Web Release Date: December 11,
2-(4,5,6,7-Tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1- H-imidazole-3-oxide-1-oxyl, A Hydrogen-Bonded Organic Quasi-1D Ferromagnet
and
Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003, Research Center for Molecular Thermodynamics, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan, and Instituto de Física, Universidade de São Paulo, Caixa Postal 66.318, 05315-970, São Paulo, SP, Brazil
Received June 9, 2007
Abstract:
The title radical (F4BImNN) is a stable nitronylnitroxide that forms hydrogen-bonded NH···ON
chains in the solid state. The chains assemble the F4BImNN molecules to form stacked contacts between
the radical groups, in a geometry that is expected to exhibit ferromagnetic (FM) exchange based on spin
polarization (SP) models. The experimental magnetic susceptibility of F4BImNN confirms the expectation,
showing 1-D Heisenberg chain FM exchange behavior over 1.8-300 K with an intrachain exchange constant
of Jchain/k = +22 K. At lower temperatures, ac magnetic susceptibility and variable field heat capacity
measurements show that F4BImNN acts as a quasi-1-D ferromagnet. The dominant ferromagnetic exchange
interaction is attributable to overlap between spin orbitals of molecules within the hydrogen-bonded chains,
consistent with the SP model expectations. The chains appear to be antiferromagnetically exchange coupled,
giving cusps in the ac susceptibility and zero field heat capacity at lower temperatures. The results indicate
that the sample orders magnetically at about 0.7 K. The magnetic heat capacity ordering cusp shifts to
lower temperatures as external magnetic field increases, consistent with forming a bulk antiferromagnetic
phase below a Néel temperature of TN(0) = 0.72 K, with a critical field of Hc 
1800 Oe. The interchain
exchange is estimated to be zJ/k 
(-)0.1 K.
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