The Haupt effect: coupled rotational and dipolar relaxation of methyl groups

A theory is described for the dynamic proton dipolar polarization observed by Haupt (1972) in 4-methylpyridine following a sudden temperature change. The theory differs from that of Haupt in assuming that transitions which change the rotational quantum number of the 4-methyl group by +or-3 occur very rapidly, maintaining thermal equilibrium within each of the three subsets of rotational levels corresponding to the three methyl group proton spin symmetry species A, Eaand Eb. The difference of A and E species populations approaches the new equilibrium value slowly and exponentially, following the temperature jump, and generates dipolar polarization in the process. Transitions between Eaand Ebspecies lead to destruction of the polarization, whose evolution from zero due to these competing processes has the simple form C(exp(-at)-exp(-bt)). This is checked by a modified version of Haupt's experiment in which the initial temperature jump is followed by a later burst of RF pulses which reduces the dipolar polarization to zero.