The reaction of an aromatic heterocycle with a proton is called a protonation. One of articles about this theory is 《Nitropyrroles. III. Ultraviolet absorption spectra and tautomeric transformations of some nitropyrroles》. Authors are Novikov, S. S.; Belikov, V. M.; Egorov, Yu. P.; Safonova, E. N.; Semenov, L. V..The article about the compound:Ethyl 3,5-Dimethyl-2-pyrrolecarboxylatecas:2199-44-2,SMILESS:O=C(C1=C(C)C=C(C)N1)OCC).Product Details of 2199-44-2. Through the article, more information about this compound (cas:2199-44-2) is conveyed.
cf. ibid. 1307. Examination of the ultraviolet absorption spectra of some nitropyrroles showed the possibility of identification of these substances and confirmed the structure of 1-methyl-3,4-dinitropyrrole. Acidity of N-unsubstituted pyrrole with nitro groups in the ring increases with increasing number of NO2 groups and with transfer of NO2 group from 3- to 2-position, probably owing to the inductive effect of the group on the NH bond. This phenomenon is discussed in connection with tautomerism of nitropyrroles. The following absolute maximum are reported: pyrrole 210 and 240 mμ; 3-methyl-4-ethylpyrrole 200; 2-ethoxycarbonyl-3,5-dimethylpyrrole 240 and 276; 3-ethoxycarbonyl-2,4-dimethylpyrrole 232 and 259; 2-acetyl-3,5-dimethyl-4-ethylpyrrole 266 and 308; 2,4-dimethyl-3-acetylpyrrole 251 and 280. Spectra of nitropyrroles are shown at various pH values. In basic media these show a maximum at about 400 mμ.
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Reference:
Thiazolidine – Wikipedia,
Thiazolidine – ScienceDirect.com