Geminal 2J(29Si‐O‐29Si) couplings in oligosiloxanes and their relation to direct 1J(29Si‐13C) couplings

Absolute values of (79) geminal ²J(²⁹Si‐O‐²⁹Si) couplings were measured in an extensive series of (55) unstrained siloxanes dissolved in chloroform‐d. Signs of ²J(²⁹Si‐O‐²⁹Si) in some (9) silicon hydrides were determined relative to ¹J(²⁹Si‐¹H) which are known to be negative. It is supposed that positive sign of the ²J(²⁹Si‐O‐²⁹Si) coupling found in all studied hydrides is common to all siloxanes. Theoretical calculations for simple model compounds failed to reproduce this sign and so their predictions of bond length and angle dependences cannot be taken as reliable. Useful empirical correlations were found between the ²J(²⁹Si‐O‐²⁹Si) couplings on one side and the total number m of oxygen atoms bonded to the silicon atoms, sum of ²⁹Si chemical shifts or product of ¹J(²⁹Si‐¹³C) couplings on the other side. The significance of these correlations is briefly discussed. Copyright © 2011 John Wiley & Sons, Ltd.     

2J(29Si-O-29Si) coupling constants in oligosiloxanes

The absolute values of spin-spin couplings between (29)Si nuclei in siloxanes, (2)J((29)Si-O-(29)Si), were determined. The couplings are small and cover a narrow range of values (0-5 Hz). The coupling constants depend on the branching or on the number of electronegative substituents on the Si-O-Si moiety.     

Measurement of long-range 29Si,13C spin-spin coupling at natural abundance

Schemes for sensitive measurements of spin-spin coupling constants between rare nuclei with low magnetogyric ratios are considered and two combinations of INEPT with gradient versions of (X,Y)HMQC and (X,Y)COSY are suggested as particularly suitable for 29Si,13C couplings. It is demonstrated that the (H,Si)INEPT-(Si,C,Si)gHMQC combination (with 29Si detection) produces excellent results even though it is theoretically inferior to the (H,Si)INEPT-(Si,C)gCOSY combination (with 13C detection).     

29Si-13C spin-spin couplings over Si-O-Carom link

29Si-13C couplings were measured in para substituted silylated phenols, X--C6H4--O--SiR1R2R3 (X = NO2, CF3, Cl, F, H, CH3, CH3O). The SiR1R2R3 silyl groups included trimethylsilyl (Si(CH3)3, TMS), tert-butyldimethylsilyl (Si(CH3)2C(CH3)3, TBDMS), dimethylsilyl (SiH(CH3)2, DMS), and tert- butyldiphenylsilyl (Si(C6H5)2C(CH3)3, TBDPS). Previously developed (Si,C,Si)gHMQC methods and narrow 29Si lines allowed the determination of coupling constants over up to five bonds. Besides the number of intervening bonds between the silicon and carbon atoms, all the measurable couplings depend also on the nature of the substituents on the silicon. The two- and three-bond couplings are not affected by ring substitution in the para position. These properties render the 29Si-13C couplings suitable for line assignment in the spectra of silylated polyphenols. The experimental results are in reasonable agreement with theoretical calculations. The calculations show, in agreement with the data reported in the literature for couplings between other nuclei, that the two-bond and three-bond couplings, which are of similar magnitudes, are of opposite signs. If the signs of these geminal and vicinal couplings could be determined experimentally, they would greatly facilitate the line assignment. The four- and five-bond couplings are affected by the substituent X in a nontrivial manner.     

29Si-13C spin-spin couplings over an Si-O-Csp3 link

(29)Si-(13)C spin-spin couplings over one, two, and three bonds as well as other NMR parameters [delta((29)Si), delta((13)C), delta((1)H), (1)J((13)C-(1)H), and (2)J((29)Si-C-(1)H)] were calculated and measured for a series of trimethylsilylated alcohols of the types Me(3)Si-O-(CH(2))(n)CH(3) and Me(3)Si-O-CH(3-n)R(n)(n = 0-3; R = Me, Ph, or Vi). The signs of the coupling constants determined for selected compounds can likely be extended to all such compounds, as supported by theoretical calculations. Similar to couplings between other pairs of nuclei, the 2-bond and 3-bond (29)Si-O-(13)C couplings are of opposite signs ((2)J > 0 and (3)J < 0), and their relative magnitudes depend on the extent of branching at the alpha-carbon.     

Improved pulse sequences for measurement of small long‐range couplings between heteronuclei (29Si?13C) at natural abundance

The gradient pulse sequences for measurement of small long‐range couplings between heteronuclei (²⁹Si? ¹³C) in natural abundance reported to date (INEPT‐(Si,C)gCOSY and INEPT‐(Si,C,Si)HMQC) suffer from significant signal loss when these nuclei (²⁹Si, ¹³C) are coupled through one‐bond couplings to protons. This negative effect can be completely eliminated by using non‐gradient versions (INEPT‐(Si,C)COSY) or by switching proton decoupling off during gradient pulses (modified INEPT‐(Si,C,Si)gHMQC pulse sequence). The beneficial effects of these two approaches on the quality of the spectra are demonstrated here. Copyright © 2009 John Wiley & Sons, Ltd.     

Solid-state NMR tools for the structural characterization of POSiSils: 29Si sensitivity improvement with MC-CP and 2D 29Si–29Si DQ-SQ at natural abundance

The ¹H–²⁹Si multiple-contact cross polarization (MC-CP) MAS NMR experiment is evaluated for the class of silicate-siloxane copolymers called POSiSils, that is, polyoligosiloxysilicones. It proves a reasonably good solution to tackle the challenge of recording quantitative ²⁹Si NMR data in experimental time much reduced compared with single pulse acquisition. In a second time, we report ²⁹Si–²⁹Si MC-CP double-quantum single-quantum (MC-CP-DQ-SQ) NMR experiment, which provides information about the through-space proximities between all silicon species despite the high degree of heterogeneity of this material. This work furthers the NMR tools for NMR crystallography for inorganic polymers, as it covers flexible polymers with different dimensionalities and long or heterogeneous relaxation characteristics at low ²⁹Si natural abundance.     

29Si NMR Shielding Tensors in Triphenylsilanes – 29Si Solid State NMR Experiments and DFT‐IGLO Calculations

²⁹Si NMR shielding tensors of a series of triphenylsilanes Ph₃SiR with R = Ph, Me, F, Cl, Br, OH, OMe, SH, NH₂, SiPh₃, C≡CPh were determined from ²⁹Si CP/MAS spectra recorded at low spinning rates. In addition the principal components of the shielding tensor were calculated employing the DFT‐IGLO method. For most silanes experimental and calculated values are in good accordance. Larger differences were observed for systems with hydrogen bridge forming substituents and the halides bromide and chloride. In some of the spectra the shielding information interfered with residual dipolar couplings. The different contributions of the various substituents to the principal components of the shielding tensor and the orientation of the tensor within the molecules are discussed and compared for the compounds under investigation.     

Relative signs of 29Si-13C couplings

Two pulse sequences applicable to the determination of relative signs of coupling constants, gHSQC-RELAY(P) and gHSQC-RELAY(D), were developed and tested. These sequences are suitable for determination of relative signs of long-range coupling constants (<2 Hz) between two heteronuclei of low abundance (such as (29)Si and (13)C), and are applicable even to cases in which one of the heteronuclei ((29)Si) does not exhibit coupling with some of the detected protons ((1)H). The two sequences differ in the manner in which they suppress undesirable homonuclear coherence transfers. Each of the sequences can be combined with an isotope filter for better suppression of the centerlines arising from more abundant NMR-inactive isotopes. The sequences were tested on ethoxytrimethylsilane and (E)-(buta-1,3-dienyloxy)trimethylsilane, and we conclude that (2)J((29)Si-O-(13)C) is positive while (3)J((29)Si-O-C-(13)C) is negative in both compounds.     

聚酰亚胺／SiO_2杂化材料的硅核磁共振波谱和电光性质的研究

以含氟的二胺5,5’-(六氟异丙基)-二-(2-氨基苯酚)(6FHP)及二酐4, 4’-(六氟异丙基)-苯二酸酐(6FDA)或均苯四甲酸酐(PMDA)为单体,以分散红 1(DR1)为活性生色分子合成具有非线性光学特性的含氟聚酰亚胺,并采用溶胶-凝胶(Sol-Gel)法合成相应的聚酰亚胺／SiO2杂化材料．采用固态29 Si MAS NMR谱研究了含氟聚酰亚胺／SiO2杂化材料的交联结构,结果表明杂化材料中是以T3、Q3、Q4结构为主,说明在杂化材料中形成了交联网状结构．采用衰减全反射(ATR)测定了聚酰亚胺和杂化材料在832 nm处的电光系数,其值分别为 32、28、34和29 pm／V,结果表明具有较高的电光系数．     

Spectra edited by relative signs of homonuclear couplings of low abundance nuclei

The proposed homonuclear coupling sign edited (HCSE) experiment can detect signed homonuclear couplings between low abundant nuclei like ¹³C, ²⁹Si and ¹⁵N in linear spin systems, that is, in systems where two nuclei are coupled by the measured coupling, and one of them is coupled by a second coupling to a nucleus of different kind. The third nucleus is usually high abundant hydrogen. Two spectra are measured during the HCSE experiment. Their weighed sum and difference yield two other spectra, one containing peaks coupled only by positive measured couplings and the other having peaks coupled by negative measured couplings. The usual E.COSY‐type experiment requires all three couplings in the three spin system (triangular spin system) and not only two couplings as the HCSE experiment. The experiment was successfully tested on known carbon–carbon and silicon–silicon two bond couplings. A set of six simple siloxanes with |²J(Si‐O‐Si)| couplings ranging from 0.5 to 9.0 Hz was measured for the first time, and all the couplings were found to be positive. Copyright © 2012 John Wiley & Sons, Ltd.     

SQ—SQ experiment for determination of relative signs of small nJ(29Si,13C) couplings in a wide variety of silicon compounds

A modification of double quantum–zero quantum (DQ—ZQ) experiment termed single‐quantum–single‐quantum (SQ—SQ) experiment is proposed for the determination of relative signs and magnitudes of coupling constants. The modification replaces the multiple‐quantum evolution period by two synchronously incremented single‐quantum periods. Similarly to DQ—ZQ experiment, the sequence requires only two coupling constants that share one nucleus, the one to be measured and a reference one. This allows application to a larger variety of molecular fragments than traditional 2D sequences producing E.COSY or TROSY pattern. The SQ—SQ experiment eliminates the effects of some other couplings during t₁, thereby simplifying the 2D pattern and increasing the signal intensity in comparison with DQ—ZQ experiment. The presented sequence is particularly designed for the determination of silicon–carbon coupling constants across several bonds at natural abundance using silicon–hydrogen couplings as the sign reference. The signs of silicon–carbon couplings across two and three bonds in dimethyl(phenoxy)silane which cannot be detected by traditional methods and which have not yet been determined are established by the SQ—SQ method here: ²J(Si,C) = +2.2 Hz and ³J(Si,C) = ?1.7 Hz. Copyright © 2009 John Wiley & Sons, Ltd.     

Solid-state NMR spectra of paramagnetic silica-based materials: observation of 29Si and 27Al nuclei in the first coordination spheres of manganese ions

Some silica-based solids, prepared by the sol/gel method in the presence of high Mn2+ concentrations, have been characterized by the 29Si, 27Al MAS NMR spectra and 29Si T1 measurements. The single-pulse 29Si and 27Al MAS NMR spectra have shown broad spinning sideband patterns that are interpreted in terms of anisotropic bulky magnetic susceptibility (BMS) and dipole-field effects. In the absence of paramagnetic isotropic shifts, the 29Si and 27Al nuclei observed in the single-pulse NMR spectra have been assigned to nuclei remote from paramagnetic centers. It has been demonstrated that the 29Si and 27Al nuclei, which are in the vicinity of the manganese ions, can be detected by the Hahn-echo MAS NMR experiments at different carrier frequencies.     

(29)Si NMR in LC-NMR analysis of silicones

It is demonstrated using a practical example that indirect detection of (29)Si NMR signals is sufficiently sensitive in LC-NMR stop-flow arrangement to analyze mixtures of siloxane polymers. New cryogenic probes with better signal-to-noise ratio will turn this version of LC-NMR into a routine method for analysis of siloxane polymers.     

硅铝酸钠溶液中27Al和29Si的核磁共振研究

本文采用原位核磁共振方法检测不同碱度硅铝酸钠溶液中²⁷Al的信号, 在比较了文献实验结果的基础上, 明确了体系中的不同铝物种的化学位移, 进一步提出了硅铝酸钠溶液中的硅铝物种的归属.     

Characterization of chemically bonded phases on precoated thin-layer plates by29Si and13C cross-polarization and magig-angle spinning NMR

Summary The paper reports results for commercial precoated plates and home-made bonded phases for TLC examined by²⁹Si and¹³C NMR of solid samples. The NMR spectra readily reveal differences in degree of silylation, details pertaining to the type of reactive silanes used, and differences in the methods of synthesis. They can also be used to explain the cause of the wettability problems encountered with some types of (precoated) TLC plates.     

Characterization of the SiOT Angle and Cation Effects on 29Si Chemical Shifts Of Zeolites

The quantitative relationships between ²⁹Si chemical shifts and (a) the SiOT (T = Al or Si) angles and (b) the nature of the cations of the alkali forms of zeolites can be account for by the changes of charge density and p electron population of the Si atoms as calculated by the extended Hückel method on cluster models.     

Improved baseline in 29Si NMR spectra of water glasses

It is shown by experiments that replacing one‐pulse sequence by RIDE (ring down elimination) pulse sequence may dramatically improve the baseline of ²⁹Si NMR spectra and eliminate the signal from the probe. Copyright © 2013 John Wiley & Sons, Ltd.     

Identification and quantification of cis and trans isomers in aminophenyl double‐decker silsesquioxanes using 1H–29Si gHMBC NMR

Cis and trans isomers of a series of double‐decker silsesquioxanes (DDSQ) were characterized by two‐dimensional NMR techniques. The ¹H NMR spectra of these species have not previously been assigned to a degree that allows for quantification. Thus, ¹H–²⁹Si HMBC correlations were applied to facilitate ¹H spectral assignment and also to confirm previous ²⁹Si assignments for this class of silsesquioxanes. With the ability to identify all the pertinent resonances of the ¹H NMR spectrum, ²⁹Si NMR is no longer required for quantification and required only for characterization. This not only saves time and material but also provides a more accurate quantification, thus allowing for the ratio of cis and trans isomers present in each compound to be determined. A more accurate measure of the cis/trans ratio enables the investigation of its influence on the physical and chemical properties of DDSQ nanostructured materials. Copyright © 2013 John Wiley & Sons, Ltd.     

29Si spin-lattice NMR relaxation in microporous silica-based materials with high Mn2+ concentrations

The 29Si T1 time measurements have been performed under magic angle spinning for supermicroporous Si/Mn materials 1-6 containing Mn2+ in concentrations of 0 (1), 1.2 (2), 2.5 (3), 3.8 (4), 15 (5) and 19.8 wt% (6). It has been found that the central lines, corresponding to isotropic chemical shifts, and their sidebands relax differently in samples 2-4 with relatively small Mn2+ contents. Because the relaxation curves for 1-6 are nonexponential and identical for 6 at spinning rates of 5, 10 and 12 kHz, dipolar relaxation via the paramagnetic centers is dominant. The relaxation data have been interpreted as a function of Mn2+ concentrations in terms of incorporation of the Mn2+ ions into the silica lattice when their concentration is small (相似文献