Are the silyl group hydrogens in peri-substituted-9-silyltriptycenes engaged in blue-shifting hydrogen bonds?

Abstract

In three 9-silyltriptycenes bearing chlorine, bromine and the methyl group in one of the peri positions, the silyl group suffers extreme hindrance of its reorientational motion. Owing to this, separate signals of the individual silyl group protons could be observed in NMR spectra at relatively high temperatures. For each compound, the measured values of J couplings of these protons to the ²⁹Si nucleus are strongly diversified, especially when a halogen atom occurs in the peri position. These differences can be qualitatively interpreted in terms of electron density transfers from the peri substituent, causing the individual Si–H bonds to differentiate. The stereoelectronic picture revealed by natural bond orbital analysis fits the pattern normally interpreted as that of the blue-shifting hydrogen bond. However, an essential similarity of the stereoelectronic effects for both the halogen and methyl substituents raises a serious question about the applicability of that concept to the present case.