INFRARED-SPECTRA OF THE DIANION AND TRIANION SALTS OF TETRACYANOETHYLENE - ASSIGNMENTS AND VIBRONIC EFFECTS

The vibrational and electronic spectra for the di- and trianion salts of TCNE, prepared in vacuo by solvent-free vapor codeposition methods, have been determined. The TCNE salts resemble the corresponding salts of TCNQ in the sense that Na2TCNE is colorless while Na3TCNE is yellow. However, the simplicity of the TCNE salt spectra permits a more thorough analysis of the infrared spectra of the di- and trianion salts. This analysis strongly suggests that, as for NaTCNE, the more intense features are produced by totally symmetric (ag) modes. A vibrational assignment based on this conclusion has permitted the evaluation of empirical stretching force constants for TCNE2- that are consistent with MO predictions, while an apparent strengthening of the central double bond, in TCNE3- vs. TCNE2-, was unexpected. Perhaps the most important and certainly the most novel observation is that the absorption intensity for the activated ag modes of Na3TCNE increases by orders of magnitude when the salt is cooled to liquid N2 temperatures while, simultaneously, the sample becomes opaque in the far-infrared. © 1979 American Chemical Society.