PHOTOOXIDATION O F PHENYLHYDRAZINE BY METHYL RED
775
3. It has been shown that, except under extreme conditions of temperature and prolonged contact, magnesium and indium(II1) oxides are unreactive toward each other. REFEREKCES (1) BARTH.4ND POSNJAK: 2. Krist. Am, 325 (1932). (2) GMELIN:Handbuch der anoroanischen Chemie, System-Piumber 37, p . 115. Verlag Chemie, G.h$.b.H., Berlin (1936). (3) HANAWALT, RINN, AND FREVEL: Ind. Eng. Chem., Anal. Ed. 10, 457 (1938). (4) MILLIGANAND WEISER:J. Am. Chem. SOC.69, 1670 (1937). (5) htoELLER: J. Am. Chem. SOC.63, 2625 (1941). (6) P.~SSERINI: Gam. chim. ital. 60, 754 (1930). (7) R E N Z :Ber. 54, 2763 (1901). (8) RESZ: Ber. 36, 4391 (1903).
T H E CHLOROPHYLL-SENSITIZED PHOTOOXIDATION OF PHENYLHYDRAZINE BY METHYL RED' ROBERT LIVINGSTOS, DOROTHY SICKLE2, . ~ N DAIJI UCHIYAMA
School of Chemistry, Institute of Technology, L'niversity of Minnesota, Minneapolis, Minnesota Received December 3, 1946
The great majority of published studies of photochemical processes sensitized by pigments or dyes are concerned with reactions between reducing agents and molecular oxygen. In t,he case of chlorophyll-sensitized reactions (in vitro), it has even been maintained by one prominent investigator ( G , 7 ) that chlorophy11 ran transfer its energy of escit)ation only to molecular oxygen. Quantitative studies on one chlorophyll-sensitized reaction which does not involve molecular oxygen, t,he oxidation of phenylhydrazine by methyl red
COOH & X = N ~ X ( C € M * Methyl red were reported by Ghosh and Sen Gupta in 1934 (4). In view of the importance of this reaction to the theory of photochemical reactions sensitized by solutions of pigments, it)is unfortunate that there appear to be three sources of uncertainty in this work. First, the source and purity of the chlorophyll used is not stated. The authors are indebted to the Graduate School of the University of Minnesota for a grant in aid which made this work possible. * Present address: Department of Chemistry, College of St. Catherine, St. Paul, Minnesota.
776
R. LIVINGSTON, D. SICKLE, AND A. UCHIYAMA
Second, it is not clear whether the authors took into account the profound effect of the alkaline substance, Phenylhydrazine, upon the extinction coefficients of the acid-base indicator, methyl red, in the green region of the spectrum. This point is especially important, since the concentration of the methyl red was determined photometrically using green light. Finally, they report that the photochemical reaction v a s ah-ays prcceded by a long induction period of unknown origin and of varying duration. The present Tvork consists of a reexamination of this photochemical system. It has been demonstrated that the induction period is caused by a side reaction with oxygen. It can be completely prevented by removing dissolved air from the system. Using solutions of pure chlorophyll A or B and making due allowance for the effect of phenylhydrazine upon the absorption of methyl red, quantum yields were obtained about one-fifth as great as those reported by the previous authors (4). These experiments Tvere performed with red light, to avoid the uncertainties due to correction for partial absorptions of the actinic light by methyl red, which is necessary when A 4358 A. is used. Otherwise, our results are in general agreement with theirs. EXPERIMENTAL METHODS
Materials
The methanol used in these experiments was treated with metallic sodium and fractionally distilled. The methyl red was of indicator grade, and was further purified by crystallization from toluene. Its corrected melting point was 182.4%. The phenylhydrazine was refluxed over zinc dust, vacuum distilled, and stored in the dark a t -2OoC., out of contact with air. The chlorophyll A and chlorophyll B solutions were prepared from market spinach by the method of Zscheile (13), using a counter-current extraction system based upon that described by Griffith and Jeffrey (5). The stock solution, in purified ethyl ether, was stored in the dark at -20°C. Solutions in methanol were prepared by adding an excess of the solvent to a sample of the stock solution, and evaporating off (at room temperature or below) the ether and part of the methanol, until a volume corresponding to the desired Concentration was obtained. Apparatus Light source: The actinic light was a red band, having a maximum at 6200 A., cutting off sharply at GOO0 A,, and tailing off gradually to about 7300 A. It was obtained from an incandescent source (100-watt, prefocussed projection lamp) with a filter consisting of a Corning filter # 241 and 2.5 cm. of 4 per cent cupric sulfate solution. A band of green light, having a maximum at 5000 A. and extremes at 4750 and 5500 A., was used to determine the concentration of methyl red photometrically. It was obtained from the incandescent lamp and a filter consisting of Corning filters # 401 and # 554, Jena # VG3, and 2.5 cm. of 4 per cent cupric sulfate solution. Optical system: The optical system was similar in principle to that describecl
PHOTOOXIDATION OF PHENYLHYDRAZINE BY METHYL RED
777
by Livingston (9). The light beam was slightly convergent and so defined by circular diaphragms that it just covered the front (plane) face of the cylindrical reaction cell. A Moll, large-surface thermopyle, used without its horn or window, was placed a few millimeters behind the second window of the cell. The galvanometer and thermopyle system was calibrated (and checked at intervals) against R U. s. Bureau of Standards radiation standard lamp, the thermopyle being without its horn or window. Two different standard lamps3were used, giving results which checked within about 1 per cent. The shutter and either of the two color filters (for actinic or analytical light) could be introduced into the light beam, quickly and reproducibly. The reaction cell and a cell of similar dimensions, but filled with solvent, were mounted on a carriage, ahich permitted either of them to be introduced into the light beam without otherwise disturbing the system. Reaction vessels: The reaction vessels were Pyrex-glass cylinders, about 2.8 cm. in diameter and provided with fused-on plane windows. Each vessel was joined through a short section of 6-mm. tubing t o a vertical tube about 2 cm. in diameter and 15 cm. long. The top of this tube was provided with a hollow ground-glass stopper, which also served as a stopcock to connect the cell to a short side tube.4 Reaction vessels either 12 or 35 mm. in length were used, depending upon the concentration of the dye in the solution. Each reaction vessel was matched with R cell of similar dimensions, but provided with a short side tube and ground-glass stopper in place of the Thunberg tube (12).
Analytical methad The change in concentration of methyl red was followed photometrically In the majority of the experiments a narrow band of green light having its maximum a t 5000 A. was used. The absorption due to the solution was determined with the thermopyle-galvanometer system. Since the light was not strictly monochromatic, calibration curves (log l 0 / l t , against the molarity of the methyl red) were prepared, using standard solutions in methyl alcohol of the dye in the presence of 0.050 M phenylhydrazine and of several concentrations of chlorophyll. A few check experiments were performed, in which the solutions were analyzed for methyl red with the Beckmann spectrophotometer. Inthese measurements, wave lengths of 5000 and of 4900A. were used, since the absorption due to chlorophyll is relatively small (although not negligible) in this region. The concentration and purity of the chlorophyll A and B solutions were determined spectrophotometrically (1) with the Beckmann instrument.
Routine procedure and computations The reaction mixtures were prepared volumetrically from stock solutions in methanol. The completed solution waa transferred to the reaction vessel, this
* The authors are indebted t o Dr. C. Stacy French of the Department of Botany for placing the second standard lamp at their disposal. 4 The design of the upper part of this cell and the technique for its use were adopted from the standard procedure of Thunberg (12).
778
R. LIVIXGSTON, D. SICKLE, AND A . UCHIYAMA
and subsequent manipulations being performed in a dim light. After the solution had been placed in the cell, it was sealed with a lightly greased, ground-glass stopcock. This stopper was turned to connect the cell to the side tube, and the dissolved gases were removed by evacuation, allowing from 5 to 10 per cent of the solvent to distill off. The second cell was filled with the solvent. Both cells were placed in the carriage on the optical bench, and allowed to come into thermal equilibrium in the dark. The intensity (lo - IC) of the absorbed, red light mas determined at the beginning and the end and at least once during the experiment. The relative transmission (Itl/Io)of the solution for the green (analytical) light was determined just before the start of the illumination (with the red, actinic light), and after each 2-min. period of illumination. The molarities (mD) of the methyl red were obtained from these measurements by the use of the calibration charts (log Zo/It, us. mD). From these molarities, the corresponding times, the intensities (in absolute units) of the absorbed red light, and the volume of the solution, the quantum yields were computed in the usual way (9). A mean wave length of the absorbed light of 6G00 A. was used in all computations. This was obtained graphically, using the energy distribution of the incandescent source and the extinction curves for the filter system, and for chlorophyll A.6 EXPERIMEXTAL RESULTS
The quantum yield with red liglzt The results of a number of determinations of the quantum yield [in terms of the change in number of molecules of methyl red) are presented in table 1. In all of these experiments, the initial concentrations of phenylhydrazine and methyl red were 0.050 M and 1.00 X M , respectively. The experiments were performed a t room temperature,which varied between 27" and 30°C. The values of the quantum yield are based upon an average value of reaction rate for approximately the first 50 per cent of the reaction, over which range the reaction was zero order (cf. curves A of figures 1 and 2). In agreement with the findings of Ghosh and Sen Gupta (9), the dark reaction is negligibly slow and there is no detectable photochemical reaction in the absence of chlorophyll. The average value of the quantum yield is 0.12 molecule of methyl red disappearing per quantum absorbed. The probable error of this mean value is very likely less than 20 per cent. Within the limits of accuracy of the experiments, the quantum yield is independent of the concentration of chlorophyll (within the range 1.0 X lo4 to 1.5 X 10-5;M).Chlorophyll A and chlorophyll B appear to be equally efficient sensitizers for this reaction. To check these results, a few experiments were performed with a modified Warburg respiration monomete9 (2), using the chlorophyll-sensitizedautooxidation of allylthiourea (3) as an actinometer. To determine the concentration of 6 The correction which should be made t o the mean wave length when chlorophyll B waa used is certainly less than 5 per cent. 8 The details of this apparatus will be described elsewhere.
779
PHOTO~XIDATIOS OF PHENYLHYDRAZINE BY METHYL RED
chlorophyll required to produce a maximum rate under the conditions of the experiment, a series of measurements were made with 0.50 M of allylthiourea in purified acetone containing different concentrations of chlorophyll. The results given in table 2 were obtained. Leaving the apparatus and light source unchanged, an oxygen-free solution, containing 0.05 M phenylhydrazine, 1.0 X lo-'M methyl red, and 7.5 X low6M chlorophyll A in methanol, was placed in the reaction vessel. The air in the TABLE 1 Summary of the determinations of. the .quantum C O N C E N I M P O N OF CHLOPOPHYLLS
moLriIy
X !Os
75.0 15.2 7.6 7.6 6.1 3.0 1.1
0.0 0.0 1.0 0.5 0.2
Q U A N m YIELD
INTENSIN OF ABSOPBED LIGHT
B
A
! Id
c
0 q *
molarily X 108
S6C.
0.0 0.0 0.0 0.0 0.0 0.0 0.0 i.5 7.5 3.8 1.9 1 .o
0.09
12.4 13.5 5.9 5.1 11.8 8.7 3.5 6.6 6.2 9.5 9.3 4.3
I
I
0.11 0.15 0.13 0.11 0.12 0.15 0.09 0.13 0.11 0.12 0.11
I
TABLE 2 Rate of the allylthhurea reaction
ConcentrationofchlorophyllA(111X lo6)/ 1 . 0 Rate (cm. of water per minute) . 1 0.62
11
,
5.0 1.12
1
i.5 1.27
'
i
9.5 1.35
i
2.0 1.32
TABLE 3 Rate of the methyl red-phenylhydrazine reaction Time (minutes) . . . . . . . . Concentration of methyl red ( X X lo6).. . Rate (moles X 106/1. per minute). . . . .
I
0 10.0
1 2.1
2 5.8 2.3
manometer and vessel was displaced with purified nitrogen. After being illuminated for a known time interval, the solution was removed from the vessel and analyzed for methyl red with the Beckmann spectrophotometer, using X 4900 and X 5000 A. The results in table 3 are an average of two such sets of determinations. When a 5-ml. volume is used, this rate corresponds to 1.1 X lo-' moles per minute. Under similar conditions, the rate (expressed as moles of oxygen consumed per minute) of the allylthiourea reaction is:
780
R. LIVINGISTON, D. SICKLE, AND A. UCHIYMdA
-APiv RTt
- 76.0 cm./atm.
1.Zcm./min. X 15.2 cc. cc. atm.
X 13.5 gm./cc. X 82.1
degree 7.7
X 28.1'
x io-'
moles per minute
Assuming that Gaffron's (3) value of 1.0 for the quantum yield of the allylthiourea reaction applies to the conditions of the present experiments, the quantum yield for the methyl red-phenylhydrazine reaction is 1.1/7.7 = 0.14. The (incomplete) data of table 2 indicated that the absorption of the actinic light by the 7.5 X M chlorophyll solution is not complete; however, this should affect both reactions equally. This value, while slightly larger than that obtained by direct measurement, is in agreement within reasonable limits of experimental error. During these measurements there was no detectable change in pressure in the manometer. This indicates that, if any nitrogen is formed as a result of the reaction, the number of moles formed is less than 5 per cent of the moles of methyl red reduced. The quantum yield with h 4358 Some preliminary measurements (not reported in table 1) gave values of the quantum yield (approximately 0.1) which were independent of the methyl red concentration in the range 5 X 10-6 to 2 X 10-4M. A few experiments were performed in which light of X 4358 A. was substituted for the red band. The values of the quantum yield, computed from this data upon the assumption that all of the light absorbed was equally effective, were approximately equal to 0.1 and were independent of the fraction of the light absorbed by chlorophyll. Since this latter result was unexpected and appears to be of considerable theoretical interest, these experiments are being repeated under carefully controlled conditions and will be reported later.
The reaction with oxygen Figures 1 and 2 illustrate the effect of oxygen upon the rate of the photochemical reaction. Each pair of curves represents reactions for which the light intensity was constant and the solutions were identical, except for the oxygen content. The reactions showing no induction period occurred in solutions w-hich had been freed from oxygen. Those exhibiting induction periods were in solutions which were saturated with air (at about 27"C.), their volumes were 8 ml., and they were confined in contact with approximately 0.3 ml. of air. In the absence of air, the rate is independent of the concentration of methyl red for values greater than 2 X M . In solutions saturated with air, the rate of disappearance of methyl red is less than 10 per cent of its normal (oxygenfree) rate. The maximum value of the rate of disappearance of methyl red attained after an induction period is always somewhat less than the normal rate (in the present cases about two-thirds of'normal). Similar results were obtained in a number of other experiments not reported here. It was also shown that the
PHOTO~XIDATIONOF PHENYLHYDRAZINE BY METHYL RED
781
length of the induction period increases with an increase in the volume of air in contact with this solution. The quantum yield for the sensitized reaction between phenylhydrazine and
782
R. LIVINGSTON, D. SICKLE, AND A . UCHIYAMA
oxygen may be estimated from the data presented in figures 1 and 2 and the solubility of oxygen in methanol. The solubility of molecular oxygen in methanol a t 27OC., expressed as the ratio of the volume of gas dissolved to the volume of the solvent, is 0.228 (8). Since the volume of the solution is 8.0 ml., the number of moles of oxygen in the reaction solution saturated with air is7
Nb, = 0.210 X (745/760) X 0.228 X 8.0 = 1.58 82.1 X 300
10-5moles
It is necessary to correct this value for the oxygen contained in an air bubble, of approximately 0.3 ml. volume, which was present in the cell.
There mere, therefore, approximately 1.83 x moles of oxygen present in the cell a t the start of the reaction. If me make the probable assumption that the maximum rate of reduction of methyl red is not attained until practically all of the oxygen is used up, we may compute the rates of disappearance of oxygen in experiments IB and IIB as 1.83 X and 1.83 moles/20 min. = 9.1 X X 10-6/32 = 5.7 X 10-7 moles per minute, respectively. The corresponding rates of reduction of methyl red in the absence of oxygen (experiments IA and IIA) are 1.13 X lo-' and 5.5 X moles per minute. Since the intensities of light and the chlorophyll concentrations were unchanged for each pair of experiments (e.g., IA and IB), the quantum yield, + o x , for the oxygen reaction must be equal to the ratio of the rates multiplied by the quantum yield for the reduction of methyl red, as follows: Figure 1:
q-o
- 9'1 - 1.13
x
lo-' io4
x
0.15
=
1.2
Similar analysis of preliminary experiments yields values of ~ - 0 % between 0 . i and 1.6. While these estimates are admittedly crude, they demonstrate that the quantum yield for the reduction of oxygen by phenylhydrazine is not the same as that for the reduction of methyl red, but is approximately equal to unity. The reaction between methyl red and allylthiourea A few experiments vere performed to determine the effect of substituting allylthiourea for phenylhydrazine in the reaction mixture. In all of these experiments, the concentrations of methyl red and allylthiourea were lo-' d l and 0.50 A t , respectively. The solvent was acetone. The cell was evacuated to remove oxygen. In two experiments the solrent contained 2 per cent pyridine (3); in a third it mas pyridine-free. X mixture of chlorophylls A and B x-as 7 It is necessary to assume that the solubility of oxygen is unaffected by the presence of 0.050 M phenylhydrazine and of the dilute pigments in the methanol.
PHOTO~XIDATIOSO F I"ESYLHYDR.AZISE
13Y METHYL RED
783
used: .5 X 10-6 M in one experiment and 5 X 10-j Jf in two others. In no case was there a detectable reduction of methyl red after 90 min. illumination with red light. With similar light intensity and chlorophyll concentration, the methyl red-phenylhydrazine reaction was more than 50 per cent complete in 10 min.
t
FIG.3. Absorption spectrum of chlorophyll l3 in inethanol
T h e absorption spectra of the chlorophylls in the presence of phenylhydrazine To determine whether 0.05 *If phenylhydrazine in methanol had any affect on the absorption spectra of the chlorophylls, the extinction coefficients of pure samples of chlorophyll A and of chlorophyll B, in the presence and absence of phenylhydrazine, were measured with the Beckmann spectrophotometer, at 50 A. in intervals from X 3800 to i500 A. The curves obtained in pure methanol compare favorably with those published by Zscheile and Harris (14). While 0.05 .l4 phenylhydrazine has no detectable affect upon the absorption spectrum of chlorophyll 4 in methanol, it distinctly changes that of chlorophyll B, as is illustrated by figure 3. The change occurs chiefly in the red end of the spectrum, it being doubtful if the slight shift which mas observed in the violet end is significant .
784
R. LIVIKGSTON, D. SICKLE, AND A. UCHIYAMA DISCUSSION
The present data, although incomplete in many respects, can be used to evaluate the merits of the several schemes (11) which have been advanced RS mechanisms of chlorophyll-sensitized photooxidations. Any mechanism for the methyl red-phenylhydrazine reaction must be consistent with the following observations: the quantum yield is small (about 0.1) and is independent of the methyl red concentration and (at least over a limited range) of the intensity of the absorbed light; the quantum yields for the oxygen-allylthiourea and the oxygen-phenylhydrazine reactions are near unity; and there is no detectablc sensitized reaction between methyl red and allylthiourea. One mechanism which is consistent with these facts, and others previously published (3, lo), can he represented by the following steps. The symbols used have the following significance: GH = chlorophyll, HG = long-lived activated chlorophyll (e.g., tautomer), GH2 = monohydrogenated chlorophyll, D = methyl red, DH2 reduced methyl red. GH
+ hv
-+
GH*
GH*
4
GH
(1)
+ hv/
(2)
GH* + H G
(3)
HG + G H
(4)
+
C ~ H ~ N N ~HG H ~ -+ GH2
+ CEHSN~HZ
(6)
D+GHt -+GH+DH 2DH
-+
D
(5)
+ DH2
(7)
2C~Hsl\'zH2-+ (CsHsN2Hz)z (8) The "follow reactions" (7 and 8) are quite arbitrary. For example, the oxidation product of phenylhydrazine has not been identified, except that its formation does not involve liberation of molecular nitrogen. Making the usual assumption about the existence of a steady state, we obtain the following relation for the quantum yield, rp.
The falling off of the yield at higher concentrations of cNorophyll is not taken into account in the preceding mechanism, since it has been adequately discussed elsewhere (11). Additional experiments are planned to test the predicted relation between rp and the concentration of phenylhydrazine. Since the sensitized reaction between oxygen and phenylhydrazine has a quantum yield near unity, it is very probable that step 5a,
+
+
(5a) Oa HG --., G HOa and the appropriate follow reactions replace step 5. Further work is required to show the relative importance of these two steps.
.
PHOTOOXIDATIOS O F PHENYLHYDRAZIKE BY METHYL RED
785
The fact that the quantum yield for the oxidation of allylthiourea by oxygen is about 1, while it is approximately 0 when methyl red is the oxidizing agent, makes it appear very probable that step 5a, rather than an analog to 5 , is the dominant one in this reaction. The shift in the red absorption band of chlorophyll B produced by moderately dilute phenylhydrazine suggests the formation of a compound between phenylhydrazine and presumably the formyl group of chlorophyll B. In this respect, it is interesting that chlorophylls A and B are equally efficient sensitizers under the conditions of our experiments. SUMMARY
1. The occurrence of the chlorophyll-sensitized photooxidation of phenyl-
hydrazine by methyl red, which was reported previously by Ghosh and Sen Gupta (4), has been confirmed. 2. In the presence of dissolved oxygen the reaction exhibits an induction period, which can be eliminated by removing the oxygen. 3. A quantum yield of 0.12 (molecules of met'hyl red disappearing per quantum absorbed) was obtained when a methanol solution containing chlorophyll (either A or B) (IOb to X ) , methyl red M), and phenylhydrazine (0,050 X)was illuminated with red light. 4. Preliminary esperiments in which light of X 4358 A. was used indicated that light which is absorbed by methyl red as well as by chlorophyll is photochemically active. This result will be studied more carefully and reported later. 5 . While the reaction products were not isolated, it was noted that the products were, compared to methyl red, colorless, and that no nitrogen mas evolved. 6. Indirect measurements indicate that the quantum yield, for the chlorophyll-sensitized photochemical reaction betiveen oxygen and phenylhydrazine, is approximately equal to unity. 7 . The photosensitized reaction with methyl red is negligibly slow when allylthiourea (3) is substituted for phenylhydrazine. 8. Phenylhydrazine, at a concentration of 0.05 M , changes the absorption spectrum of chlorophyll B in methanol, but does not appreciably affect that of chlorophyll A. REFERENCES
COMAR, C. L., A N D ZSCHEILE,F. P.: Plant Physiol. 17, 198 (1942). DIXOK, 11.:Manometric Methods. Cambridge University Press, London (1934). GAFFRON, H.: Ber. WB,755 (1927). GHOSH,J. C., A N D SENGI-PTA, S . B.: J. Indian Chem. Soc. 11, 65 (1934). ( 5 ) GRIFFITH,R. B., AND JEFFREY,R . h-.:Ind. Eng. Cheni., Anal. Ed. 17, 448 (1015). (6) KACTSKY, H., HIRSCH,A , , A N D FLESCH,W . : Ber. 68, 152 (1935). W . : Biochem. Z. 284, 412 (1036). (7) KAUTSKY, H . , ASD FLEBCH, (8) LEVI,M.G.: Gaze. chim. ital. 31, 11, 513 (1901). R . : J. Phys. Chem. 44, 601 (1940). (9) LIVIKGSTON, (10) LIVINGSTON, R . : J. Phys. Chem. 46, 1312 (1941). (11) RIBINOWITCH,E. I.: Photosynthesis, Vol. I, Chap. XVIII. Interscienoe Publishers, (1) (2) (3) (4)
Inc., S e x York (1945).
(12) THL-ABLRG, T . : IIundhitcA der hioloyincheTi :l,CpifsI,lelhotloi, Teil I, Heft i . Urban and Schwarzenbcrg, Berlin (1020). (131 ZSCHGILE,F. l'., AND COYAR,C. L.: Botan. Gaz. 102,463 (1941). (14) ZSCHPILE,F. P., A X I > HARRIS, D. G.: B o t a n Oaz. 104, 515 (1943).
EFFECTS OF HIGH-ESERGT RADIATIOS OK ORGANIC COVPOUSDP1 RI ILTOX BURTOW l l o n s a n t o Cheruical Company, Clinton Laboratories, Oak Ridge, Tennessee
Received November 8 , 1946 I. IKTRODUCTION
Organic compounds are hydrogenous materials. Thus, they are particularly effective as moderators for fast neutrons and at the same time become particularly susceptible to their influence. Quite apart from possible usefulness in the production of atomic energy, they definitely belong among those materials which might be introduced in or near a pile for the production of new and interesting chemical effects and chemical products. Furthermore, since piles and other portions of atomic energy plants are operated more or less remotely by human beings, discovery of the effects of all high-energy radiations on materials of byhich biological systems arc composed has become an increasingly important matter. The practical value of solution of the radiation-chemical problems of organic compounds is by no means the sole justification for their investigation. Just, as in reaction kinetics of thermal and photochemical systems, organic compounds nffer a very fruitful field for study because SO many different aspects of bond type and bond number and of molecular size, complexity, and relative stability (in the thermodynamic sense) can be independently and gradually varied in a manner advantageous for detailed study, for determination of general rules and correlations, and for discovery of underlying principles. The essential information required is knowledge of relative reactivity (under irradiation) of compounds and bonds of various types, of the relative effects of different kinds of radiation, and of the nature of the compounds produced. Much of this information is in the literature prior to t.he establishment of the Atomic Energy Projects, and some of it has been obtained outside of the projects, notably by the Massachusetts Institute of Technology group (7, 8, 14, 15),even during World War 11. 'Paper presented before the Symposium on Radiation Chemistry, which was held under the auspices of the Division of Physical and Inorganic Chemistry at the 110th hleeting of the American Chemical Society, Chicago, Illinois, September, 1946. 2Present address: Department of Chemistry, University of Xotre Dame, Notre Dame, Indiana. The work reviewed is in part taken from studies performed at the Metallurgical Laboratory, University of Chicago.
