Spin gap formation in the quantum spin systems TiOX, X=Cl and Br
  In the layered quantum spin systems TiOCl and TiOBr the magnetic susceptibility shows a very weak temperature dependence at high temperatures and transition-induced phenomena at low temperatures. There is a clear connection of the observed transition temperatures to the distortion of the octahedra and the layer separation. Band structure calculations point to a relation of the local coordinations and the dimensionality of the magnetic properties. While from magnetic Raman scattering only a small decrease of the magnetic exchange by -5-10% is derived comparing TiOCl with TiOBr, the temperature dependence of the magnetic susceptibility favors a much bigger change. 
P. Lemmens, K.Y. Choi, R. Valenti, T. Saha-Dasgupta, E. Abel, Y.S.   Lee, and F.C. Chou