RELATION BETWEEN PHOTO-CHEMICAL AND IONIZATION REACTIONS BY S. C. LIND
Both of these classes of reactions may be profitably considered from the standpoint of their quantum relations, the former through the Einstein Equivalence Law, the latter through the principle of ionic-chemical equivalence. The primary step of activation may be regarded in each case as involving a single electron in a single molecule. While the activity of an ion is due to the complete removal of an electron from the molecule acted on, photochemical activation is attributed to the shift of an electron from the normal orbit to one of higher energy, or to a kind of internal ionisation. The energy necessary to ionize is greater than that to excite, accordingly the latent energy possessed by an ion is greater than that of the excited molecule, and one should expect the activity of ions to be more universal and possibly to extend to reactions requiring more energy. Nevertheless the energy of excitation is quite sufficient to accomplish many vigorous and also quite unusual reactions. Qualitatively one does find some differences of the character just indicated, though perhaps to a less degree than might be expected. Quantitatively, the question presents itself in the form of comparing the yields per quantum and per ion. As is well known, the yield per light quantum often exceeds the direct prediction of Einstein's law by very large amounts.' It has long been stressed by Bodenstein, Stark, Warburg and others that this does not constitute a ground for the rejection of the the Einstein law but is merely an indication that the law holds for the primary step which may be and often is then followed by a chain or cyclical reaction series which may multiply the quantum yield by a large number of cycles. Until recently this assumption had not been tested experimentally, as there was no independent means of determining the primar! action or number of cycles. 1Jsing two different activating agencies for the H2 C12 reaction, light for the production of quanta and a radiation for the generation of ions, Porter. Bardwell and Lind2 recently showed that the quantity of HC1 formed varies through a wide range of sensitivities of reaction mixture but always in exactly the same degree for the two forms of radiation, hence the length of reaction chains seems to be identical for both. I n absolute terms it was found that each chlorine ion (positive or negative) produced about twice as many HC1 molecules as one light quantuma (blue). This result shows that, tracing back
+
' A . J. hllmand: Trans. Faraday SOC.,21,446 (1926). J. Am. Chem. SOC.,48,2603 (1926). "here was even some indication that this difference at zj" might disappear a t though this requires further substantiation.
100'1
574
S. C. LIND
to the primary unit from two independent directions, its reality is established in terms of reaction yield, thus answering one of the most persistent criticisms of the Einstein Law. Unfortunately there is not a large number of reactions that have been examined for both photo-and ionic activation in the same systems and none, perhaps, where the identity of conditions was insured to the same degree as in Hz - Clz reaction. One is therefore not justified in concluding that the parallelism holds for all or even a large number of reactions. This reaction (H2+ Cll) is the only one known where anything approaching so large a yield per ion (+ MHcL/IL' = 5 X IO^) has been found. The next largest, that of polymerization of acetylene ( - hfc,H,/N = 20) is much smaller and has none of the characteristics of a chain mechanism, since it remains constant under all circumstances even in the presence of inhibitive oxygen' and also of positive catalytic gases.* Unlike the ion yield, departure of quantum yield (M/hv) from the Einstein equivalence seem to be the rule rather than the exception. Taylor3 has suggested that the long chain mechanism of photo-reactions is due to the potency not only of free atoms but of free radicals (like C2Hsand CHO -) in maintaining reaction chains. The reactions which Marshall4 has examined, however, thought to be long-chained, have proved to be rather short with a yield of the same general order as the M/N yield of the ionic reactions; for example: for H Q CO: -hI(H, +co,/hv = 4.8; - M(H*+co,," = 3 . ~ . ~ For H 2 02: MHIOZ/hv= 6.6;4 MH~O/?;= 4; for zHBr = HP Br2R -MHBr/hv = 2 ; - 1 L f ~ ~ , / r u "= 2.6; for NH3 decomposition - ?vI"2/hV8 = 0 2 5 ; - MxH,/ru" = 0.8, Other points of similarity exist with respect to the decompositionof NHsby light and by a particles-for example, a positive temperature coefficient due to an increase of quantum yieldlo: - MNHI/hv = 0.4 a t zoo, but 3.3 a t 500'; while - h-INHJ/S= 0.8 at 18'but 2 . 5 a t 3 5 5 O . " It is not only the energy relations which strike one in the photo-and radio-chemical actions but-the fact that reactions ordinarily requiring high temperatures are brought about readily a t room temperature. This is no longer surprising when the energy expense per unit of action is considered.
+
+
+
+
Mund and Koch: J. Phys. Chem., 30, 289 (1926). J. Am. Chem. Soc., 48, 1575 (1926). aH. S. Taylor: Trane. Faraday Soc., 21, 560 (1926). 'A. L. Marshall: J. Phys. Chem., 30, 1078, 1634 (1926). Lind and Bardwell: J. Am. Chem. Soc., 47, 2688 (1925). 6 A. L. Marshall: loc. cit. E. Warburg: Sitsungsber. Berlin G a d . , 13, 314 (1916). Lind: Le Radium, 8 , 289 (1911). E. Warburg: Sitsungsber. Berlin Akad., 1911, 746; 1912, 216 1°E. Wourtsel: Le Radium, 11, 289, 332 (1919). I 1 K . Kuhn: Compt. rend., 117, 956; 178, 708 (1924).
* Lind and Bardwell:
+
PHOTO-CHEMICAL AND IONIZATION REACTIONS
515
But the directness is surprising with which certain final products are obtained which would usually be possible only through several successive steps; for example, the direct addition’ of H I 02 to form H202 or the splitting out of Hz from two hydrocarbons under ionizing influence to give the hydrocarbon with double the number of C atoms as: 2C2H6 = CJLO Hz2. I n this same connection it is interesting that dhile ammonia shows no such tendency as do the hydrocarbons to double up with elimination of hydrogen under ionizing influence, Taylor3 has found evidence of the formation of hydrazine when ammonia is acted on by resonated Hg atoms. On the whole there are some great similarities, but no less striking differences between the photo- and ion reaction mechanisms. I n ionisation, clustering seems to be the first step. Theoretically this would be expected from the strong electrostatic attraction exerted by the free charge. It may be this force which prevents disruption and shattering of the molecule under ionizing forces, which is assumed to occur under collisions with photo-sensitized mercury atoms.4 Clustering thus prevents chain mechanism and by the same argument we conclude that the chain mechanism does not begin in the HB C1 (or C1 i) reaction until electrical neutrality has been re-established.6 A new and striking case in point may be gained by comparing the recent results of hIcDonald6 on the decomposition of NzO by ultraviolet light (A1860 to 1990 -&.I?.) with the earlier ones of Wourtzel under cr radiation. hlcDonald finds the quantum yield -MNIO/hv = 3.9, while Wurtzel’ found-MN20,” = 1.74 to 2.55. If we use the Mund Equation8 for calculating ionization (N), we are probably justified in multiplying Wourtzel’s yield by 1.56, which brings it into close agreement with McDonald’s quantum yield. The fact that both are in accord with the stoichiometric formula 4 NzO = 3 NZ 2 NO2 (or 2 NO 02)must suggest “stoichiometric” clusteringg about an ionized N20 molecule on the one hand and an excited one on the other, which is a much more direct mechanism than the steps that have been proposed. Of course a t ordinary pressure the excited molecule will exist long enough for the successive clustering collisions to occur. It is to be emphasized that a quadruple collision is not necessary. From this review, it appears that there is great need for the examination of more reactions by both methods of activation, ionization and excitation, under identical conditions before it can be stated how far the parallel between hl/hv and M/Pi yields will extend.
+
+
+
+
Cnzeersitu of .Mznnesota.
‘E. Wourtzel: Le Radium, 11, 342 (1919). * H. S.Taylor: Ioc. cit. 3Lind and Bardwell: J. Am. Chem. SOC.,48, 2335 (1926). ‘Taylor and Bates: Proc. Nat. Acad. Sci., 12, 714(1926). H. S. Taylor: loc. cit. McDonald: J. Chem. SOC.,1928, I. Wourtzel: Le Radium, 11, 289 (1919). * J. Phys. Chem., 30,894 (Footnote) (1926). See Lind: “Chemical Effect of Alpha Particlea,” 2nd Ed., 968, p. 144 (1928).
’
+
