A strong absorption at 1720 cm-1 in the IR spectrum is indicative of a carbonyl group C=O present in the compound. However, this information alone is not sufficient to determine the exact structure of the unknown organic compound. We need more information from the IR spectrum or other spectroscopic techniques such as NMR or mass spectrometry to further narrow down the possibilities.Nonetheless, based on the presence of a carbonyl group, the unknown compound could belong to one of the following functional groups:1. Aldehydes R-CHO 2. Ketones R-CO-R' 3. Carboxylic acids R-COOH 4. Esters R-COO-R' 5. Amides R-CONR'R'' To propose a possible structural formula, we would need more information about the compound, such as additional IR peaks, molecular weight, or other spectroscopic data.