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The most probable conformation of the isomeric thienylpyrroles has been investigated using the HOFFMANN’S variant of the Extended HÜCKEL Theory. Planar syn conformation is predicted slightly more stable than anti for 2,2′-, 2,3′-, and 3,2′-isomers, while the 3,3′-isomer is predicted to have anti conformation slightly more stable than syn. For the two energetically most favourable conformations of each molecule the charge distribution and dipole moment have been calculated using the CNDO/2 method. The relative stability of these compounds has been also discussed.

Comparative calculations by the self-consistent perturbed INDO method on thiophene and by the unrestricted HF INDO method on the 2-thenyl radical show that satisfactory reproduction of nuclear spin coupling constants and hyperfine coupling constants for sulphur heterocyclics can be obtained by considering the sp interaction only.