Pyrroles protonation site, calculated

The phosphorus analogue of pyrrole, phosphole, has a degree of aromatic character, according to molecular orbital calculations and nmr spectra (Brown, 1962 Chuchman et al., 1971). 1-Methyl-phosphole has a p/fg-value of 0-5 (Quin et al., 1969), much higher than that of pyrrole. It polymerizes rapidly in aqueous acid. The site of protonation of 1,2,5-triphenylphosphole is phosphorus according to the infrared spectra of some of its stable salts (Chuchman et al., 1971). 

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