Silicon decoupling

Fig. 14-8 Circuit diagram for a dc decoupling device with silicon diodes. (KE) insulated cable end sealing, (E) grounding installation, (1) silicon power diodes.

Fig.39a-c Silicon-29 and proton spectra of diphenylsilane in C6D6. a INEPT spectrum with complete proton decoupling, b proton-coupled INEPT spectrum ( h 201 Hz) the fine structure is due to coupling with thearomatic protons, c proton spectrum showing 29Si satellites for the SiH protons)... 

NMR analyses were done on an IBM Instruments NR-300 spectrometer and an Oxford 7 Tesla superconducting narrow-bore magnet. Silicon-29 (Si-29) NMR spectra were recorded at 59.6 MHz and hydrogen (also commonly called proton or H-l) NMR spectra at 300.13 MHz. Spectra were recorded using conventional single-pulse techniques with proton decoupling for Si-29 acquisitions. Si-29 experiments were structured so as to suppress nuclear Overhauser enhancement (NOE). For Si-29 acquisitions, spectral widths were 50 kHz and Fourier transform (FT) sizes were 4K points. For protons, spectral widths were 7.5 kHz and FT sizes were 16K points. Si-29 rf pulse widths were approximately 12 fits and proton rf pulse widths were 8 jj.s. 

The description of the chemisorption in terms of cycloaddition reactions is useful if it leads to reliable predictions of the reaction products for most reactions, a variety of products are possible, yet only one will result from a particular cycloaddition mechanism. Central to the applicability of such schemes is the notion that the silicon dimers contain a weak 7r bond responsible for the enforced concerted motion of the two electrons involved. However, in reality there is little evidence to support the presence of even a weak 7r bond within the dimers. While DFT calculations that enforce spin pairing depict the bond as a singlet biradical [32], spin-polarized calculations predict a triplet ground state for the unbuckled dimer [33] with no 7T character whatsoever. The decoupling of the two silicon electrons means that their motion is not likely to be concerted so that a [2+2] cycloaddition reaction becomes better represented as an independent [1 + 2+1] process, a notation that recognizes the independence of the silicon free radicals. This mechanism is also illustrated in Fig. 3. In practice such a reaction is unlikely to proceed in a concerted fashion, and a key signature for it would be the... 

These sequences produce enhanced proton decoupled 29Si NMR spectra. Coupled spectra can be measured by the same sequences as described below. For the simple case of one silicon coupled to n equivalent protons, the optimum settings of delays r and A and pulse QO is99,106,107. 

These simple formulas get considerably involved when all of the n protons coupled to silicon are not equivalent, have different couplings with silicon, 7(SiI F ), and exhibit homonuclear 1H—1H couplings, 7(1 V q). Formulas describing the enhancement were derived numerically by Schenker and von Philipsbom108 and analytically by Blechta and Schraml109. Repeated analysis confirmed the formulas110. The derived formulas for the decoupled variants are ... 

FIGURE 16. 29Si INEPT spectra (not refocused) of bis(trimethylsilyl)adenine 11 measured without decoupling (bottom) and with selective decoupling of NH proton (top) during acquisition. The selective decoupling identifies the silicon with chemical shift S = 6.88 as Si—NH while that at iS — 14 77 js ]—silicon. Reproduced by permission of John Wiley Sons, Ltd... 

FIGURE 54. 29Si NMR spectra (IGD) of the 3-deuterio derivative 39 under conditions of proton noise decoupling (a) and with selective proton decoupling of (CH3)3Si protons (b). The line due to silicon from the 2HC(3)—O—Si fragment is the line that does not exhibit any splitting in spectmm b. Reproduced by permission of John Wiley Sons, Ltd from Reference 167... 

Silicon NMR measurements were made at 39.5 MHz on a Jeol FX200WB spectrometer using the combined techniques of magic-angle sample spinning and proton dipolar decoupling. All samples were examined in the fully hydrated state. Chemical shifts were referenced to sodium 4,4-dimethyl-4 silapentane sulfonate taken to... 

Other nuclei besides H or have been used to monitor enantiomeric discrimination with chiral lanthanide chelates. This includes the NMR spectra of chiral 2-thiabicyclo[4.3.0]nonane 2,2-dioxides (14) and 8,8-dioxides (15) with Pr(hfc)3 . The Si NMR spectrum of a-C-silylated amines and alcohols (38) in the presence of Eu(tfc)3 was used to monitor the optical purity of these compounds . A refocused-decoupled INEPT (insensitive nuclei enhanced by polarization transfer) pulse sequence was used to circumvent the long spin-lattice relaxation times of the silicon. 

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