Far infrared spectra of charge-transfer complexes between iodine and substituted pyridines, dissolved in cyclohexane, have been measured in the region 20 to 200 cm
-1
. Bands have been assigned to the modified iodine molecule stretching vibration near 180 cm
-1
, and to the stretching of the intermolecular bond in the range 65 to 95 cm
-1
. The shifts of the vibration frequencies, and force constants calculated using a simple valency force field, have been discussed in relation to the mass, electronic and steric effects of substituent groups. Equilibrium constants for the formation of the complex have been determined. From the intensities of the absorption bands, further evidence has been found for a vibronic interaction, leading to a delocalization of the transition moment in the vibrations of the complexes, and to an enhancement of the intensity of absorption.
