A comprehensive and diverse benchmark set for the calculation of 29 Si NMR chemical shifts is presented. The SiS146 set includes 100 silicon containing compounds with 146 experimentally determined reference 29 Si NMR chemical shifts measured in nine different solvents in a range from −400 to +828 ppm. Silicon atoms bound to main group elements as well as transition metals with coordination numbers of 2−6 in various bonding patterns including multiple bonds and coordinative and aromatic bonding are represented. The performance of various common and specialized density functional approximations including (meta-)GGA, hybrid, and double-hybrid functionals in combination with different AO basis sets and for differently optimized geometries is evaluated. The role of scalar-relativistic effects is further investigated by inclusion of the zeroth order regular approximation (ZORA) method into the calculations. GGA density functional approximations (DFAs) are found to outperform hybrid DFAs with B97-D3 performing best with an MAD of 7.2 ppm for the subset including only light atoms (Z < 18), while TPSSh is the best tested hybrid functional with an MAD of 10.3 ppm. For 29 Si cores in the vicinity of heavier atoms, the application of ZORA proved indispensable. Inclusion of spin−orbit effects into the 29 Si NMR chemical shift calculation decreases the mean absolute deviations by up to 74% compared to calculations applying effective core potentials.
A new benchmark set termed SnS51 for assessing quantum chemical methods for the computation of 119Sn NMR chemical shifts is presented. It covers 51 unique 119Sn NMR chemical shifts for a selection of 50 tin compounds with diverse bonding motifs and ligands. The experimental reference data are in the spectral range of ±2500 ppm measured in seven different solvents. Fifteen common density functional approximations, two scalar- and one spin–orbit relativistic approach are assessed based on conformer ensembles generated using the CREST/CENSO scheme and state-of-the-art semiempirical (GFN2-xTB), force field (GFN-FF), and composite DFT methods (r2SCAN-3c). Based on the results of this study, the spin–orbit relativistic method combinations of SO-ZORA with PBE0 or revPBE functionals are generally recommended. Both yield mean absolute deviations from experimental data below 100 ppm and excellent linear regression determination coefficients of ≤0.99. If spin–orbit calculations are not affordable, the use of SR-ZORA with B3LYP or X2C with ωB97X or M06 may be considered to obtain qualitative predictions if no severe spin–orbit effects, for example, due to heavy nuclei containing ligands, are expected. An empirical linear scaling correction is demonstrated to be applicable for further improvement, and respective empirical parameters are given. Conformational effects on chemical shifts are studied in detail but are mostly found to be small. However, in specific cases when the ligand sphere differs substantially between conformers, chemical shifts can change by up to several hundred ppm.
Silafulleranes with endohedral Cl − ions are a unique, scarcely explored class of structurally well-defined silicon clusters and host-guest complexes. Herein, we report regioselective derivatization reactions on the siladodecahedrane [nBu 4 N][Cl@ Si 20 (SiCl 3 ) 12 Cl 8 ] ([nBu 4 N][1]), which has its cluster surface decorated with 12 SiCl 3 and 8 Cl substituents in perfect T h symmetry. The room-temperature reaction of [nBu 4 N][1] with excess iBu 2 AlH in ortho-difluorobenzene (oDFB) furnishes perhydrogenated [nBu 4 N][Cl@Si 20 (SiH 3 ) 12 H 8 ] ([nBu 4 N][2]) in 50% yield; the non-pyrophoric [2] − is the largest structurally authenticated (by X-ray diffraction) hydridosilane known to date. A simple switch from pure oDFB to an oDFB/Et 2 O solvent mixture suppresses core hydrogenation and results in the formation of [nBu 4 N][Cl@Si 20 (SiH 3 ) 12 Cl 8 ] ([nBu 4 N][3]). In addition to the exhaustive Cl/H exchange at all 44 Si−Cl bonds of [1] − and the regioselective 36-fold silyl group hydrogenation, we achieved the simultaneous introduction of Me substituents at all 8 SiCl vertices along with the conversion of all 12 SiCl 3 to SiH 3 groups by treating [nBu 4 N][1] with Me 2 AlH/Me 3 Al in oDFB ([nBu 4 N][Cl@ Si 20 (SiH 3 ) 12 Me 8 ], [nBu 4 N][4]; 73%). Quantum-chemical free-energy calculations find an S N 2-Si-type hydrogenation of the exohedral SiCl 3 moieties in [1] − (trigonal-bipyramidal intermediate) slightly preferred over metathesis-like S N i-Si substitutions (four-membered transition state). Cage hydrogenation likely occurs via S N i-Si processes. The experimentally demonstrated influence of an Et 2 O co-solvent, which drastically increases the respective reaction barriers, is attributed to the increased stability of the resulting iBu 2 AlH-OEt 2 adduct and its higher steric bulk compared to free iBu 2 AlH.
The “Swiss army knife” composite density functional electronic-structure method r 2 SCAN-3c ( J. Chem. Phys. 2021 , 154 , 064103) is extended and optimized for the use with Slater-type orbital basis sets. The meta generalized-gradient approximation (meta-GGA) functional r 2 SCAN by Furness et al. is combined with a tailor-made polarized triple-ζ Slater-type atomic orbital (STO) basis set (mTZ2P), the semiclassical London dispersion correction (D4), and a geometrical counterpoise (gCP) correction. Relativistic effects are treated explicitly with the scalar-relativistic zeroth-order regular approximation (SR-ZORA). The performance of the new implementation is assessed on eight geometry and 74 energy benchmark sets, including the extensive GMTKN55 database as well as recent sets such as ROST61 and IONPI19. In geometry optimizations, the STO-based r 2 SCAN-3c is either on par with or more accurate than the hybrid density functional approximation M06-2X-D3(0)/TZP. In energy calculations, the overall accuracy is similar to the original implementation of r 2 SCAN-3c with Gaussian-type atomic orbitals (GTO), but basic properties, intermolecular noncovalent interactions, and barrier heights are better described with the STO approach, resulting in a lower weighted mean absolute deviation (WTMAD-2(STO) = 7.15 vs 7.50 kcal mol –1 with the original method) for the GMTKN55 database. The STO-optimized r 2 SCAN-3c outperforms many conventional hybrid/QZ approaches in most common applications at a fraction of their cost. The reliable, robust, and accurate r 2 SCAN-3c implementation with STOs is a promising alternative to the original implementation with GTOs and can be generally used for a broad field of quantum chemical problems.
Starting from the perhydrogenated silafullerane [nBu4N][Cl@Si20(SiH3)12H8], treatment with BBr3 leads to partially and exhaustively brominated clusters, [nBu4N][Cl@Si20(SiBr2H)12Br8] (120 eq. BBr3, room temperature) and [nBu4N][Cl@Si20(SiBr3)12Br8] (300 eq. BBr3, 130 °C). Perbromination...
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