Pyridine frequency shift

FIGURE 1. Linear enthalpy-frequency shift plot for t-BuOH adducts (from Table 7). 1 Pyridine, 2 Et3N,... 

In their studies of the effect of solvent upon the N—H stretching frequency in pyrrole, Fuson and Josien [1] have shown the distinction between the solvent-solute interaction which is a function of dielectric constant alone [2, 3] and that which is more specific, involving N—H hydrogen bonding. The most pronounced frequency shifts are those caused by pyridine [4] (K—M N bonding) and by acetone (N—H 0 bonding). The choice of pyrrole for these studies was presumably partly governed by convenience since the N—H band in pyrrole is considerably more intense than in the more basic secondary amines. We have attempted an extension of this work in two directions ... 

Fig. 8). There is an evident increase in both acetone and pyridine with decreasing basicity. We suggest, however, that the frequency shift is not so much a measure of the basicity of the amine in aqueous solution as of the acidity in the appropriate solvent. Thus, if the lowering of the frequency corresponds to a weakening and lengthening of the N—H bond, the limit of this process is the proton transfer ...

The question of the acidity of silica, alumina and silica-alumina surfaces has always been of great interest to catalytic scientists. Previously, transmision infrared spectroscopy, particularly of pyridine adsorption, has been used to distinguish the presence of Lewis and Bronsted acid sites on oxide surfaces (24). The frequency shift of the surface OH group during adsorption now... 

A comparative study using IR spectroscopy can rank solid acidity by determining the frequency shift of the adsorption band of pyridine [102], the band shift of OH groups due to the adsorption of benzene or CD3CN [103, 104], and the shift of the adsorbed CO on Lewis sites [105]. These IR techniques, however, are not widely used for evaluation of solid acidity. 

Cations derived from azole structures by protonation of pyridine-type nitrogen atoms, e.g. [31], show an appreciable low-frequency shift... 

In a unique experiment Wetzel et determined the dependence of SERS of pyridine on a silver sol on the electric potential, which was maintained by a reversible redox system (europium +3, +2) added to the suspension. The relative concentration of the europium ions was maintained by means of an electrode whose potential was controlled with standard electrochemical equipment. They found that the ratio between the two main bands behaved similarly on an electrode and on a sol. Also frequency shifts were seen. They interpreted these results as indicating that the same SERS mechanisms are operative on the sol and in electrochemical systems. 

FIG. 9. The frequency shift versus the surface charge density in four systems pyridine (5 mM) in water [111], pyridine (5 mM) in butanol [111], -butanol (5 mM) in water [111], and tert-butanol (1 M) in water [108]. Supporting electrolyte 0.1 M LiC104. Solid circles represent measurements in the supporting electrolyte. Open circles represent measurements in the presence of the adsorbate. (Fig. 9a-c from Ref 111.)... 

FIG. 10. Frequency shift due to pyridine adsorption, corrected for the shift of frequency in the supporting electrolyte (0.1 M L1C104), on gold (a) and silver (b) electrodes versus potential. (From Ref 98.)... 

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