FT-Raman Study on Charge-Transfer Polyiodide Complexes and Comparison with Resonance Raman Results

In resonance Raman (RR) spectroscopy, the laser excitation sources have often been found to be destructive towards polyiodides if compared with the milder conditions under which the Fourier transform Raman (FT-R) technique operates. In fact, our FT-R spectra of some model polyiodides—[(CH₃)₄N]I₅ (I₅⁻ bent), [(C₂H₅)₄N]I₇, and [(CH₃)₄N]I₉—are significantly different from the literature RR data, give evidence of decomposition of the samples in RR, and are in agreement, respectively, with the I⁻ · 2I₂, I₃⁻ · 2I₂ and (I⁻ · 2I₂) · 2I₂ descriptions. In addition to the above-cited cases, the FT-R spectra of (Mn(modtc)₃]I_s (modtc = morpholine carbodithioato) and (moH]I₅ (moH = morpholinium) are reported. The crystal structures indicate that in these two compounds the I₅ anions can be properly described as I⁻ · 2I₂ and I₃⁻ · I₂, respectively, and FT-R spectra agree well with this formulation. Moreover, the first FT-R spectrum of an I₁₆^4– anion in [mo₂ttl]₂I₁₆, ([mo₂ttl]²⁺ = 3,5-di(N-morpholinio)-1,2,4-trithiolane), whose X-ray structure shows a sequence of two I₃⁻ … I₂ … I⁻ ·I₂ (I₈^2–) interacting anions, is reported. A close correlation of the FT-Raman peaks with the molecular species, identified by the interatomic distances, is also observed in this case. Thus, a combination of X-ray structural data and FT-R data can provide a reasonable interpretation of the nature of the acceptor iodine moiety in charge-transfer polyiodide complexes.