ELECTRON-SPIN ECHO ENVELOPE MODULATION STUDY ON OXOVANADIUM(IV)-PORPHYRIN COMPLEXES - REINVESTIGATION OF HYPERFINE AND QUADRUPOLE COUPLINGS OF PYRROLE NITROGEN

The hyperfine and quadrupole couplings of the pyrrole nitrogen in oxovanadium(IV) octaethylporphyrin (VO-(OEP)) and oxovanadium(IV) tetraphenylporphyrin (VO(TPP)) were investigated by angle-selected N-14 ESEEM spectroscopy. Computer simulation of the ESEEM results provides \A(1)\, \A(2)\, \A(3)\ = 6.7, 7.2, 7.7 MHz (VO-(OEP)) and \A(1)\, \A(2)\, \A(3)\ = 6.8, 7.3, 7.8 MHz (VO(TPP)) for the hyperfine coupling (the A(1) axis is parallel to the V-N bond direction, the A(3) axis is perpendicular to the porphyrin plane, and the A(2) axis is perpendicular to the former two axes) and e(2)Qq = 2.0 MHz, eta = 0.5 (VO(OEP)) and e(2)Qq = 1.9 MHz, eta = 0.58 (VO(TPP)) for the quadrupole coupling. Noteworthy is the nearly isotropic nature of the hyperfine coupling, which argues against the previous report on N-14 ENDOR study [Mulks, C. F.; van Willigen, H. J. Phys. Chem. 1981, 85, 1220]. The hyperfine parameters presented here can be well understood on the basis of indirect spin transfer plus dipole-dipole coupling mechanism, which implies that the in-plane pi interaction between the vanadium d(xy) and the nitrogen p orbitals contributes little to the hyperfine coupling.