Anisotropic Molecular Reorientation in Methyl-d3-acetylene in the Liquid State

Abstract

The temperature dependence of deuteron and proton spin–lattice relaxation times, T1, has been measured in methyl-d3-acetylene and methylacetylene-d1 using the adiabatic fast passage method. The difference in activation energies obtained from deuteron T1 data for acetylenic deuteron (Ea = 1.7 kcal/mole) and methyl-d3 deuterons (Ea = 1.1 kcal/mole) demonstrates that the reorientation is anisotropic without assumption of any model for reorientation. Assuming the validity of the rotational diffusion equation the deuteron T1's due to quadrupolar interactions were analyzed in terms of rotational diffusion constants D⊥ and D|. At −30°C D⊥ = 1.3 ± 0.15 × 1011sec−1 and D| = 18.3 ± 1.2 × 1011sec−1 in methyl-d3-acetylene. The applicability of the rotational diffusion mechanism for the description of the reorientation about the main symmetry axis is questionable as is shown by a comparison of τ| with the time for the mean period of rotation of a free rotor. At temperatures above the bp the spin–rotation interactions contribute to the relaxation rates of protons, in addition to dipolar interactions which are predominant at lower temperatures. The spin–rotation interaction constants were approximately estimated to be 0.6–4.0 kHz for the acetylenic proton in methyl-d3-acetylene and 6–25 kHz for the methyl group in methylacetylene-d1. The results indicate the importance of inertial effects.