The effect of axial Mg ligation on the geometry and spin density distribution of chlorophyll and bacteriochlorophyll cation free radical models: A density functional study

Density functional calculations are performed on models of chlorophyll and bacterio chlorophyll to examine the effect of Mg ligation on the geometry a nd spin density distribution of the cation free radicals formed. It is show n that, whereas the properties of the bacteriochlorophyll model can be expl ained on the basis of the electron density distribution of the highest occu pied molecular orbital (HOMO), for the chlorophyll model the geometry and s pin density properties of the ligated species do not follow this trend. For the ligated chlorophyll models it is shown that, due to the closeness in e nergy of the HOMO and HOMO-1 orbitals, a Jahn-Teller distortion occurs on o ne-electron oxidation, leading to an admixed hybrid orbital for the cation radical form. Orbital mixing is shown to lead to significant changes in the geometry and spin density distribution of the cation free radical formed. It is also shown that orbital mixing does not lead to an increase in the ma gnitude of the N-14 hyperfine couplings thereby invalidating reports in the literature which have dismissed mixed orbital states for the primary donor cation radicals of photosynthetic reaction centers based on this criterion .