29Si NMR Chemical Shifts Research Articles

Cl@Si20X20 cages: evaluation of encapsulation nature, structural rigidity, and 29Si-NMR patterns using relativistic DFT calculations.

The experimental characterization of Cl@Si20 endohedral clusters, featuring different ligands such as [Cl@Si20H20]- (1) [Cl@Si20H12Cl8]- (2), and [Cl@Si20Cl20]- (3), provides insight into the variable encapsulation environment for chloride anions. The favorable formation of such species enables the evaluation of the encapsulation nature and the role of the inner anion in the rigidity of the overall cluster. Our results show a sizable interaction which increases as -66.7, -100.8, and -130.3 kcal mol-1 from 1 to 3, respectively, featuring electrostatic character. The orbital interaction involves 3p-Cl → Si20X20 and 3s-Cl → Si20X20 charge transfer channels and a slight contribution from London dispersion-type interactions. These results show that the inner bonding environment can be modified by the choice of exobonded ligands. Moreover, 29Si-NMR parameters are depicted in terms of the chemical shift anisotropy (CSA), leading to a strong variation of the three principal tensor components (δ11, δ22, δ33), unraveling the origin of the experimental 29Si-NMR chemical shift (δiso) differences along the given series. Thus, the Si20 cage is a useful template to further evaluate different environments for encapsulating atomic species.

Cover Feature: An Anionic Amido‐Substituted Seven‐Vertex Siliconoid Cluster (Chem. Eur. J. 45/2022)

Breakdown of a record breaker: An anionic seven-vertex amido-substituted siliconoid cluster with four unsubstituted silicon atoms and a record upfield 29Si NMR chemical shift at −414 ppm is described. Magnetically induced current density calculations were performed to understand the record upfield chemical shift. The result of this calculation is illustrated here. More information can be found in the Research Article by R. J. F. Berger, F. Lips, and co-workers (DOI: 10.1002/chem.202201473).

Lead silicate glass structure: New insights from diffraction and modeling of probable lone pair locations

AbstractStructures of binary PbO‐SiO2 glasses have been studied in detail over the compositional range 35 to 80 mol% PbO using high‐resolution neutron diffraction, high‐energy X‐ray diffraction, static 207Pb NMR, and structural modeling. The changes in the local environment of Pb(II) are subtle; it has a low coordination to oxygen (∼3 to 4) plus a stereochemically active electron lone pair and, thus, behaves as a glass network forming (or intermediate) cation over the entire composition range. This conclusion contradicts previous reports that Pb(II) is a network modifier at low concentrations, and is supported by an analysis of lead and alkaline earth silicate glass molar volumes. The Pb‐O peak bond length shortens by 0.04 Å with increasing PbO content, indicating stronger, more covalent bonding, and consistent with an increase in the number of short (≤ 2.70 Å) Pb‐O bonds, from 3.3 to 3.6. This is accompanied by increased axial symmetry of the Pb(II) sites, and is interpreted as a gradual transition toward square pyramidal [PbO4] sites such as those found in crystalline PbO polymorphs. An attendant decrease in the periodicity associated with the first sharp diffraction peak (FSDP) toward that of β‐PbO, accompanied by increases in the correlation lengths associated with the plumbite network (FSDP) and silicate anions (neutron prepeak), provides evidence of increased intermediate‐range order and has implications for the glass forming limit imposed by crystallization. Pb(II) electron lone pairs occupy the natural voids within the silicate network at low PbO contents, while at high PbO contents they aggregate to create voids that form part of the plumbite network, analogous to the open channels in Pb11Si3O17 and the layered structures of α‐ and β‐PbO. Si‐O and Pb‐O bond lengths have been correlated with 29Si and 207Pb NMR chemical shifts, respectively. This is the first time that such correlations have been demonstrated for glasses and attests to the accuracy with which pulsed neutron total scattering can measure average bond lengths.

A theoretical and experimental investigation of hetero- vs. homo-connectivity in barium silicates

Abstract Barium silicates may be found in contact aureoles and are used in several important technologies (e.g., LEDs). The BaO-SiO2 system stabilizes 13 crystalline phases with different silicate tetrahedral and connectivity profiles. Aside from phases composed of a single structural unit (isolated or homo-connected tetrahedra), one encounters the relatively rare case of hetero-connected tetrahedra in which varying proportions of several Qn species are linked together. Here, we analyze the 29Si MAS NMR and Raman spectroscopic manifestations of the connectivities in seven barium silicates: Ba2SiO4, high-BaSiO3, Ba4Si6O16, Ba5Si8O21, Ba6Si10O26, high-BaSi2O5, and sanbornite (low-BaSi2O5). The structures and purity of these phases were confirmed by Rietveld refinement. From a Raman spectroscopic database of 144 predominantly homo-connected crystalline silicates, the mean Qn mode frequencies νQn (±1σ) are found at 828 (±14) cm−1 for Q0, 905 (±22) cm−1 for Q1, 994 (±26) cm−1 for Q2, and 1068 (±18) cm−1 for Q3 units. Experimentally, homo-connected barium silicates show good agreement with these values, whereas the hetero-connected phases show a wider range of νQ2 than of νQ3 frequencies. While the 29Si NMR chemical shifts of the barium silicates are in agreement with known structural trends, those measured for the Q2 resonances remain essentially constant, which may be caused by the lattice distortion around the large Ba2+ cations. To complement and rationalize experimental measurements, first-principles calculations at the density functional theory level have reproduced measured frequencies within a mean absolute deviation of <7 cm−1. Our work highlights how the results provided by 29Si NMR and Raman spectroscopies and ab initio calculations can be combined to rationalize the structure of complex systems. The present findings also shed light on the vibrational modes that may be used to track bond lengths in situ at extreme conditions and the behavior of homo- vs. hetero-connectivity, revealing clear implications for evaluating silicate glasses and melts where hetero-connectivity is the rule rather than the exception.

Comprehensive Benchmark Study on the Calculation of 29Si NMR Chemical Shifts

A comprehensive and diverse benchmark set for the calculation of 29Si NMR chemical shifts is presented. The SiS146 set includes 100 silicon containing compounds with 146 experimentally determined reference 29Si 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 29Si cores in the vicinity of heavier atoms, the application of ZORA proved indispensable. Inclusion of spin-orbit effects into the 29Si NMR chemical shift calculation decreases the mean absolute deviations by up to 74% compared to calculations applying effective core potentials.

Exploring the Substituent Еffect on the Structure and Еlectronic Рroperties of Si2(para-C6H4X)2 Мolecules

Substituent effect on the structure and electronic properties of the Si2(para-C6H4X)2 molecules (X = NH2, OH, Me, H, F, Cl, CN, NO2) were studied at wB97XD/6-311G(d,p) level of theory. The substituent effect on the structural parameters, reactivity parameters (hardness, chemical potential and electrophilicity) and 29Si NMR chemical shift was explored. Stability of the Si(para-C6H4X) fragments were investigated in the doublet and quartet states for illustration of the Si≡Si bond character. Substituent influence on the stability of these different spin multiplicities of Si(para-C6H4X) fragments was investigated. The Si–Si bonding situation in the studied molecules was examined via the Shubin Liu’s energy decomposition analysis (EDA-SBL), adaptive natural density partitioning (AdNDP), and Quantum theory of atoms in molecules (QTAIM) schemes.

Spectroscopic and Computational Assessment of Silicon’s Electrophilicity in Phosphinosilylium Cations

An experimental method of determining the electrophilicity of silicon in phosphinosilylium cations is reported and compared to Djukic’s DFT method of computing relative intrinsic silylicity, Π. We also establish linear correlations between silicon electrophilicity and 29Si NMR chemical shifts, DFT-computed silylicities, and the Tolman electronic parameter of the phosphine. These correlations were not universal, as deviations were observed for the most sterically hindered phosphines. Intermolecular silylium transfer experiments between phosphinosilyliums and added phosphines provided a thermodynamic assessment of the electrophilic character at silicon. This confirmed that relative intrinsic silylicity (Π) and 29Si NMR chemical shifts are useful parameters for semiquantitatively determining the electrophilicity of the silyl group in a Lewis pair.

1H, 13C and 29Si magnetic shielding in gaseous and liquid tetramethylsilane

Tetramethylsilane (TMS) is well-known as a reference standard of 1H, 13C and 29Si NMR chemical shifts. In the present study, we have observed TMS molecules in gaseous and liquid solutions. In the gas phase, the shielding parameters of TMS are monitored as the functions of density when xenon and krypton are applied as the buffer gases. It permits the evaluation of shielding in an isolated TMS molecule which is determined from the measurements of frequency and available nuclear magnetic moments. Having the shielding constants of an isolated TMS molecule, it is possible to proceed with the evaluation of 1H, 13C and 29Si TMS shielding in liquid state, which is extremely useful for the complete referencing of NMR spectra for protons, carbon-13 and silicon-29 nuclei. Consequently, the readings of chemical shifts and shielding parameters can be practically performed in the same experiment.

Origin of the 29Si NMR chemical shift in R3Si-X and relationship to the formation of silylium (R3Si+) ions.

The origin in deshielding of 29Si NMR chemical shifts in R3Si-X, where X = H, OMe, Cl, OTf, [CH6B11X6], toluene, and OX (OX = surface oxygen), as well as iPr3Si+ and Mes3Si+ were studied using DFT methods. At the M06-L/6-31G(d,p) level of theory the geometry optimized structures agree well with those obtained experimentally. The trends in 29Si NMR chemical shift also reproduce experimental trends; iPr3Si-H has the most shielded 29Si NMR chemical shift and free iPr3Si+ or isolable Mes3Si+ have the most deshielded 29Si NMR chemical shift. Natural localized molecular orbital (NLMO) analysis of the contributions to paramagnetic shielding (σp) in these compounds shows that Si-R (R = alkyl, H) bonding orbitals are the major contributors to deshielding in this series. The Si-R bonding orbitals are coupled to the empty p-orbital in iPr3Si+ or Mes3Si+, or to the orbital in R3Si-X. This trend also applies to surface bound R3Si-OX. This model also explains chemical shift trends in recently isolated tBu2SiH2+, tBuSiH2+, and SiH3+ that show more shielded 29Si NMR signals than R3Si+ species. There is no correlation between isotropic 29Si NMR chemical shift and charge at silicon.

Is hydrogen electronegativity higher than Pauling’s value? New clues from the 13C and 29Si NMR chemical shifts of [CHF3] and [SiHF3] molecules

Abstract We previously demonstrated that the δ NMR chemical shift of central NMR active atoms (A), in simple halido [AXn] (A=C, Si, Ge, Sn, Pb, Pt; Xn = combination of n halides, n = 4 or 6) derivatives, could be directly related to X radii overall sum, Σ(r L ). Further correlation have also been observed for tetrahedral [AX4] (A=C, Si; X4 = combination of four halides) compounds where the X Pauling electronegativities sum, Σ ( χ L Pau ) , $\Sigma (\chi _L^{{\rm{Pau}}}),$ exceeds a specific value (≈12.4). In this work, we focused on these latter systems considering the H vs. X substitution. The analysis of the literature reported δ(13C) and δ(29Si) NMR chemical shift for the mono hydrogenated derivatives and in particular for [CHF3] and [SiHF3], characterized by the lowest Σ(r L ) and the highest Σ ( χ L Pau ) , $\Sigma (\chi _L^{{\rm{Pau}}}),$ suggests a revised value for the H electronegativity ranking with respect to Pauling’s.

Weak Interactions Get Strong: Synergy between Tetrel and Alkaline-Earth Bonds.

Weak and strong noncovalent interactions such as tetrel bonds and alkaline-earth bonds, respectively, cooperate and get reinforced when acting together in ternary complexes of general formula RN··· SiH3F···MY, where MY is a Be or Mg derivative and RN is a N-containing Lewis base with different hybridization patterns. Cooperativity has been studied in the optimized MP2/aug'-cc-pVTZ ternary complexes by looking at changes on geometries, binding energies, 29Si NMR chemical shifts, and topological features according to the atoms in molecules theoretical framework. Our study shows that cooperativity in terms of energy is in general significant: more than 40 kJ/mol, and up to 83.6 kJ/mol in the most favorable case. The weakest the isolated interaction, the strongest the reinforcement in the ternary complex; in this sense, the tetrel bond is shortened enormously, between 0.3 and 0.6 Å. This dramatic reinforcement of the tetrel bond is also nicely reflected in the positive variations of the 29Si chemical shifts in all the ternary complexes. At the same time the ternary complexes are characterized by the presence of totally planar silyl group, due to the pentacoordination of the Si atom. Both the hybridization of the N base and the geometry imposed by the alkaline-earth ligands have a strong influence on the binding energies, as they modify the donor ability of N and the Lewis acid character of the alkaline-earth metal.

Mono-C,O-chelated bromo- and triflatosilanes with an amino acid moiety: salts or covalently bonded complexes?

Derivatives of the amino acids sarcosine and proline – triflates TsN(Me)CH2C(O)N(Me)-CH2SiMe2OTf and Ns–Pro–N(Me)CH2SiMe2OTf (Ns is 4-NO2C6H4SO2) – were synthesized by the reaction of trimethylsilyl triflate with halosilanes prepared previously. Bromide NsNHCH(Me)C(O)N(Me)CH2SiMe2Br was synthesized by the cleavage of the Si–O–Si moiety of the appropriate disiloxane with excess acetyl bromide. The X-ray diffraction study of these compounds and of the previously characterized bromide TsN(Ac)CH2C(O)N(Me) CH2SiMe2Br showed that the Si–O coordination bond lengths in the triflates are 1.7692(14)– 1.8623(14) A. The interatomic distances between the bromine and silicon atoms are 2.7095(8) and 2.9704(7) A, which indicates that these bonds are weak. To elucidate the nature of Si…X bonding (X = OTf, Br), the topological analysis of electron density was performed and 29Si NMR chemical shifts were calculated. The interatomic Si…X interaction in triflate TsN(Me)CH2C(O)- N(Me)CH2SiMe2OTf and bromide TsN(Ac)CH2C(O)N(Me)CH2SiMe2Br is weak. In triflate Ns–Pro–N(Me)CH2SiMe2OTf and bromide NsNHCH(Me)C(O)N(Me) CH2 SiMe2Br, the Si…X interaction is significantly stronger and corresponds to a weak coordination bond. Hence, the tosylates can be considered as salts, whereas the para-nitrobenzenesulfonyl derivatives are monochelated five-coordinate silicon complexes.

A quantum chemical investigation of the influence of solvent polarity on the structural, electronic, spectroscopic properties and hyperpolarizability in Molybdenum Silylidyne complex CpMo(CO)2([tbnd]Si[sbnd]Ph)

In this study, quantum chemical calculations using MPW1PW91 method was applied to analyze of solvent effect on the structural, 13C and 29Si NMR chemical shifts, vibrational analysis, thermochemical parameters and first hyperpolarizability for CpMo(CO)2(SiPh) complex. The solvent effects were examined by the self-consistent reaction field theory (SCRF) based on Polarizable Continuum Model (PCM). The correlations between these parameters and solvent polarity functions, involve both the dielectric constant (ε) and refractive index (nD) of the liquid medium were explored. Correlations of the calculated spectral parameters (υ(CO), δ(13C) and δ(29Si)) with the Kirkwood–Bauer–Magat equation (KBM) and improved form of this equation were explored.

Realistic Atomistic Structure of Amorphous Silicon from Machine-Learning-Driven Molecular Dynamics.

Amorphous silicon ( a-Si) is a widely studied noncrystalline material, and yet the subtle details of its atomistic structure are still unclear. Here, we show that accurate structural models of a-Si can be obtained using a machine-learning-based interatomic potential. Our best a-Si network is obtained by simulated cooling from the melt at a rate of 1011 K/s (that is, on the 10 ns time scale), contains less than 2% defects, and agrees with experiments regarding excess energies, diffraction data, and 29Si NMR chemical shifts. We show that this level of quality is impossible to achieve with faster quench simulations. We then generate a 4096-atom system that correctly reproduces the magnitude of the first sharp diffraction peak (FSDP) in the structure factor, achieving the closest agreement with experiments to date. Our study demonstrates the broader impact of machine-learning potentials for elucidating structures and properties of technologically important amorphous materials.

Pauling Electronegativity On/Off Effects Assessed by 13 C and 29 Si NMR Spectroscopic Analysis.

In carbon and silicon tetrahalide compounds, the experimental 13 C and 29 Si NMR chemical-shift values are known to increase or decrease on increasing the overall sum of the ionic radii of the bonded halides Σ(rh ) (normal and inverse halogen dependence (NHD and IHD, respectively)). Herein, we extrapolate the main factors responsible for such NMR chemical shifts. Intriguingly, we found a characteristic value for the overall sum of the Pauling electronegativities of the bonded halides Σ(χh ), which works as a triggering factor to determine the transition from the NHD to IHD. Below this Σ(χh ) value, the chemical shift of the central atom was strictly related to only the Σ(rh ) value, thus producing a NHD trend. Conversely, above this value, the chemical shift of the central atom was dependent on both the Σ(rh ) and Σ(χh ) values, thus producing a IHD trend. A simple model, in which the effect of the Σ(χh ) value on 13 C and 29 Si NMR chemical shifts is related to an apparent increase in the Σ(rh ) value, is deduced.

Identification of double four-ring units in germanosilicate ITQ-13 zeolite by solid-state NMR spectroscopy

Well-crystallized Ge-free and Ge-ITQ-13 were successfully obtained by solid state synthesis method. The Ge/Si ratio and the water content that are the two important factors in the synthesis of germanosilicate zeolites were explored for the formation of ITQ-13. The effect of the mineralizing agents (NH4F and NH4Cl) on the ITQ-13 synthesis was investigated as well. The obtained pure silica ITQ-13 and Ge-ITQ-13 were characterized by one- and two-dimensional solid- state NMR techniques. One-dimensional (1D) 19F MAS, 1H→29Si CP/MAS and 19F→29Si CP/MAS NMR spectroscopy evidenced the formation of pure Si-D4R (double four ring) and Ge-D4R units, with the latter being generated by substitution of Si atom from the former units. The detailed configurations of the Ge-D4R units in ITQ-13 was revealed by two-dimensional (2D) 29Si{19F} HETCOR NMR spectroscopy. With the help of theoretical calculations on the 19F and 29Si NMR chemical shifts, six types of D4R units were determined. The formation of the specific D4R unit confirms the structural directing roles of Ge atom and F ions in the formation of the D4R units in zeolite framework. The identification of the SiOSi bonds in the D4R units that connects the layers of ITQ-13 framework provided rationale for the high stability of the ITQ-13 in the degermanation treatment.

A Stable Silylene with a σ2, π- Butadiene Ligand

The synthesis of a new type of silylene 1 is reported. It adopts a bicyclo[2.1.1]hexene structure in which a hafnocene group is incorporated. The silylene is stabilized by homoconjugation with the remote C═C double bond. This is indicated by its highly shielded 29Si NMR chemical shift (δ29Si = -155) and is firmly established by its experimental molecular structure from XRD analysis. The results of a detailed bonding analysis based on DFT calculations suggest for model compounds of silylene 1 and for its heavier germanium, tin, and lead homologues uniformly electronic structures of carbene analogues that are stabilized by homoconjugation. This stabilization mode is equivalent to a σ2, π-coordination of the butadiene ligand to the element atom as it is typical for zirconocene or hafnocene butadiene complexes.

Quantum-chemical calculations of NMR chemical shifts of organic molecules: XV. Relativistic calculations of 29Si NMR chemical shifts of silanes

Calculations of 29Si NMR chemical shifts of 68 silanes possessing various substituents, in particular, with heavy halogens attached to silicon atom, were carried out applying an efficient calculation scheme of locally dense basis set in the framework of the electron density functional theory utilizing the Keal–Tozer functional combined with relativistic Dyall basis sets on a four-component relativistic level. The main factors of calculation accuracy of silicon chemical shifts were analyzed including the relativistic effects, environmental impact, and vibrational corrections. The mean absolute calculation error for the studied compounds series accounting for all mentioned factors was 14.0 ppm for the nonrelativistic calculation and 6.7 ppm for the four-component relativistic calculation at the range of silicon chemical shifts variation of ~250 ppm.

NMR effective radius of hydrogen in XIV group hydrides evaluated by NMR spectroscopy.

In the [ABrnIm] (A = C, Si, Ge, Sn; n + m = 4) compounds, with the heavier halides bonded to the central IV group elements, the experimental 13C, 29Si, 73Ge and 119Sn NMR chemical shifts of the central atoms were found to be strictly linearly proportional to the bonded halides ionic radii overall sum ∑(rh). Based on this, calibration lines relating the chemical shifts to ∑(rh) could be built for the considered subgroup of [ABrnIm] compounds. Using such calibration lines we could calculate the equivalent NMR radius, NMRrH-A, attributable to each of the bonded hydrogens in [AH4] species, according to the overall NMR shielding produced on the central A atom. Interestingly, the calculated NMRrH-A value resulted to be almost constant in all [AH4] examined systems (A = 13C, 29Si, 73Ge, 119Sn) with an average NMRr[combining macron]H-A value equal to 194.6 ± 1.6 pm. Based on this approach, we could calculate the 207Pb NMR chemical shift of the unstable [PbH4] complex using the value of 192.7 pm calculated for NMRrH-Sn in the stable closest hydride [SnH4]. The obtained unprecedented NMR value is in accord with the 207Pb NMR chemical shift estimation, independently calculated for [PbH4] from the [SnH4] data, using the Pb/Sn chemical shift correlation defined in the Mitchell equation.

Planar PtPd3 Complexes Stabilized by Three Bridging Silylene Ligands.

A heterobimetallic PtPd3 complex supported by three bridging diphenylsilylene ligands, [Pt{Pd(dmpe)}3 (μ3 -SiPh2 )3 ] (1, dmpe=1,2-bis(dimethylphosphino)ethane), has been synthesized from mononuclear Pd and Pt complexes. The hexagonal core composed of Pt, Pd, and Si atoms is slightly larger than that of the tetrapalladium complex, [Pd{Pd(dmpe)}3 (μ3 -SiPh2 )3 ] (2). Reaction of PhSiH3 with complex 1 in the presence and absence of Ph2 SiH2 results in the formation of a tetranuclear complex with silyl and hydride ligands at the Pt center, [PtH(SiPh2 H){Pd(dmpe)}3 (μ3 -SiHPh)3 ] (3), and an octanuclear complex, [{Pt{Pd(dmpe)}3 (μ3 -SiHPh)3 }2 (κ2 -dmpe)] (5), respectively. Both M-Si (M=Pt, Pd) bond lengths and the 29 Si NMR chemical shifts of 1 and 2 are located between those of mononuclear late transition-metal complexes with a silylene ligand and complexes with donor-stabilized silylene ligands. CuI and AgI adducts of 1 and 2, formulated as [M(μ-M'I){Pd(dmpe)}3 (μ3 -SiPh2 )3 ] (M=Pt, Pd; M'=Cu, Ag), undergo elimination of CuI (AgI) and regenerate the tetrametallic complexes upon heating or addition of a chelating diphosphine. Elimination of AgI from 2-AgI occurs more rapidly than elimination of CuI from 2-CuI, as determined from the results of kinetics experiments.