Methyl and Tert-Butyl Reorientation and Distributions of Activation Energies in Molecular Solids: A Nuclear Spin-Relaxation Study in 2,4- and 2,5-Di-Tert-Butylhydroxybenzene

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 (j)/21T = 8.50,22.5, and 53.0 MHz. The Tranges 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 (j) can be interpreted qualitatively in terms of three Bloembergen-PurcellPound (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 Frolich 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 (j) for 2,4- DTHBwhereas good quantitative fits are obtained using a Frolich spectral density. The distributions of activation energies characterizing the three rotor types are so wide for the Frolich spectral density fit of the 2,4-DTHB data that the individual rotor types lose their identity