INFRARED-SPECTRA AND THEORETICAL-STUDIES OF BRIDGED DIQUATERNARY 2,2'-DIPYRIDINIUM COMPOUNDS

IR spectra have been measured for bridged diquaternary 2,2'-dipyridini um species in which the two quaternary nitrogens are linked by a satur ated (CH2)(n), bridge: diquat (n = 2) DQ(2+), triquat (n = 3) TQ(2+), tetraquat (n = 4) TTQ(2+), and AQ(2+) the analogue of DQ(2+) which con tains an unsaturated (CH)(2) bridge. IR band assignments for AQ(2+), D Q(2+), TQ(2+) and TTQ(2+) were obtained by comparison with 2,2'-dipyri dine and other structurally related compounds. The IR spectra of DQ(.) and TQ(.+) generated electrochemically in aqueous solution were reco rded by using the potential subtraction technique. These spectra are d ominated by a single intense band at 1534 cm(-1) (DQ(.+)) and 1520 cm( -1) (TQ(.+)). The intensity of this band relative to those of the othe r bands in the spectrum decreases from DQ(.+) to TQ(.+). Molecular orb ital calculations show that the dihedral angle between the two pyridin e rings increases with the number of methylene units in the quaternizi ng alkyl bridge, and that this angle undergoes a significant decrease from the dication to the corresponding monocation. The observed trends in the reduction potential and electronic (ultraviolet/visible) spect ra correlate well with changes in the molecular orbital energies. The calculated IR intensity of the asymmetric ring breathing mode increase s considerably upon reduction of the dications to the cation radicals, and decreases with increasing dihedral angle, in agreement with exper iment.