Density functional theory study of the β-carotene radical cation

The hybrid density functional theory method B3LYP is employed to study the beta -carotene radical cation. The radical is characterized by means of its geometry, spin distribution, and isotropic and anisotropic hyperfine coupling constants. It is shown that the spin is delocalized over the whole pi -conjugated system, including the double bonds of the headgroups. This delocalization results in methyl hyperfine coupling constants lower than 9 MHz, in excellent agreement with recent experimental couplings of the carotene radical in Photosystem II and in vitro, but in conflict with previous theoretical calculations. It is also demonstrated that rotation of the headgroups can affect the properties of the radical, in particular the spin delocalization to the ring.