Journal of Physical Chemistry A, Vol.119, No.15, 3663-3673, 2015
Analysis of the Hypersensitivity of the Si-29 NMR Chemical Shift of the Pentacoordinate Silicon Compounds to the Temperature Effect. N-(Silylmethyl)acetamides
Theoretical investigation of the phenomenon of hypersensitivity of the Si-29 NMR chemical shift, delta, in the pentacoordinate silicon compounds to the temperature effect has been performed by the example of N-(silylmethyl)acetamides MeC(O)NMeCH2SiX3 (X = Me, 1; OMe, 2; F, 3) and MeC(O)NMeCH2SiMe2F (4) with the use of experimental dynamic NMR (DNMR) Si-29 data. It is based on the following: (i) the analysis of the potential energy surface of molecules 1-4 in polar solvents and the energetics of interconversion between their possible isomeric forms; (ii) the calculations of delta at different temperatures taking into account the dependence of the dielectric constant (epsilon) of the medium on T, and (iii) the isolation of dynamic, geometrical, and polar contributions to the temperature drift of d. The results obtained allowed us to give a consistent explanation of the DNMR Si-29 spectra of acetamides 1-4 and to elucidate the nature of an unusual effect of T on delta.