Hyperfine and nuclear quadrupole couplings of the coordinated nitrogen of oxovanadium(IV) phthalocyanines studied by electron spin-echo envelope modulation spectroscopy

Electron spin-echo envelope modulation (ESEEM) measurements were made for t hree substituted oxovanadium(IV) phthalocyanines ([VO(tcpc)], [VO(tbpc)], a nd [VO(tapc)]), where the phthalocyanine rings are modified with four carbo xyl, t-butyl, and amino groups, respectively. The N-14 hyperfine coupling ( HFC) and nuclear quadrupole coupling (NQC) tensors of the phthalocyanine is oindole nitrogens were determined by computer simulations of orientationall y selected ESEEM spectra. The HFC tensors of these three complexes were fou nd to be almost identical, and no appreciable effects from the substituents were detected. The isotropic HFC constants, as determined by the simulatio ns, are \A(iso)\ = 7.08, 7.08, and 7.05 MHz for [VO(tcpc)], [VO(tbpc)], and [VO(tapc)], respectively. The NQC constants were also found to be very sim ilar to one another: e(2)qQ/h = 2.2, 2.1, and 2.0 MHz for [VO(tcpc)], [VO(t bpc)], and [VO(tapc)], respectively. However, the simulations revealed a di fference in the orientation of the NQC unique axis, where the unique axis i s parallel to the V-N bond in [VO(tcpc)] and [VO(tbpc)], whereas this axis is perpendicular to the V-N bond within the phthalocyanine plane in [VO(tap c)]. An analysis of the NQC parameters based on the Townes-Dailey model ind icates an increase in the electron population of the nitrogen Je orbital in [VO(tapc)], which is consistent with the expected effects of the amino gro up.