An unknown compound gives a molecular ion of m/z 70 in the mass spectrum. It reacts with...

An unknown compound gives a molecular ion of m/z 70 in the mass spectrum. It reacts with semicarbazide hydrochloride to give a crystalline derivative, but it gives a negative Tollens test. The NMR and IR spectra follow. Propose a structure for this compound, and give peak assignments to account for the absorptions in the spectra. Explain why the signal at 1790 cm ^-1 in the IR spectrum appears at an unusual frequency.