Methylsilane, rotational barrier

The rotational barrier in methylsilane (Table 3.4, entry 5) is significantly smaller than that in ethane (1.7 versus 2.88 kcal/mol). This reflects the decreased electron-electron rqjulsions in the eclipsed conformation resulting from the longer carbon-silicon bond length (1.87 A) compared to the carbon-carbon bond length (1.54 A) in ethane. 

Some of the interesting features in silanes which are well reproduced by the calculations include the rotational barriers (T7 kcal mole-1 in methylsilane as compared with 2-9 kcal mole-1 in ethane, and correspondingly low numbers for other methyl silanes). While the torsional function about the 2,3-bond in 1-sila-butane looks almost like that in butane, the corresponding function... 

Schwendemann, R. M., and G. D. Jacobs Microwave Spectrum, Quadrupole Coupling Constants, and Barrier to Internal Rotation of Chloro-methylsilane. J. chem. Physics 36, 1251—1257 (1962). 

Tetramethylsilane.—The torsional frequencies for tetrahedral (CHa)4X molecules are forbidden in the i.r. spectra and the only source of information on the barrier heights is a comparison of measured and calculated thermodynamic functions, on the assumption that the internal rotations are independent. For tetramethylsilane such a comparison has been made for the entropy at 227 and 298.16 K, and affords barrier height values of 6.91 and 6.69 kJ mol S respectively. These are to be compared with values from microwave spectroscopy of (7.11 0.42) kJ mol for methylsilane and of (6.97 0.04) kJ mol for dimethylsilane. ... 

---