The single-crystal electron spin resonance spectra of dichloro- and dibromo-oxobis(N,N,N′,N′-tetramethylurea)vanadium(IV); weak two-dimensional exchanging systems

Abstract

The single-crystal e.s.r. spectra of [VOX₂(tmu)₂], where X = Cl or Br and tmu =N,N,N′,N′-tetramethylurea, at Q-band frequencies and over the temperature range 300–4.2 K are reported. Unusual fine structure, which does not correlate with the presence of an integral number of strongly interacting vanadium centres, was observed in the range ca.170–120 K. The spectra have been successfully simulated on the basis of a weak isotropic electronic exchange interaction, J₀, within a two-dimensional layer. Parameters used were g_∥= 1.941 ± 0.002, g_⊥= 1.979 ± 0.002, A_∥=–203 ± 2 G, A_⊥=–69 ± 2 G, and J₀= 130 ± 2 G (antiferromagnetic).