Argon Cluster-Mediated Trapping and Vibrational Spectroscopic Characterization of an OH - ‚HCH 2 • Intermediate in the O •- + CH 4 Reaction Eric G. Diken, Gary H. Weddle, ‡ Jeffrey M. Headrick, J. Mathias Weber, † and Mark A. Johnson* Sterling Chemistry Laboratory, Yale UniVersity, P.O. Box 208107, New HaVen, Connecticut 06520-8107 ReceiVed: June 21, 2004 We isolate an [O‚CH 4 ] •- intermediate in the reactive O •- + CH 4 encounter using an argon cluster-mediated trapping technique and characterize it using vibrational predissociation spectroscopy. The spectra of the argon- solvated complexes establish that only the OH - ‚CH 3 • ion-radical adduct is prepared. Its formation is firmly established by the appearance of the signature OH - stretching band close to that of the free hydroxide ion. The band origin locations and partially resolved rotational spacings indicate that hydroxide binds onto one of the methyl hydrogen atoms, much like the motif observed previously in the I - ‚HCH 2 • ion-radical complex. This OH - ‚CH 3 • species is best regarded as an entrance-channel complex in the secondary (endothermic) OH - + CH 3 • f H 2 O + CH 2 - proton transfer reaction. These observations indicate that the initial H-atom abstraction step (O •- + CH 4 f OH - + CH 3 • ) occurs too quickly to enable capture of the intermediates directly associated with this process. I. Introduction The chemistry of the O •- radical anion ( 2 P) with alkanes and alkenes (RH 2 ) has been extensively studied over the past 30 years, 1-9 largely because it presents several unique features nicely summarized in the review article by Lee and Grabowski. 10 Its pattern of reactivity derives from the fact that the proton transfer channel (leading to OH • + RH •- ) is endothermic, while H-atom abstraction occurs with a low barrier and is typically exothermic. Interest- ingly, when the very basic hydroxide ion product of reaction 1 remains in the vicinity of the nascent hydrocarbon radical, the proton transfer process then becomes operative in a second step leading to the production of carbene anions. 5,10 This propensity was, in fact, exploited 11,12 to great effect as a preparative route to the vinylidene anion, CH 2 dC - , by the reaction of O •- with ethylene. In this paper, we focus on the simplest of these reactions 7 where, in the case of methane, the second, proton transfer step (leading to H 2 O + CH 2 - ) is endothermic and, therefore, is not available at the low collision energies explored in this work. Viggiano and co-workers 3,5 have recently reported the kinetics of reaction 3 and discussed their results in the context of the potential surface illustrated in Figure 1. This is a standard double-minimum construction 13 that supports two local minima. These correspond to the entrance- and exit-channel complexes of reaction 3, which are separated by the transition-state barrier. Like the case of the classic S N 2 reaction (e.g., Cl - + CH 3 Br), the barrier is calculated to lie below the asymptotic energy of the reactants, 3 presenting the possibility of trapping reaction intermediates in these minima and characterizing them using photoelectron and vibrational spectroscopies. 14,15 Here, we exploit the argon-mediated synthetic methodology developed at Yale 16,17 to capture [O‚CH 4 ] •- intermediates in reaction 3 This approach has significant advantages in that the intermedi- ates are vibrationally cooled by the evaporation of weakly bound argon atoms, and when argon atoms are retained upon conden- sation of methane, the trapped species can be characterized using argon predissociation vibrational spectroscopy 18 One expects that the structures of the entrance- and exit- channel intermediates (O •- ‚CH 4 and OH - ‚CH 3 • , respectively) * To whom correspondence should be addressed. E-mail: mark.johnson@ yale.edu. † Present address: Institut fu ¨r Physikalische Chemie, Universita ¨t Karlsruhe, Kaiserstr. 12, D-76128 Karlsruhe, Germany. ‡ Present address: Fairfield University Department of Chemistry, Fair- field, CT 06430. O •- + RH 2 f OH - + RH • (1) OH - + RH • f H 2 O + R - (2) O •- + CH 4 f OH - + CH 3 • + 0.26 eV (3) Figure 1. Potential energy curve (schematic drawing) depicting the reaction coordinate (O •- ‚‚‚H‚‚‚CH3) for the O •- + CH4 f OH - + CH3 • hydrogen atom abstraction reaction. The minima correspond to the O •- ‚CH4 and OH - ‚CH3 • entrance- and exit-channel reaction intermedi- ates, respectively. O •- ‚Ar n + CH 4 f [O‚CH 4 ] •- ‚Ar m + (n - m)Ar. (4) [O‚CH 4 ] •- ‚Ar m + hν f [O‚CH 4 ] •- + mAr. (5) 10116 J. Phys. Chem. A 2004, 108, 10116-10121 10.1021/jp0404403 CCC: $27.50 © 2004 American Chemical Society Published on Web 10/19/2004