29Si Shifts Research Articles

Dynamics and Disorder in Surfactant-Templated Silicate Layers Studied by Solid-State NMR Dephasing Times and Correlated Line Shapes

Surfactant-templated layered silicates are shown to possess complex compositional, structural, and dynamic features that manifest rich and interrelated order and disorder at molecular length scales. Temperature-dependent 1D and 2D solid-state 29Si NMR measurements reveal a chemical-exchange process involving the surfactant headgroups that is concomitant with reversible broadening of 29Si NMR line shapes under magic-angle-spinning (MAS) conditions at temperatures in the range 205−330 K. Specifically, the temperature-dependent changes in the 29Si transverse dephasing times T2′ can be quantitatively accounted for by 2-fold reorientational dynamics of the surfactant headgroups. Variable-temperature analyses demonstrate that the temperature-dependent 29Si shifts, peak broadening, and 2D 29Si{29Si} correlation NMR line shapes are directly related to the freezing of the surfactant headgroup dynamics, which results in local structural disorder within the silicate framework.

High-performance liquid chromatography with nuclear magnetic resonance detection applied to organosilicon polymers: Part 2: Comparison with other methods

LC–NMR utilizing 1H and 29Si NMR spectroscopy is ideally suited for the analysis of silicones. It is shown that reversed phase gradient LC–NMR surpasses standard gel permeation chromatography (GPC) and diffusion ordered spectroscopy (DOSY) in the analysis of model hydride terminated polydimethylsiloxane. 1H and 29Si NMR in the stopped-flow arrangement leads to full identification of the components. Concentration gradient introduces a dependence of the 29Si shifts on solvent composition, this dependence can be substantially reduced by a proposed method of referencing. It is shown that the ADEQUATE version of powerful but insensitive 2D INADEQUATE experiment can be used for complete line assignment.

Silyl modification of biologically active compounds. 11. Synthesis, physico‐chemical and biological evaluation of N‐(trialkoxysilylalkyl)tetrahydro(iso,silaiso) quinoline derivatives

AbstractN‐(trialkoxysilylalkyl) derivatives of 1,2,3,4‐tetrahydroquinoline, 1,2,3,4‐tetrahydroisoquinoline and 4,4‐dimethyl‐4‐sila‐1,2,3,4‐tetrahydroisoquinoline were prepared and characterized by elemental analysis, 1H, 13C and 29Si NMR spectroscopy. In vivo psychotropic properties and in vitro cytotoxic effects of 3‐[N‐(1,2,3,4‐tetrahydroisoquinolyl)]propyltriethoxysilane methiodide and 3‐[N‐(1,2,3,4‐tetrahydroisoquinolyl)]propylsilatrane are reported. Comparative study of 29Si shifts in newly synthesized compounds suggested donor–acceptor interaction between nitrogen and silicon atom, which increased electron density at Si nuclei, revealing a stronger increment of N → Si transannular bond in comparison with N → Si α‐effect. The molecular structure of 3‐[N‐(1,2,3,4‐tetrahydroisoquinolyl)]propylsilatrane features a penta‐coordinate silicon atom having CSiO3 pattern and Si…N intramolecular interaction. Copyright © 2005 John Wiley & Sons, Ltd.

Calculation of 29Si NMR shifts of silicate complexes with carbohydrates, amino acids, and muhicarboxylic acids: potential role in biological silica utilization

The existence of ether or ester-like complexes of silicate with organic compounds has long been debated in the literature on biological utilization of silicon. Comparison of theoretically calculated 29Si NMR chemical shifts for such complexes with experimentally measured values in biological systems could provide a diagnostic tool for identifying which, if any of these molecules exist under physiological conditions. Results are presented here for ab initio molecular orbital calculations of 29Si NMR shifts and formation energies of silicate complexes with polyalcohols, sugar-acids, pyranose sugars, amino acids and multicarboxylic acids. The effects of functional group and molecular structure including ligand size, denticity, ring size, silicon polymerization and coordination number on calculated 29Si shifts were considered. The potential role of such compounds in biological silica utilization pathways is discussed. 29Si NMR shifts and energies were calculated at the HF/6-311+G(2d,p)//HF/6-31G* level. The main result is that only five-membered rings containing penta- and hexa-coordinated Si can explain experimentally observed resonances at ∼ −101 and −141 ppm. Further, the heptet observed in 1H- 29Si coupled spectra can only be explained by structures where Si bonds to oxygens atoms in H-C-O-Si linkages with six symmetrically equivalent H atoms. While compounds containing quadra-coordinated silicon may exist in intracellular silicon storage pools within diatoms, calculated reaction energies suggest that the organism has no thermodynamic advantage in taking up extracellular organ-silicate compounds, instead of silicic acid, from the ambient aqueous environment. Hyper-coordinated complexes are deemed unlikely for transport and storage, though they may exist as transient reactive intermediates or activated complexes during enzymatically- catalyzed silica polymerization, as known previously from sol-gel silica synthesis studies.

Silylation of substituted benzhydroxamic acids: NMR spectra (13C, 15N and 29Si) and structure of tert‐butyldimethylsilyl derivatives

Twelve para- and meta-substituted benzhydroxamic acids were subjected to exhaustive silylation with N-(tert-butyldimethylsilyl)-N-methyltrifluoroacetamide (with 1% of tert-butyldimethylsilyl chloride as a catalyst). In all cases only one product was isolated. According to the assigned NMR spectra (13C, 15N, and 29Si), the product is the Z-O1, O4-bis(tert-butyldimethylsilyl) derivative of substituted benzhydroximic acid, independently of the nature of the para or meta substituent. For structure determination, the 29Si shifts and 1J(13C, 13CN) coupling constants are decisive. The chemical shifts (13C, 15N, 29Si) show dependences on the substituent constants of various kinds: even the 13C shift of the sixth atom from the benzene ring varies with substitution. The 13C and 15N chemical shifts of the CN moiety exhibit opposite dependences on the substituent; the 29Si shifts of the two silicon atoms are almost equally sensitive to the substituent effects despite their different distances from the substituent. Copyright © 1999 John Wiley & Sons, Ltd.

29Si NMR Chemical Shifts and Energetics of Silica-Serine and Silica-Polyalcohol Complexes as Indicators of Silica Biomineralization Mechanisms

AbstractSerine- and polysaccharide-enriched organic matrix is associated with biogenic silica such as diatom tests, sponge spicules, and phytoliths. We have used molecular orbital theory to determine the relative stability and 29Si NMR shifts of direct Si-O-C ester-like bonds versus hydrogen bonds between the monomeric silicic acid and the alcohol group on aliphatic organics such as serine and threitol (a polyacohol as proxy for polysaccharides). Preliminary results suggest that at neutral pHs, H-bonds and ester-bonds of four-fold coordinated silicon are of comparable stability. Formation of ester-like bonds with five-fold coordinated silicon is endothermic at neutral pHs but is stabilized at higher pHs. 29Si shifts of the H-bonded and ester-bonded complexes of four-fold coordinated silicon range from −55 to −73 ppm similar to monomeric inorganic silicic acid but far more positive than the −92, −102, and −110 ppm values observed experimentally in biogenic silicas. The five-coordinated silicon complexes yield shifts of −96 to −107 ppm. The latter range is within the range of inorganic, polymerized silica. If five-fold coordinated Si with direct Si-O-C bonds is present as a precursor or intermediate or stable species in biogenic silica, it could have escaped detection due to overlap with inorganic polymerized silica. Thus, 29Si NMR shifts are not necessarily diagnostic of the presence or absence of Si-O-C bonds in biogenic silica.

Empirical Correlations between 207Pb NMR Chemical Shifts and Structure in Solids

Using 207Pb magic angle spinning and static NMR, we have resolved and assigned different lead sites in crystalline lead oxides and lead silicates to their isotropic chemical shifts. Chemical shift anisotropies were also obtained for lead sites from the intensities of spinning sidebands. Empirical correlations between 207Pb isotropic chemical shifts and structural parameters are proposed. For ionic compounds, we show good correlations between chemical shift and coordination number or mean bond length. For more covalent compounds, the best empirical correlation has been obtained by using the degree of oxygen s−p hybridization and second neighbor electronegativity similar to that previously used to characterize 29Si shifts in aluminosilicates. The Pb2+ chemical shift anisotropies increase with more positive chemical shifts. These correlations, established for simple crystalline compounds, should allow better characterization of lead environments in disordered materials of complex composition.

29Si and 13C NMR spectra of tert‐butyldimethylsilyl derivatives of amino acids

Abstract29Si and 13C NMR chemical shifts are reported for tert‐butyldimethylsilyl (TBDMS) derivatives of ten common amino acids. The 29Si chemical shifts of TBDMS derivatives of various functionalities encountered fall into distinct spectral regions. The 29Si chemical shifts correlate linearly (r = 0.999) with the 29Si shifts reported earlier for the trimethylsilyl (TMS) derivatives of the same amino acids, the 29Si chemical shift values in the TBDMS derivatives being larger than those in the TMS derivatives (in SiN groups this downfiled shift varies between 3.8 and 5.8 ppm and in SiO groups between 1.4 and 1.7 ppm).

A multinuclear NMR study of N-(chlorodimethylsilylmethyl) amides

A series of N-(chlorodimethylsilylmethyl) amides of the type RCONR′(CH 2Si(CH 3) 2Cl), where R=H, R′=CH 3; R = CH 3, R′ = CH 3; R = H, R′ = C 6H 5; and R = CH 3, R′ = p-XC 6H 4 (X = H, OCH 3, Cl, Br, CF 3, NO 2) were prepared by amination or transsilyation. The 1H, 13C, 14N, 17O, and 29Si NMR spectra were obtained and used t to determine the structure of the compounds. The 14N and 17O shifts are indicative of the amide structure reported previously. The 17O and 29Si shifts can be related to the extent of dative interaction between the carbonyl oxygen and the silicon and correlate with the carbonyl stretching frequencies. The 1H spectra of most of the acetanilides in CDCl 3 contain a broad peak for the Si(CH 3) 2 protons. At lower temperatures this peak separates into two sharp peaks of unequal intensities. The two peaks can be attributed to rotamers, with the large peak at higher frequencies being due to the rotamer with the CH 2Si(CH 3) 2Cl group cis to the carbonyl oxygen. In toluene, CH 2Cl 2 and THF, the Si(CH 3) 2 protons appear as a sharp peak which does not broaden upon heating to the boiling point of the solvent. The free energies of activation for rotation about the carbon-nitrogen bond were determined by the approximate method of Shanan-Atidi and Bar-Eli and show a rough correlation with the sigma constant of the substituent.

29Si and 13C nmr spectra of trimethylsiloxy and hydroxy adamantanes reinvestigated at low concentration in deuteriochloroform solutions

29Si and 13C NMR chemical shifts are reported for mono- and bis(trimethylsiloxy)adamantanes as well as for mono- and dihydroxyadamantanes dissolved in deuteriochloroform. The chloroform induced changes in 29Si and 13C chemical shifts can be qualitatively accounted for by hydrogen-bonding considering basicity and solvent accessibility of oxygen atoms in the solute molecules. The steric effects observed on the 29Si shifts are not due to an interaction of the terminal hydrogen atoms of the trimethylsilyl groups but are due to proximity of two oxygen atoms on one cyclohexane ring of the adamantane skeleton.

A multinuclear, high-resolution solid-state NMR study of sorensenite (Na4SnBe2(Si3O9)·2H2O) and comparison with wollastonite and pectolite

This paper presents a solid state 1H, 23Na, 29Si, and 119Sn nuclear magnetic resonance (NMR) study of sorensenite. New 29Si NMR data for two related pyroxenoids, wollastonite and pectolite, are included for comparison. By more sensitively probing chemical bonding, the element-specific NMR observations both complement and reinforce the x-ray diffraction results. The role of hydrogen and its interactions with neighboring atoms in sorensenite is further clarified. The wollastonite and pectolite 29Si spectra resolve for the first time all three crystallographic sites and their comparison with sorensenite reveals subtle, systematic differences in sites along the pyroxenoid chains. Effects of corner-shared BeO4 tetrahedra on 29Si shifts show that sorensenite may also be viewed as a more polymerized silicoberyllate.

Observation of Silicon-29 Lanthanoid-Induced Shifts and Its Use for Estimates of Equilibrium Constants for the Complex of an Organosilane with a Lanthanoid Shift Reagent

Abstract On the successive addition of Pr(fod)3(L) to a 0.53 M CDCl3 solution of diethoxydimethylsilane(S) at 25 °C, the large lanthanoid-induced 29Si shifts were observed and analyzed in terms of the least squares fits to one step equilibrium between L and S to estimate the intrinsic shift of the 1:1 complex and the equilibrium constant.

High-resolution solid-state 29Si NMR of polymorphs of Ca 2SiO 4

High-resolution solid-state 29Si NMR spectra of four ( γ, β, α L 1 and α) polymorphs of Ca 2SiO 4 have been measured. The isotropic silicon 29Si chemical shifts lie between −73.5 and −70.1 ppm, and increasing coordination number of calcium leads to low-field 29Si shifts. The 29Si shifts in combination with the linewidth information allow to identify the C 2S phases by 29Si NMR.

Linear solvation energy relationships VIII—solvent effects on NMR spectral shifts and coupling constants

AbstractThe solvatochromic comparison method is used to unravel solvent polarity and hydrogen bonding effects on a variety of NMR spectral shifts and coupling constants. Solvent effects are rationalized in terms of the solvatochromic parameters π*, δ, α and β. Properties analyzed include 19F shifts of 5‐fluoroindole, 1H shifts of fluorodinitromethane, tert‐butanol, phenol, 2‐methylbut‐1‐en‐3‐yne, and thioacetamide, 1H and 13C shifts and J(13C1H) coupling constants of chloroform, 13C shifts of acetone, 15N shifts of pyridine, 15N and 29Si shifts of 1‐methylsilatrane, and some J(119Sn,C,19F) coupling constants of polyalkyltin compounds.