Full 1H and 13C NMR Chemical Shift Assignment of 1-Pyrenyl Substituted Oligosilanes as a Tool to Differentiate between Intramolecular "Through Space" and "Through Bond" Ground State Interactions

Abstract

A detailed analysis strategy is developed to realize a full proton and carbon NMR chemical shift assignment of a series of substituted 1-pyrenylsilane derivatives mainly based on direct and long-range proton<arbon chemical shift 2D-correlation spectroscopy. The proton and carbon chemical shifts of two dimethylaniline substituted 1-pyrenylsilane derivatives (P2D and P3D) and a pentafluorophenyl substituted 1 -pyrenylsilane derivative (P2F) are compared to nonsubstituted 1-pyrenylsilane derivatives of different silane chain lengths (P2, P3, and P6). Accurate I3C shifts of these compounds are shown to be an important tool to study the character of the ground state interactions in these compounds. They are more sensitive than 1H shifts, and the interpretation is more straightforward since there is no confusion with possible chemical shift anisotropy effects. The analysis leads to the conclusion that changes in local electron density, caused by a donor group as dimethylaniline or by an acceptor group as pentafluorophenyl, are transferred to the pyrene moiety primarily via “through space” electrostatic polarization effects rather than donoracceptor effects. The observed changes in chemical shift point to the presence of a significant contribution of folded conformations in the ground state for the a,w-substituted silane derivatives.