Diffusion and Reorientations in Crystalline Acetylene

Abstract

Crystalline acetylene $(\mathrm{HC}\equiv \mathrm{CH})$ in its two solid modifications was studied by means of temperature dependent static and rotating frame spin‐lattice relaxation time measurements, T₁ and $T_{1_{\rho }}$ respectively, between 188–122°K. The presence of molecular motions was established in both phases. Two types of motions were observed in the high temperature cubic phase. One of the motions was associated with a minimum in the $T_{1_{\rho }}$ curve below the melting point and also gave rise to a marked increase in the length of the free induction decay and was assigned to the onset of self‐diffusion. The other motion gave a minimum in the T₁ plot near 170°K and a corresponding decrease with temperature in the $T_{1_{\rho }}$ curve. Calculations of the longitudinal relaxation time behavior, as well as second moment considerations, for models of twofold flipping (assumes independent motion with two correlation times τ_c and τ_c/2) and spherical rotations (assumes a single correlation time τ_c) of the acetylene molecule, are both in agreement with the experimental data. However, the $T_{1_{\rho }}$ results are in better agreement with the spherical reorientation model which, actually, may approximate to a good degree a twofold flipping motion about two mutually perpendicular axes. Discontinuities in the T₁ and $T_{1_{\rho }}$ plots were observed at the phase transition. In the low temperature phase relatively slow twofold flips or, less likely, spherical reorientation occurs. Motional parameters such as activation energies, pre‐exponential factors, etc., were deduced from our data, and it was found that they vary only slightly at the transition. In light of the results obtained in this study it is evident that the solid phase of acetylene stable immediately below the melting point is of a plastic crystalline nature.