Microwave Spectrum and Barrier to Internal Rotation in Methylsilylacetylene

The microwave spectrum of CH3CCSiH3 shows a complex fine structure caused by internal rotation with a very low potential barrier. The major part of this structure can be explained on the basis of completely free internal rotation. However, small perturbations are found which suggest that the barrier may not be identically zero. Subject to reasonably firm assumptions, an upper limit of 3 cal/molecule can be placed on the barrier height. The theory for treating this type of spectrum is developed, and some implications of the results are discussed.