THE DISSOCIATION THEORY AND PHOTOCHEMICAL THRESHOLDS.

BY EDMUND JOHN BOWEN (OXFORD).

Received August I oih, I 9 2 5 .

I t is the purpose of this paper to show that although the dissociation theory is capable of explaining a very large variety of photochemical re- actions, certain attempts to extend it are probably to be regarded as of little value. According to this theory, after absorption of a light-quantum dis- sociation of the molecule into free atoms or radicles occurs. The observed chemical change is then determined by the subsequent reactions of these free atoms.

In many photochemical reactions the relationships between the number of molecules changed and the number of quanta absorbed have been ex- plained in terms of ‘( chain reactions.” A similar explanation could be given to the results of Bonhoeffer on the photosensitising of ozone by chlorine and bromine if unstable intermediate oxides are assumed. Such unstable oxides have been assumed to explain other reaction^.^ The hypo- thesis of “ chain reactions ” involving free atoms and not activated mole- cules is strengthened by the work of Weigert and Kellerman4 on the “ after effect ” of hydrogen-chlorine mixtures ; and if atoms occur in the re- action chains it is not unlikely that the primary chemical change initiated by light is the formation OF atoms. I t has not yet been decided whether dissociation, if it occurs, is a unimolecular process or whether collisions of the activated molecules with unactivated ones is necessary. In any case, the dissociation theory is remarkably elastic and is capable of co-ordinating very diverse phenomena. I t has been recently suggested that an explanation of the characteristic effect of minute traces of impurities on photoreactions with long ‘‘ chains ” can be found in terms of the catalytic activity of the walls of the vessel in controlling the recombination of the free atoms present.

Assuming the dissociation theory to be applicable to photochemical processes many workers have attempted to find a connection between heats of linking and the activation energies corresponding to the absorption spectra. The heat of linking between two atoms is the energy given out when the two free atoms or radicles combine to give the molecule. The heats of linking H-H, Cl-Cl, Br-Br, 1-1, are the heats of dissociation of the diatomic molecules of these substances, and are now fairly closely fixed at go,ooo, 55,000, 46,200, 34,500 calories per gram-molecule re- spectively. By combining these values with the ordinary thermochemical

Nernst, Warburg, etc.

2. physikal Chem., 1923, 107, I. ”. Physik, 1923, 13, 91.

3 Hinshelwoodand Prichard,J.C.S., 1923,2730; Bowen and Booth, Y.C.S. , 1925,510,

6 Chapman and Burgess, Y.C.S . , 1906, 1399 ; Coehn and Jung, 2. physikal Cherrt., Bowen, Y.C.S., 1924, 1233.

I9249 110, 705. 543

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544 DISSOCIATION AND PHOTOCHEMICAL THRESHOLDS

data for the formulation of the hydrogen halides, for example, we can ob- tain the values of the heats of linking H-Cl, H-Br, H-I.

Very interesting results can be obtained by calculating the heats of linking of carbon compounds. For example, the heat of linking C-H is one-quarter of the energy of combination of one gaseous atom of carbon with four hydrogen atoms to give methane, and can be obtained by com- bining the ordinary heat of formation of methane with the heat of dissocia- tion of hydrogen and with the heat of vaporisation of carbon. Older values for this latter quantity were much too high, and from recent experi- ments on the electric arc the heat of volatilisation of carbon appears to be about 140,000 calories, from which the heat of linking C-H is calcu- lated to be 85,400 calories. Assuming that the value for C-H is the same in ethane, the heat of linking C-C appears as 66,000 calories.

These two quantities can be calculated making similar assumptions but in an entirely independent manner from the accurate thermochemical data for the heats of formation of octane and propane, and we obtain C-H= 84,400, and C-C = 66,000, in agreement with the values for methane and ethane. In a similar manner we obtain for ethylene C = C = 116,000, and for acetylene, C=C = 15 1,400 ; while the C, C link in benzene is 97,500, almost exactly 3/2 C-C. Making the same as- sumptions, the heat of linking C-I is 40,000 calories when calcu- lated both from the heat of formation of ethyl iodide and from the heat of formation of tetraiodoethylene. We may state then, on the basis of the meagre experimental data available, that heats of linking appear remark- ably constant in different compounds. The actual energy values should throw light on the quantised electron orbits of a carbon atom in combina- tion with other atoms.

Heats of linking represent merely the dzyererzce between the energies of the lowest electron orbits in the free atoms and in the molecule. There is consequently no strong justification for connecting these energies with absorption spectra, which are characteristic of the molecule alone. Never- theless many attempts have been made to connect the two ; the most im- portant case being that of chlorine. The heat of linking was at one time supposed to be of the order of IOO,OOO calories, which was reduced later to 70,000, and it was identified with the point of maximum absorption of chlorine, corresponding to about 85,000 calories. From more recent work it appears that the heat of linking is about 55,000 calories, and it has been identified with the lower limit of the absorption spectrum, which corre- sponds to about 52,500 calories. The basis of this kind of calculation is purely empirical. on the photolysis of organic compounds. Here the calculations and re- sults are no longer acceptable owing to the use of an old value for the heat of vaporisation of carbon. Another example is a paper by Job and Emschwiller.3 From an investigation of the products of the photochemical decomposition of ethyl iodide, they concluded that the primary process was C,HJ + C,H, + I, They then assumed that the lower limit of the absorp- tion spectrum would correspond to the energy of the co-valency C-I, and obtained a value 69,300 calories in this way. We have seen, however, that from purely thermal calculations the energy of the C-I link is probably 40,000 calories. Clearer conceptions have been held back by our incom- plete knowledge of the physical meaning of absorption spectra. The bands

Kohn and Guckel, NatzriwisselLsclinften, 1924, 12, I39 ; Fajans, 2. ELektrochem,

We may refer also to an interesting paper by Volmar

1925: 31, 63. - Compt. R e d , 1924, 178, 697. 3 Comfit. Rend., 1924, 179, 5 2 , 168.

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E. J. BOWEN 545

important photochemically are those associated with electronic activation ; and in gases these bands usually have a fine structure of sharp lines. A beginning has been made of an interpretation of the physical processes involved in absorption by the theory of band-spectra, elaborated by Heurlinger, Lenz, Kratzer, and others. In no case yet has a complete interpretation of the spectrum of a photoactive substance been worked out, but we have to recognise from the general theory that firstly no significance can be attached to arbitrary ‘lower limits’ of absorption or to maxima of absorption; secondly that direct calculation from the equation Q = Nhv may not give the energy level to which the molecule is raised because of the participation of vibrational quanta changes in the activation ; and thirdly that because of these changes the energy is divided between vibrational and electronic energy. Until the theory of band-spectra is further advanced we cannot discover the relationships between chemical activation and the processes of physical activation.

Closely connected with what has previously been discussed is a con- ception which has been introduced into photochemistry known as the Photochemical Threshold ; this representing the minimum of photochemical energy a molecule must have in order to become reactive. Job and Emschwillerl identify this with the lower limit of the absorption spectrum, a theory for which there is no foundation. Other investigators have attempted to associate it with the heat of linking. Warburg, for example, made this assumption in his classical work on the application of the photochemical-equivalence law to gaseous photochemical reactions. There seems, however, no reason why these two quantities should be directly equated. We may note in this connection the work of Coehn and Jung who found that hydrogcen and chlorine do not combine in light of wave-length less than 5400 A. They present their results on a graph together with the logarithm of the extinction coefficient of chlorine, and $t first sight it appears as if there is a real photochemical threshold at 5400 A, corresponding to the heat of linking C1-C1. As chlorine absorbs feebly, the actual light absorption is proportional to the extinction coefficient itself and not to its logarithm ; and plotting the results anew with this correction we find that all the experiments show is that the amounts of combination are merely proportional to the number of quanta absorbed.

At present, in fact, there is no clear experimental case of a ‘photo- chemical threshold ’ cutting across an absorption band of any substance. On the uncertain hypothesis that chemical activation is chiefly determined by the total energy of the molecule, photochemical sensitisation has been assumed to provide a hopeful means for measuring thresholds, but here intermediate compound formation seems to be common, if not universal. Even in the dissociation of hydrogen by activated Hg atoms an inter- mediate compound seems to be formed, if we take into account the work of B~nhoeffer.~ We have already referred to the photodecomposition of ozone by the halogens.

There is only one field which has thrown light on the question of the minimum energy required to make a molecule reactive, namely the investi- gation of the rate of bimolecular thermal gaseous reaction^.^ An examina- tion of the temperature coefficients and of the actual rates seems to show that after practically every collision in which the total energy of the

1 L O C . cit . 2 L O C . cit. Cario and Franck, 2. Physik, 1922, 12, 162. 2. Physik. Chem., 1925, 116, 391. Lewis, Y.C.S., 1g18,113,471; Hinshelwood and Hughes, Y.C.S., 1924,125, 1841.

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546 DISSOCIATION AND PHOTOCHEMICAL THRESHOLDS

colliding molecules exceeds a certain value, chemical reaction occurs. These critical energies of activation for four well investigated cases of bimolecular reactions are compared in Table I. with the heats of linking, and with the electronic activation energies of the absorption spectra, which are taken to be greater than the energies corresponding to the lower limits of the bands. In each case we note that the heats of activation for the bimolecular processes are considerably less than the activations obtainable from the absorption spectra, and are unconnected with the heats of linking.

TABLE I.

Decomp. of HI . 9 , N2O *

9 9 C1,O - Combin. of H, and I,

(I, photoactive) }

Energy of Activation from

Absorption Band.

> 81,000 > 100,000 ? > 56,500 50,000 approx.

Heat of Activation of Bimolecular

Thermal Reaction.

43,900 55,000 22,000

40,000 I

Heat of Linking.

63,700 ~

? 34,500 for I, go,ooo for H,

From these results we see how difficult it is at present to form any clear conception of what constitutes an active molecule. The only apparently definite measures we have of the critical energies required to make a mole- cule reactive are obtained from bimolecular thermal reactions, and these energies are very much less than the minimum electronic energies of the molecules, as obtainable from the absorption spectra. Are we to interpret thermal activation as a kinetic energy effect, or as the possession by the molecule of a number of the order of several tens of vibrational quanta, or is the theory of gas reactions incorrect ? However that may be, there are indications that photochemical activation is something different from thermal activation. For example, practically every collision of hydrogen and iodine molecules in which the total thermal energy is greater than 40,000 calories leads to combination, but when a mixture of hydrogen and iodine is ex- posed to light, whereby iodine molecules receive an activation equal to about 50,000 calories, no trace of combination can be detected.l Again, the bimolecular thermal activation heat of nitrous oxide is 55,000 calories. I t might be expected that when nitrous oxide is mixed with a little bromine or nitrogen peroxide to act as a photosensitiser and exposed to blue light, the energy of which is greater than 55,000 calories, that some nitrous oxide molecules would be decomposed through the transfer of energy by collision. Experiments carried out with the help of Mr. Raikes showed no decomposition under these conditions. We are still far from understanding, therefore, the quantitative significance of chemical activation.

'Lemoine, Compt. Rend., 1877, 85, 144; 1881, 93, 514; Bowen, Y.C.S., 1924, 1233.

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