The infrared spectra of two oxoanions are summarized in the table below. Each ion is planar and contains three C–O bonds. Assume the highest‐frequency absorption in the table corresponds to the stretching of the shortest C–O bond in each ion.
Ion
Prominent IR stretch (cm−1)
Formate, HCOO−
1600
Carbonate, CO32−
1415
Additional data: a typical C=O double bond stretches near 1715cm−1, whereas a C–O single bond stretches near 1100cm−1.
(a) Draw all resonance contributors for the formate ion and for the carbonate ion. Indicate formal charges on each contributor.
(b) Using the resonance contributors from part (a), determine the average (fractional) bond order for the C–O bonds in each ion.
(c) Explain how the average bond orders you obtained in part (b) account for the difference in the highest‐frequency IR absorption between the two ions.
(d) Predict which ion will have the shorter average C–O bond length and justify your prediction based on your analysis.