Forbidden transitions

Definition

Forbidden transitions refer to electronic transitions in a molecule that have very low probabilities of occurring due to quantum mechanical selection rules. These transitions typically involve changes in molecular symmetry or spin states that do not conform to the allowed transitions under conventional spectroscopic rules, leading to their rarity in processes such as UV-visible spectroscopy and Raman spectroscopy.

5 Must Know Facts For Your Next Test

1. Forbidden transitions are less likely to occur than allowed transitions, making them typically weaker and less intense in spectroscopic measurements.
2. The probability of a forbidden transition can be increased through factors like higher temperatures or the presence of certain catalysts.
3. In UV-visible spectroscopy, forbidden transitions often result in spectral bands that are either very weak or require specific conditions to be observed.
4. Raman spectroscopy can detect forbidden transitions if the vibrational mode associated with the electronic transition is Raman active, despite being an otherwise forbidden electronic transition.
5. Certain molecular symmetries can lead to specific selection rules that govern which transitions are allowed or forbidden, influencing the observable spectra.

Review Questions

• How do selection rules impact the occurrence of forbidden transitions in spectroscopy?
  • Selection rules are critical because they outline the conditions under which electronic transitions can occur. Forbidden transitions arise when these rules dictate that a transition involving certain changes in angular momentum or symmetry is not allowed. As a result, when examining spectra, the presence of forbidden transitions indicates that while they may exist, they will be significantly weaker and less likely to be detected compared to allowed transitions.
• Discuss how temperature affects the observation of forbidden transitions in UV-visible spectroscopy.
  • Temperature plays a crucial role in the observation of forbidden transitions because higher temperatures provide sufficient energy to excite molecules into states that might otherwise be unreachable. This increased thermal energy can enhance the likelihood of these rare transitions occurring by promoting population of excited states and can also increase the intensity of spectral lines associated with forbidden transitions. Thus, measuring spectra at elevated temperatures can reveal previously undetectable features attributed to these otherwise weak processes.
• Evaluate the significance of forbidden transitions in understanding molecular symmetry and its implications for spectroscopic techniques.
  • Forbidden transitions serve as a significant indicator of molecular symmetry and the limitations imposed by selection rules in spectroscopic techniques. By analyzing these transitions, researchers gain insight into the underlying symmetry properties of molecules and how they influence electronic structure. The ability to identify and study forbidden transitions enhances the understanding of molecular behavior, interactions, and reactivity, ultimately refining analytical methods like UV-visible and Raman spectroscopy for more accurate characterization of complex systems.

Related terms

Selection Rules: Criteria that determine whether a transition between quantum states is allowed or forbidden based on properties like symmetry and angular momentum.

Oscillator Strength: A measure of the probability of absorption or emission of light by an electron transitioning between energy levels; forbidden transitions have very low oscillator strengths.

Raman Activity: A property that determines whether a vibrational mode can be observed in Raman spectroscopy, which is often linked to molecular symmetry and allowed or forbidden transitions.