Effect of pressure on the charge-transfer band of the [Fe(CN)6]4−·dimethyl viologen ion pair

The effect of pressure on the optical charge-transfer (CT) band of the ion pair (Fe(CN)/sub 6/)/sup 4 -//center dot/DMV/sup 2 +/ (DMV = dimethyl viologen) has been studied in aqueous solution. The most significant effects of pressure were twofold: (1) In the pressure range 0.001 to 10.0 kbar (1 kbar = 986.92 atm = 0.1 GPa) the CT band maximum shifted to higher energy by /approximately/1700 cm/sup -1/. (2) In this same pressure range the integrated intensity of the CT band increased by /approximately/35%. There were no changes in the peak's full-width at half maximum (FWHM). Qualitatively, these phenomena are in accord with a vibronic coupling model used to describe mixed-valence metal compounds and also with a general theory of absorption spectra of ions in solution, as well as with Mulliken's theory of charge transfer. We also noted that the oscillator strength of the CT band decreased by over a factor of 2 at ambient pressure with increasing concentration of the ion pair (from 1.0 to 50 mM). This change correlates with the change of the ionic strength over the same concentration range. 28 refs., 3 figs.