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

Abstract

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 |D| 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 π 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.

Six-Membered Aromatic Polyazides: Synthesis and Application

“…Due to the presence of two heavy bromine atoms on the pyridine ring, this trinitrene showed the record value of | D | = 0.297 cm −1 among all organic septet hexaradicals [26]. Recent high-level ab initio calculations have proved that trinitrene 80a has the negative sign of D and can be assigned to strong organic magnetic molecules [120]. The finding that heavy atoms drastically strengthen the magnetic properties of organic polyradicals opened up principally new prospects in design of organic magnetic materials.…”

Cyclic Triradicals Composed of Iminonitroxide–Gold(I) with Intramolecular Ferromagnetic Interactions

“…The small spin density thus induced on gold also provides significant effect on the zero‐field splitting (ZFS) in the ESR spectrum (| D / hc |=0.0414 cm −1 ; see Figure S1). The large observed | D/hc |values cannot be explained by the spin–spin dipolar interaction and requires large contribution of the spin–orbit coupling (SOC) induced by the heavy gold atom (see Table S3) . In addition, we calculated the intermolecular exchange interaction ( J calc‐inter / k B =−5.1 K) between the IN moieties (IN‐ C and IN‐ D , taken from top and bottom 5 s, respectively, in Figure b).…”

Molecular Magnets: The Synthesis and Characterization of High‐Spin Nitrenes

“…In general, the contribution of the SOC-term to D Tot of high-spin molecules built from light atoms is less than 12 %. [29][30][31] However,i nc ase of some bromine-containing quintet and septet nitrenes, the contributions of the SOC-terms to D Tot are dominant and much surpass the contributions of the SS-terms. [19,23] Nowadays, all effects of the molecular structure on magnetic parameters of high-spin nitrenes are reliably and precisely predicted by DFT and ab initio calculations.…”

“…Furthermore, the introduction of heavy atoms in appropriate sites of quintet and septet nitrenes may change the sign and increase several times the D values of the molecules [19, 29] . According to theory, [30, 31] the total tensor D Tot of high‐spin molecules is the sum of two tensors, namely, a first order term D SS describing the dipolar spin‐spin (SS) interactions between unpaired electrons and a second term D SOC arising from spin‐orbit coupling (SOC) as the result of interactions of spins with excited electronic states. In general, the contribution of the SOC‐term to D Tot of high‐spin molecules built from light atoms is less than 12 % [29–31] .…”