PHOTOCI-IEMICAL ISOMERIZ~WIOX OF AQUEOUSCINSAMIC ACID
Feb., 1962
rcaction 49 and the observed rate constant; and (3) the fact that the observed D-€I fractionation factor was thc expcctcd order of magnitude. Although we cannot deduce unambiguously from our studics the type of free radical with which the D r molccule rcacts, ,the evidcncc indicatcs rcactioiis with both thc RCHR’ and RCHv frec radicals. Thc results of the delayed-exchange cxperimcnts definitely prove a higher rate of frec radical decay in the prcsrncc of 8 cm. pressure of deuterium than in vacuo. Thus, thcre arc strong indications that frec radical migration occurs as a result of the exchange reactions 1 and 2. Sincc the D-H exchange did riot proceed until isotopic equilibrium had bccome established, the cessation of exchange was not the result of saturation of the frec radical centers with deuterium (as would hnvc occurred without free radical migration).
20 1
Finally, we conclude that the exchange probably involvcd a nearest neighbor H-atom or one closc t o thc initial free radical center. I n othcr words we postulate that thc 11-atom of reaction 1 did not migrate far through the solid polycthylene beforc reacting. Evidcncc for this conccpt of “nearest ncighbor migration” conies from the fact that ‘y” did not decline continually with dose and that some reverse exchange was detected in the special reverse cxchangc experiment. This work was supported by the U. S. Atomic Encrgy Commission. The usc of the mass spcctromcter was made possible by a grant from the National Science Foundation. Grateful acknowledgment is made for a I;ulbright travel grant (to I?. C.). This research also was supported by the Advanced Research Projcct Agency of the U. S. Department of Defense.
PHOTOCHEMICAL ISOMERIZATION OF CINNAMIC ACID IN AQUEOUS SOLUTIONS BY BERTH. CLAMPITTIAND JAMES W. CALLIS Applied Phvsics StuD; Boeing Airplane Company, Wichita Disision, Wichita, Kansas Received Mag 80, 1961
The photochemical isomerization of cis-trans organic acids in aqueous solutions is complicated by the ionization of the acids. This gives rise to a number of absorbing species possessing different, photochemical properties all of which are related by the ionization constants. A theoretical treatment of the process is presented and the particular case where the ions do not isomerize is solved. The resulting equation is tcstcd for cis-trans-cinnamic acid, and shows good agreement with the experimental data.
Introduction A mcthod of conversion of solar to electrical energy employing the geometrical inversion of stercoisomers recently was proposed by this Laboratory.* I n this conncction, a study of the rate of isomrrization of various cis-trans organic acids under the influence of ultraviolet light in aqueous solution was undertaken. Whilc JTaidya3and 01son4 invcstigated sevcral aspects of the photochemical isomerization, a detailed analysis of the kinetics of this process have not been reported previously, and it is the purpose of this paper t o present a theorctical approach to the problem togethcr with somc experimental data which may bc used t o compare to the theory. Previous investigators have failed t o take into account that aqueous organic acid solutions are ionized and indeed the amount of ionization is changed as the photochemical reactions proceed. This is equivalent, to assuming that the ions and undissociated acids h a m idcntical photochemical propcrtirs, which in the gcnernl casc appcars t o bc invalid. When the trans-(FIB) and cis-(HA) forms of the stereoisomers of an organic acid are added t o water, they dissociate to somc extent to give the respective trans-(B) and cis-(A) ions. ( 1 ) Spencer Chemical Company, Xlerriam, Kansas. (2) B. 11. Clampitt and ( 1
