Infrared absorption spectra solvent effects

Another consequence of the strong absorption properties of water is the spectral impact of the displacement of water by dissolved solutes. Generally, in absorption spectroscopy, the solvent is selected not to absorb over the wavelength range of interest. When the absorption properties of the solvent are negligible, any displacement of solvent molecules from the optical path by the dissolution of solute molecules has a negligible effect on the measured spectrum. For near-infrared spectra of aqueous solutions, however, the absorption spectrum depends heavily on the degree of water displacement by solutes in the sample. 

The problem of finding a suitable solvent is more serious in the infrared region, where it is difficult to find one that is completely transparent. The use of either carbon tetrachloride or carbon disulfide (health effects aside) will cover the most widely used region of 2.5 to 15 tm (see Figure 16.4). Water exhibits strong absorption bands in the infrared region, and it can be employed only for certain portions of the spectrum. Also, special cell materials compatible with water must be used rock salt is usually used in cells for infrared measurements because glass absorbs the radiation, but rock salt would dissolve in water. The solvents must be moisture-free if rock salt cells are used. 

Interestingly, an unusual mixed-valence state of a stacked tetrathiafulvalene (TTF) dimer was observed within the coordination host 1 at ambient temperature in an aqueous solution [10], When excess TTF was added to an aqueous solution of 1, the colorless solution became dark green in color because of the formation of a 1 D (TTF)2 complex (Fig. 4.3a). Electrochemical studies revealed that an initial one-electron oxidation occurred at -150 mV that led to the mixed-valence dimer, whereas a second one-electron oxidation at -300 mV afforded the cation radical dimer. The mixed-valence state was also indicated by the appearance of a broad absorption band in the near-infrared region ( max = -2000 nm) of the UV-vis spectrum (Fig. 4.3b). The host framework effectively forced the two molecules of TTF into close proximity in the cavity. As a result, the labile mixed-valence dimer was protected from oxygen and solvent molecules. 

E. N. Vasenko, Effect of the Solvent on the Position of the Absorption Rands in the Infrared Spectrum of Amides, Fix. Sbornik Uvav. Univ. 1957, No. 3, 192-195. 

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