Silanol heterogeneity

Silanol Heterogeneity. The i/OH absorbance profile of each dehydrated silica is shown in Figure 2. A narrow absorption at 3747 cm-1 dominates the spectrum of silica A and characterizes isolated silanols on surfaces that are not completely hydroxylated. This last statement is also... 

Ln = Sc, Y, La), and have been tested as heterogeneous catalyst in the Diels-Alder cyclization of terH-methoxy-3-trimethylsilyloxy-l,3-butadiene with benzaldehyde (Danishefsky transformation of 1,3-dienes), and proved active [51]. The surface yttrium bis-dionate has also been synthesized directly from the molecular precursor [Y( Bu-COCHCO-"C3F7)3] by reaction with MCM-41 surface silanols [51]. 

The silica gel used in TLC has the same properties as that used in LSC in columns, and the discussion about silica in Chapter 9 is relevant. In brief, the silica has a heterogeneous energy surface and many very active silanol groups. It picks up water from the atmosphere very readily and will preferentially adsorb the most polar component in a mobile phase mixture, as just described. Most of the discussion about LSC is also applicable to TLC. 

For the moment we have no clear model to relate these low and high energetic adsorption sites to chemical groups such as silanols or the oxide backbone. Such an interpretation will need further investigations in order to achieve more quantitative data describing this picture of a heterogeneous silica surface. 

---