SO4
The Journal of Physical Chemistry, Vol. 82, No. 5, 1978
generous financial support, and to the Army Research Office (Durham) for instrumentation funding. References and Notes (1) Collisional Activation and Metastable Ion Characteristics. 58. For part 57, see D. J. McAdoo, D. N. Witiak, and F. W. McLafferty, J. Am. Chem. Soc., 99, 7265 (1977). (2) H. M. Rosenstock, M. B. Wallenstein, A. L. Wahrhaftig, and H. Eyring, Proc. Natl. Acad. Sci. U.S.A., 38, 667 (1952). (3) A. L. Wahrhaftig in "Mass Spectrometry", MTP Int. Rev. Sci., 5, 1 (1972). (4) F. W. McLafferty, T. Wachs, C. Lifshitz, G. Innorta, and P. Irving, J. Am. Chem. Soc., 92,6867 (1970); F. W. Mdafferty, D. J. McAdoo, J. S. Smith, and R. Kornfeld, ibid., 93, 3720 (1971); P. J. Derrick and A. G. Loudon, ibid., 98, 2362 (1976); W. J. Chesnavich and M. T. Bowers, ibid., 99, 1705 (1977). (5) H. M. Rosenstock, K. Draxl, B. W. Steiner, and J. T. Herron, J. Phys. Chem. Ref. Data, 6, Suppl. 1, 26 (1977). (6) G. Herzberg, "Electronic Spectra of Polyatomic Molecules", Van Nostrand, Princeton, N.J., 1966. (7) Direct dissociation of excited unstable states was offered as an explanation for the disagreement of early QET calculations with experiment: A. B. King and F. A. Long, J . Chem. Phys., 29, 374 (1958); see also C. Lifshitz and F. A. Long, J. Phys. Chem., 67, 2463 (1963). (8) There is welldocumented evidence for decompositions from repulsive electronic states of small ions such as H,+ and C2HZt.' (9) J. Durup In "Recent Developments in Mass Spectroscopy", K. Ogata and T. Hayakawa, Ed., University Park Press, Baltimore, Md., 1969, p 921; H. Yamaoka, P. Dong, and J. Durup, J. Chem. Phys., 51, 3465 (1969). (10) I.0. Simm, C. J. Danby, and J. H. D. Eland, Int. J. Mass Spectrom. Ion Phys., 14, 285 (19741, and references cited therein. (1 1) It Is also possible that an isomeric C6He+ion is involved instead of an Isolated state.lZc (12) (a) B. Andlauer and Ch. Ottinger, J. Chem. Phys., 55, 1471 (1971); (b) R. D. Smith and J. H. Futrell, Org. Mass Spectrom., 11, 445 (1976); (c) J. H. D. Eland, R. Frey, H. Schulte, and B. Brehm, Int. J. Mass Spectrom. Ion Phys., 21, 209 (1976); (d) M. L. Gross, D. H. Russell, R. J. Aernl, and S. A. Bronczyk, J. Am. Chem. Soc., 99, 3603 (1977). (13) C. Lifshitz, J. Chem. Phys., 62, 1602 (1975). (14) R. Gooden and J. I. Brauman, J. Am. Chem. Soc., 99, 1977 (1977). (15) (a) P. P. Dymerski, E. Fu, and R. C. Dunbar, J. Am. Chem. Soc., 96, 4109 (1974); (b) E. W. Fu, P. P. Dymerskl, and R. C. Dunbar, ibid., 98, 337 (1976); (c) R. C. Dunbar and R. Klein, ibid., 98, 7994 (1976).
G. Perez (16) B. S. Freiser and J. L. Beauchamp, J . Am. Chem. SOC.,99, 3214 (1977). (17) F. W. McLafferty, P. F. Bente, 111, R. Kornfeld, S . 4 . Tsai, and I.Howe, J . Am. Chem. Soc., 95, 2120 (1973). (18) C. Koppel, C. C. Van de Sande, N. M. M. Nibbering, T. Nishlshita, and F. W. McLafferty, J. Am. Chem. Soc., 99, 2883 (1977), and references cited therein. (19) M. Medved, R. G. Cooks, and J. H. Beynon, Chem. Phys., 14, 345 (1976). (20) T. Wachs, C. C. Van de Sande, P. F. Bente, 111, P. P. Dymerski, and F. W. McLafferty, Int. J . Mass Spectrom. Ion Phys., 23, 21 (1977). (21) R. G. Cooks, J. H. Beynon, R. M. Caprioli, and G. R. Lester, "Metastable Ions", Elsevier, New York, N.Y., 1973. (22) The magnitudes of the curves were not corrected for trapping times or other mass effects.14 Although no evidence was found for the formation of the benzoyl ion (5) between 2.0 and 3.5 eV," there are substantial experlmental difficulties associated with the measurement of 5, such as its possible fast photodissociation: R. Gooden and J. I.Brauman, private communication. (23) D. A. Shirley and S. T. Lee, University of California, Berkeley, unpublished results. (24) However, as noted by Dunbar," it is possible that an accessible excited state of the ion would not give a significant peak in the PD spectrum due to the lack of an allowed dissociation mechanism competitive in rate with other deexcitation processes. (25) The CA excitatlon probabilities shown in Figure 1 were calculated by a method described separately: M. S. Kim and F. W. McLaflerty, J . Am. Chem. Soc., accepted for publication. (26) An energy release >1 eV would be unusual for metastable (threshold energy) ion decompositions. The formation of 3 from 1 is a simple cleavage reaction, and so its reverse activation energy should be low;*' the release of 0.4 eV for metastable ions decomposing by methyl loss appears to be the largest value reported for such a case, and this reaction involves the formation of a particularly stable ion; see R. G. Cooks, J. H. Beynon, M. Bertrand, and M. K. Hoffman, Org. Mass Spectrom., 7, 1303 (1973). (27) We thank Dr. James D. Dill for prelimlnary extended Huckel calculations on the electronic levels of the 1 Ion. (28) However, Dr. Randy1G. A. Flynn of this laboratory found no slgnificant increase in the relative abundance of m l e 71 in the normal E1 mass spectrum of butyrophenoneon making the ion source repeller potential so negative (-10 eV) that the total ion abundance was reduced to -0.1 % of normal. Tsuchiya has found that this technique can give dramatic changes in the relative abundances of "ions of excess kinetlc energy": M. Tsuchiya and K. Tamura, Org. Mass Spectrom., 11, 1281 (1976). (29) M. S. Kim, R. C. Dunbar, and F. W. McLafferty, J. Am. Chem. Soc., submitted for publlcation.
Isomerization of Ethyltoluenes in the Gas Phase G. Perez Laboratorio di Chimica Nucleare CNR, C.P. IO, 00016 Monterotondo Stazione, Rome, Italy (Received August 11, 1977) Publication costs assisted by Laboratorio di Chimica Nucieare del CNR
Ethyltoluenes were isomerized in the gas phase using D3+ions as protonating agents. The results show that isomerization proceeds by 1,2 alkyl shifts. The isomerization rate decreases in the order o-ethyltoluene > p-ethyltoluene > m-ethyltoluene. A comparison with the isomerization of xylenes has been reported and confirms the higher ability of ethyl group to migrate. A protodeethylation reaction, thermodynamically favored with respect to the protodemethylation, explains toluene formation during the isomerization reaction. The yields of the products were determined as a function of the pressure of the substrate. The mechanisms of gas phase electrophilic attack and of isomerization and dealkylation of the gaseous arenium ions are discussed. The results are compared to those of the analogous Friedel-Crafts reactions occurring in solution. Introduction As reported by Brouwer et a1.l the reaction between strong acids, such as HF-BF3 or HF-SbF6, and arenes leads to arenonium ions. Cations of the less basic isomers of dialkylarenes show their tendency to isomerize to the more stable isomeric cations. These isomerizations proceed by successive intramolecular 1,2 hydrogen and 1,2 alkyl shifts. Furthermore, dealkylation reactions may lead to 0022-365417812082-0504$01 .OOlO
irreversible conversion of alkylarenium ions. The stability of the leaving carbonium ions is undoubtedly1 responsible for the dealkylation reaction. Also in the gaseous phase, as in the chemical ionization mass spectra,2dealkylation reactions were observed. Several gas phase alkylations of arenes were reported in which the proportion of meta substitution obtained was anomalously large for an electrophilic r e a ~ t i o n It . ~was ~~ 0 1978 American
Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 5, 1978 505
Isomerization of Ethyltoluenes in the Gas Phase
TABLE I: Radiolysis of Ethyltoluene-Helium Mixturesa Substrate
Concn, pmol L-I
o-Ethyltoluene
38.0 2.7
m-Ethyltoluene
37.7
p-Ethyltoluene
3.4 36.1 1.8
a
Yields of aromatic hydrocarbons, mol % Benzene
Toluene
Ethylbenzene
0.01 0.01 0.01 0.01 0.01 0.01
0.03 0.04
0.01 3
IBDl,x+
(3)
The Journal of Physical Chemistry, Vol. 82, No. 5, 1978
506
G. Perez
TABLE 11: Ethyltoluenes t Deuterium Ionsa Concn, pmol €-I
Substrate
Ortho
Final svstem comDosition. mol % Ethyltoluene Meta Para
Toluene
o-Ethvltoluene
3.71 7.98 19.15 25.02 47.70
95.15 96.25 96.71 97.76 98.17
4.24 3.33 3.10 2.08 1.70
0.41 0.30 0.13 0.11
