Advanced Topic 969 of 1102 Analytical, Lab & Spectroscopy
Selection Rules
Rules determining which spectroscopic transitions are allowed.
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In-depth explanation of Selection Rules
Selection rules are quantum mechanical constraints that determine which transitions between energy states are allowed and which are forbidden, and they set the intensities of spectral lines. For infrared absorption a vibration must change the dipole moment, while for Raman scattering it must change the polarizability. Electronic transitions in atoms obey delta l equal to plus or minus 1 and the spin selection rule delta S equal to zero. In centrosymmetric molecules the Laporte rule forbids transitions between states of the same parity, so d-d transitions are weak. Forbidden transitions are not absolutely impossible but appear with low intensity or only through vibronic coupling.
Going deeper: Selection Rules
Key ideas: Key rules: electronic dipole transitions need delta l = plus or minus 1, delta S = 0 and Laporte (g to u). Vibrational IR: delta v = plus or minus 1 and dipole change. Pure rotational: delta J = plus or minus 1 and permanent dipole. Raman rotational: delta J = 0, plus or minus 2. Rules rest on the transition dipole moment integral, which must be non-zero, and the result of symmetry analysis.
Common mistakes and connections: Students call forbidden transitions impossible, when they appear weakly through vibronic coupling, spin-orbit coupling or tunnelling. Selection rules apply to single-photon transitions, and two-photon processes follow different rules. Using IR rules for Raman gives wrong predictions. Tetrahedral complexes lack inversion centres, so d-d bands are stronger than in octahedral ones. Related to Beer-Lambert, spectroscopy and group theory.
Visualization of Selection Rules
In the hydrogen atom only transitions obeying the selection rules, such as delta l of plus or minus 1, appear strongly in the spectrum. The interactive lesson shows this as a live animation or chart.
Worked example and use case
The d-d transitions of octahedral transition metal complexes are weak because of the Laporte selection rule.
Frequently asked questions about Selection Rules
What is Selection Rules?
Rules determining which spectroscopic transitions are allowed.
Is Selection Rules a beginner, intermediate or advanced topic?
Selection Rules is a advanced topic in Analytical, Lab & Spectroscopy. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Selection Rules?
Study these earlier topics in Analytical, Lab & Spectroscopy first: Rigid Rotor Model, Molecular Symmetry, Group Theory in Chemistry.
What comes after Selection Rules?
The next topics in Analytical, Lab & Spectroscopy are Franck-Condon Principle and Beer-Lambert Limitations.
Can I practise Selection Rules with quizzes?
Yes. The interactive lesson includes a self-check question on Selection Rules, and the Analytical, Lab & Spectroscopy quiz draws on every topic in this area.
Where Selection Rules fits in Analytical, Lab & Spectroscopy
Selection Rules is topic 93 of 99 in Analytical, Lab & Spectroscopy.