Carbon-13 chemical shift tensors of halobenzenes

Reactivity of Substituted Benzenes toward Oxidative Addition Relates to NMR Chemical Shift of the Ipso-Carbon

The oxidative addition of benzene derivatives to Pd⁰ catalysts is a key step in cross-coupling reactions. In this work, we show that the ipso-carbon chemical shift of substituted benzenes, and in particular the δ₂₂ component of the chemical shift tensor, correlates with the free energy barrier for oxidative addition. This correlation is traced back to the electron density in the p_z orbital of the ipso-carbon (perpendicular of the ring-plane), with high electron densities favoring oxidative addition. The correlation between chemical shift and free energy barrier holds true for a variety of substituted benzenes, making chemical shift a useful descriptor for predicting the reactivity of aromatic substrates in oxidative addition.

On Librational and Rotational Motions of Aromatic Rings in Layered Sn(IV) and Zr(IV) Phosphonate Materials: A Variable-Temperature 13C, 31P Solid-State NMR Study

According to the solid-state ¹³C, ³¹P NMR study and ¹³C chemical shift anisotropy (CSA) measurements, aromatic rings in the layered metal(IV) phosphonate materials behave as low-energy rotors at rotation activation energy, E_act, of 1.4-3.0 kcal/mol. The rotational mechanism consists of 180° flips and librations around C(1)-C(4) axis. The amplitude of the librations, added to the flips, grows with temperature, shifting the reorientations toward rotational diffusion at high temperatures.