ESR Study of (C_5H_{12}N)_2CuBr_4
  ESR studies at 9.27, 95.4, and 289.7 GHz have been performed on (C$_5$H$_{12}$N)$_2$CuBr$_4$ down to 3.7 K. The 9.27 GHz data were acquired with a single crystal and do not indicate the presence of any structural transitions. The high frequency data were collected with a polycrystalline sample and resolved two absorbances, consistent with two crystallographic orientations of the magnetic sites and with earlier ESR studies performed at 300 K. Below $B_{C1}=6.6$ T, our data confirm the presence of a spin singlet ground state. 
S. Zvyagin, B. C. Watson, Ju-Hyun Park, D. A. Jensen, A. Angerhofer,   L.-C. Brunel, D. R. Talham, M. W. Meisel