A theory is described for the dynamic proton dipolar polarization observed by Haupt 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 + 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, E" and 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 E" and Eb species 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. The subsequent evolution of the polarization always has the above form. The theory accounts for the dependence of C on the temperature jump, and of a and b on the final temperature. The A to E conversion rate a is obtained also by a calorimetric method which measures the rate at which rotational energy is transferred to the lattice. The result is in good agreement with the rate obtained from the dipolar polarization experiment.
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