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

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, ED=3.5 kcal/mole and ΔVΔ=31 cm3/mole tetramethyl silane, ED=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.