with C=C stretching modes.The absence of strong Raman bands characteristic of carbonyl stretching modes normally found near 1700 cm-l provides further evidence for the delocalized structure. Two strong carbonyl bands at 1686 and 1711 cm-1 are found in the Raman spectrum of free rhodizonic acid. The available evidence therefore fully supports a planar, resonance stabilized structure for the C₆O₆^2- ion in the barium salt.