Ultraviolet spectra, optically active

The detection of optically active absorption bands is usually prevented by technical barriers, for they generally lie in the far ultraviolet region of the spectrum. Yet, largely as a result of instrumental improvements, the first measurements of Cotton effects associated with helical polypeptides have very recently been achieved. Simmons and Blout (1960) first measured a minimum in the rotatory dispersion of tobacco mosaic virus protein at 232... 

Bis(4-imino-2-pentanonato)nickel(II) crystallizes from a benzene-petroleum ether mixture as dark red needles or as fine red-orange needles. The two forms have identical melting points. The compound is very soluble in chloroform, but less soluble in benzene, pyridine, and carbon tetrachloride, and very insoluble in water. The compound crystallizes from pyridine without adduct formation. The compound is diamagnetic and apparently has the trans configuration. Partial resolution in optically active fractions has been achieved by means of a chromatographic technique. Molecular weight determinations indicate that the compound is monomeric in chloroform and benzene solution. The visible absorption spectrum of this compound in chloroform is characterized by a band centered at 552 m/i (e = 43). The ultraviolet absorption maxima for solutions in 1 1 benzene-petroleum ether occur at 298, 348, and 364 m x (e = 4150, 4760, and 4460, respectively). ... 

Figure 10. CD spectra of poly(n-hexylisocyanate) (poly-85) dissolved in optically active solvents at 20 °C. Ultraviolet spectrum (bottom) shown only for (A)-2-chlorobutane (polymer concentration 1.9 mg/mL). (Reprinted with permission from ref 171. Copyright 1993 American Chemical Society.)...

The significance of vibrational optical activity becomes apparent when it is compared with conventional electronic optical activity in the form of optical rotatory dispersion (ORD) and circular dichroism (CD) of visible and near-ultraviolet radiation. These conventional techniques have proved most valuable in stereochemistry, but since the electronic transition frequencies of most structural units in a molecule occur in inaccessible regions of the far-ultraviolet, they are restricted to probing chromophores and their immediate intramolecular environments. On the other hand, a vibrational spectrum contains bands from most parts of a molecule, so the measurement of vibrational optical activity should provide much more information. 

An interesting series of degradation products has been reported recently by Muller et al. (356). In addition to allocolchicine and some recovered starting material, a desacetylthiocolchicine, CjoH23N04S, has been obtained from the action of sodium methyl mercaptide on methylthiocolchicide. This substance was optically active and showed an ultraviolet absorption spectrum that was different from that of methylthiocolchicide. The infrared spectrum showed carbonyl absorption at 5.93 fi. This was interpreted by the authors as indicative of a y-lactam, and desacetylthiocolchicine was assigned the tentative structure CXII. 

The rotatory dispersion of amino acids in the visible range of the spectrum has been studied by several workers (Karrer and Kaase, 1919 Waser, 1923 Pertzoff, 1927). Recently, Patterson and Erode (1943) have published detailed results with thirteen a-amino acids. With most compounds studied, the dispersion is normal, i.e., the specific rotation [a] at a particular wave length can be expressed by a one-term Drude equation which has the form a = a/(X — Xo ), where a and Xo are constants. It is likely that, if measurements were extended to the far ultraviolet, amino acids, like many other optically active compounds, would exhibit anomalous dispersion, especially in those parts of the spectrum in which absorption of amino and carboxyl groups becomes significant. It appears, especially from the work of Patterson and Erode (1943) that the l forms of amino acids fall into three classes Group I consists of those amino acids which have a normal and positive dispersion for this class, to which most purely aliphatic amino acids belong, Xo has... 

The most common form of optical activity was first measured at a constant wavelength by the angle of rotation of linearly polarized light. More recently the measurements have been extended to the entire range of visible and attainable ultraviolet regions where electronic transitions are observed, giving rise to the ORD technique (Optical Rotatory Dispersion). The Cotton effects appear in the region of optically active absorption bands outside of these bands the plain curve spectrum is also dependent on all the electronic transitions of the chromophores. 

Insulators provide a useful trap for optically active ions due to their transparency in the visible region. Nuclear (vibrational) resonances with electromagnetic radiation take place in the infrared part of the spectrum whilst resonances with bound electrons (from the valence band to the conduction band) are usually in the ultraviolet. Visible radiation therefore... 

---