J. Phys. Chem. 1992, 96, 3247-3258 support for the view that the colored species is peroxonitrite since loss of 0 2 N 0 0 - would require 0 atom transfer to NO2- and introduce a higher order kinetic process. Although the radiation-chemical studies of the 1950-1980 period focused upon nitrite as the photolysis product, we see that instead peroxonitrite predominates a t short irradiation times. Some nitrite accumulates in the solids but much of the nitrite reported in previous mechanistic studies appears to have resulted from systematic errors in the analytical method. We have found that dissolution in neutral H 2 0 , as done by previous workers, converts a significant portion of the ONOO- to NO2-.I6The rate constants in the kinetic model described here are undoubtedly sensitive to material-specific properties such as crystal packing efficiency but the formation of the yellow color and hence the peroxonitrite accumulation is common to most solid nitrates. NO3- is also photoisomerized to peroxonitrite in aqueous solutionI2 but solution studies have been troubled by the same problems which interfered with solid-state studies. The buildup of ONOO- is only observed in alkaline media because of the rapid
3241
thermal isomerization to nitrate when protonation occurs. It has been observedI5 that the quantum yield of NO2- from NO; increases concurrently with the ONOO- as pH increases. The dependence of the rate of NO2- formation upon pH is readily explicable as due to (1) the longer lifetime of the ONOO- photoproduct which may be photolyzed to NOT and ' / 2 0and 2 (2) the larger proportion of the ONOO- which undergoes thermal decay to NO, at higher pH values. This explanation is at variance with earlier studies in which it was reasoned from flash photolysis studieslSthat ONOO- and NO, are formed by independent paths. Although some NO, may be formed by an independent path, as in the solid, the categorical statement can no longer be accepted because the argument was based on the incorrect premises that ONOO- decays only by isomerization and that ONOO- does not interfere with NO, analyses.
Acknowledgment. We appreciate the assistance of F. Greenaway (Clark University) and A. Rieger (Brown University) with the ESR measurements.
Electronic Spectroscopy of Jet-Cooled Half-Sandwich Organometallic Free Radicals. 1. Laser-Induced Fluorescence Study of the Cyciopentadienyl Complexes of Zinc and Cadmium Eric S. J. Robles,+ Andrew M. Ellis,*and Terry A. Miller* Loser Spectroscopy Facility, Department of Chemistry, The Ohio State University, 120 West 18th Avenue, Columbus, Ohio 43210 (Received: November I, 1991; In Final Form: January 2, 1992)
Laser excitation and dispersed fluorescencespectra of the monocyclopentadienylcomplexesof zinc and cadmium are reported. These organometallic free radicals were prepared in a free jet environment using a laser vaporization/photolysis technique recently developed in our laboratory. Guided by ab initio calculations on zinc cyclopentadienyl, the electronic transitions observed here are tentatively assigned to a metal ligand excitation. On the basis of such a transition, we have tentatively assigned the symmetriesof the ground and first excited states of the metal cyclopentadienyls. Furthermore, extensive vibrational structure, originating from both metal-ring and intraring vibrational modes, in both excitation and dispersed fluorescence spectra of these molecules, has been observed. An assignment of these vibrational features is presented. We note that our observation of ZnCSHSis the first spectroscopic study of this molecule.
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I. Introduction In recent years, numerous spectroscopic studies of free radicals in the gas phase have been reported, most having been concerned with small organic radicals. (For a recent summary, see ref 1). In contrast, little spectroscopic work on organometallic radicals has been undertaken, principally because of the technical difficulties encountered in preparing these radicals in the gas phase. This is unfortunate for a number of reasons, not least because of the central role played by organometallic radicals in the mechanisms of many organometallic reactions2 Furthermore, studies of these radicals would allow us to extend our understanding of the various phenomena derived from the presence of unpaired electrons in molecules. Included in this category are spin-orbit coupling and the Jahn-Teller effect. In the past, such information has been derived mainly from studies of small inorganic or organic radicals. There is also the interesting problem of elucidating the nature of the bonding in organometallic radicals. Detailed spectroscopic studies should help to provide this information. Recently, we have developed a rather general laser vaporization/photolysis technique for preparing organometallic radicals 'Rohm and Haas Predoctoral Fellow. INATO/SERC and Ohio State Postdoctoral Fellow. Present address: Department of Chemistry, University of Leicester, University Road, Leicester LEI 7RH. U.K.
in the ultracold environment of a supersonic jet expansi0n.j The idea behind this preparative method is simple. Metal atoms, produced by laser vaporization, are reacted with organic fragments which have been prepared by photolysis of an organic precursor. The crucial feature is that the same laser pulse, derived from an excimer laser with an output wavelength in the far-UV region, is used both to vaporize the metal and to photolyze the organic precursor. Reactions between the metal atoms and organic fragments take place prior to expansion into vacuum and so any organometallic radicals formed can be entrained and cooled in a supersonic expansion. As detailed in ref 3, this technique has a number of important advantages over earlier preparative methods. Most notably, since any metal can be vaporized in principle using laser vaporization, radicals containing any metal can be prepared. In our first reported spectroscopic study of organometallic radicals prepared in this manner, laser excitation spectra of the monomethyl and monocyclopentadienyl complexes of calcium and cadmium were pre~ented.~ The only spectroscopic observation of metal cyclopentadienyl radicals in the gas phase prior to our (1) Foster, S.C.; Miller, T. A. J . Phys. Chem. 1989, 93, 5986. ( 2 ) Journal of Organometallic Chemistry Library 2 2 Organometallic Radical Processes; Trogler, W. C . , Ed.; Elsevier: Amsterdam, 1990. ( 3 ) Ellis, A. M.; Robles, E. S. J.; Miller, T. A. J. Chem. Phys. 1991, 94, 1752.
0022-3654/92/2096-3247$03.00/00 1992 American Chemical Society
3248 The Journal of Physical Chemistry, Vol. 96, No. 8, 1992 work was a study of calcium and strontium cyclopentadienyls, CaC5H5and SrC5HS(CaCp and SrCp) by OBrien and Bernathe4 However, since O'Brien and Bernath employed a Broida oven technique to prepare these molecules, which are relatively hot under these circumstances, the vibrational bands they observed were very broad (fwhm N 50 cm-I). Under such conditions, any closely spaced vibrational structure will not be resolved. In our preliminary work on metal cyclopentadienylcomplexes we reported vibrational bands with fwhm N 2.5 cm-I, a dramatic narrowing resulting from the low rotational temperatures (
