Dispositions en matière de droit financier de la loi n° 99-532 du 25 juin 1999 relative à l'épargne et à la sécurité financière

International audience(JO 29 juin 1999, p. 9487 ; D. affaires 1999. 1057 et 1121

Heavy Atom Effect on Magnetic Anisotropy of Matrix-Isolated Monobromine Substituted Septet Trinitrene

The heavy atom effect on the magnetic anisotropy of septet trinitrenes is reported. Septet 1-bromo-3,5-dichloro-2,4,6-trinitrenobenzene (<b>S-1</b>) was generated in a solid argon matrix by ultraviolet irradiation of 1,3,5-triazido-2-bromo-4,6-dichlorobenzene. This trinitrene displays an electron spin resonance (ESR) spectrum that drastically differs from ESR spectra of all previously studied septet trinitrenes. The zero-field splitting (ZFS) parameters, derived from the experimental spectrum, show the parameter |<i>D</i>| = 0.1237 cm^–1 and the unprecedentedly large ratio of <i>E</i>/<i>D</i> = 0.262 that is close to the rhombic limit <i>E</i>/<i>D</i> = 1/3 for high-spin molecules. The CASCI (based on state-averaged CASSCF) and DFT methods were applied to calculate the ZFS tensor focusing on the heavy (bromine) atom effects on the spin–orbit term. These calculations show that the multiconfigurational ab initio formalism and the CASCI method are the most successful for accurate predictions of the spin–orbit term in the ZFS tensor of high-spin nitrenes containing heavy bromine atoms. Due to the presence of the bromine atom in <b>S-1</b>, the contribution of the spin–orbit term to the total parameter <i>D</i> is dominant and responsible for the unusual orientation of the easy <i>Z</i>-axis lying in the molecular plane perpendicular to the C–Br bond. As a result, the principal values <i>D</i>_<i>XX</i>, <i>D</i>_<i>YY</i>, and <i>D</i>_<i>ZZ</i> of the total tensor <i>D̂</i>_Tot have such magnitudes and signs for which the ratio <i>E</i>/<i>D</i> is close to the rhombic limit, and the total parameter <i>D</i> is large in magnitude and positive in sign