title: X-ray crystallography and magnetic studies of a stable macrocyclic tetranitroxide. Intramolecular dimer of nitroxides in a constrained geometry of the upper rim of calix[4]arene.
abstract: Synthesis, crystallography, and magnetic characterization of a stable macrocyclic tetranitroxide 1, a calix[4]arene, which is functionalized with four tert-butylnitroxides at the upper rim, is described. In solution, 1 has a 4-fold symmetric fixed cone conformation on the NMR time scale and small, but nonnegligible, exchange interactions between the radicals (30 K > /J/k/ >>1.8 mK). In the solid state, dimerization of one diagonal pair of nitroxides leads to a pinched cone conformation for 1 with strong intradimer antiferromagnetic coupling with /J/k/ = 200-300 K (singlet-triplet energy gap, DeltaE(ST) approximately 1 kcal/mol).
authors.affiliation: Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588-0304, USA. arajca1@unl.edu
authors.forename: Andrzej
authors.initials: A
authors.lastname: Rajca
authors.forename: Maren
authors.initials: M
authors.lastname: Pink
authors.forename: Teerapat
authors.initials: T
authors.lastname: Rojsajjakul
authors.forename: Kan
authors.initials: K
authors.lastname: Lu
authors.forename: Hua
authors.initials: H
authors.lastname: Wang
authors.forename: Suchada
authors.initials: S
authors.lastname: Rajca