Self-Diffusion of Nearly Spherical Molecules. Neopentane and Tetramethyl Silane

Abstract

Self‐diffusion coefficients have been measured for neopentane and tetramethyl silane by means of the nuclear magnetic resonance spin‐echo technique. Both pressure and temperature dependences have been determined, thus allowing evaluation of energies and volumes of activation; neopentane, E_D=3.5 kcal/mole and ΔVΔ=31 cm³/mole tetramethyl silane, E_D=1.16 kcal/mole and ΔVΔ=22 cm³/mole. The numbers given for neopentane are average values as the lnD vs 1/T and lnD vs p plots were not straight lines. Kirkwood's theoretical relation for the self‐diffusion coefficient of spherical molecules has been tested for the quasi‐spherical neopentane. At 0°C the theoretical value is 6×10^—5 cm²/sec while the experimental value is 2.6×10^—5 cm²/sec. The Kirkwood equation for the self‐diffusion coefficient is discussed in detail.