Origin of the shielding effect in the 29Si NMR spectra of silatranes

Abstract

AbstractThe diamagnetic and paramagnetic terms of the 29Si nuclear shielding constant of silatranes, XSi(OCH2CH2)3N, and the related four‐coordinated silicon compounds, triethoxysilanes, XSi(OC2H5)3, have been estimated to explain the experimental peculiarities of their 29Si NMR spectra. High‐field shifts of the 29Si NMR signals of silatranes relative to triethoxysilanes are of diamagnetic origin, arising mainly from the existence of the Si←N bond and pentacoordination of the silicon atom in silatranes. At the same time, the decrease in the positive charge on the silicon atom upon Si←N interaction increases the absolute value of the negative paramagnetic term. When the Si←N bond in silatrane molecules becomes stronger through introduction of a more electronegative substituent X, the absolute value of the difference in the paramagnetic terms between silatranes and the related triethoxysilanes increases to a greater extent than that in the diamagnetic terms. This reduces the difference in 29Si chemical shifts of the corresponding four‐ and five‐coordinated silicon compounds with increasing electronegativity of the substituent X.