1ST EVIDENCE FOR PI PI-DIMERIZATION OF TH E STABLE 1,4-DIALKYLQUINOXALINIUM RADICAL CATIONS - STRUCTURE, SPECTROSCOPY AND MAGNETISM

Crystal and molecular structure analysis of 1,4-diethylquinoxalinium i odide shows a virtually planar quinoxaline ring with 11 conjugated pi- electrons. In contrast to the triiodide of the 1,4,6,7-tetramethyl der ivative or to the tetraphenylborate salt of 1,4-diethylquinoxalinium c ation radical the iodide exhibits pi-pi-dimerized radical cations in t he solid state with synplanar ethyl groups and a rather small intermol ecular distance of about 315 pm between the pi-planes of the primarily interacting 1,4-diazine rings. Solid state magnetic measurements betw een 2 and 300 K show considerably diminished magnetic moments due to p artial spin-pairing, and UV/VIS spectroscopic measurements in acetonit rile reflect the pi-pi-interaction in solution through the appearance of a long-wavelength absorption band.