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J. H. BEYNON,R. A. SAUNDERS, A. TOPHAM AND A. E. WILLIAMS
Vol. 65
tion and dependent, in the case of the alcohol, on steric hindrance. While the alcohol oriented polar end down, both in physical and chemical adsorption, the chloride did not. This compound was predominantly physically adsorbed and ori$nted flat-wise with the very large C.S.A. of 46.2 A.2. Obviously, the assumptions made by the workers utiliiaing single heat measurements are unsound. Many moire criticisms could be leveled. Unfortunately, it appears that the practice of drawing conclusio.ns from heat of immersional values of clean samples originated with Harkins and has persisted too long. For example, HarkinP con-
cluded that physical adsorption alone occurred on Anatase (TiOz) activated a t 500". The Russian School headed by 11'"' were also guilty of assuming, amongst other things, only physical adsorption, occurred on high temperature activated BaSOd and proceeded to make extensive calculations of physical interaction energies. Perhaps their assumptions were correct but the work presented here points out the dangers of such assumptions as well as the conclusion that in all likelihood they were incorrect. Acknowledgment.-The authors greatly appreciate the support provided by the Office of Ordnance Research, U. S. Army.
(16) W. D. Harkins, "The Physical Chemistry of Surface Films," Reinhold Publ. Corp., New York, N. Y., 1952.
82, 85 (1952).
(17) B. V. Il'in and V. F. Kiselev, Dokladg Akad. Naulc, S. S. 5'. R.,
THE STUDY OF THE FRAGMEKTATIOX OF LONG-CHAIK PARAFFINS UNDER ELECTRON B0,MBARDMENT USING ISOTOPICALLY LABELLED CORIPOUNDS BY J. H. BEYNON, R. A. SAUNDERS, A. TOPHAM AND A. E. WILLIAXS I.C.I. Dyestufs Division,Research Department, Manchester 9, England Recezued July 7, 1960
Study of '3C abundance in the various fragments obtained from labelled compounds enables various hypotheses a8 to the mode of fragmentation to be tested. It is shown that results obtained on samples of n-heptadecane-1-W and n-eicosane4-13C do not support the postulate that abundant peaks are formed by simple carbon-carbon bond rupture. It appears that a multi-stage fragmentation process involving ring formation may be occurring.
Introduction The factors important in determining the route of fragmentation of an isolated positively-charged ion have been studied by many workers. Attempt,s to explain the fragmentation pattern of paraffinic ions in terms of the electron densities at the various bonds1 give results which conflict with experimentally observed intensities especially for C3 and Ch ions from long-chain molecules.2 , 3 Attempts have also been made to relate t,he fragmentation pattern with the bond strength in the parent molecule4 but here, too, it is not possible to explain the very large peaks occurring at small numbers of carbon atoms. The quasi-equilibrium theory of mass spect.ra6-' provides the only means of calculating complete mass spectra but has not been employed in this way for molecules larger than Cg paraffins although it has been used in comparative st,udies of more complicated molecules.8-12 (1) J. Lennard-Jones and G. G. Hall, Trans. Faraday Soc., 48, 581 (1952). (2) N. I). Coggrshall, J . Chem. Phys., 30, 595 (1959). (3) W. M. Fairbairn, Nature, 186, 151 (1960). (4) G. R. Lester, "Advances in Mass Spectrometry," Pergamon Press, London, 1969. p. 287. (5) H. M. Rosexistock, M. B. Wallenstein, A . L. Wahrhaftig and H. Eyring, PTOC. Natl. Acad. Sri. 17. S., 38, 667 (1952). (6) H. M. Rosenstock, A. L. Wahrhaftig a n d H. Eyring, "The Mass Spectra of Large Molecules: Part 11, The Application of Absolute Rate Theory," Tech. Rept. No. 2, Institute for the Study of R a t e Processes, University of Utah, June 25, 1952. (7) M. Krsus. A. L. Wahrhaftig a n d H. Eyring, Ann. Ren. Nuclear Sci., 6, 241 (1955). (8) J. Collin, BLIZZ.SOC.Rou. Sei. Liege, 25, 520 (1920).
The present' work was carried out in order to obt'ain informat,ion concerning the relative import,ance of different mechanisms in t,he formation of the various fragments and in particular, to obt,ain information about t'he very abundant CaH,+ and CdHs+ ions in long-chain paraffin spectra. The method employed was to prepare a hydrocarbon isotopically labelled with I3C in a single position and to deduce from the degree of labelling found in various fragments, which processes could have been important in the formation of the fragment. The simplifying assumption had to be made that the probability of fracture of a C-C bond was equal to t'hat,of a C J 3 C bond. (The convention is employed that the symbol C is used to represent the isotope 12Cand H to represent the isotope IH.) Since t'he work described here was complet,edwe have learned of some work carried out by Gur'ev, Tikhomiroland Tunit8skiP3using n-nonane-5J3C, who have investigated 13C-labellingof Cz,C3 and C4fragment's. Let' us consider hypothetical paraffinic compounds, the molecules of which contain only C and H atoms except at one position where the C atom is entirely replaced by 13C. We shall be particularly concerned with the molecules n-heptade~ane-l-l~c and n-eico(9) L. Friedman, F. .4. Long and 11. Tolfsber:, J . Chem. Z'hgs., 27, 613 (1957). (101 L. Friedman, F. A. Long and M. Wolfsberg, ibid., 30, 1605 (1959). (11) .2. B. King and F. A . Long, ibid., 29, 374 (1958). (12) J. H. Beynon. G . R. Lester and A . E. \Tilliams, THISJOURKAL, 63, 1861 (1959). (13) M. V. Gur'ev, M. V. Tikhomirov and N. N. Tunitskil, Zhur. Fiz. K h i n . , 32, 2847 (1958).
Jan. , 1961
FRAGMENTATION OF LONG-CHAIN PARAFFINS UNDFR ELECTRON BOMBARDMENT 115
sane-4-'3C. The argument presented below is concerned entirely with the relative numbers of unlabelled and labelled ions in each group of peaks (which ratio is (designated R), and not with the 8 absolute abundaiices of the peaks. In the following section the percentage labelling for each carbon number corresponding to a variety 6 of hypothetical modes of breakdown of the parent ion have been considered and the results compared with the actual pattern of labelling found in practice. Consider the following specific modes of 4 breakdown. (1) Simple Cleavage of a Single CarbonCarbon Bond in the Parent Ion to Give an Ion and a 2 Neutral Fragment.-If we consider an end-labelled paraffin such as n-heptadecane-l-W, since we are assuming the probability of fragmentation to be the same from both ends of the chain, fragment ions 0 5 10 15 20 will have a 50'g probability of containing the Carbon no. labelled atom, i.,~., R = 1 for all carbon numbers Fig. 1.-Plot of R versus carbon number assuming formaexcept C1,; for the parent ion R = 0. If w-e con- tion of a cyclic form of parent ion or complete randomization sider instead n-e~cosane4-'~C,by a similar reason- before fragmentation: 0, n-eicosane-41*C; 0,n-heptaing the groups (h,C,, C19 and CZomust always decane-1-lac. contain the 13C atom so that R = 0 for these groups; at all other groups down to C4 R = 1. Below Cr, R = E . (2) Rearrangement of the Parent Ion into a is Ring Structure Cationated to a Proton.-It assumed that if this process occurred a fragment ion would be formed by breaking any pair of bonds in this rearranged ion. Then calling Rn the ratio of unlabelled to labelled ions a t a carbon number n,we have for any straight chain paraffin containing N carbon atoms that the total number of ways of forming a Cn fragment is N and that of these, ( N n) are unlabelled. Thus
Rn
=
(N - n) N
The values of R which would be obtained if such a mechanism of fragmentation were dominant are shown graphically in Fig. 1. (3) Complete Randomization of the Carbon Skeleton before Fragmentaton.-The chance of choosing n carbons under these circumstances all of which are unlabelled is also given by (N - n),". Thus in an n-carbon fragment the ratio of unlabelled to labelled material is the same as in ( 2 ) above, viz., R, = ( N - .iz)/N,so that fragmentation modes ( 2 ) and (3) cannot be distinguished by measurements of R when only a single carbon atom is labelled. (4) The Selection of Any Given Number of Adjacent Carbon Atoms from the (Linear) Parent Ion.-In this fragmentation scheme it is necessary t o break two C-C bonds in order t o remove a fragment from the inner carbon atoms of the ion and only one for removal of an end. For reasons of simplicity the probabilities of these two processes have been assurried identical. The number of ways of choosing n atoms from n-eicosane-4J3C in this way is (21 - n). The number of these which are labelled is 1 when n = 1 or 20,2 when n = 2 or 19,3 when n = 3 or 18 and 4 when 4 6 n 17, enabling R to be calculated.
