Analysis of the Hypersensitivity of the ²⁹Si NMR Chemical Shift of the Pentacoordinate Silicon Compounds to the Temperature Effect. <i>N</i>‑(Silylmethyl)acetamides

Abstract

Theoretical investigation of the phenomenon of hypersensitivity of the ²⁹Si NMR chemical shift, δ, in the pentacoordinate silicon compounds to the temperature effect has been performed by the example of <i>N</i>-(silylmethyl)­acetamides MeC­(O)­NMeCH₂SiX₃ (X = Me, <b>1</b>; OMe, <b>2</b>; F, <b>3</b>) and MeC­(O)­NMeCH₂SiMe₂F (<b>4</b>) with the use of experimental dynamic NMR (DNMR) ²⁹Si data. It is based on the following: (i) the analysis of the potential energy surface of molecules <b>1</b>–<b>4</b> in polar solvents and the energetics of interconversion between their possible isomeric forms; (ii) the calculations of δ at different temperatures taking into account the dependence of the dielectric constant (ε) of the medium on <i>T</i>, and (iii) the isolation of dynamic, geometrical, and polar contributions to the temperature drift of δ. The results obtained allowed us to give a consistent explanation of the DNMR ²⁹Si spectra of acetamides <b>1</b>–<b>4</b> and to elucidate the nature of an unusual effect of <i>T</i> on δ