Silicon spin-lattice relaxation time

Measurement of the silicon spin lattice relaxation times also indicates a large difference between the local chain motions of phase I and phase II. The T1 value is 5.5 s for phase II silicon nuclei, whereas phase I nuclei have a Ti value of 3.2 h (iO), one of the longest relaxation times observed for silicon nuclei. Unfortunately, as a result of these tremendous differences in motional properties, quantitative data for the entire PDHS sample cannot... 

Spin-lattice relaxation times have also been reported for PhgSiHg, namely ortho-czxhon 5.5 sec, Twcta-carbon 5.5 sec, and/>ara-carbon 3 sec. These data were interpreted as showing that there is preferential rotation about the silicon-phenyl bond 131). 

Silicon only has one naturally occurring isotope ( Si) with a nonzero nuclear spin and Si NMR spectroscopy has become one of the most widely used techniques for the identification of silicon compounds. Unfortunately, the natural abundance of Si is only 4.7%, which combined with its long spin-lattice relaxation times, means that relatively long acquisition times may be needed to obtain high quality spectra. Si NMR spectroscopy has been the subject of a number of reviews and tables of chemical shift data and coupling constants are available. " ... 

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. 

The total integrated intensities of all the silicons in the sample were usually reproducible to within 3% over the course of the reaction. The Si spin-lattice relaxation time, Ti, was 4.2 1.0 s, with a slight tendency for the more highly condensed species to exhibit times toward the lower limit of the uncertainty range. The pulse repetition period of 17.7 s ensured that the magnetization has recovered to 98 2% of its equilibrium value. 

The spin-lattice relaxation times in solids can be very long and the recording time for a simple NMR spectrum of a solid could be extremely long without any experimental improvement On the other hand, the transversal relaxation times T2 are usually shmt Since controls the recycle time between the experiments a method improving the sensitivity (i.e. the signal-to-noise ratio) of the spectra of rare spins (like silicon-29 or carbon-13) has been developed Cross polarization (CP), which does, however, not influence the resolution of the spectrum. 

To gain additional insight into the nature of the phase of PM-co-ES, the spin-lattice relaxation times, Ti, of the silicon and carbon nuclei were measured. These data are reported in Table 17.5. 

Si spin lattice relaxation times. Direct evidence of pairing between alkali metal cations and silicate anions has recently been presented by McCormick et al. (14) using alkali metal NMR spectroscopy (14). The formation of cation-anion pairs was found to influence both the chemical shift and spin-relaxation time of the cation. Additional manifestations of ion pairing were observed in studies of the silicon spin relaxation and exchange between silicate anions (L5). This work revealed that cation-anion contacts affect the silicon T2 values, an effect also seen in Figure 1. 

Particle Microstructure (NMR Spectroscopy and Elemental Analysis). The 29Si CP NMR spectra were much easier to obtain than the direct excitation spectra, because of the long spin-lattice relaxation time of the silicon nuclei. Nevertheless, direct excitation was used to obtain the quantitative results of the siloxane structure, because not all Q4 nuclei were detected with CP (Figure 3 and Table III) (47). 

NMR measurements The high resolution, solid state NMR measurements were performed at 30 C on a JEOL JNM 400 spectrometer. Sample membranes were cut into small pieces having an area of about 5x5 mm and packed into each sample tube, llie spin lattice relaxation time (T ) of carbons and silicon were obtained by the CPT, pulse sequence, operating at a spinning speed of 6 kHz. 

Figure 34.3 shows the results of a variable contact time CP experiment, in which the Si CP contact period (tcp) is varied in order to elucidate the Si CP (relaxation) time constant (ThsO for each Si peak. The early (small tcp) part of such curves is typically dominated by the rate of CP transfer, as characterized by the rate constant and the latter part of such curves is usually determined by the rate constant of the rotating frame spin-lattice relaxation of the protons responsible for polarization transfer to the observed silicons, as characterized by the time constant, (assuming > Thsi). These curves can be analyzed mathematically in terms of well-known equations [17]. 

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