Nonadiabatic screening of proton charges in the case of tunneling methyl groups in external magnetic field

The equation of motion corresponding to methyl groups embedded in solid lattices and reorienting around their symmetry axes is considered in the presence of external magnetic field. It is known that the Born-Oppenheimer approximation does not describe the electronic screening of the magnetic field as seen by the nuclei, i.e., the protons interact with the magnetic field with their bare charges. To obtain the screening, one has to include the non-adiabatic terms in the analysis. Using the rotating coordinate system fixed to the methyl group, we have shown that the effective chargeq, describing the interaction of the protons with the magnetic field, satisfiesq>0.24e (–e is the electron charge). The possibility of experimental verification is also discussed.