Infrared pyrrole oxidation

A poly(pyrrole) film was deposited on a Pt electrode from potentiostatic conditions at 0.8 V vs Ag/AgCl. The film was colorless, its presence was verified by oxidation and reduction of the film in plain electrolyte solution. The infrared spectrum of the electrochemically prepared poly(pyrrole) is similar to the catalytically prepared films indicating the two films are structurally similar. 

The ubiquitous porphyrin homodimers are only of interest in their oxidized form. The zinc octaethylporphyrin cation radical, for example, forms a diamagnetic dimer with an extremely strong charge transfer band in the near infrared (Figure 5.21). In crystalline material, all four pyrroles lie exactly on top of each other with an interplanar distance of 0.42 nm. The tt, Tr -dimer behaves like a... 

Ludwig. This compound has an empirical formula corresponding to structure (1) and shows the ultraviolet and infrared" absorptions of a pyrrole-3-carboxylic ester. Its acetylation gives a tetra-O-acetyl derivative." Oxidation with lead tetraacetate yields ethyl 5-formyl-2-methyl-pyrrole-3-carboxylate (4), identical with the compound prepared in a different way. Oxidation with potassium permanganate in alkaline solution at low temperature yields 3-(ethoxycarbonyl)-2-methylpyrrole-5-carboxylic acid (7) which can be transformed " into the diethyl ester (8), identical... 

The adsorption of pyrrole on a series of different oxides and zeolites showed that the shift of the NH infrared vibration to lower values could be used as a measure of the increased basic strength of the sites (65). Infrared spectra can be recorded 10-15 min. after pyrrole is contacted with the zeolite. This is a great advantage over calorimetry for instance which requires a long time to reach equilibrium, which may favor the pyrrole transformation. 

The nature of acid sites was at the center of several studies. Barthomeuf discussed spectroscopic methods applied to study hydroxyl groups, Bronsted and Lewis acidities and basicities, and the effects of extraframework oxides on acidity or basicity. These are IR, which is the most important here, NMR, XPS, UV- and visible, Raman and Mdssbauer spectroscopies. Far infrared technique may reveal cation locations. The shift of the band associated with the NH of pyrrole depends on the charge of 0, which in turn determines the basicity. Some progress has been made in the last few years in the identification of different OH groups. An interesting observation was that sU adsorbed benzene molecules are disturbed by the 0-s, even at high benzene coverages. 

Further support for the coupling of pyrrole through the 2-,5-positions comes from solid-state nuclear magnetic resonance (NMR) [33], infrared (IR) spectroscopy [34,35], and x-ray photoelectron spectroscopy (XPS) [36] experiments. The presence of pyrrole units in both the oxidized and neutral forms of polypyrrole is inferred from NMR ( C) spectra [37]. After polymerization, a downfield shift is observed in the a-carbon resonance that is consistent with the formation of an a-substituted pyrrole. The IR spectra exhibit strong similarities between a pyrrole dimer, trimer, and polymer and polypyrrole in its neutral form. Finally, the XPS carbon h and nitrogen Is polypyrrole spectra have features similar to that of a pyrrole monomer [38,39]. 

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