Frequency shift-acid strength

The parent quinolinium cation unexpectedly shows two inflections, at 5 8 and —1-6, in the fluorescence intensity curve as the solution acidity is increased from neutral pH to H0 = —4 the absorption spectrum remains unchanged, however. Capomacchia and Schulman (1973) discount further protonation at a carbon atom as an explanation of this effect, on the grounds that there is no shift in the frequency of the band. They suggest that the nearly degenerate 1 L and 1 Lb states may have very different acid strengths, an idea that must ultimately be reconciled, at least approximately, with the Forster cycle. 

Other adsorbents have been used in an effort to measure the acid strength of the sites or eliminate diffusion limitations. Kubelkova et al. used low temperature adsorption of CO on H-ZSM-5, H-Y, NaH-Y, and various AlPO sieves to measure the shift in the acidic OH stretching frequency upon CO adsorption. The authors argue that this shift is related to the proton affinity of the zeolites and thus to the Brpnsted acid strength. Tvaruzkova et al. used d3-acetonitrile to characterize both the Brpnsted and Lewis acidity of a number of zeolites. Using the band intensities and the frequency of the C-N band they obtained relative concentrations and strengths of the various acid sites. 

In many cases multiple probe molecules are used in order to deduce the existence of acid strength distributions. For example, the heterogeneity of the acid sites in H-ZSM-5 was studied by monitoring the adsorption of a series of weak electron donor molecules, such as hexane, fluorobenzene, chlorobenzene, benzene, toluene, p-xylene, and ethylene, to the acid sites. The frequency shift of the acidic OH group was taken as a... 

Y, H-Y, H-MOR, H-ZSM-5) and correlated especially the shift, Av (OH), of the (high-frequency) OH stretching bands to the acid strength (Sanderson s charge on the H atom). The shifted OH bands appeared as broad signals between 3025 (AN H-X) and 2540 cm (AN - H-ZSM-5) for the neutral complexes. Also, the in-plane, 6(OH) and out-of-plane, y(OH), bending vibrations were measured and compared with the results of Fermi resonance calculations. Interaction of di-acetonitrile with NH4-, H, Na-, H-, and Co-Beta led to the detection of Brqnsted and Lewis sites through the appearance of bands at 2297 and 2325 cm", respectively. Na+ and Co + were identified by bands at 2284 and 2308 cm, respectively. 

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