JOURNAL OF T H E AMERICAN CHEMICAL SOCIETY 0 Copyright 1987 by the American Chemical Society
FEBRUARY 18, 1987
VOLUME 109, NUMBER 4
A Stepwise Mechanism for Gas-Phase Unimolecular Ion Decompositions. Isotope Effects in the Fragmentation of tert-Butoxide Anion William Tumas, Robert F. Foster, Mark J. Pellerite, and John I. Braurnan* Contribution from the Chemistry Department, Stanford University, Stanford, California 93405. Received March 24, 1986
Abstract: Infrared multiple photon (IRMP) photochemical activation of gas-phase ions trapped in an ion cyclotron resonance (ICR) spectrometer has been used to study the mechanism of a gas-phase negative ion unimolecular decomposition. Upon irradiation with a C 0 2 laser (both high-power pulsed and low-power continuous wave (CW)), tert-butoxide anion, trapped in a pulsed ICR spectrometer, decomposes to yield acetone enolate anion and methane. The mechanism of this formal 1,2-elimination reaction was probed by measuring competitive hydrogen isotope effects (both primary and secondary) in the IR laser photolysis of 2-methyl-2-propoxide-Z,Z,l-d3 (1) and 2-methyl-2-propoxide-Z,Z,~,3,3,3-d6 ( 2 ) anions. Unusually large secondary isotope effects (pulsed laser, 1.9 for 1 and 1.7 for 2 ; cw laser, 8 for 1) and small primary isotope effects (pulsed laser, 1.6 for 1 and 2; cw laser, 2.0 for 1) were observed. These isotope effects, particularly the large difference in energy dependence of the primary and secondary effects, are consistent only with a stepwise mechanism involving initial bond cleavage to an intermediate ion-molecule complex followed by a hydrogen transfer within the intermediate complex. The observed secondary isotope effects have been modelled by using statistical reaction rate (RRKM) theory. The implications of this study for several previously reported unimolecular ion decompositions are also discussed.
Advances in ion generation and trapping techniques as well as ion detection methods have stimulated interest in understanding the dynamics and mechanisms of gas-phase ion reactions.'-3 The availability of such techniques has lead to rapid development in the field of negative ion mass ~ p e c t r o m e t r y . ~In spite of the increasing number of reports on negative ion chemistry, no clear mechanistic picture has emerged for even simple negative ion unimolecular decompositions. Efforts in our laboratory have been directed toward the use of photochemical activation of gas-phase ions trapped in an ion cyclotron resonance (ICR) spectrometer to induce negative ion fragmentations and to investigate reaction mechanisms. In this paper,5 we report the details of our investigations into the infrared multiple photon6 (IRMP) photochemically induced unimolecular ( I ) Gas Phase Ion Chemistry; Bowers, M. T., Ed.; Academic Press: New York, 1979. (2) Nibbering, N. M. M. Mass Spec. Rev. 1984, 3 , 445. Dannacher, J. Org. Mass Spec. 1984,19,253. Gas Phase Ion Chemistry: Ions and Light;
Bowers, M. T., Ed.; Academic Press: New York, 1983. ( 3 ) Howe, I.; Williams, D. H.; Bowen, R. D. Mass Spectrometry: Principles and Applications; McGraw-Hill: New York, 1981. McLafferty, F. W.; Interpretation of Mass Spectra; University Science Books: 1980. Current Topics in Mass Spectrometry and Chemical Kinetics; Beynon, J. H., McClashan, M. L., Eds.; Heyden: London, 1982. Holmes, J. L. In Isotopes in Organic Chemistry; Buncel, E., Lee, C. C., Eds.; Elsevier: Amsterdam, 1975. (4) Bowie, J. H. Mass Spec. Reo. 1984, 3, 161. Budzikiewicz, H. Angew. Chem., Int. Ed. Engl. 1981, 20, 624. Dougherty, R. C . Anal. Chem. 1981, 53, 625. (5) A preliminary report of this work has appeared: Tumas, W.; Foster, R. F.; Pellerite, M. J . ; Brauman, J . I. J . A m . Chem. S o t . 1983, 105, 7464.
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elimination of methane from tert-butoxide anion, which is a prototypical example of 1,2-elimination of a neutral fragment from a negative ion.' We present competitive intramolecular kinetic hydrogen isotope effect measurements (both primary and sec~ n d a r y ) *which > ~ support a stepwise mechanism for methane elimination from tert-butoxide anion that involves initial cleavage to an intermediate ion-molecule cornplex.l0 The observed secondary isotope effects have been modelled by using statistical reaction rate theory (RRKM theory) and are consistent with initial cleavage of the carbon-methyl bond as the rate-determining step. The implications of these results are discussed for several unimolecular decompositions reported in the literature, particularly ~~~~
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(6) Reviews on IRMP photochemistry: Golden, D. M.; Rossi, M.J.; Baldwin, A. C.; Barker, J. R. Acc. Chem. Res. 1981, 14, 56. Schultz, P. A,; Sudbo, A. S.; Kranovich, D. J . ; Kwok, H. S.; Shen, Y. R.; Lee, Y. T. Ann. Rev. Phys. Chem. 1979, 30, 379. Thorne, L. R.; Wight, C . A,; Beauchamp, J. L. In Ion Cyclotron Resonance Spectrometry I& Hartmann, H., Wanczek, K. P., Eds.; Springer-Verlag: Berlin, 1982; Vol. 31. (7) Melander, L.; Saunders, W. H. Reactions of Isotopic Molecules; Wiley: New York, 1980. (8) Derrick, P. J. Mass Spec. Rev. 1983, 2, 285. (9) Elimination of alkanes and/or molecular hydrogen from alkoxide anions has been reported in several studies.g,'OTo date, however, there have been few studies which have provided much insight into how these simple negative ions dissociate: Boand, G.;Houriet, R.; Gaumann, T.; Lect. Notes Chem. 1982,31, 195. Houriet, R.; Stahl, D.; Winkler, F. J . Environ. Health Perspec. 1980, 36, 63. Roy, T. A,; Field, F. H.; Lin, Y. Y.: Smith, L. L. Anal. Chem. 1979, 51, 272. ( I O ) For reviews on stepwise decompositions of positive ions, see: Bowen, R. D.; Williams, D. H . In de Mayo, P., Ed.; Rearrangements in Ground and ExcitedStates; Academic Press: New York, 1980; Vol. 1 . Williams, D. H. Arc. Chem. Res. 1977, 10, 280. Morton, T. H. Tetrahedron 1982, 38, 3195.
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those involving a p p a r e n t four-center 1,2-eliminations fragments.
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Experimental Section Materials. Nitrogen trifluoride (Ozark Mahoning Company), tertbutyl peroxide (MCB), acetone-d6 ( M S D Isotopes, 99.7% atom D), and i0d0methane-d~(Aldrich, 99 % atom D) were obtained commercially and were used without further purification. Prior to use, tert-butyl alcohol was distilled under nitrogen from sodium metal. Dimethyl peroxide was prepared by the reaction of basic hydrogen peroxide with dimethyl sulfate following the method of Hanst and Calvert." The gaseous product was collected in a trap at -78 " C and then dried by bulb-to-bulb distillation from activated 4-A molecular sieves at 0 OC on a vacuum line. Deuteriated tert-Butyl Alcohols. 2-Methyl-2-propanol-Z,Z,Z-d3 was prepared12 by the addition of trideuteriomethylmagnesium iodide, generated from iodomethane-d, and magnesium turnings in ether, to acetone followed by hydrolysis with saturated aqueous ammonium chloride. After separation, numerous extractions of the aqueous layer with ether, and drying over anhydrous sodium sulfate, the alcohol was isolated by fractional distillation under an atmosphere of nitrogen (bp 82-84 "C). The fractions of high purity (>99%) were used in the ICR experiments. The d6 analogue, 2-methyl-2-propanol-1,1,1,3,3,3-d6, was synthesized in an analogous manner from methylmagnesium bromide (Aldrich) and perdeuteriated acetone (Aldrich). N M R and I R spectra were consistent with those in the literature. T h e isotopic purity of the alcohols was analyzed by ICR negative ion studies by using methoxide anion as a base for proton abstraction. N o isotopic impurities could be detected (
