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 three substituted oxovanadium(IV) phthalocyanines ([VO(tcpc)], [VO(tbpc)], and [VO(tapc)]), where the phthalocyanine rings are modified with four carboxyl, t-butyl, and amino groups, respectively. The 14N hyperfine coupling (HFC) and nuclear quadrupole coupling (NQC) tensors of the phthalocyanine isoindole nitrogens were determined by computer simulations of orientationally selected ESEEM spectra. The HFC tensors of these three complexes were found to be almost identical, and no appreciable effects from the substituents were detected. The isotropic HFC constants, as determined by the simulations, are |Aiso| = 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 similar to one another: e2qQ/h = 2.2, 2.1, and 2.0 MHz for [VO(tcpc)], [VO(tbpc)], and [VO(tapc)], respectively. However, the simulations revealed a difference in the orientation of the NQC unique axis, where the unique axis is 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(tapc)]. An analysis of the NQC parameters based on the Townes-Dailey model indicates an increase in the electron population of the nitrogen π orbital in [VO(tapc)], which is consistent with the expected effects of the amino group.