Capping, with trimethylsilyl

Residual silanol groups on the silica surface are capped with trimethylsilyl groups by reaction with CISi(CH3)3 to eliminate polar adsorption sites that cause tailing. 

In this technique, labile sllanol groups are rendered unreactive by capping with trimethylsilyl (TMS) groups ... 

The chiral recognition abilities of these materials were investigated using racemic molecules, e.g., frans-stilbene oxide (Figure 4) by HPLC. Consequently, the enantiomers eluted at different retention times, indicating that racemic frans-stilbene oxides were completely separated. In addition, the chiral discrimination ability depended on both the DP of amylose and the preparation method. The chiral recognition ability increased with an increase in the DP of amylose, and the CSPs prepared by approach (II) were superior to those by approach 0) if the DPs were similar. Methods I and II differed only in the surface treatment processes. The silica surface in approach 0) has been end-capped with trimethylsilyl chloride, while that in approach 01) has been end-capped with 3,5-dimethylphenyl isocyanate. 

Chemical polynKrizations were carried out in 10 ml Schlenck tub. The tubes were treated with trimethylsilyl chloride, washed with three S ml portions of methanol, dried at 120 in a oven for 12 h, flame dried and kept in a desiccator to cool down to room temperature. In a glove bag maintained under nitrogen atmosphere the lactone mixture and the catalyst (0.1 molar solution in toluene, monomer to catalyst ratio was 1 2(X)) were transferred into the polymerization tube ami capped with a rubber septum. The tubes were degassed by several vacuum purge cycles to remove the solvent in the catalyst solution and then placed in an oil bath maintained at 120 for 12 h. At the end of die reaction period, the crude sample was collected to estimate the conversion and the contents of the tubes woe dissolved in chloroform (0.5 ml) and precipated in methanol (30 ml) by vigorous stirring and methanol decanted. The precipitate was further washed with methanol (2 X 20 ml). The polymer was dried in a vacuum oven at 40 °C for 24 h and GPC data were recorded. 

The reaction of l,4-bis(trimethylsilyl)-2,5,7-cyclooctatriene (37) with two equivalents of n-butyhithium in a mixture of hexane and 1,2-dunethoxyethane (DME) gives the dilithi-ated species 38 in a yield of 78% (Scheme 14) . The compound could be isolated by Edelmann and coworkers as single crystals and thus the molecular structure in the solid state could be determined. It reveals an almost planar eight-membered ring which is capped by two lithium-DME units in a /j -manner from both sides of the ring plane. 

After evaporation of the alcohol, the trimethylsilyl ether was prepared by dissolving the residue or the standard acid (after methylation) in 0.2 cm3 of dioxane, 0.2 cm3 of hexamethyldisila-zane, and 0.1 cm3 of trimethylchlorosilane and mixing thoroughly. The preparation in a 2-cm3 test tube with Teflon lined cap was left for 30 min at room temperature. After centrifugation, 0.5 pi of the supernatant can be injected into the gas chromatographic instrument. The derivatives are stable if kept in a desiccator. It is recommended that the analysis be carried out the same day. 

With R = —(CH2)17CH3, the final product is octadecyl silica stationary phase (ODS, Cl8, RP 18). Steric hindrances or other reasons leave some fraction of the silanol groups unreacted and additional ones are produced by hydrolysis of superfluous SiOCH3 groups. The silanol groups from both sources can be subject to capping reactions with N,0-bis(trimethylsilyl) acetamide, Cl—Si(CH3)3, or (CH3)3Si—NH—... 

Calibration Standards Prepare four HQ calibration standards as follows Add 0.50, 1.00, 2.00, and 3.00 mL of HQ Stock Solution into separate 10-mL volumetric flasks, then add 2.00 mL of Methyl Benzoate Stock Solution (internal standard) to each, dilute to volume with pyridine, and mix. In the same manner, prepare four DTBHQ Stock Solution calibration standards. Prepare the trimethylsilyl derivative of each standard as follows Add 9 drops of calibration standard to a 2-mL serum vial, cap the vial, evacuate with a 50-mL gas syringe, add 250 p,L of A.O-bistrimethylsilylacetamide, and heat at about 80° for 10 min. 

A 0.05-ml aliquot of the dried ether solution is pipetted into a small glass vial containing a few drops of pyridine and 0.1 ml of N,0-bis(trimethylsilyl)acetamide (Pierce Chemicals). The vial is stoppered with a Teflon-lined cap and the TMS derivatization is performed at room temperature for 1 h. The TMS derivatives are separated and determined by gas chromatography. 

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