Conformational studies by dynamic NMR, 69 - Conformation and stereodynamics of aliphatic triisopropylsilanes in solution and in the solid state

A dynamic process has been detected in the solution C-13-NMR spectra of tBu -Si(iPr)3 (4) in the temperature range -145 degrees C to -163 degrees C, th e corresponding free energy of activation being 5.5 kcal mol(-1). Molecular mechanics calculations have identified three types of conformations that a re markedly more stable than all the others, but the one with the highest s ymmetry cannot be populated, as is evident from the appearance of the spect rum at low temperature. This spectrum can be interpreted in terms of restri cted rotation of the tert-butyl group and of the isopropyl groups, leading to a structure centered around the (a,g,-g) conformation. In the case of th e analogous (iPr)3SiCl derivative (5), this dynamic process was not observe d in solution, but an appropriate spectrum for the same (a,g,-g) conformati on was obtained by the solid-stale CP-MAS NMR spectroscopy at -100 degrees C.