Discrepancy between the Spin Distribution and the Magnetic Ground State for a Triaminoxyl Substituted Triphenylphosphine Oxide Derivative

Abstract

The magnetic interaction and spin transfer via phosphorus have been investigated for the tri-tert-butylaminoxyl para-substituted triphenylphosphine oxide. For this radical unit, the conjugation existing between the pi* orbital of the NO group and the phenyl pi orbitals leads to an efficient delocalization of the spin from the radical to the neighboring aromatic ring. This has been confirmed by using fluid solution high-resolution EPR and solid state MAS NMR spectroscopy. The spin densities located on the atoms of the molecule could be probed since (1)H, (13)C, (14)N, and (31)P are nuclei active in NMR and EPR, and lead to a precise spin distribution map for the triradical. The experimental investigations were completed by a DFT computational study. These techniques established in particular that spin density is located at the phosphorus (rho=-15x10(-3) au), that its sign is in line with the sign alternation principle and that its magnitude is in the order of that found on the aromatic C atoms of the molecule. Surprisingly, whereas the spin distribution scheme supports ferromagnetic interactions among the radical units, the magnetic behavior found for this molecule revealed a low-spin ground state characterized by an intramolecular exchange parameter of J=-7.55 cm(-1) as revealed by solid state susceptibility studies and low temperature EPR. The X-ray crystal structures solved at 293 and 30 K show the occurrence of a crystallographic transition resulting in an ordering of the molecular units at low temperature.

Towards a better understanding of photo-excited spin alignment processes using silole diradicals

“…1,2 Since most studies are limited to the magnetic ground state, synthetic efforts are currently devoted to the design of high-spin compounds, in which an appropriate topology gives rise to ferromagnetic interactions between the spin bearing units. [3][4][5] Therefore, topologies expected to allow antiferromagnetic interactions, yielding a singlet or low spin ground state, have been generally discredited. The recent observation, for purely organic pconjugated diradicals, of photo-excited quintet (S = 2) states starting from singlet ground state has relaunched the interest in designing systems with the ''wrong'' topology.…”

Advanced Magnetic Resonance Techniques for the Structural Characterization of Aminoxyl Radicals and Their Inorganic–Organic Nanocomposite Systems

“…Their schemes were found to be consistent with those expected for ferromagnetic interactions, however, their actual strengths were found to be too weak to be detectable by magnetic susceptibility measurements . NMR and EPR studies of other, more complex situations have been reported as well …”

Galvinoxyl-inspired dinitronyl nitroxide: structural, magnetic, and theoretical studies

“…1b. However, since both nitrones and nitroxides have an N–O length of about 1.3 Å, 16,19,37,48–85 the bond length between the N and neighbouring sp 2 carbon atoms is important to determine the electron state of the N–O site. In fact, there is a difference in the N–C sp 2 length; the average lengths for nitrones are 1.31 Å (N–O: 1.29 Å), 48–53 those for aryl tert -butyl nitroxides are 1.43 Å (N–O: 1.29 Å), 16,19,37,45,54–71 and those for 10-oxy-9(10 H )acridines are 1.41 Å (N–O: 1.29 Å).…”