SpinselectionRegeln

SpinselectionRegeln, often translated as spin selection rules, are fundamental principles in spectroscopy that dictate which transitions between energy levels are allowed or forbidden based on the change in the spin quantum number of an electron or nucleus. These rules are derived from the conservation of angular momentum and the nature of the electromagnetic radiation that interacts with the system.

In electronic spectroscopy, such as UV-Vis absorption, transitions are generally governed by the selection rule that the change in the spin quantum number, ΔS, must be zero. This means that transitions are most probable between states that have the same total spin multiplicity. For example, a transition from a singlet ground state (S=0) to a singlet excited state (S=0) is allowed, while a transition to a triplet excited state (S=1) is forbidden. Violations of this rule can occur through mechanisms like spin-orbit coupling, which mixes states of different spin multiplicities, leading to weaker, "forbidden" transitions.

Similar spin selection rules apply in nuclear magnetic resonance (NMR) spectroscopy. Here, the spin quantum number of nuclei, such as protons (¹H) or carbon-13 (¹³C), is relevant. Transitions between nuclear spin states, which are induced by radiofrequency pulses, are governed by the change in the nuclear spin quantum number, ΔI. For many common nuclei, the allowed transitions involve a change of ±1 in the spin quantum number.

The understanding of spin selection rules is crucial for interpreting spectroscopic data, identifying molecular structures, and understanding the mechanisms of photochemical and photophysical processes. They provide a framework for predicting the intensity and appearance of spectral lines, distinguishing between different types of electronic or nuclear transitions.