Resonance Raman Spectroelectrochemistry. I. The Tetracyanoethylene Anion Radical

A new spectroelectrochemical technique based on the resonance Raman effect is described whereby high resolution vibrational spectra can be obtained for electrogenerated intermediates and products. Both controlled potential coulometric electrolysis in bulk solution and cyclic potential step electrolysis under semiinfinite diffusion mass transport conditions have been tested as electrogeneration modes. Resonance Raman spectra of the tetracyanoethylene anion radical (TCNE•^-), which exhibits perfectly reversible electrogeneration behavior, have been obtained in both of these electrogeneration modes with 4880, 4765, and 4579 Å Ar⁺ laser excitation. In addition resonance Raman intensity (RRI) vs. time transients produced by single-shot, double-potential steps at a microelectrode have been obtained for the 2194 cm^-1 v₁(A_g) C≡N and 1421 cm^-1 v₂(A_g) C≡C stretching modes. The Raman transients correlated exactly in time with the corresponding chronoamperometric and transmission chronoabsorptometric signals indicating that the Raman transients and spectra obtained under cyclic potential step conditions result from TCNE•^- in the electrochemical diffusion layer.