 Solution

The acceptor-strength trend is amide oxygen and amine nitrogen strongest, ketone oxygen weaker, hydrocarbon essentially nonaccepting. trimethylamine and isopropylamine each provide a relatively available nitrogen lone pair; adding N–H bonds changes donor ability much more conspicuously than it changes the acceptor ranking in this comparison. Alkyl substitution and solvation modify the precise values, so the matching amine parameters should be read as the stated empirical strengths rather than a universal equality of all amines.

In acetamide, nitrogen lone-pair donation contributes an ${\mathrm{O}}^{-}-\mathrm{C}={\mathrm{N}}^{+}$ form contributing to resonance in chemistry, increasing electron density at oxygen and making it a strong hydrogen bond acceptor. The amide acceptor is oxygen, not nitrogen: the nitrogen lone pair is delocalized into the carbonyl. acetone also accepts at oxygen, but lacks this nitrogen-to-carbonyl donation and has the smaller hydrogen-bond acceptor parameter. isobutane has neither a heteroatom lone pair nor a comparable acceptor site; its zero value is consistent with the absence of an ordinary directional hydrogen bond acceptor.