We have analyzed the infrared properties of a 1-D dimerized tight binding system with linear coupling to one inter- and one intramolecular phonon. The infrared conductivity has been calculated for a quarter-filled system both in the small U and large U limits. A suitable choice of the model parameters allows the comparison with the experimental infrared conductivity of representative (TMTTF)2X and (TMTSF)2X salts. The reseults indicate that the infrared properties are quite sensitive to the effects of the gap open by the chain dimerization. This can only be explained if a large on-site correlation U is included. Previously noted correlations between d.c. conductivity and infrared properties are easily explained with the interpretative scheme suggested here.