title: Thermal averaging of the spin-rotation coupling in small molecules leads to an isotropic NMR shielding.
abstract: 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.
authors.affiliation: Department of Chemistry, University of Nebraska at Lincoln, Lincoln, NE 68588-0304, USA. gerry@setanta.unl.edu
authors.forename: Gerard S
authors.initials: GS
authors.lastname: Harbison
mesh_terms: D000465:Algorithms; D016880:Anisotropy; D005740:Gases / Q000737:chemistry; D016014:Linear Models; D060526:Magnetic Fields; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D013696:Temperature
chemical_list: D005740:Gases