The effect of spin polarization on zero field splitting parametersin paramagnetic π-electron molecules

摘要

Spin polarization effects play an important role in the theory of isotropic hyperfine interactions foraromatic protons. The spin polarization gives rise to significant isotropic proton hyperfineinteractions—spin-dependent one-electron properties—smaller than 0 MHz and the effect has beentheoretically described [H. M. McConnell and D. B. J. Chesnut, Chem. Phys. 28, 107 (1958)]. Theinfluence of spin polarization on the zero field splitting parameters, which are spin-dependenttwo-electron properties, has not been clearly identified yet. A phenomenological equation isproposed here for the contribution of spin polarization to the zero field splitting parameter D inanalogy to McConnell's equation for hyperfine interactions. The presence of the effect isdemonstrated in a series of calculations on polyacenes in the triplet state and turns out to beresponsible for up to 50% of the D parameter in the case of naphthalene! It is found thatspin-unrestricted single-determinant methods, including the widely used density functional theorymethods, do not accurately reproduce the two-electron reduced electron density required for theevaluation of two-electron spin-dependent properties. For the accurate calculation of zero fieldsplitting parameters by quantum chemical methods, it thus seems necessary to resort to correlatedab initio methods which do not give rise to spin contamination and which do provide an accuratedescription of the two-electron reduced electron density.