The Radical Pair Mechanism (RPM) is the foundational theory in spin chemistry that explains how extremely weak magnetic fields, such as the Earth's geomagnetic field, can alter the outcome of chemical reactions.

Here is a brief breakdown of how it works:

1. Formation of a Radical Pair When certain molecules are energized (often by absorbing light), an electron is transferred from one molecule to another. This creates a "radical pair"—two molecules that each possess an unpaired, magnetic electron. Because these two electrons originally shared a chemical bond, their spins remain quantum entangled even after they separate.

2. Singlet vs. Triplet States The radical pair exists in one of two quantum spin states:

• Singlet: The electron spins point in opposite directions (anti-correlated).
• Triplet: The electron spins  ...