Current density maps, magnetizability, and nuclear magnetic shielding tensors of bis-heteropentalenes. III. Thieno-thiophene isomers

Near Hartree–Fock values of the magnetic susceptibility and nuclear magnetic shielding of bis-heteropentalenes consisting of two thiophene units ([2,3-b], [3,2-b], [3,4-b], and [3,4-c] isomers) have been estimated via computational schemes relying on continuous transformation of the origin of the current density within the coupled Hartree–Fock approximation and extended gaugeless Gaussian basis sets. The results are compared with those obtained via London gauge-including orbitals. Maps of streamlines and the modulus of the ring current density induced by a magnetic field normal to the molecular plane are reported for the three isomers of higher symmetry, showing that the intense diamagnetic flow of the π electrons causes strong magnetic anisotropy and a down-field chemical shift of the out-of-plane component of the proton shielding tensor in thieno-thiophenes. ‘Diatropicity matrices' for the series of bis-heteropentalenes formed by two fused five-membered heterocyclic systems with different heteroatoms can be constructed by choosing the out-of-plane component of either the magnetic susceptibility or the proton magnetic shielding as indicator. Alternative measures of relative aromaticity are obtained by the modulus of the induced π current densities.