Abstract:
IR spectra have been measured for bridged diquaternary 2,2′-dipyridinium species in which the two quaternary nitrogens are linked by a saturated (CH2)n bridge: diquat (n = 2) DQ2+, triquat (n = 3) TQ2+, tetraquat (n = 4) TTQ2+, and AQ2+, the analogue of DQ2+ which contains an unsaturated (CH)2 bridge. IR band assignments for AQ2+, DQ2+, TQ2+ and TTQ2+ were obtained by comparison with 2,2′-dipyridine and other structurally related compounds. The IR spectra of DQ·+ and TQ·+ generated electrochemically in aqueous solution were recorded by using the potential subtraction technique. These spectra are dominated 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 other bands in the spectrum decreases from DQ·+ to TQ·+. Molecular orbital calculations show that the dihedral angle between the two pyridine rings increases with the number of methylene units in the quaternizing 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) spectra correlate well with changes in the molecular orbital energies. The calculated IR intensity of the asymmetric ring breathing mode increases considerably upon reduction of the dications to the cation radicals, and decreases with increasing dihedral angle, in agreement with experiment.
