THERADIATION CHEMISTRY OF ACETAMIDE ing infrared-active band at 210 cm-l, we will not make any general statement regarding the intensity data. However, the present data seem to suggest one important conclusion about the chemical effect. The beautiful agreement of the intensity data of the gas and the liquid phase for the three bands suggests that any chemical effect which may be present is not vibronic in type or vibronic interaction quite different from that of benzer~e.~According to the theory of Brown,4
2181 the vibronic effect should be observed for v18 and vI9 of the el, species. Acknowledgments. We thank Professor W. B. Person for suggesting hexafluorobenzene as a good example to study. We wish to acknowledge helpful correspondence with Professor W. Steele of The Pennsylvania State University. We are grateful to the National Science Foundation for financial support through their Grant GP-6541.
The Radiation Chemistry of Acetamide' by K. Narayana Rao and A. 0. Allen Chemistry Department, Brookhaven National Laboratory, Upton, New Yorlc 11978 (Received December 4, 1067)
The two crystalline forms of acetamide give quite different product yields under y rays. In particular, acetonitrile, the major product from the thermodynamically stable form, is produced in much smaller yield from the labile form. Doses of several megarads produce some change in the labile form that results in a large decrease in hydrogen yield. Liquid acetamide gives smaller nitrile yields and much larger yields of gaseous products than either solid form. To the radiation chemist, acetamide is of interest as one of the simplest compounds containing the amide bond. Beyond this, acetamide is one of the few simple organic compounds which can be obtained at room temperature in two different crystalline forms. Comparison of their behavior under radiation shows how the radiolytic decomposition of a molecule may be affected by small changes in the detail of its molecular environment.
Experimental Section Stable rhombohedral acetamide can be prepared by crystallization from a solvent or by sublimation. Material prepared by crystallization was found not to give a reproducible decomposition under y rays, probably because traces of the solvent persisted even after pumping under vacuum for many hours. The material used in the experiments reported here was prepared by subliming under vacuum. It melted at 82". Metastable or "labile" orthorhombic acetamide was prepared by a method similar to that of Kahm2 Ordinary acetamide was sealed in a tube under vacuum and was melted in a bath of hot water. The material was then allowed to cool slowly; it supercooled to 45" and then crystallized suddenly. That the product was in the labile form was confirmed each time by placing in a bath at 70" and noting that it started to melt. The capillary melting point of this form was 68-69",
Deuterated acetamide CHICOND~was prepared by repeated equilibration with heavy water. The final product contained 95-980/, of deuterium on the nitrogen. CD3CONH2was prepared by conversion of CD3COOD (Merck, 99% D) to CD3COONH4, followed by heating with additional CDaCOOD to drive off water; the resulting amide was recrystallized and purified by several vacuum sublimations. Irradiations were made under vacuum in 6oCoy-ray sources having intensities of the order of 3 X 10'' eV/ ml min. For irradiations at temperatures other than 23", the specimen was immersed in a dewar of Dry Ice or inside an electrical furnace. Temperatures were maintained to within =t2". Most of the radiation cells were of 15-ml capacity and fitted with a break-seal. The dose rate was determined with the usual ferrous sulfate dosimeter. After irradiation the cells were sealed to the vacuum line. The break-seal was opened, the acetamide was melted, and the products were pumped off by an automatic Toepler through ice traps and liquid-nitrogen traps. Hydrogen, methane, and carbon monoxide did not condense in the traps but passed into a McLeod gauge where their volume was measured; the mixture was later analyzed by combustion. Ammonia, ace(1) Research performed under the auspices of the U. S. Atomic Energy Commission. (2) E. Kahrs, 2. Kryst. Mineral., 40, 476 (1905).
Volume 7.9, Number 6 J u n e 1968
K. NARAYANA RAOAND A. 0. ALLEN
2182 tone, acetonitrile, and water condensed in the liquidnitrogen trap and this mixture was analyzed by gas chromatography. The small amount of material retained by liquid nitrogen was taken up in 0.1 ml of methanol, and 5 pl of this solution was injected into a Perkin-Elmer 154C gas chromatograph using a W column (Carbowax supported on Teflon) maintained at 94". The elution times for ammonia, acetone, methanol, acetonitrile, and water were 1.6, 4.4, 8.2, 14.0, and 20.2 min, respectively. The products were also examined for COZ, CZHe, K:2H4, and CH3COCOCHa. None of these was present, except for a very small trace of To determine succinic and malonic amides in the product, 80 g of irradiated acetamide was converted to the ethyl ester by refluxing for 24 hr with 200 ml of absolute ethanol and 52 ml of concentrated sulfuric acid. About two-thirds of the alcohol and ethyl acetate was distilled off; the residue was mixed with water, the organic materials were extracted with ether, the ether solution was washed and dried, the ether was distilled off, and the residue concentrated to about 500 mg. A sample of 50 ml was injected into the gas chromatograph using either a Perkin-Elmer I< column or a UCOK 5% column. The elution times of the unknowns matched (on both columns) those of authentic samples of ethyl malonate and diethyl succinate. The succinate and malonate were presumably present originally in the form of amides, but this could not be definitely established. When an unirradiated sample was treated by the above procedure, no succinate or malonate peak was seen. For esr studies, acetamide crystals were irradiated in one end of a long silica tube; the tube was then inverted and the crystals fell to the other end, which had been heated to destroy any paramagnetic centers in the silica. Unfortunately, the esr spectrum for the labile form could not be obtained, as it was not possible to transfer the acetamide from one end of the tube to the other after irradiation. Attempts to dig out pieces of the labile form from the solidified mass after irradiation invariably resulted in the conversion of the whole mass into the stable form as a result of the mechanical shock.
r
I
I
1
I
1
I
I
coz.
Results Table I shows, as a function of the total radiation dose, the 100-eV yields of the most important products found from the two crystalline forms irradiated at room temperature. The variations of yield with dose found for the stable form are within experimental error. The labile form, however, undergoes some change during the irradiation which results in an increase of the yield of acetonitrile and a decrease in the yield of hydrogen. Since at the highest dose only about 0.1% of the acetamide appears to have decomposed, the large The Journal of Physical Chemistry
TEMPERATURE,T
Figure 1. Product yields from acetamide: circles, CHaCN; squares, CHa; triangles, NHI; open symbols, stable solid; filled symbols, labile solid; half-filled symbols, liquid.
0
-0 0
80
1
I I60
TEMPERATURE, 'C
Figure 2. Product yields from acetamide: circles, Hz; squares, CO; triangles, acetone. The meaning of open, filled, and half-filled symbols is the same as in Figure 1.
changes in yield with increasing dose are quite unexpected. The yields of the various products from both crystalline forms and from liquid acetamide as well are shown in Figures 1 and 2 as a function of temperature. The total doses used in all these runs were in the region of 20 RiIrads. Not only were the G values quite different for the two crystalline forms, but in the case of hydrogen, their temperature coefficients were of opposite sign. Yields of succinic and malonic amides were determined only at 23". The data on water yields are not shown because they were much less reproducible than
THERADIATION CHEMISTRY OF ACETAMIDE
2183
Table I: Yields of the Major Radiolysis Products of Solid Acetamide a t 23" -----a--------------------,
I-----
Dose, Mrads
Form
Stable
Labile
Ha
CHI
CHsCN
Hi0
0.5 0.78 6,5 16.6 23.16
0.261 0.246 0.249 0.238 0.245
0.149 0.146 0.141 0.134 0.140
,..
...
1.o 6.0 15.06 23.16
0.159
...
...
0 * 122
0.185 0.175 0.177
0.270 0.395 0.740
0.093 0.075
the others, The values obtained were relatively quite high. As between different samples, they seemed to show some parallelism to the yields of acetonitrile, generally running 1-2 units higher. It seems probable that the CH&N is accompanied in the products by an equimolar quantity of water, as suggested by simple stoichiometry, and that the excess water was introduced either from the glass, while sealing off the tubes, or by condensation from the air during handling of the cold methanol solution preparatory to injection into the chromatograph. The labile form appeared as a shiny mass adhering to the walls, while the stable form was microcrystalline powder. To see if the observed differences in the G values might be due to the state of aggregation of the solid rather than the crystal structure, acetamide was melted under vacuum and seeded with a crystal of the stable form. This shiny mass, similar in appearance to the labile form but actually in the stable form, gave yields under irradiation identical with those given by the stable form when irradiated as a crystalline powder. In general, G values for the various products from the labile form tended to lie between those for the stable form and those for the liquid. We found the same three-line esr spectrum in irradiated acetamide as reported by Gordy and coworker^.^^^ I n order to locate the position of the unpaired electron, the compounds CH3COiliD2and CDICONHz were irradiated and the esr spectra were obtained. The compound deuterated on the nitrogen gave the same spectrum as that from CH3CONH2,but the compound deuterated on the carbon gave a five-line spectrum. The results indicate that the radical is *CHzCONH2. The
... 1.04 1.36 1.21
(CHzC0NHz)z
CHz(CONH1)s
0.05
0.006
0.03
0.004
... ...
2.38 2.35
yield of this radical was estimated for a sample of acetamide irradiated with 0.7 Mrad by comparing the total integrated peak area of the triplets with that of a standard DPPH sample. The G value obtained was 0.25. This method is not considered to be very accurate, and the result could be off by a factor of 2. Some data on radiolysis of acetamide deuterated on the nitrogen are given in Table 11.
Discussion The radiolysis yields are indeed quite different for the two crystalline forms, although the forms can be interconverted by only a small rotation of the molec u l e ~ and , ~ ~the ~ densities differ only by about 3%. The result illustrates the great sensitivity of net radiolytic processes to the details of the molecular environment. An even larger difference in yields is reported to occur at the crystal transition temperature of choline chloridef** in that case, one crystalline modification apparently can carry a radical chain reaction which does not occur with the other crystalline modification. I n general the majority of radiolysis products of organic vapors and liquids can be accounted for in terms of simple breakage of single bonds in the molecule followed by known reactions of the resulting free radicals, Below the freezing point the yields are generally assumed to become smaller because many of the free radicals are unable to escape from the cage in which they are formed and, accordingly, recombine to regenerate the parent molecule. Looking at the radiolysis products of acetamide, we see that this picture seems to be borne out with the exception of the decomposition to acetonitrile and water, which obtrudes into the picture as an element foreign to the usual reaction types. The esr data and the hydrogen yield data in Table I1
Table 11: Hydrogen and Methane Yields from Radiolysis of CHaCONDz
Form
Stable Labile Liquid
Temp, Dose, O C Mrads
23 23 85
25 15 15
+
G(Hz HD)
0.22 0.078 ,
..
G(CH4 4Ha:HD CHaD)
2.0 1.3 2.2
0.12 0.14
...
CH4:
CHsD
0.91 0.77 10.05
(3) C. F. Luck and W. J. Gordy, J . Amer. Chem. Soc., 78, 3240 (1956). (4) I. Miyagawa and W. Gordy, ibid., 83, 1036 (1961). (5) F. Senti and D. Harker, ibid., 62, 2008 (1940). (6) W. C. Hamilton, Acta Crystallogr., 18, 866 (1965). (7) I. Serlin, Science, 126, 261 (1957). (8) R. Collin, J . Amer. Chem. SOC,7 9 , 6086 (1957).
Volume 73, Number 6 June 1968
K. NARAYANA RAOAND A. 0 . ALLEN
2184 show that an important net reaction is the loss of an H atom by breakage of the C-H bond or by abstraction, which is more likely to occur from a C-H than from an N-H bond.g A reaction which is even more frequent in the liquid phase, but less frequent in the solid phase, is the breaking of the C-C bond with the formation of a methyl radical which then attacks another molecule to form methane. Apparently the H atom finds it much easier to escape in the solid than does the methyl radical, which usually recombines with its partner radical CONH2. Occasionally, however, the CH, will have enough energy to extract an H atom from a neighboring molecule in the lattice. This it does about equally often by reaction with a neighboring N-H or C-H bond (Table 11). In the liquid the methyl radical becomes freed and then is found to react much more readily by abstracting H from the carbon presumably because the C-H bond is weaker than the IT-H bond. A similar result was found by Spall and Steacieg in the photolysis of acetamide vapor. From the separation of the methyl group, a small yield of amide radicals CONH2 must remain in the crystal, as shown by the formation of a small yield of malonamide, although only the amidomethyl radicals are seen in the esr spectrum. Succinamide, malonamide, ammonia, and perhaps some of the other products are presumably formed in part when trapped radicals are released by melting the irradiated material during the analytical procedure. As often happens in the radiolysis of simple organic compounds, yields of hydrogen and methane are greater than expected on the basis of material balance with the other products found. Of the radicals remaining from the separation of Ha and CH3*, some decompose to yield CO and NH3, either immediately when they are formed or later when the crystal is melted in order to extract products for analysis; others recombine with one another to form succinamide or malonamide. The amounts of all these products are too small to balance all the hydrogen and methane formed, and it seems probable that some products of higher molecular weight are also being formed. The small yields of acetone may be regarded as indicating the attack of an energetic methyl radical or ion on the C-N bond of a neighboring molecule. The formation of acetonitrile represents a more unusual type of reaction, since it cannot be formed from acetamide by simple bond-breaking processes. This product was also found by Spall and Steacie in photolysis of the vapor at temperatures below 200". They found that addition of nitric oxide has little effect on the yield of this product, which, therefore, does not have free-radical precursors. They were not sure whether it was formed by intramolecular decomposition of an excited molecule or by reaction of an excited molecule with another molecule of acetamide. The quantum yield of CH3CN they found to lie between 0.1 The Journal of Physical Chemistry
and 0.2. Our G values for the liquid are much smaller than might be expected from this value of the quantum yield from the vapor, indicating that many of the excited and ionized states formed by high-energy radiation do not lead to the formation of this product with as much efficiency as the low-lying state produced in photolysis. However, the radiolytic yield becomes larger in the solid state, especially with the stable rhombohedral form. I n fact, the thermodynamically stable form seems to be definitely less stable toward radiation. Structural evidence6 indicates that the hydrogen bonding is stronger in the stable form than in the labile form, and, in fact, the energy difference may be largely ascribed to the energy of the hydrogen bonds. The degree of hydrogen bonding in the liquid must be still smaller. The yield of nitrile would appear to be correlated with the amount and strength of hydrogen bonding. It may be suggested that in certain excited or ionized states the CO bond is weakened and the oxygen atom can combine with a hydrogen from a neighboring molecule with a probability which is greater the nearer the hydrogen atom is to the oxygen as a result of the hydrogen bonding occurring in the ground state. Since the oxygen atoms are bonded to hydrogen atoms of neighboring amino groups, this reaction would result in the neighboring molecule losing a hydrogen atom, and that molecule could then be regarded as having more tendency to continue a similar reaction on its own account, with its oxygen abstracting a neighboring hydrogen to form the imino compound ?JH=C(CH3)0H. A short chain of hydrogen shifts could thus, perhaps, be propagated through the crystal. The imino compound would no doubt readily lose water to form the finally observed product CH3CN. The probability of occurrence of such a chain should clearly be quite sensitive to the degree and strength of the hydrogen bonding in the particular crystal. The marked change in yields with increasing dose shown by the labile form but not by the stable form (Table I) suggests that the structure of the solid is gradually changing under irradiation. y rays deliver energy to material in scattered events ranging in size from a few electron volts upward, but averaging about 50 eV. Heat is liberated in the immediate neighborhood of the excitation, and in the labile form there will be a tendency for the molecules to move into the stable configuration, with evolution of more heat. A wave of phase change would be expected to begin, but it evidently dissipates before getting very far, since no macroscopic phase change is noticeable. Mechanical disturbances initiate strain in regions of much larger volume, which can nucleate a continuing wave of phase change. Within the submicroscopic regions of energy release ("spurs") , we can, however, expect the crystalt6
(9)
B. C. Spall and E. W. R. Steacie, Proc. Rou. SOC.(London), A239,
l(1957).
THEPREEXPONENTIAL FACTORS FOR SOLID-STATE THERMAL DECOMPOSITION line configuration to move toward that of the stable form. If the affected regions average about 15 A in radius, each would contain about 200 molecules; and if two such regions form for each 100 eV, a dose of 20 Mrads should produce alteration of about one-third of the total material. If the altered material on further irradiation produces CHaCN with a yield characteristic of the stable form, the observed increase of G(CH3CN) with dose is not unreasonable. The altered material must then, however, be supposed to exhibit a G(H2)much smaller than that of either pure crystalline form. We have no specific suggestion as to how this might be possible. Many different mechanisms have been proposed for radiolytic Hz formation from organic compounds: (1) reaction of a positive ion or excited molecule with a neighboring molecule, with or without cross linking; (2) separation of H2from an excited state of a single molecule; (3) splitting out
2185
H:, on ion neutralization; (4)separation of atomic H, followed by abstraction of a second H from another molecule; (5) trapping or “solvation” of a free electron, followed by protonation to form atomic H, then by the abstraction reaction. The data in Table I1 on the ratio H2:HD show that different mechanisms predominate in Hz formation from the two crystalline forms. The peculiar effect of dose on G(H2) from the labile form points up how little we understand about the formation of this most common and prominent product of radiolysis. The apparent negative temperature coefficient of G(H2) for the labile form at 20 Mrads total dose presumably indicates that the alteration in crystalline form occurs to a much lesser extent a t lower temperatures, as would be expected since it is essentially a thermal effect. The low value of G(CH3CN) for the labile form a t -78” tends to confirm this explanation.
The Preexponential Factors for Solid-state Thermal Decomposition by Herman F. Cordes Michelson Laboratories, Naval Weapons Center, China Lake, California 03666
(Received December 11, 1067)
The preexponential factors for both unimolecular and bimolecular solid-state thermal decompositions are analyzed from the point of view of activated complex theory. The effect of molecular rotation in both the reactants and the activated complex is considered. If the activated complex has freer rotation than the reactant, the first-order preexponential factor is high. In the bimolecular case, the activated complex is likely to have restricted rotations leading to very low pseudo-first-order preexponential factors. This approach is compared with other treatments.
Introduction It is the general nature of solid-state decompositions
Several applications have been made of the theory of the activated complex to solid de~ompositions.~-~~ Most of these treatments have assumed a monomolecular reaction, although Schultz and Dekkar considered a bimolecular reaction to form a surface
that the observed macroscopic rates are extremely difficult to interpret in terms of elementary steps. One reason is that in contrast to gas-phase and liquid-phase reactions the time behavior is not controlled by the molecularity or chemistry alone, but also is controlled (1) W.E.Garner, P. W. M. Jacobs, and F. C. Tompkins, Ed., “The Chemistry of the Solid State,” Butterworth and Co. Ltd., London, by the geometry of the system, its topochemistry.l 1955, Chapter 7. Nearly all solid decompositions have rate “constants” (2) W.Gomes, Nature, 192, 865 (1961). that are given in units of reciprocal time regardless of (3) J. H.Taplin, ibid., 194, 471 (1962). the behavior of the rate with time. There is some (4) 8. Glasstone, K.J. Laidler, and H. Eyring, “The Theory of Rate confusion as to the interpretation of such constants.2*a Processes,” McGraw-Hill Book Co., Inc., New York, N. Y., 1941. (5) S. S. Penner, J . Phys. Chem., 52, 949 (1948). Consequently the question of the molecularity of (6) 8. 5. Penner, ibid., 52, 1262 (1948). solid-phase rate constants has been neglected. The (7) 8. S. Penner, ibid., 56, 475 (1952). topochemistry will not be considered here. The (8)R. D. Schultz and A. 0. Dekkar, J . Chem. Phys., 23, 2133 constants to be considered here are the ones to go into (1955). the topochemical solutions. Electron-transfer reac(9) R. D. Schultz and A. 0. Dekkar, J . Phys. Chem., 60, 1095 (1956). tions are excluded from this discussion. (10) R. D.Shannon, Trans. Faraday Soc., 60, 1902 (1964). Volume 72,Number 6 J u n e 1068
