Quantum-chemical study of the protonation of pyrrolo[2,1-b]thiazole and its selenium and tellurium analogs

The possibility of proton attack on Various centers in pyrrolo[2, 1-b]thiaz ole (1) has been evaluated. The results of semiempirical (MNDO, AMI, and PM 3) and ab initio (6-31G*) calculations were compared. The MNDO and 6-31G* m ethods give "chemically proper" and qualitatively coincident results. Analy sis of the intramolecular (geometric and electronic) reorganization of mole cule 1, depending on the protonation center, has been carried out. The most probable attack centers, depending on the mechanism of electrophilic react ion, have been recognized. The energy parameters of intramolecular prototro pic rearrangements in cation I and the "blocking" factor value of methyl gr oups reducing the corresponding complex stability have been evaluated. It h as been established that the relative stability of the protonated forms doe s not change on going to pyrrolo[2,1-b]selenium- and telluriumazoles, but t he range of variations is considerably narrowed in the series S > Se > Te.