FIGURE 1. C–P–F triad.

Structure (top) and reaction scheme (bottom) of the C–P–F triad used to demonstrate the principle of a chemical compass. The interconversion of the singlet (S) and triplet (T) states of the radical pair [ C ⁺–P–F ^-] is driven by magnetic hyperfine interactions and is modulated by the Zeeman interaction with an external magnetic field. [ C ⁺–P–F ^-] can recombine spin-selectively, with rate constants k_S and k_T. As the hyperfine interactions are anisotropic, and k_S  k_T, the lifetime of the radical pair reaction depends on its orientation with respect to the external magnetic field. The inset at top left is a representation of the anisotropic hyperfine interactions in C ⁺; 22 of the 46 protons in this radical have isotropic hyperfine couplings larger than 100 T. F ^-, by contrast, is almost devoid of hyperfine couplings. h: light excitation.