Origin of the magnetic couplings for the weak ferromagnet Li+[TCNE]•- (TCNE = Tetracyanoethylene)

Abstract

Based upon the 16-K crystal structure, the nearest-neighbor spin couplings (J; H = −2JSa•Sb) for the weak ferromagnet (=canted antiferromagnet) Li+[TCNE]•- (TCNE = tetracyanoethylene) (Tc = 21 K), which possesses two interpenetrating diamondoid sublattices with two layers of parallel [TCNE]•-s canted with respect to each other by ∼60°, are computed at the B3LYP/aug-cc-pV-TZ-level. Li[TCNE] has computed interlattice, through-space interactions that exceed the intralattice, through-bonding to Li+ interactions, that are antiferromagnetic. The interlattice interactions dominate the spin coupling leading to the observed weak ferromagnetic behavior. The computed J's based upon the 50-K structure have the strongest ferromagnetic intralayer interlattice interactions increase by 2.9 ± 0.6% while the antiferromagnetic interlayer interlattice interaction increases by 23% with respect to the 16 K results and are in accord with the observed initial increase and subsequent decrease in the temperature dependence of the canting angle that is a consequence of the these changing opposed interactions arising from the change in structure as a function of temperature.