Compare electronic, vibrational, rotational and nuclear energy quantisation

A molecule can change its electronic arrangement, vibrate, rotate, or change the orientation of a suitable nuclear spin in a magnetic field. Each kind of motion or state has its own quantised energy levels.

GCE A-Level H3 Chemistry 9813-2027

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Learning objectives

Compare electronic, vibrational, rotational and nuclear energy quantisation

A molecule has several kinds of energy

The important comparison is the typical size of the gaps, because a transition occurs only when radiation supplies a matching photon energy.

Four quantised energy families

Electronic levels describe different electron configurations or molecular-orbital occupations. Their gaps are generally the largest of the four considered here and are probed by UV/visible radiation.

Vibrational levels describe quantised bond stretching and bending, commonly probed by infrared radiation. Rotational levels describe whole-molecule rotation and have smaller gaps, commonly in the microwave region.

For nuclei with non-zero spin in a magnetic field, nuclear-spin levels split by a very small amount. Radiofrequency radiation can drive transitions between them in NMR.

Energy scale and nested levels

A useful qualitative order is electronic gap > vibrational gap > rotational gap. Nuclear-spin gaps used in NMR are smaller still under typical conditions.

These energy types are not four unrelated molecules. One electronic state contains several vibrational levels, and each vibrational level contains several rotational levels. A spectrum may therefore show structure arising from more than one type of energy change.

The radiation-region matches are useful guides, not universal labels for every possible transition. Always use the stated molecular process and energy gap as the main evidence.

Worked example: match the technique to the change

A C=O stretching absorption changes the vibrational state without normally promoting an electron to a different molecular orbital. It is therefore associated with infrared radiation.

A π→π* transition changes electronic occupation and usually needs the higher photon energies found in the ultraviolet or visible region.

Try this

Match C=O stretching and a π→π* excitation to the energy type and radiation region.

Check your answer

C=O stretching is vibrational and usually infrared. A π→π* excitation is electronic and usually UV/visible.

Compare rotational and nuclear-spin changes

Rotational spectroscopy changes the rotational state of the whole molecule. NMR changes the state of a suitable nucleus in an applied magnetic field.

Do not describe NMR as the molecule physically spinning faster; nuclear spin is an intrinsic quantum property.

Try this

Why is a radiofrequency NMR transition not a molecular rotational transition?

Check your answer

It changes the nuclear-spin state in a magnetic field, not the rotational energy of the whole molecule.

Use gap size to predict radiation

If two transitions are otherwise comparable, the larger energy gap requires the higher-frequency, shorter-wavelength photon.

Explain the conclusion with ΔE = hf rather than memorising radiation names alone.

Try this

Which normally needs the higher-frequency photon: an electronic transition or a vibrational transition? Explain.

Check your answer

The electronic transition, because its energy gap is generally larger and ΔE = hf.

Common mistake: one radiation region does everything

Infrared radiation does not usually cause a nuclear-spin flip, and radiofrequency radiation does not usually promote a valence electron across a large electronic gap.

A transition occurs when the photon energy and an allowed energy-level difference match; intensity alone cannot compensate for the wrong frequency in this model.

Try this

Correct: ‘A very intense radio wave can cause a normal π→π* transition.’

Check your answer

A normal π→π* gap requires a much higher-frequency UV/visible photon. Greater radio intensity supplies more low-energy photons, not a matching high-energy photon.

Check your understanding

You should be able to name each quantised energy type, order typical gap sizes and connect each to the appropriate region of the electromagnetic spectrum.

Next, follow one photon through an absorption or emission event between two of these discrete levels.

Try this

Write the usual decreasing order of energy gaps for the four types considered.

Check your answer

Electronic > vibrational > rotational > nuclear-spin under typical spectroscopic conditions.