Abstract
A theory is presented for nuclear spin conversion of methyl groups, where the total spin of the three protons changes fromI=1/2 toI=3/2. The transition may be mediated by the magnetic dipolar interaction of the protons among themselves or with a nearby unpaired electron. In general the excess energy, i.e. the tunnelling energy Δ, is transferred from the spin system to the lattice via the rotor-phonon coupling; for the case of an almost free rotor in the magnetic field of an unpaired electron spin, the direct coupling of the electron-proton interaction to the lattice motion may be the more efficient mechanism. AtT=0 the rate is found to be finite, at high temperatures it shows an Arrhenius behaviour. In the intermediate range, two different power laws may govern the temperature dependence, namely 1/τ∝T or 1/τ∝T 7; the latter is due to two-phonon scattering.
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Würger, A. Nuclear spin conversion of methyl groups. Z. Physik B - Condensed Matter 81, 273–279 (1990). https://doi.org/10.1007/BF01309359
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DOI: https://doi.org/10.1007/BF01309359