H3 Chemistry 9813 · Study focus: H3 Chemistry: Explain nuclear spin
H3 Chemistry: Explain nuclear spin
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Explain nuclear spin
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
A proton behaves as a tiny magnet
A 1H nucleus has spin quantum number I=1/2 and an associated magnetic moment.
Without an applied field its orientations have equal energy; field B0 splits them into lower-energy aligned and higher-energy opposed states.
Nuclear spin states
Nuclear spin is intrinsic angular momentum of a nucleus; non-zero spin gives magnetic behaviour relevant to NMR.
For 1H in B0 there are two allowed spin states, conventionally α lower and β higher.
At thermal equilibrium slightly more protons occupy the lower α state than β. The energy separation grows with field strength.
NMR observes transitions between nuclear-spin states, not electron promotion between molecular orbitals.
Text alternative: Text alternative names axes/field condition, state order, population relation and the meaning of ΔE.
Population difference creates a signal
At thermal equilibrium slightly more protons occupy the lower α state than β. The energy separation grows with field strength.
NMR observes transitions between nuclear-spin states, not electron promotion between molecular orbitals.
Place 1H states in a field
At B0=0, draw one degenerate line. At B0>0, draw α below β and label ΔE.
Show a slightly larger α population; do not calculate ΔE quantitatively.
- Draw the 1H nuclear-spin energy diagram before and after applying B0.
Open the feedback checkpoint after attempting
- Credit degeneracy at zero field, two split levels in field, α lower than β and a small population excess in α.
Choose an NMR-active nucleus
1H has I=1/2 and is NMR active. A supplied isotope with I=0 has no nuclear magnetic moment for this experiment.
Use supplied spin information; do not infer activity only from atomic number.
- Compare 1H and a supplied I=0 nucleus in B0.
Open the feedback checkpoint after attempting
- Only 1H supplies split magnetic spin states under the stated data.
Separate nuclear from electron spin
A proton NMR line reports a proton environment through nuclear shielding.
Electron-spin resonance is a different technique; do not describe HOMO–LUMO excitation.
- Correct an explanation that attributes 1H NMR to electron transitions.
Open the feedback checkpoint after attempting
- Replace electron energy levels with nuclear-spin states split by B0.
Spin is not literal orbital motion
Intrinsic spin is a quantum property; the nucleus is not modelled as a classical ball physically rotating on an axis.
The useful consequence is its magnetic moment and quantised orientations in B0.
- Repair: “A proton produces NMR because it orbits around the molecule.”
Open the feedback checkpoint after attempting
- State intrinsic nuclear spin, magnetic moment and field-split states.
From spin to resonance
Complete twelve fixed spin-state checks, then unseen 13C and different 19F re-test cases.
Next objective: explain energy absorption at /learning/h3-nmr-energy-absorption-lesson.html.
- Explain the two-level 1H diagram without instrumentation detail.
Open the feedback checkpoint after attempting
- Credit I=1/2, α/β splitting in B0, population difference and nuclear—not electronic—states.