edited by SUSAN H. HIXSON National Science Foundation Arlington, VA22230 CURTIS T SEARS, JR. Georgia State University Atlanta. GA 30303
Projects supported b y the NSF Division of Undergraduate Education Chiroptical Spectroscopy
ORD of Cmrvones
Jerome E. Gurst University of West Florida Pensacola. FL 32514
Abrief review of the literature, and Chemical and Engineering News in particular, reveals that the determination and use of optical activity is of increasing importance in today's commercial and research laboratories. The classical technique is to measure [ a l using ~ a manual or recording polarimeter to provide a single value, the specific rotation a t 589 nm. A spectropolarimeter can be used to determine optical activity through the UV-V~siblespectrum (Optical Rotatory Dispersion [ORDI). At wavelengths far removed from electronic absorption bands, optical activity arises from circular birefringence, or the difference in the refractive index for let%-and right-circularly polarized light; i.e., nL nK t 0 for chiral materials. If the optical activity is measured through an absorption band, complex behavior is observed (a Cotton Effect curve). At an absorption band, c h i d materials exhibit circular dichroism (CD), or a difference in the absorption of lefb and right-circularly polarized light; EL - EK # 0. If the spectmpolarimeter is set for the measurement of CD sDectra. one observes what appears tobe a UV- is spectrumexcept that some absomtion hands are ~ositivewhile others may ie negative. Just as enantiomers have specific rotations that are equal and opposite at 589 nm (sodium D line), rotations are equal and opposite a t all wavelengths, and CD measurements are equal and opposite a t all wavelengths. Figure 1shows the ORD curves for the enantiomeric carvones while Figure 2 contains the CD curves. The enantiomer of ~ ob- R-(-)-Cawone carvone that has the positive [ a l is tained fmm caraway seeds and is known to have the S-configuration while the R-nantiomer is found in spearmint oil. While little can be done to wmlate sterwchemistrywith [ a l ~ values, chimptical spectmsmpy (ORD andlor CD) often can be used to assien absolute or relative confirnuation, or it can be useful in w~ormationalanalyses (1).Ex&heni are being develo~edfor undermaduates that involve the svnthesis of chiral m a k a l s , or the &dution of chiralmaterials,kcludmgorganic compounds, inorganic complexes and organometallic compounds. Both classical and c h i d HPLC resolutions are being tested. Once prepared, these c h i d materials are studied by various techniques includmg NMR, Raman, IR, W-VIS,differential scanning calorimetry (DSC), and chimptieal techniques. Molecular mechanics calculations are included (using PC~odel)'when appmpriate. Examples include some traditional experiments; i.e., the preparation and resolution of the tris-ethylenediaminwbaltwmplexes as well as some not now 'Available from Serena Sohare. Bloomington, IN.
300
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800
Wcveleneth [nm]
Figure 1. ORD of S(+)and R-(-)-cawones.
h
>le -.2 300
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800
Wsvel*ngth [nm]
Figure 2. CD os S(+)and R-(-)-carvones. found in typical undergraduate labratory manuals. For and example, the resolution of ham-1,2diaminocyclohexane subsequent conversion to the bis-Schiffbase withpam-dimethylamin~bemaldehyde.These SchiEbases have been studied by Nakanishi (2)using the exdton coupling method. Acknowledgment This work was supported partially under the award DUE-9351122 from the National Science Foundation Division of Undergraduate Education Instrumentation and Laboratory Improvement Program. Literature Cited 1. Eliel, E.; wlenlen,S. H. Stemmkmistry of Organic Compounds; J. Wiiey & Sons, h e . : New York, 1994; Djerssai. C. O p t i d Rofov Disprmion; MeGraw-Hill Book Comp a w Inc, New York, 1960.:Crabbe, P Optlml Rotary Disprmion and Cireuior Diehmism in Org.nie Ckmisfry: Hoiden-Day: San Francisco. 1965. 2. Gawulo, D.; cai, G.; Ikemoto, N.; Borhkova, N.; Odingo,J.Berow. N. Nakanishi, K Angem. C k m i Inf. ~ Ed. Engl. 1993,32,888-891.
Volume 72 Number 9 September 1995
827
