Thermal averaging of the spin-rotation coupling in small molecules leads to an isotropic NMR shielding

It has been known for over 70 years that nuclear spins couple to molecular rotation via a Zeeman interaction. This spin-rotation coupling can be observed as a discrete splitting in molecular beam magnetic resonance experiments, but is quenched by molecular collisions at higher pressures. We show that because of differential thermal population of M-J levels at high magnetic fields, the spin rotation coupling retains a small isotropic component at high field. For all but the smallest molecules at very low temperature, the residual coupling is temperature independent and linear in the magnetic field; it therefore closely mimics the chemical shift. The 'super spin rotation' shift may in the future be a necessary correction to ultra - high precision computations of the NMR chemical shielding of small molecules in gases and liquids. (C) 2011 Published by Elsevier Inc.