IR and NMR: two complementary views of a structure
Combine functional-group clues with atom-environment evidence, while keeping observations separate from interpretation.
Ask each method the right question
IR examines absorption associated with molecular vibrations. NMR provides information about suitable nuclei and their environments or interactions. These approaches look at different features of the same candidate structure, so neither needs to carry the entire interpretation alone.
For a carbonyl-containing candidate, IR may support the presence of a carbonyl group while NMR helps evaluate the expected carbon and proton environments.
Test competing candidates
Write several plausible structures and list what each predicts. Look for observations that disagree with your favourite candidate as well as those that support it. A single band or shift rarely establishes all connectivity. Overlap, sample composition and conditions can complicate the interpretation.
NIST Chemistry WebBook provides IR and mass-spectrometric data where available; it should not be described as a general NMR database. Read the technique and conditions attached to the record.
Use a three-column note
Separate observation, compatible interpretation and open question. Put measured information in the first column and your inference in the second. Record which additional check could resolve the question. This prevents a tentative assignment becoming an unsupported fact.
