Proton Magnetic Resonance Studies of Solid Tetramethyls of Silicon, Germanium, Tin, and Lead

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Abstract
Forty‐megacycle‐per‐second proton magnetic resonance studies of solid Si(CH3)4, Ge(CH3)4, Sn(CH3)4, and Pb(CH3)4 have shown that methyl group reorientation occurs in all four compounds at temperatures as low as 77°K. T1 measurements show that the energy barrier to methyl reorientation in the X(CH3)4 family goes roughly as the inverse sixth power of the methyl separation. From linewidth measurements we find general molecular reorientation only in solid Si(CH3)4 and Ge(CH3)4 and only at temperatures just below the melting point. The activation energies are 7 and 10 kcal/mole, respectively. Self‐diffusion was found only in the metastable α phase of Si(CH3)4—an interesting result since the entropy of melting of α‐phase Si(CH3)4 is 8.34 eu, somewhat high for a plastic crystal. The experimental energy barrier to self‐diffusion in α‐phase Si(CH3)4 is about 6.2 kcal/mole. Proton magnetic resonance and thermal studies indicate that the X(CH3)4 family obeys the qualitative features of the Pople—Karasz theory of melting. A number of methods for calculating rigid‐lattice and methyl‐reorientation second moments are considered. A means is given for estimating the intermolecular contribution to the rigid‐lattice second moment when the crystal structure is unknown but the density of the solid is known. The method agrees well with other calculations.