Chemistry · Grades 11–12
Spectroscopy: combine clues carefully
Use simplified provided evidence instead of treating one peak as a full identification.
Learn the idea
IR
Infrared absorptions can suggest bond types; positions shift with molecular environment.
NMR
Chemically equivalent nuclei can give a shared signal; symmetry affects the count.
Mass
A correctly assigned molecular-ion signal can inform molecular mass, but not all spectra have an obvious intact molecular ion.
Evidence
Several structures can fit one clue, so combine independent observations and stated reference ranges.
Worked examples
What does a strong absorption in a supplied carbonyl reference range primarily suggest?
A characteristic absorption is a structural clue, not a full identity.
A carbonyl group may be present.
What can symmetry do to the number of distinct NMR environments?
Equivalent nuclei need not produce separate environments.
Make several nuclei chemically equivalent.
Check your understanding
In an idealized proton-decoupled ¹³C spectrum of ethane, how many carbon environments are expected?
In the same simplified ¹³C model for propane, how many carbon environments are expected?
Why should a missing visible molecular-ion peak not be taken as proof that the molecule has no mass?
Answers and marking notes
Answers and marking notes
- 1
The two carbons are equivalent by molecular symmetry.
- 2
The two end carbons are equivalent, while the middle carbon is different.
- Ionization and fragmentation can obscure the intact molecular ion.
The instrumental evidence has limits.
Further reading and review references
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.