Single silanol

FIGURE 52. 29Si CPMAS NMR spectra of the Zorbax octadecyl RP-HPLC phases. Both spectra are on the same intensity scale. M = dimethyl-n-octadecylsiloxane, M = diisobutyl-n-octadecylsiloxane, Q3 = single silanol, Q4 = siloxane. Reproduced by permission of Elsevier Science from Reference 152... 

In 29 Si NMR spectroscopy, silanols are not referred to as free, bridged and geminal silanols, but as Q sites. Siloxane bridges are called Q4 sites , single silanols Q3 sites and geminal silanols Q2 sites . 

Figure 5.10 shows the structural resolution obtainable in a 29Si CP MAS NMR experiment in the most favourable cases. The low intensity peak at -89 ppm is assigned to geminal silanol sites, the peak at -100 ppm to single silanol sites and the peak at -109 ppm to siloxane bridges. 

Experiments generating sulfate radicals, SO/", by UV photolysis of S2O82 in aqueous suspensions of silica nanoparticles showed a fast disappearance of the aqueous sulfate radicals yielding two transient species with absorption maxima around 320 and 600 nm, respectively [20]. The results indicated that at pH 3-9 S04 radicals build up an adduct on the surface with maximum absorption at 320 nm. This adduct shows similar reactivity to that observed for the sulfate radical in aqueous solution. The transients with absorption maximum at 600 nm were identified as SiO surface defects formed from the reaction between the adduct and deprotonated geminal and single silanols. Other less oxidative radicals lead to different radical-silica interactions. For example, thiocyanate radicals react with deprotonated silanols, not involving silanol oxidation. 

As pointed out above, 1-3 silanol groups are in the immediate vicinity of the photogenerated pentacaibonyl. Consequently, the pentacarbonyl readily completes its coordination shell by binding to a single silanol group, and as found, is relatively stable on PVG (18,32). The... 

In principle, TiCU might react with single silanols, vicinal pairs, or geminal pairs of silanols on silica as shown in reaction 2, 3, and 4. Although these reactions are not shown, it is possible that only one or both of the OH groups of a vicinal pair or geminal pair of silanols might react as depicted for an isolated silanol. 

Thus, the isolated single silanols on thermally treated sample ACi would be located on the ex-GP sites (Table I). The density of these hydroxyls gives an estimate of the density of sites SP-GP on silica A to be about 0.65 per square nanometer. As a consequence, the OH density on initial sample ACi would be about 1.3 per square nanometer. The sum of this value and 2.9 (the methoxyl density) gives a total larger than the surface OH density on silica A. Thus when sample ACi is prepared, some methanol is added to siloxane bonds according to... 

Surface Models. A surface corresponding to the 111 faces of / -cristobalite would bear single silanols on alternate Si atoms (36). The 0-0 distance of two fn silanols is about 0.51 nm in an idealized structure of / -cristobalite with the orientation Fd3m (38). The corresponding surface... 

Sm is a single silanol, somewhere on the middle of a surface 111 (Figure 6a). The idealized 0-0 distance with a fn silanol, which is 0.51 nm, only applies to one-fourth of the silanols in a more realistic model of / -cristobalite in the orientation P2i3 (33). The three other silanols, which are... 

Geminal sites are also involved in steps between parallel surfaces 111. Such steps between two successive planes can be built either with one site Ge (Figure 6b) or with one geminal site GP and one single silanol SP (Figure 6a). The O(H) atoms of Ge are parallel to the step, whereas they are perpendicular in a site Sp-Gp. 

A tentative assignment of the main features of the infrared spectrum of silica A is summarized as follows 3500 cm-1, mainly SP 3620 cm-1, closest Sm in triplet sites i-j-k 3670 cm-1, internal silanols 3680 cm-1, weak proton donors Sm, Se, and Ge in pairs 3715 cm-1, terminal Gp 3736-3742 cm-1, weakly perturbed Gp, Sm, Se, and Ge 3747 cm-1, isolated Sm, Se, and single silanols ex-GP and ex-Ge. This assignment gives a possible explanation of the Raman components appearing at about 3685 and 3615 cm-1 by rehydroxylation of a silica gel pretreated at 600 °C (14). 

A priori, one way to correlate H and X NMR spectra is to examine the effect of some systematic chemical treatment on both types of spectra. Maciel and coworkers [59] have explored the effects of silylation with the reagent CHD (CD3)2SiCl on the H CRAMPS and Si CP-MAS spectra of silica gel. The spectra are shown in Fig. 23. After deconvolution and an attempt to correct the intensities of the deconvoluted peaks for differences in relaxation properties, it appeared that the H CRAMPS peak identified with hydrogen-bonded silanols correlates chemically with the Si CP-MAS peak identified with single silanols. In addition, the H CRAMPS peak identified with isolated (non-hydrogen-... 

Figure 25 Plots of deconvoluted peak integrals of the 39.75 MHz Si CP-MAS NMR spectra of Fisher S-679 silica gel vs. H- Si dipolar-dephasing time up to four rotor perkxJs. CP contact time, 5 ms magic-angle spinning speed, 2.0 kHz. Vertical da.shed lines show odd numbers of rotor periods, and vertical solid lines show even numbers of rotor periods, (a) - 89 ppm peak (geminal silanols) (b) -99 ppm peak (single silanols) (c) - 109 ppm peak (siloxane silicons). (From Ref. 60.)...

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