Methyl and t e r t-butyl reorientation and distributions of activation energies in molecular solids. A nuclear spin-relaxation study in 2,4- and 2,5-di-t e r t-butylhydroxybenzene

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Abstract
We have measured proton Zeeman relaxation rates R in the 2,4- and 2,5-isomers of di-tert-butylhydroxybenzene (DTHB) in the solid state. R was measured as a function of temperature T at proton Larmor frequencies of ω/2π=8.50, 22.5, and 53.0 MHz. The T ranges were from 78 K to just below the melting points of 2,4- and 2,5-DTHB, 385 and 323 K, respectively. The 2,5-DTHB R vs T and ω can be interpreted qualitatively in terms of three Bloembergen–Purcell–Pound (BPP) spectral densities, one for each of the three types of rotors in the molecule. The quantitative agreement is poor but a good fit is obtained using either a Davidson–Cole (DC) or Frölich spectral density, still preserving the three rotor types. The implications of this are discussed. The BPP and DC spectral densities fail completely in interpreting R vs T and ω for 2,4-DTHB whereas good quantitative fits are obtained using a Frölich spectral density. The distributions of activation energies characterizing the three rotor types are so wide for the Frölich spectral density fit of the 2,4-DTHB data that the individual rotor types lose their identity.