“Unexpected” ²⁹Si NMR Chemical Shifts in Heteroatom-Substituted Silyllithium Compounds:  A Quantum-Chemical Analysis

Abstract

Previous 29Si NMR spectroscopic investigations of various heteroatom-substituted silyllithium compounds revealed “unexpectedly” high silicon chemical shifts. To find explanations for these observations, the 29Si chemical shift tensors of various methoxy-, dimethylamino-, and methylthio-substituted chloro and lithiosilanes have been evaluated by quantum-chemical calculations. Substituent effects on shielding have been analyzed by IGLO-DFT calculations. Popular notions on the influence of atomic charge and substituent electronegativity on the central-atom shifts are not applicable. The combination of very electronegative and electropositive substituents leads to large deshielding contributions in heteroatom-substituted silyllithium compounds. As shown by analyses of localized molecular orbital (LMO) contributions to shielding, this is due to a coupling of bonding and antibonding orbitals with large silicon character and small energy differences. After a comparison of IGLO-LMO results and GIAO-based natural chemical shielding analyses, the LMO interpretations have been augmented further by an inspection of natural bond orbitals, to provide a consistent interpretation of the substituent effects on chemical shifts. The presented results should be transferable also to other heteroatom-substituted p-block main-group compounds.

Molecular and Silica-Supported Molybdenum Alkyne Metathesis Catalysts: Influence of Electronics and Dynamics on Activity Revealed by Kinetics, Solid-State NMR, and Chemical Shift Analysis

“…[78][79] An orbital analysis, closely related to the original Natural Chemical Shift (NCS) analysis, [80][81][82] of the computed shielding provides detailed insight into the nature of the frontier orbitals, helping to relate the electronic structure of reaction intermediates with activity. [83][84][85][86][87][88][89][90][91][92][93][94][95][96][97][98][99][100][101][102] Applying this approach to alkyne metathesis catalysts, the chemical shift tensor components ( 11 >  22 > model compound with three tris(tert-butoxy)siloxy ligands, at low spinning rates (Table 4) (Table 4) with a maximum deviation of less than 20 ppm, while the calculated and experimental principal components differ more, possibly due to the presence, at least in part, of dynamics (vide infra).…”

β-Amino- and Alkoxy-Substituted Disilanides

“…Later on, α-alkoxy- [12,13,14] and fluorosilanides [15,16,17] were introduced, and these compounds (silylenoids) turned out to be ambiphilic with a tendency to self-condensation. This was not observed for aminosilanides, although by NMR spectroscopy they clearly showed a relationship to those more reactive compounds [18].…”

The calculation of 29Si NMR chemical shifts of tetracoordinated silicon compounds in the gas phase and in solution

“…The measurement of 29 Si NMR chemical shifts is exceedingly helpful in elucidating the identity of silicon-containing molecular systems. [1][2][3][4][5][6][7][8][9][10][11][12][13] This is not only true for stable reactants and products of well-defined transformations, but also for silicon-based species generated as transient intermediates in the course of a reaction. 14 In this latter case the combination of theoretically predicted and experimentally measured 29 Si NMR chemical shifts is particularly helpful, as was amply demonstrated in detailed studies of, for example, silylenes 15 and disilenes.…”