THE JOURNAL OF
PHYSICAL CHEMISTRY (Registered in U. 8. Patent O5oe)
VOLUME62
(0Copyright, 1968, by the Ameriaan Chemioal Society)
JANUARY 23, 1958
NUMBER 1
INTRODUCTION TO THE SYMPOSIUM: MECHANISM IN RADIATION CHEMISTRY1 BY MILTONBURTON Department of Chemistry, University of Notre Dame, Notre Dame, Ind. &sceived June $4, 1967
I #
I
some rather large molecules (e.g., benzene, etc.) values are unknown in many cases of interest to the radiation chemist. There is a particularly glaring absence of such information regarding simple hydrocarbons. The energy required t o produce an ion pair, W , is known for a number of gases. Many important substances still require investigation. The value is known with exceptional accuracy in several cases as a result of the recent work of Jesse and Sadauskis. In general, W = 21 in gases. From our knowledge of ionization potentials and excitation potentials in gases, it follows that for each ionization act in radiation chemistry enough energy is left over to produce about 1.5 excited molecules. It is generally assumed that 1.5 excited molecules are actually produced per ionization act in the radiation chemistry of gases but t o the author’s knowledge there seems to be no real verification of this assumption for polyatomic molecules. Platzman has made calculations of W from theory for helium and the other noble gases, in which known cross-sections of excitation processes are employed. The calculated values seem in these cases to be as good as the experimental ones. However, there is no present likelihood of such a calculation for larger molecules. The value of W is in principle determinable for solids in which the electrons can be trapped and quantitatively measured. The experimental difficulties are so great that, so far as the author knows, there are no reliable estimates of W in solids. Experiments have been performed in liquids but, in this case also, all that can be said for the resultant values is that they are near the right order of magnitude. Nevertheless, it is customarily assumed that the ratio of ionization acts to excitation acts is very nearly the same in liquids as in gases; the value usually employed for liquids is
If we examine our knowledge of what actually happens when high-energy radiation interacts with matter, we find ourselves woefully ignorant. Only a few facts are really known; some might be established provided the proper experiments were done; most of the so-called facts are inferences based on rather courageous speculation and extrapolation. On the physical side, the stopping powers of many elements have been measured and the additivity of stopping powers has been established to a reasonable degree. I n some cases the experimental values reported for condensed systems or for certain compounds differ from the calculated values by amounts up to 15%. However such differences are small enough and infrequent enough to raise a legitimate question regarding the experimental errors. More work of this kind and more accurate work is required. Ionization potentials, I, are known very accurately for a large number of gaseousmolecules. Such ionization potentials, measured in various ways, generally agree within 10% and frequently much better. Nothing is known about ionization potentials in liquids although it is generally assumed that such ionization potentials are lower in condensed systems. This is a field which does seem open to the experimental approach. The excitation potentials, E, of gases are known in a number of cases, particularly for optically permitted transitions. Forbidden transitions (e.g., singlet-triplet) can be very important when excitation is by low-energy electrons as it can be to a rather significant degree in radiation chemistry. Although excitation potentials for such transitions are known for several simple cases and even for (1) Contribution from the Radiation Project, operated by the University of Notre Dame and supported in part under Atomia Energy Commission contract AT-(ll-1)-38.
1
SEYMOUR MEYERSON AND PAUL N. RYLANDER
2
1:2 and some theoretical support for this view has been proposed, notably by Fano. There have been a number of attempts at relating quantitative knowledge of the primary physical effects to the resultant chemical effects. It is not the purpose of this introduction to survey these attempts. Some of them have been very useful in guiding further experimentation. These essentially speculative treatments have been concerned with a number of factors, the possible effects of which have been considered individually and also in their relationships to other factors. They include: mechanism of neutralization; the contribution of negative ions and negative ion formation; the fate of excited molecules and the effect of excitation level, state of aggregation and of other molecules, surfaces, etc., on the fate of such excited molecules; the phenomenon of energy transfer; Stern-Volmer reactions of excited molecules and of excited free radicals; and, rather recently, ion-molecule and electron-ion-molecule reactions. From a more chemical point of view the speculations extend to
Vol. 62
the fate of free radicals, both “cold” and “hot,” and to the effect of temperature on that fate. With all this speculation and incipient theory it is apparent that much more extensive and reliable information is required before one can expect to make reasonable predictions in radiation chemistry. In the radiation chemistry of organic compounds especially, a strictly pragmatic approach to radiation chemistry may be particularly useful. Not only can the observations and correlations resultant from careful, detailed work give useful information, the immediate applications of which would be readily apparent, but, much more importantly the subtle variations of study possible in the radiation chemistry of organic compounds (e.g., by detailed studies of a homologous series) may eventually give us a clearer understanding of the physical processes precedent to the chemistry and thus in turn give us a sound basis for prediction. In this Symposium, there are represented in a systematic way some of the purely chemical approaches t o the solution of some fundamental problems of radiation chemistry.
ORGANIC IONS I N THE GAS PHASE. VI. THE DISSOCIATION OF p-XYLENE UNDER ELECTRON IMPACT BY SEYMOUR MEYERSON AND PAUL N. RYLANDER Research Department, Standard Oil Company (Indiana),Whiting,Indiana Received June 84, 1867
The most abundant ions obtained from xylenes and other olymethylbenzenes by electron impact result from the loss of Although other alkylbenzenes preferentiafy cleave a bond once removed from the ring, the polymethylbenzenes appear to cleave a ring-to-alkyl bond. Mass spectra of a series of labeled p-xylenes confirm the conclusion, based on a study of appearance potentials, that the resulting ions do not have the tolyl structure. Further, these spectra indicate drastic rearrangement before dissociation. Consequently, the methyl group lost does not contain solely the elements of an original side-chain. a methyl radical.
The mass spectrum of a compound gives the distribution by mass of the ionized products that result from electron impact. Unfortunately, the spectrum tells nothing about the identity of an ion except its mass-or, more accurately, the ratio of mass to charge. The masses of the more abundant ions from most compounds can be accounted for by postulating cleavage of one or more bonds in the original molecule. Such a simple explanation is attractive and has been the basis for many assumptions about the nature and origin of ions produced by electron impact. Although such assumptions have facilitated empirical correlation of mass-spectral data, without supporting evidence they may be no more than convenient fictions and have little significance for problems involving actual chemical processes. For example, chemical reactions induced by electron impact seem to have important implications for radiation chemistry and perhaps other areas. Also, appearance potentials of ions, interpreted in terms of reactions more often assumed than established, have been widely used to compute bond-dissociation energies and other thermochemical values. Although the simplest assumptions continue to be useful in providing initial hypotheses, such assumptions must be proved valid by independent observa-
tions before they can be accepted as descriptions of actual chemical particles and events. The most abundant ions obtained from xylenes and other polymethylbenzenes by electron impact have masses 15 units lower than the parent molecules.lV2 These masses correspond to loss of a methyl group and suggest cleavage of a ring-tomethyl bond. When the process is considered to be such cleavage, however, appearance potentials of the ions derived by loss of methyl from toluene, the three xylenes and mesitylene are not mutually consistent.a These potentials lead t o heats of formation of the ions from the xylenes and mesitylene far lower than that of the ion from toluene, and close to that of the ion derived from ethylbenzene by the loss of methyl. This finding has led various investigators to suggest that the ion derived from toluene is actually phenyl,4 but that those derived from the other compounds have rearranged to benzy13or tropylium5ions. Regardless of the structure attributed to the ions, the methyl lost was assumed (1) I. W. Kinney and G. L. Cook, Anal. Chem., 84, 1391 (1952). (2) S. Meyerson, Appl. Spectroscopu, 9,120 (1956). (3) F. H.Field and J. L. Franklin, J . Chem. Phya., 2 2 , 1895 (1054). (4) However, see P. N. Rylander and S. Meyerson, ibid., 27, 1116
(1957). (5) P. N. Rylander, S. Mcyerson and H. M. Grubb, J . A m . Chsm. Sac., l e , 842 (1957).
t ’
.
