Hindered rotation of the silyl group in liquid-phase NMR spectra of 9-silyltriptycene derivatives: a comparison with the methyl analogues

Three 9-silyltriptycene derivatives, 1,4-dichloro- (DCST), 1,4-dibromo- (DBST), and 1,4-dimethyl-9-silyltriptycene (DMST) were synthesized, and temperature-variable (1)H NMR spectra thereof were measured. Below 220 K for DMST, and below 250 K for DCST and DBST, rotation of the silyl group becomes practically frozen on the NMR time scale. Iterative line shape analysis of the silyl proton spectra reveals that the familiar Alexander-Binsch line shape equation, employing only one rate constant for the observed rate process, is adequate in these cases. This is at a striking variance with the behavior of the methyl group in the analogous compounds, investigated by us recently, where fingerprints of the damped quantum rotation effect, a phenomenon once predicted by us, are clearly visible in the experimental spectra of the methyl protons. In the damped rotation approach, the relevant dynamics are described in terms of two quantum rate processes characterized by two coherence-damping constants, and the Alexander-Binsch model is obtained as a limiting case where these two constants become equal. The possible reasons of the differences between the dynamics of the silyl and methyl groups in the same molecular environment are discussed.