Silicate solution: absorption spectra

The absorption spectrum of nitrobenzene molecules adsorbed on silica gel from the vapor phase has been recorded by Okuda (55) (Fig. 7), and that from a cyclohexane solution in a slurry of silicic acid by Robin and Trueblood (46). The first author has found a shift from 240 in the gas to 260, and the second authors from 253 in cyclohexane to 270 mp, which is equivalent to about about 3000 cm-. Such a large displacement does not necessarily indicate a chemisorption, since the position of the 260 mju. band of adsorbed nitrobenzene is between those of its aqueous and ethanol solutions. However, Fig. 7 shows that after desorption the most firmly held molecules display a broad absorption band with a maximum at 300 m/a, which might indicate a kind of chemisorption. 

In order to increase the transparency of silicic acid powder Robin and Trueblood suspended it as a slurry in cyclohexane, possessing approximately the same refractive index and thus obtained absorption spectra of adsorbed benzene and its derivatives (46). However, the adsorption from even dilute solutions produced poorly resolved broad bands, contrasting with the sharp spectrum of the cyclohexane solution itself with the same total number of dissolved molecules. J In comparison with the cyclohexane solution, the maxima of the broad absorption bands of adsorbed benzene and its derivatives are seen to be shifted to the higher frequencies. However, relative to the gaseous state, the shift is to the red by < 130 cm . ... 

---