An ab initio MO study of heavy atom effects on the zero-field splitting tensors of high-spin nitrenes: how the spin-orbit contributions are affected

The CASSCF and the hybrid CASSCF-MRMP2 methods are applied to the calculations of spin-spin and spin-orbit contributions to the zero-field splitting tensors (D tensors) of the halogen-substituted spin-septet 2,4,6-trinitrenopyridines, focusing on the heavy atom effects on the spin-orbit term of the D tensors (D-SO tensors). The calculations reproduced experimentally determined vertical bar D vertical bar values within an error of 15%. Halogen substitutions at the 3,5-positions are less influential in the spin-spin dipolar (D-SS) term of 2,4,6-trinitrenopyridines, although the D-SO terms are strongly affected by the introduction of heavier halogens. The absolute sign of the D-SO value (D = D-ZZ - (D-XX + D-YY)/2) of 3,5-dibromo derivative 3 is predicted to be negative, which contradicts the Pederson-Khanna (PK) DFT result previously reported. The large negative contributions to the D-SO value of 3 arise from the excited spin-septet states ascribed mainly to the excitations of in-plane lone pair of bromine atoms -> SOMO of pi nature. The importance of the excited states involving electron transitions from the lone pair orbital of the halogen atom is also confirmed in the D-SO tensors of halogen-substituted para-phenylnitrenes. A new scheme based on the orbital region partitioning is proposed for the analysis of the D-SO tensors as calculated by means of the PK-DFT approach.