Thermomagnetic properties of the Finland trityl radical.

The Finland trityl paramagnet is characterized by magnetic susceptibility and a new form of quantitative electron paramagnetic resonance (EPR) that utilizes a superconducting quantum interference device (SQUID) as a detection method. This radical is of interest due to its use as a dynamic nuclear polarization agent as well as a potential magnetic refrigerant and quantum computing bit. The SQUID-EPR data show that the EPR linewidth of a concentrated trityl powder decreases dramatically from 4.4 to 1.4 mT as the temperature is increased from 1.8 to 10 K. The quantitative nature of SQUID-EPR is used to thermodynamically quantify the EPR energy transfer times and saturated fractions. At 95 GHz and 1.8 K, only 40% of the spins are in resonance at the onset of saturation. Conventional dc magnetic susceptibility over 1.8-150 K indicates an S=12 Curie-Weiss relationship with little long range interaction. Magnetization versus applied field at 1.8 and 4 K fits a Brillouin function with >80% electronic polarization at a normalized field of gmu(B)mu(0)HkT approximately 3. These results provide information required for theoretical modeling and engineering of the trityl radical for a wide range of applications.

Studies on TMPD:TCNB; a donor-acceptor with room temperature paramagnetism and n-pi interaction.

A donor-acceptor compound based on N,N,N',N'-tetramethyl-p-phenylene-diamine and 1,2,4,5-tetracyanobenzene (TMPD:TCNB) has been synthesized. The crystal structure of the black 1:1 complex formed between TMPD and TCNB has been determined by single crystal X-ray diffraction at room temperature. The compound crystallizes in the triclinic space group P-1 with cell dimensions: a = 7.4986(15) å, b =7.6772(11) å, c = 8.0764(15) å, alpha = 78.822(12) degrees, beta = 83.3779(19) degrees, gamma = 86.289(17) degrees .TMPD and TCNB molecules are stacked alternately in infinite columns along the a-axis. The structure does not seem to show the usual pi-pi interaction between the two aromatic rings, but indicates an n-pi interaction localized between the nitrogen atoms of the donor and the cyano groups of the acceptor.

Observation of symmetry lowering and electron localization in the doublet-states of a spin-frustrated equilateral triangular lattice: Cu3(O2C16H23) x 1.2C6H12.

Cu3(O2C16H23)6.1.2C6H12, containing a Cu36+ core in an equilateral triangle geometry, has been found to be a versatile model system for investigating the spin-frustration phenomenon in a triangular lattice. It affords well-resolved EPR spectra from both of the two possible (Stotal = 1/2 and 3/2) spin states of the Cu36+ core. From 295 to 100 K, the spectra consist of a triplet, but with the central line overlapped by an additional, sharp peak, which replaces the triplet at 30 K and below. The triplet was thus assigned to the excited state with Stotal = 3/2, located at 324 +/- 5 K ( approximately 225 cm-1), with the zero-field parameters D = -535 G, E = 0, g parallel = 2.209 and g perpendicular = 2.057. The singlet was attributed to the Stotal = 1/2 state, with gxx = 2.005, gyy = 2.050, gzz = 2.282, and, surprisingly, a hyperfine splitting arising from a single Cu2+ nucleus, with Azz = 157 G. The detailed magnetic measurements on a three-electron, equilateral triangular system, and the observation of symmetry lowering in the doublet ground state, should be of broad theoretical and experimental interest in molecular magnetism.