SYMMETRY SPECIES EXCHANGE IN ROTATIONAL TUNNELING SYSTEMS

The rate for the exchange of symmetry species due to nuclear dipolar i nteraction between the protons of two rotational tunnelling systems is calculated taking the example of methyl groups. Its temperature depen dence is related to the broadenings of the tunnelling lines and is cau sed by the same phonons that determine the temperature dependence of t he rotational tunnelling. Equivalent rotors show rapid exchange at low est temperatures which decreases with increasing temperature while a n on-monotonic temperature dependence is predicted for inequivalent roto rs.