J . Phys. Chem. 1990, 94, 4769-4772
of the bands assigned to the nitroso compound overlap with the azoxy compound. Transient bands are found at 966, 1233, and 1580 cm-l. These bands appear slightly earlier than those bands for the final products and then disappear about 70 ms later. The experimental curves of the intensities of the transient 1233- and 1580-cm-I bands versus time are shown in Figure 3b. These transient bands must correspond to some kind of intermediate. A reasonable interpretation suggests that this intermediate is a free-radical anion, Ar-NO-*, produced during the electrochemical oxidation of the hydroxylamine compound: Ar-NHOH
+ 20H-
i-
Ar-NO-*
+ 2 H 2 0 + e-
(4)
+
Ar-NO-* e Ar-NO e(5) The lifetime of this species on the Ag electrode surface is about 70 ms. ESR studies provide evidence for the existence of nitroso radical anion but the lifetime of this free radical in solution is much longer, about 2 min.19 The shorter lifetime in our experiment is understandable since the species is not free in solution but is situated in the electrical field of the double layer at the Ag surface. The 1580-cm-l band of the transient species may be assigned as the benzene ring stretching mode. The resonance effect of the
4769
free radical with the benzene ring would tend to stabilize the radical and thus cause a shift of the ring stretch band from 1600 to 1580 cm-I. Furthermore, the transient band at 1233 cm-l may be related to the carbon-nitrogen vibration since the resonance effect favors the double-bond structures and the carbon-nitrogen single-bond vibration band is located at about 1145 cm-I.
Conclusion It is clear from the results presented that time-resolved surface-enhanced Raman spectroscopy can provide a valuable tool for detailed studies of short-lived intermediates in electrode processes. Further time-resolved S E R S studies of the PNBA electrochemistry are in progress.23
Acknowledgment. This work was supported primarily by the National Science Foundation (CHE-8711638) with supplementary support by the PSC-BHE Research award program of the City University of New York (666367,667261, and 66875) and the National Institutes of Health MBRS program (RR-08168). (23) Shi, C.; Zhang, W.; Birke, R. L.; Lombardi, J. R. To be submitted for publication.
Photoinitiated Charge Transfer and Dissociation in Mass-Selected Metalloorganic Complexes K. F. Willey, P. Y. Cheng, K. D. Pearce, and M. A. Duncan* Department of Chemistry, University of Georgia, Athens, Georgia 30602 (Received: March 28, 1990)
Metalloorganic ion-molecule complexes of silver and aluminum ions with benzene and acetone molecules are prepared in a supersonic molecular beam with pulsed laser vaporization. Complexes are mass-selected in a reflectron time-of-flight spectrometer and excited with ultraviolet laser radiation in the 385-266-nm region. Aluminum complexes dissociate to produce the isolated metal ion and a neutral organic, while silver complexes undergo dissociative charge transfer, yielding charged organic fragments and neutral metal atoms. The mechanism and energetics of this new photochemistry are discussed.
Introduction
New techniques in molecular beam mass spectrometry have made it possible to synthesize metal-rare gas or metal-molecular complexes in the gas phase.I-I9 These complexes are interesting (1) Smalley, R. E.; Auerbach, D. A.; Fitch, P. S.; Levy, D. H.; Wharton, L. J. Chem. Phys. 1977,66, 3778. (2)Kowalksi, A.; Czajkowski, M.; Breckenridge, W. H. Chem. Phys. Lett. 1985. 121. 217. (3) Kowalski, A.; Funk, D. J.; Breckenridge, W. H. Chem. Phys. Lett. 1986,132, 263. (4)Kvaran, A.; Funk, D. J.; Kowalski, A,; Breckenridge, W. H. J. Chem. Phys. 1988,89.6069. ( 5 ) Funk. D. J.; Kvaran, A.: Breckenridee. - W. H. J. Chem. Phvs. 1989, 90,'2915. (6)Gardiner, J. M.; Lester, M. I. Chem. Phys. Lett. 1987, 137, 301. (7)Schriver, K. E.;Hahn, M. Y.; Persson, J. L.; LaVilla, M. E.;Whetten, R. L. J. Phys. Chem. 1989,93,2869. (8)Lessen, D.; Brucat, P. J. Chem. Phys. Lett. 1988,149,IO, 473. (9)Lessen, D.; Brucat, P. J. Chem. Phys. Lett. 1988,152,473. (IO) Lessen, D.;Brucat, P. J. J . Chem. Pbys. 1989,90,6296. ( I I ) Callender, C. L.; Mitchell, S. A.; Hacket, P. A. J. Chem. Pbys. 1989, 90,2535,5252. (12)(a) Castleman, A. W.; Holland, P. M.; Lindsay, D. M.; Peterson, K I. J. A m . Chem. SOC.1978,100,6039.(b) Castleman, A. W. Chem. Phys. Leu. 1978,53,560. (c) Holland, P. W.; Castleman, A . W. J . Chem. Phys. 1982,76, 4195. (d) Glein, K. L.; Guo, B. C.; Keesee, R. G.; Castleman, A. W. J. Phys. Chem. 1989,93,6805. (13)Shaw, M. H.; Farrar, J. M. J . Phys. Chem. 1989,93,4386. (14)Magnera. T.F.:David, D. E.; Michl, J. J . Am. Cbem. SOC.1989,1 1 1 , 4100.
0022-3654/90/2094-4769$02.50/0
models for the interactions at metal surfaces and for condensed-phase organometallic chemistry. In the gas phase, spectroscopy and photochemistry can be studied without the influence of the solvent. Alternatively, systematic studies of different sized complexes can probe the effects of solvation. We describe here the production of jet-cooled metal-organic ion-molecule complexes (Al'acetone, Ag+-benzene, etc.) and their study by mass-selected photochemistry. We obtain the first evidence for photoinduced "metal-to-ligand" charge transfer in these gas-phase metalloorganic complexes. Ion-molecule complexes containing metals have been generated in a variety of mass spectroscopy experiments using high-temperature ovens, sputtering, or laser However, spectroscopic or photochemical studies of these complexes are limited.9*'0.'3J6Previous spectra appear to result from excitation (15) Marinelli, P. J.; Squires, P. R. J. Am. Chem. SOC.1989,1 1 1 , 4101.
(16)(a) Lessen, D.; Brucat, P. J. Chem. Phys. Lett. 1988,149, IO. (b) fbid. 1988,149,473. (c) fbid. 1988,152,473. (d) Lessen, D.; Brucat, P. J. J . Cbem. Phys. 1989,90,6296.
(17)Bouchard, F.; Hepburn, J. W.; McMahon, T. B. J. Am. Chem. Soc. 1989,111,8934. (18) El-Shall, M. S.; Schriver, K. E.; Whetten, R. L.; Mautner, M. J. Phys. Chem. 1989,93,7969. (19)Hettich, R. L.; Freiser, B. S. J. A m . Chem. SOC.1987,109,3543. (20)Gantefor, G.;Siekmann, H. R.; Lutz, H . 0.;Meiwes-Broer, K. H . Chem. Phys. Lett. 1990,165, 293. (21)(a) Bauschlicher, C. W.; Partridge, H.; Langhoff, S. R. J . Chem. Phys. 1989,91,4733.(b) Rosi, M.: Bauschlicher, C. W. J . Chem. Phys. 1989, 90,1264.
0 1990 American Chemical Society
4770 The Journal of' Physical Chemistry. Vol, 94, No. 12, 1990 e
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