  Results of relativistic calculations of nuclear magnetic resonance shielding
tensors ($\sigma$) for the thallium monocation (Tl$^+$), thallium hydride (TlH)
and thallium halides (TlF, TlCl, TlBr, TlI, and TlAt) are presented as obtained
within a four-component polarization propagator formalism and a two-component
linear response approach within the zeroth-order regular approximation.
Additionally, some quantum electrodynamical (QED) effects on those NMR
shieldings are estimated. A strong dependence of $\sigma$(Tl) on the bonding
partner is found, together with a very weak dependence of QED effects with
them. In order to explain the trends observed, the excitation patterns
associated with relativistic $ee$ (or paramagnetic-like) and $pp$ (or
diamagnetic-like) contributions to $\sigma$ are analyzed. For this purpose,
also the electronic spin-free and spin-dependent contributions are separated
within the two-component zeroth-order regular approximation, and the influence
of spin-orbit coupling on involved molecular orbitals is studied, which allows
for a thorough understanding of the underlying mechanisms.
