Experimental Considerations and Precautions

Crystalline or solid samples (ca. 10 mg) are ground with 5 drops of nujol in an agate mortar to form a paste for subsequent CD or MCD measurement. Perfluoro-butadiene can also be used instead of nujol. Otherwise, the KBr pellet technique, for which KBr and sample should be completely dried in vacuo and ground into a fine powder, can be used conveniently. On using the KBr technique, one can often observe a different CD spectrum from that of the nujol-mull method. 

Many types of cells having a variety of path lenths are commercially available. In our laboratory, various types of well-designed cells are used for CD measurements at lower temperatures. For example, a cell having a long guide tube and a reaction vessel capable of evacuation, is very useful for CD and MCD measurements at the 

An alternative type of cell, which consists of two parts of optically flat windows, is suitable for CD and MCD measurements of small-volume samples. One of the window affords a trough for filling in the sample. Otherwise, a well-calibrated spacer is inserted to a conventional cell for adjusting its path length. The light path length is calibrated by using the absorbance of an appropriately diluted solution of benzene or toluene. 

The magnitude of the CD has commonly been calibrated at 290 nm using ( + )-camphor-10-sulfonic acid as a standard. Because of its hygroscopic nature, the water content has led to some confusion. Tsujimura et al. have recommended to use a molar ellipticity of +7.78 x 103 for the compound in an aqueous solution 280). The concentration can be calibrated by the ORD of the same sample, which gave a molar rotation of +4.28 x 103 at 305 nm and of —5.44 x 103 at 270 nm. 

In order to eliminate difficulty in the preparation of (+)-camphor-10-sulfonic acid, a proposal has been made to use D-pantolactone and (+)-tris(thylenediamine)-cobalt(III) iodide monohydrate as calibration standards at 220 and 490 nm, respectively. For the former having [a]23 = —51.0° (c = 2 in water), the molar ellipticity at 220 nm is —13.1 x 103 in methanol (—17.3 x 103 in water) and the [0] value at 490 nm for the latter in water is +6.24 x 103 ([a]23 = +89° in water). Andro-sterone, having a molar ellipticity at 304 nm of 11.2 x 103 degree cm2 decimol 1 in dioxane, is also used as a standard. 

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In contrast to the SNMR method, it is crucial to measure an undistorted polycrystaUine static spectrum in order to have a reUable spectral analysis when the effects of finite RF pulses and detection bandwidth are ignored in the numerical simulation. Since Co static powder spectra in general have well-defined singularities and therefore are suitable for lineshape simulation, the issue of spectral distortion seems to be particularly important for Co NMR practitioners. In this section we will give a brief account of lineshape analysis, including experimental considerations and precautions for numerical simulations. 

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