OBSERVATION OF A MAGNETIC DIPOLE ORIGIN IN THE SPECTRUM OF THE SQUARE-PLANAR CUCL42- ION

The low temperature d-d absorption spectrum of bis(2-aminobenzothiazol ium) copper tetrachloride is reported. The lowest energy B-2g(xy) <-- B-1g (x(2) - y(2)) transition has a relatively sharp electronic origin which has not previously been observed in a CuCl42- ion. The polarisa tion properties of this electronic origin show it to be magnetic dipol e allowed. The transition also has extensive vibrational structure and several false origins due to coupling with odd-parity vibrations. The direct observation of the electronic origin allows an accurate determ ination of the excited state geometry.