207Pb NMR study of novel Pb–Pb chemical bonding in lead monoxides, α-PbO and β-PbO

α-PbO and β-PbO have been studied with 207Pb NMR-MAS spectroscopy in the solid state. The 207Pb NMR chemical-shift tensor in α-PbO is axial, with principal values δ⊥=3030±5, δ‖=−270±5, and δiso=1930±10 ppm. In β-PbO, the 207Pb NMR powder spectrum is approximately represented by a single non-axial tensor with principal values δ11=2820, δ22=2760, δ33=−1000, δiso=1525±10 ppm and η=0.024, although there appears to be an overlap of two very similar patterns. Using normal-coordinate analysis, the 207Pb magnetic shielding tensor is represented as a sum of contributions from separate Pb–O and Pb–Pb interactions. It is shown that Pb–O and Pb–Pb contributions are of the same order of magnitude. A potential underlying relativistic reason for the abnormal Pb–Pb interaction is revealed.