Vibrational analysis of spectra of quinonoid molecular ions. Part 3.—Vibrational spectra and assignment of 7,7,8,8-tetracyanoquinodimethane radical anion

Abstract

The Raman and infrared spectra of TCNQ and [²H₄]TCNQ radical anions in solution are presented for the first time. The successful assignment of most of the totally symmetric a_g modes, obtained directly from the experimental Raman depolarization ratios, corroborates the assignment of the infrared active modes which is based on comparison with the neutral molecules. Radical–radical dimerization in water solution has no relevant influence on the intramolecular fundamentals. The Raman and infrared powder spectra of LiTCNQ and RbTCNQ and of the analogous fully deuterated salts are also reported, and the solid state–solution shifts in the band positions are found not to exceed 15 cm^–1. Once the effect of environment on TCNQ^– vibrational frequencies had been identified, we were able to evaluate the frequency shifts due to the addition of the extra electron. These shifts of up to about seventy wave numbers are reproduced by a normal coordinate analysis, based on a set of judicious adjustments of the neutral molecule empirical Valence Force Field. The vibrational spectra have been used successfully, as previously with Cs₂(TCNQ)₃, to demonstrate that in crystalline Cu(TCNQ)₂ both neutral and negatively charged TCNQ units are present.