Silanols signals from

To confirm the possible formation of macrocycles, otherwise saying the absence of chain-ends, - Si NMR was carried after the end of reaction. The spectrum (Fig. 11) shows signals for D and M (hybrid segment) units and arising from L H self-condensation. Neither methoxy nor silanol end-groups could be traced on this spectrum, nor remaining L H at -5.2 ppm or precursor M, around 15 ppm (see for instance [20]). The residual peak at 4-2.5 ppm could not be ascribed. 

FiC5. 14. NMR spectra of Ru/SiOi. The initial number of silanol protons has been reduced by exchange with deuterium. Both traces are difference spectra with respect to the state after initial evacuation. The continuous line represents a sample under 20 Torr of H, gas, and the dashed line represents a sample after pumping away the reversible hydrogen. There is both reversible and irreversible spillover to the support (signal at. 3 ppm), and ther e is rever sible and irreversible chemisorption on the metal (sigiral at 65 ppm). [Reproduced with permission from Uner et al. (47). ... 

Fk . 18. II NMR spectra of a Pt/SiO, catalyst in a rcvcrsc-spillovcr experiment. Initially (trace a), the sample was evacuated and only the silanol II NMR signal deserved at about 0 ppm. Next, deuterium gas was admitted to the sample. Traces b, c. and d were taken alter 15 min, 15 h, and 200 li, respectively. The presence o the deuterium gas leads to the appearance of H on the metal (signal at approximately 50 ppm). It is also known from other expe.rimcnts that, if the rovcrsiblo deuterium gas is pumped off, the process becomes unmeasurably slow. [Reproduced with permission from Sheng and Gay (52).]... 

This example is illustrated in Figure 13.9 in which the MAS spectrum (a) is represented with the spectrum obtained from cross polarisation (b). The comparison of the two clearly shows a relative increase in certain peaks these are the Si (OH) (Si)4, , n taking the values shown on the spectrum (b). The Si (OSi)4 signal is still present this is simply due to an extended contact time which permits the polarisation to be extended over 3 bonds. It may thus be necessary to vary the contact time in order to fully discriminate the atoms bearing hydroxyls, Quantitative analysis is not possible but the experiment can be used to determine the presence or absence of silanol groups. 

Although both H and Si MAS NMR of silica gel indicate the presence of significant Si-OH intensity, the presence of silanol groups was difficult to discern from the O spectra (van Eck et al. 1999). By using echoes with very short delays and determining the relaxation time, a liquid-like signal with essentially no quadrupole interaction was detected. An MQ MAS NMR experiment with a very short z-filter delay... 

A better solution is to couple the method with mass spectrometry. Here, the signal for water molecules is clearly separated from the combustion products, leading to a better quantification. Measurement of the silanol group density by deuterium exchange with CF3COOD followed by H NMR spectroscopy is a chemical method combined with a spectroscopic measurement and gives reliable values. 

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