Selective deuteration and the anomalous temperature dependence of proton spin–lattice relaxation in polycrystalline p-t e r t-butyltoluene

Abstract

Proton spin–lattice relaxation times T₁ are reported for p‐t‐butyltoluene (I); the derivatives having the toluene methyl group relaced with CH₂D (II), CHD₂ (III), and CD₃, (IV); and the derivative having all of the ring positions deuterated (V). The measurements were made on polycrystalline samples in the temperature range 4–260 K at both 17.1 and 36 MHz. Raman spectra are also reported for these compounds at 77 K. All of the compounds show T₁ minima between 100 and 200 K which are associated with motions of the protons in the t‐butyl group and, except for compound V, with protons attached to the ring. Compounds I, II, and V exhibit two T₁ minima below 10 K which are presumably associated with motions of the protons in the toluene methyl group. The Raman peaks do not permit the form of the potential function for internal rotation of this methyl group to be established. It is argued that a potential having both three and sixfold contributions is consistent with the low temperature T₁ data. For compound II, which contains CH₂D, the 17.1 MHz experiment gives frequency dependent T₁ minima at 13.5 and 52.6 K which show apparent activation energies of 80±20 and 640±10 cal/mol, respectively. If these energies are associated with the torsional splitting E₁−E₀ and the activation energy V_max−E₀, respectively, the required ratio of V₆ to V₃ contributions in the potential function is 0.71.