Understanding Substituent Effects on²⁹Si Chemical Shifts and Bonding in Disilenes. A Quantum Chemical Analysis

Abstract

The 29 Si chemical shift tensors in both symmetrically and unsymmetrically substituted disilenes have been analyzed by quantum chemical calculations. A detailed breakdown of the paramagnetic contributions to nuclear shielding into individual sum-over-states terms allows new insights into the interrelations between bonding and chemical shifts in these unsaturated compounds. While the substituent effects in symmetrically substituted systems may be understood to a large extent from the energy denominators of the perturbation expression, the very different shifts for the two silicon nuclei in unsymmetrically substituted systems are mainly due to the spatial extent and orientation of the occupied and unoccupied molecular orbitals, as sampled by the matrix elements of nuclear magnetic moments at both silicon sites. The unequal shifts may thus be understood and could serve as a diagnostic tool to distinguish between symmetrically and unsymmetrically substituted disilenes. Furthermore, unequal charge transfer and an alternating charge distribution can rationalize unexpectedly larger pyramidalization of the silyl-substituted silicon center in unsymmetrically substituted systems. † Dedicated to Professor Wolfgang Malisch on the occasion of his 60th birthday.

Origin of the Conformational Modulation of the 13C NMR Chemical Shift of Methoxy Groups in Aromatic Natural Compounds

“…Because the contributions of the C6 AOs to MO 53 are smaller for the out-of-plane orientation of the methoxy group (7.3% for the out-of-plane vs 22.9% for the in-plane), the corresponding contribution to the deshielding is reduced from −10.2 to −3.6 ppm (Table 5). Two significant contributions come from the ΣBD C6−H → MO 43 and ΣBD C6−H →MO 47 interactions, both of which contribute non-negligibly only to the out-of-plane conformation. For example, whereas the C6 AOs contribute 17.0% to MO 47 , the same contributions are vanishingly small for the inplane orientation (0.1%).…”

“…For example, whereas the C6 AOs contribute 17.0% to MO 47 , the same contributions are vanishingly small for the inplane orientation (0.1%). MO 43 and MO 47 are shown for both orientations in the Supporting Information ( Figure S9 and Tables S10 and S11).…”

The Versatile Chemistry of Disilenides: Disila Analogues of Vinyl Anions as Synthons in Low-valent Silicon Chemistry

“…30,42 The at first sight surprising deshielding of the formally negatively charged silicon atom in the 29 Si NMR (in the case of silyl anions usually strongly shielded signals are observed) 31 might be attributed to the increased mixing of · orbitals with the ³* orbital of the Si=Si bond. 32 The UVvis data of disilenides remains somewhat inconclusive at this point, but it is safe to say that a substantial difference between aryl-, alkyl-, and silylsubstituted derivatives can be expected such as it is well established for neutral disilenes. 4 The UVvis spectroscopy of aryl-substituted disilenides at least give a first indication that the HOMO may indeed be the nonbonding orbital.…”

Phosphide Delivery to a Cyclotrisilene

“…Strongly polarized Si=Si bonds with their characteristically wide dispersion of 29 Si NMR shifts have been subject to detailed experimental and computational studies. [24], [25]…”