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464
true in some respects, but rather seems to be due to a case of field-ionization experiments, because secondary perfect misunderstanding about the present stage of dissociation is not important.6 There are other factors which make difficult a direct investigation, which assumes the hypothesis that primary scission probability of the skeletal bond of alkanes comparison between the electronic charge distribution is in proportion to the charge density in the highest calculated for the molecular ion in its ground state and the experimental mass spectrum. Excited states of occupied level of their molecular orbitals. the molecular ion, as well as autoionizing states of the For instance, the contribution of the “refragmentaneutral molecule, certainly play an important role, tion” was already concluded to be important in the cases of large alkanes,’ and taking it into consideration, even if it is assumed that most of them decay radiationof theory with experiment was carried in lessly into the ground state of the molecular i ~ n . ~ ~ comparison ~ our paper on cyclododecane.2 According to this paper, Another important factor is the rearrangement underwhen the ionizing voltage Vi was decreased sufficiently gone by the electronic structure of the ion during the low, c6 fragments became maximum, instead of C3 dissociation process, which we believe to be of crucial fragments in the ordinary mass spectra.’ Similar i m p o r t a n ~ e . ~This factor can be studied by constructresults were obtained recently on normal he~adecane,~ ing the potential energy hypersurface of the molecular in whose spectra the maximum shifted to the central ionlo or by cruder meth0ds.l’ groups at low Vi. Figure 1is a part of the unpublished Photoionization experiments and the Spiteller techdata, from which Figure 1 of ref 4 was drawn. Similar nique are probably the best available methods to deterresults can be obtained from the data on n-triacontane mine the initial fragmentation pattern of a molecular ion. The processes involved in radiolysis experiments C30H62 reported by Remberg, et aE.5 Since excess energy of the primary fragments is too small to producc in the liquid phase seem to us to require a far more comrefragmentation in such cases, the above results may be plicated analysis than those occurring in the ion source expected from the standpoint of the theory, based of a mass spectrometer. Finally, it should be noticed that Lennard-Jones and on the hypothesis, because it predicts that the maximum Hall never proposed that there should be a direct relaprobability of primary scission of %-alkanes lies at or nearly at the central bond of the chain. tionship between the probability of fragmentation and A similar tendency of the effect of Vi on the spectra the positive charge distribution. Quoting Hall: “I would like to add that the simple theory of mass spectra which has been attributed to Lennard-Jones and myself was not, in fact, one that we proposed. It may have been suggested by our calculations, but it was not con50v tained in our paper. It is too simple to expect the 15v ground state of the parent ion to be the only relevant 1ov factor.”12 2 -.
I. Guyon, ibid., 11, 17 (1966). (9) J. C. Lorquet, A. J. Lorquet, and J. C. Leclerc in “Advances in Mass Spectrometry,” Vol. IV, The Institute of Petroleum, London, 1968, p 569. (10) J. C. Leclerc and J. C. Lorquet, J . Phys. Chem., 71, 787 (1967). (11) J. C. Lorquet, Mol. Phys., 10, 489 (1966). (12) J. D. Waldron, Ed., “Advances in Mass Spectrometry,” Pergamon Press, London, 1959, p 306.
INSTITUT DE CHIMIE DE LILGE UNIVERSIT~ LILGE,BELGIUM
J . C. LORQUET
RECEIVED SEPTEMBER 9, 1968
Comment on the Paper “Charge Distribution in Some Alkanes and Their Mass Spectra”
Sir: The general conclusion in the Santoro-Spadaccini (SS) com.rnunication on ordinary mass spectrometry is The Journal of Physical Chemistry
4 2o 5 , a
h
8 10
Ci-Frag me nts Figure 1. Percentage yield of fragments of normal hexadecane us. carbon number of t h e fragments; ionization voltages are not corrected with a standard gas.
(1) K. Fueki and K. Hirota, N i p p o n Kagaku Zasshi, 81, 212 (1960). (2) K. Hirota and Y. Niwa, J. Phys. Chem., 72, 5 (1968); Tetrahcdron Letters, 5767 (1966). (3) The last column of Table I11 in ref 1 shows the calculated values. (4) Y. Niwa and K. Hirota, BUZZ. Chem. Soc. Jap,, 41, 1746 (1968). (5) G. Remberg, E. Itemberg, M. Spiteller-Friedmann, and G. Spiteller, O T ~Mass . Spectrosc., 1, 87 (1968).
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tions with the methylsilanes. The minor effect of 0 2 on yields and the excess reactivity were recorded for all three methylsilanes, with the latter almost entirely contributed by very much higher yields of H T than in reactions with the corresponding hydrocarbons. These higher yields were attributed for mono- and trimethylsilane to an expected relatively greater ease of abstraction of H from weak Si-H bondsb8 Our experiments with tetramethyl- and trimethylsilane also showed the same four products, HT, CH3T, RCH2T, and RT, with reasonable agreement on yields, except for that of HT. However, in both systems, we have found (a) a very substantial effect of 02 on the H T yields and (b) no unusually high yield of HT, or (6) Discussion of the following papers: (a) R. Thompson, “Conference on Mass Spectrometry,” Institute of Petroleum, London, total reactivity of the methylsilanes us. hydrocarbons. 1953, p 164; (b) G. G. Hall, Bull. SOC.Chenz. BeEges, 73, 306 (1964). The data on the 02-scavenger effect with tetramethylsilane are shown in Figure 1. I n one set of our samples, Kozo HIROTA more heavily irradiated than the other, we observed DEPARTMENT OF CHEMISTRY YOSHIONIWA FACULTY OF SCIENCE substantial depletion of 02 during irradiation, accomMASAOYAMAMOTO OSAKA UNIVERSITY panied by an increase in HT yield and the appearance TOYONAKA, OSAKA,JAPAN of additional labeled products, not found in the wellRECEIVED SEPTEMBER 23, 1968 scavenged samples. The agreement between our poorly scavenged samples and the earlier experiments is reasonable, especially when differences in irradiation conditions are c~nsidered.~ We have also carried out experiments under constant moderator conditions (excess N2) in order to compare Recoil Tritium Reactions with Methylsilanes directly the reactivity toward hydrogen abstraction of methyl C-H groups in (CHI) $i and (CHI)4C. From Sir: While detailed and thorough studies have been these experiments, we have found that the yield of H T made of the reactions of recoil tritium atoms with many from (CH3)4Si is only 0.8 as large as from (CH3)4C hydrocarbons and with other carbon-skeletal molecules, under comparable conditions. This observation implies relatively few experiments have been reported for such that the methyl C-H bond in the former is slightly reactions with molecules containing other skeletal stronger than in the latter,lOJ1consistent with greater atom^.^-^ Consequently, we initiated a study of the energetic reactions of tritium with the methyl derivatives of silicon, germanium, and tin.5re As our work was in progress, we learned of earlier experiments (1962) (1) R. Wolfgang, Progr. Reaction Kinetics, 3, 97 (1965). on niethylsilanes, whose details have now been pub(2) R . Wolfgang, A n n . Rev. Phys. Chem., 16, 15 (1965). lished.’ Since we find important quantitative dis(3) F. Schmidt-Bleek and F. S. Rowland, Anuew. Chem., Intern. Ed. Engl., 3, 769 (1964). crepancies with this work and draw some qualitatively (4) T. Tominaga and F. 9. Rowland, J. P h y s . Chem., 72, 1399 different conclusions, we are reporting here a brief (1968). description of our experiments with tetramethyl- and (5) Recoil tritium reactions with silanes have been reported by G . Cetini, et al., J. Chem. Phys., 46, 89 (1967); Atli Acad. Sci. Torino: trimethylsilane. Further details, including those for Classe Sci. Fis., Mat. N u t . , 97, 1137 (1963). the germanium and tin compounds, will be published (6) Reactions of energetic silicon atoms with silanes have been reported by P. P. Gasper, B . D. Pate, and W. Eckelman, J . Amer. shortly. Chem. SOC.,8 8 , 3878 (1966). By analogy with similar hydrocarbon systems, the (7) J. Witkin and R . Wolfgang, J. P h y s . Chem., 72, 2631 (1968). following tritiated products should be expected in hot (8) The bond strength of Si-H bonds is frequently cited to be -70 kcal/mol, in comparison to 99 kcal/mol for comparable C-H bonds. reactions of tritium with a methylsilane (RCHS) : HT, However, experimental measurements with silanes show wide CHIT, RCH2T, and RT. Each of these products was variations, and a value of SiHs-H of 9 4 1 3 kcal/mol is quoted in N. J. Friswell and B. J. Gowenlock, Adsan. Free Radical Chem., 1, observed for each of the systems mono-, tri-, and tetra39 (1965). methylsilane, together with some additional products (9) The two most prominent additional products are CxHsT and of less certain id en tit^.^ In addition to the hydrocarCsH7T. The latter elutes in almost the same position as the tentatively identified (CHs)&iHT in the earlier experiments. A yield bon analogy, two further conclusions were drawn from of (CHa)&iHT has been undetectable in all of our tetramethylsilane these experiments by the authors? (a) the total experiments. (10) J. W. Root, W. Breckenridge, and F. S. Rowland, J. Chem. reactivity of silane systems is much higher than that of Phys.,43, 3694 (1965). the analogous hydrocarbons; and (b) oxygen scavenger (11) E. Tachikawa and F . S. Rowland, J. Amer. Chem. Soc., 9 0 has relatively little effect on the yields from the reac4767 (1968).
of large branched alkanes can be concluded by the data of Remberg, et al. The discrepancy found by Santoro and Spadaccini on large branched alkanes, therefore, is not significant. Considering this situation, their conclusion has to be restricted within the mass spectra measured at ordinary conditions. I n order to minimize confusion which might occur in future discussions, it might be noted that to cite the above hypothesis as a proposal by Lennard-Jones and Hall is not proper, because Hall himself said6 that he did not even suggest such a hypothesis, when Thompson proposed it on octane.
Volume 73, Number # February 1969
