  Utilizing infrared vibrational spectroscopy we have investigated dimerized
two-dimensional organic salts in order to search for possible charge
redistribution that might cause electronic dipoles and ferroelectricity: the
quantum spin liquid \kappa-(BEDT-TTF)2Cu2(CN)3, the antiferromagnetic Mott
insulator \kappa-(BEDT-TTF)2$Cu[N(CN)2]Cl, and the superconductor
\kappa-(BEDT-TTF)2$Cu[N(CN)2]Br. None of them exhibit any indication of charge
disproportionation upon cooling down to low temperatures. No modification in
the charge distribution is observed around T = 6 K where a low-temperature
anomaly has been reported for the spin-liquid \kappa-(BEDT-TTF)2$Cu2(CN)3. In
this compound the in-plane optical response and vibrational coupling are rather
anisotropic, indicating that the tilt of the BEDT-TTF molecules in c-direction
and their coupling to the anion layers has to be considered in the explanation
of the electromagnetic properties.
