Giles Lee Henderson
Eastern Illinois University Charleston, 61920
Optical Rotary Dispersion in Transparent Media
I n the typical undergraduate curriculum, the student acquires an understanding of the molecular symmetry requirements for optical activity. H e learns and, in many instances, uses the relationship of activity to path length and concentration. However, his only experimental contact with optical activity usually employs D line measurements with a conventional polarimeter. Frequently the dependency of optical activity on wave length (rotatory dispersion) is overlooked. In view of the relative availability of nearly monochromatic sodium light sourccs as compared with spectropolarimeters, it is apparent that ORD (optical rotatory dispersion) studies have not been convenient nor practical for thc undergraduate. It is the purpose of this paper t o describe a more complete analysis of optical activity in the visible spectrum, which utilizes readily available materials and retains a high degree of simplicity. The experiment as described might be appropriate for use in the undergraduate physical or physical organic laboratory. Theory of Optical Activity
n'loscowitz (I) has described the theory of optical activity by considering plane polarized light as the vector sum of two components: right and left circularly polarized. This concept of plane polarized light is not superficial and indeed it has been experimentally demonstrated (3). The two circularly polarized components arc dissymmetric in that their electric (or magnetic) vectors describe right or left helical paths about the axis of propagation. Sincc thc helix has no center or plane of symmetry, the right and left mirror image components of plane polarized radiation are not superimposable. These two dissymmetric components of plane polarized light will interact (by electromagnetic induction) equally with a system of symmetrical electron oscillators. Hence, no optical activity is observed in molecules possessing a plane or center of symmetry. However, the right and left components will not interact equally with a system of asymmetric electron oscil-
lators. Thus any material which is optically active is circularly birefringent (i.e., the phase velocities for right and left circularly polarized light of the same frequency differ; or stated differently, the indices of refraction for left circularly polarized light is different than that of right polarized light). Thus if plane polarized light propagates through such a medium, one of its components will have a different velocity than the other and the plane of polarization will be rotated (note that the observer only sees the resultant of the components). The magnitude of activity is frequency dependent, giving rise to rotatory dispersion. Every molecule has certain natural frequencies in accord with the Planck Statement AE = hu. As the frequency of a propagating wave approaches a resonance condition with the natural frequencies of a molecule, the electromagnetic inductive interactions are enhanced. Thus the magnitude of optical properties such as indices of refraction, absorption, and optical rotation become more pronounced in t,hese spectral regions. I n the case of optical rotation this effect with its associated anomaly is lcnown as the "Cotton effect," (8). Not only do the two circular components of plane polarize8 light travel with different speeds in an optically active medium, but thcy arc also absorbed to different extents. This phcnomcnon is known as circular dichroism. The circular dichroism is largest in the region of the Cotton effect. It should be noted that circular birefringence is related to circular dichroism in the same way that the index of refraction is related t o absorption. Every natural frequency or electron transition may not necessarily contribute to the optical activity of an asymmetric molecule. Only those. transitions which are dissymmetric result in circular birefringence and circular dichroism. The valence bonds associated with an assymmetric carbon atom will constitute a dissymmetric o -+ u* transition in the far ultraviolet, region (around 135 mp). Since the usual measurements in thc visible spectral region are far removed
Volume 45, Number 8, August 1968
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from this opticdly act,ive :d~sorptionband, the opt,ical rot,ntions of i~ctivehydrocarbons are t,ypicnlly small. Absorpt,io~i~ in alcohols and halidcs result from t,hc corresponding n --* o* transitions in the ult,raviolct rcgion. The c:~rljo11y1chromophore has recently hccn of p:lrticulnr inlcrcst (4). The lcctones and aldehydes show intcnsc :~bsorptionin the far ultrnviolet due t,o :L a --* s* t,musit.ion. All s:ltnrated ketol~es and aldclrydes :~lso show weak absorpt,ion in the ncer ultrnviolct region (:wound 280 mp) from n --, s* transitions. I t is clc:~r that the cnrhonyl chromophore, per sc, is synnnct,ric:ll. However, it has been known for some time t,h:rt mhcri it is couplcd to n proper dissymmctric:ll cnvironnm~t (such :IS an :mymnietric c:~rl,on :&t,om):rsymn~et,ryis inducctl in thc clcctron distribution of the cnrbonyl chromophore ( 6 ) . Thus, Cotton ef'fcots m:qr hc observed in t,hc rcgion of thc 7~ + a* tlmnsition (:LII~ 31~0,a t least ill principle, in t,hc region of a + a* tmnsitions). As expcctctl, the Cotton effect curve is sensitive to changes in the dissymmetric nrvirom~rcnt.and in t,hedcgrcc of coupling of the ehromopliorc to t,hc tlissymmct,ric ccntcr. Thc dcpcndcncc of rot,:~tory power on m:~velengt,h was first cxpresscd empirically by Drude (ci):
rvhcrc K , is an cmpiricnl constant, nssociatcd wit,h 1111 :~bsorption1):~nd: ~ Xt.I t I t should he noted t,h:~t: ~ any t rn:lvclcngth A , [a] dcpcnds on contributions of n optic:rlly :~ct,ive:~bsorptionhands locntcd a t A,, A?, . . . A,. Also it is :lpp:lrent t h t t,he Ilrude equation is :l hyprrholic function mit,h ali asymptote located : ~ each t X i (i.e., c o ~ ~ s i d the e r hch:~viorof [ a ]as X :y)pro:lchcs :L A,). Obviously, thc Ilrrvlc rcl:ltionship is only valid in the t,r:~~lsl,arct~l rcgion. 111the region of nhsorptio~~ (X = Xi) it docs not consider energy dissil~at,ion:~nd thcrcby f;llscIy rlcscribcs rot,ntions of plos and minus infinit,y r:lthw th:~nt,hc :lctuid familiilr maxim~lm:LIIII minimnm on either sido of A, (scc Fig. 1). Dispersion ill tlic tr:rusparcnt region is usunlly noi.rim1 (i.c., tho i~ctivityincn::lses progrcssivcly :ts the w:lvelrngtli ~lccrc:~scs).Kornud dispersio~t t h t c:m he ilrscribetl by one tcrm of Drudc's cquation (LC.,
Figure 2. Experimental equipment for meowing ORD in the visible spectral range. M-Hitochi Perkin-Elmer Model 139 rpectrophotometer adapted or o monochromobr. Px, Pr-Polorizer and anolyrer consisting of poloroid filters. C-Somple cell.
[ a ] = K/AS - A,,') is s:rill to 11c simple. If :l multitcrmed Dru
