Comparative analysis of 13C shielding constants stereospecificity in the silicon and germanium derivatives of acetylenic aldehyde and ketone oximes based on the 13C NMR spectroscopy and GIAO calculations

Abstract

In the acetylenic aldehyde oximes with substituents containing silicon and germanium, the (13)C NMR signal of the C-2 carbon of triple bond is shifted by 3.5 ppm to lower frequency and that of the C-3 carbon is moved by 7 ppm to higher frequency on going from E to Z isomer. A greater low-frequency effect of 5.5 ppm on the C-2 carbon signal and a greater high-frequency effect of 11 ppm on the C-3 carbon signal are observed in the analogous acetylenic ketone oximes. The carbon chemical shift of the C=N bond is larger by 4 ppm in E isomer relative to Z isomer for the aldehyde and ketone oximes. The (29)Si chemical shifts in the silicon containing acetylenic aldehyde and ketone oximes are almost the same for the diverse isomers. The trends in changes of the measured chemical shifts are well reproduced by the gauge-including atomic orbital (GIAO) calculations of the (13)C and (29)Si shielding constants.