Spectroscopic Structure Analysis
Using mass spectra, infrared spectra and carbon-13 and proton NMR evidence together to determine molecular formula, functional groups and structure.
How to study A-level Chemistry
Represent the species and process correctly, conserve atoms, charge and energy, calculate with units, then test the result against observations and chemical evidence.
Core concepts
Concept 1
Mass spectrometry provides molecular and fragment mass evidence; accurate mass can distinguish possible formulae.
Exam cue: Establish molecular formula and unsaturation before proposing detailed structures.
Concept 2
Infrared absorption identifies bonds and functional groups while the fingerprint region supports comparison.
Exam cue: Treat each spectrum as a constraint and cross-check all evidence against the same structure.
Concept 3
NMR chemical shift, number of environments, integration and splitting constrain carbon-hydrogen environments and connectivity.
Exam cue: For proton NMR, use chemical shift, integration and n+1 splitting together.
Risk pitfalls and guardrails
Assigning a structure from one IR peak alone.
Guardrail: Do not substitute a memorised equation, mechanism or trend until you have identified the species, conditions and chemical model required.
Treating the tallest mass-spectrum peak as always the molecular ion.
Guardrail: Do not substitute a memorised equation, mechanism or trend until you have identified the species, conditions and chemical model required.
Using proton integration as the exact atom count without scaling to the molecular formula.
Guardrail: Do not substitute a memorised equation, mechanism or trend until you have identified the species, conditions and chemical model required.
Memory anchors
Molecular Ion
The molecular-ion peak can provide the relative molecular mass of the intact molecule.
Infrared Spectrum
IR absorption at characteristic wavenumbers indicates particular bonds.
Chemical Shift
NMR chemical shift reflects the electronic environment of a nucleus.
Integration
Proton-NMR integration gives relative numbers of protons in each environment.
n+1 Rule
A proton environment is commonly split into n+1 peaks by n equivalent neighbouring protons.
Checkpoint rule
Do the check-up only after you can summarize each concept in one sentence and identify one dangerous pitfall from memory.
Knowledge Check (after reading)
Short check-up to confirm understanding of this module.
Check-up Questions
Electron impact gives a singly charged intact molecular ion at m/z 72. Which relative molecular mass follows?
An organic compound shows molecular-ion peaks M and M+2 in an intensity ratio close to 3:1. Which atom is strongly indicated?
Answer all questions to submit.
Next step personalized recommendations
Continue learning
Move forward only after this module is stable.
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