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J . Am. Chem. S o t . 1989, I l l , 892-899
Gas-Phase Reactions of the Methyl Anion Susan T. Graul and Robert R. Squires* Contribution from the Department of Chemistry, Purdue University, West Lafayette, Indiana 47907. Received May 31, 1988
Abstract: We report the first direct observations of ion/molecule reactions of the methyl anion CH3- in the gas phase. Methyl anions a r e generated by means of collision-induced dissociation (CID) of CH3C02-or CH3CH20-in a flowing afterglow-triple quadrupole instrument. Ion/molecule reactions of CH3- can be observed when C I D is effected in t h e presence of reactive gases under appropriate conditions of collision energy and reactant gas pressure. Following fragmentation of the precursor ion, methyl anion reactions take place within the collision cell under nonthermal conditions. For a given neutral reactant, only those product ions t h a t a r e unique to CH3- a r e reported. Reflecting its strongly basic character [AHoaCid(CH4)= 416.6 kcal/mol] and its low electron-binding energy [EA(CH3) = 1.8 kcal/mol], CH3- abstracts a proton from CH2=CH2 and N H , a n d transfers a n electron to 02,OCS, CS2, and C6F6. Nucleophilic addition/elimination reactions a r e observed both with aprotic reagents such as N 2 0 and with protic substrates such as propionaldehyde. Results of reactions with a variety of other substrates a r e also presented.
The m e t h y l anion CH3- arguably can be considered the arc h e t y p e of organic anions, but virtually nothing is k n o w n of its gas-phase chemistry. The major obstacle encountered in attempts to study the chemistry of CH3- is t h e difficulty of generating this ion under conditions appropriate for the observation of ion/ molecule reactions. Although CH3- can be observed in the lowpressure negative-ion mass spectra of several organic numerous other reactive anions are also formed and CH3- is but a minor constituent of the resulting complex mixtures. Chemical ionization methods such as proton abstraction, fluorodesilylation, and other methods developed to generate carbanions512 fail utterly t o produce CH3-. Even t h e amide ion NH2-, t h e most basic reagent ion readily available [L~H~,,~~("~), = 403.6 k ~ a l / m o l ] , ~ is far too weak to d e p r o t o n a t e CHI [AHoaCid(CH4) = 416.6 k ~ a l / r n o l ] . " - ~ Moreover, ~ it is likely that collisional detachment of the weakly bound electron of CH3- [EA(CH3)= 1.8 k ~ a l / m o l ] ' ~ would occur under t h e high-pressure conditions or at the long interaction times t h a t are usually required for the study of ion/molecule reactions and t h e r e b y preclude t h e observation of CH3-.I4 Recently, we described how CH3- can be produced by collision-induced decarboxylation of CH3C02- in a flowing afterglow-triple quadrupole instrument (eq Herein w e report t h a t ion/molecule reactions of CH3- can be observed w h e n de( 1 ) (a) Trepka, L.; Neuert, H. 2. Nuturforsch. 1963, 18u, 1295. (b) Jager, K.; Henglein, A. Z . Nuturforsch. 1967, 22u, 700. (c) Jager, K.; Henglein, A.; Doumont, M. Z . Nuturforsch. 1970, 25u, 202. (2) Dressler, R.; Allan, M. Chem. Phys. Lett. 1985, 118, 93. (3) Compton, R. N.; Bouby, L. C. R . Acud. Sci., Ser. C 1967, 264, 1153. (4) Dorman, F. H. J . Chem. Phys. 1966, 44, 3856. (5) (a) Bartmess, J. E.; McIver, R. T., Jr. In Gus Phase Ion Chemistry; Bowers, M. T., Ed.; Academic Press: New York, 1979; Vol. 2, Chapter 11. (b) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J . Phys. Chem. ReJ Dura 1988, 17, Suppl. 1. (6) DePuy, C. H.; Bierbaum, V. M.; Flippin, L. A,; Grabowski, J. J.; King, G. K.; Schmitt, R. J. J . A m . Chem. SOC.1979, 101, 6443. (7) DePuy, C. H.; Bierbaum, V. M.; Damrauer, R.; Soderquist, J. A. J . A m . Chem. SOC.1985, 107, 3385. (8) DePuy, C. H.; Bierbaum, V. M.; Damrauer, R. J . A m . Chem. SOC. 1984, 106, 4051. (9) DePuy, C. H.; Bierbaum, V . M. Acc. Chem. Res. 1981, 14, 146. ( I O ) Bohme, D. K.; Lee-Ruff, E.; Young, L. B. J . A m . Chem. SOC.1972, 94, 5153. ( 1 1) Lindinger, W.; Albritton, D. L.; Fehsenfeld, F. C.; Ferguson, E. E. J . Chem. Phys. 1975, 63, 3238. (12) (a) Schiff, H. I.; Bohme, D. K. Int. J . Muss Spectrom. Ion Phys. 1975, 16, 167. (b) Payzant, J. D.; Tanaka, K.; Betowski, L. D.; Bohme, D. K. J . A m . Chem. SOC.1976, 98, 894. (13) Ellision, G. B.; Engelking, P. C.; Lineberger, W. C. J. A m . Chem. SOC.1978, 100, 2556. ( 1 4) However, we note that weakly bound anions have been observed under flowing afterglow conditions; for example, NO- in an Ar buffer: Fehsenfeld, F. C.; Ferguson, E. E.; Schmeltekopf, A. L. J . Chem. Phys. 1966,45, 1844. (15) Graul, S. T.; Squires, R. R. J . A m . Chem. SOC.1988, 110, 607.
0 0 0 2 - 7 8 6 3 / 8 9 / 151 1-0892$01.50/0
carboxylation is effected in the presence of reactive gases at appropriate pressures. We describe an experimental protocol for identifying CH3- reactions and present results from the first systematic experimental investigation of the gas-phase chemistry of CH3-. Experimental Section The experiments described in this paper were carried out in a flowing afterglow-triple quadrupole instrument (Figure l).'6J7 Briefly, the instrument consists of a I-m-long, 7-cm-diameter stainless steel flow tube, through which a constant flow of helium buffer gas is maintained by a Roots blower. For these experiments, the buffer gas pressure was 0.30 Torr and the buffer flow rate was 130 (STP) cm3/s. Primary ions generated in the upstream ion source (electron ionization, EI, at 70 eV and emission current of 0.40-0.50 mA) are carried downstream by the helium buffer, where they can react with neutral reagents added through fixed or movable inlets. The resulting ions are sampled through a 1mm-diameter orifice in the nose cone partition separating the flow tube from the differentially pumped detection chamber, which is maintained Torr. A low, attractive potential (0.1-1.5 V) is at less than 5 X applied to the molybdenum disk that contains the sampling orifice in order to optimize collection of the negative ions. The sampled ions are focused into the triple quadrupole analyzer and examined by standard quadrupole mass analysis or by tandem mass spectrometry.I8 The ions are detected at the exit of the third quadrupole (Q3) by a conversion dynode and Channeltron particle multiplier. Pressures in the collision chamber ( 4 2 ) were measured with an MKS Baratron capacitance manometer. Collision energies (laboratory frame) are given by the Q2-rod offset voltage, and the energy axis origin is located by retarding potential analysis. Because of the complex trajectories described by ions in the quadrupole collision cell and the fact that collision cross sections for the ion/ molecule pairs considered here are unknown, we are unable to define rigorously the upper bound of pressure for single-collision conditions. However, we have developed a number of empirical tests that signal the onset of effects due to multiple collisions.'6 The tests involve monitoring the appearance of secondary ion/molecule reaction products when a reactive gas is present in 4 2 or, alternatively, the onset of nonlinearity in the pressure dependence of sequential dissociation reactions. For parent ion axial kinetic energy in excess of -1 eV (lab), the various diagnostic tests indicate that effects of multiple collisions become manTorr, depending ifest at collision gas pressures greater than (5-10) X upon the collision gas used and the collision energy. Quantitative measurements of threshold energies for C I D in the second quadrupole region are carried out at collision gas pressures corresponding to single-collision conditions (PQz < 5 X IO-' Torr). Ion/ molecule reactions of the methyl anions produced by C I D can be ob(16) (a) Lane, K. R.; Lee, R. E.; Sallans, L.; Squires, R. R. J . A m . Chem. R.; Lee, R. E.; Wright, L. G.; Wood, K. V.; Cooks, R. G. Int. J . Mass Spectrom. Ion Processes 1985, 64, 185. (c) Squires, R. R.; Brickhouse, M. D.; Graul, S. T.; Hajdasz, D. J.; Marinelli, P. J.; Wang, D.; Workman, D. B., manuscript in preparation. (17) Graul, S. T.; Squires, R. R. Mass Spectrom. Reu. 1988, 7 , 263. (18) (a) McLafferty, F. W., Ed. Tandem Mass Spectrometry; Wiley: New York, 1983. (b) Dawson, P. H.; French, J. B.; Buckley, J. A.; Douglas, D. J.; Simmons, D. Org. Mass Spectrom. 1982, 17, 205.
Sot. 1984, 106, 5767. (b) Squires, R. R.; Lane, K.
0 1989 A m e r i c a n C h e m i c a l Society
J . Am. Chem. Soc., Vol, 1 1 I , No. 3, 1989 893
Gas-Phase Reactions of the Methyl Anion Roots Blower Tuple Ouodrupole Moss Analyzer
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Figure 1. The flowing afterglow-triple quadrupole instrument. served only in multiple-collision conditions, and most were studied at reactant gas pressures of (3-5) X IO-, Torr. It should be noted that reaction conditions within 42 are nonthermal and not well-characterized. Particularly under multiple-collision conditions, the distribution of collision energies is broad and indeterminate, and the actual number of collisions suffered by ions will also be described by an unknown distribution function. Methyl anions were generated in the central quadrupole by CID of either CH3C0
