J . Phys. Chem. 1984,88, 5285-5290
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Figure 11. Stability o f j with time, under a polarization of -0.4 V/SCE in an aqueous electrolytic medium, pH 0: (-) platinized Au electrode, (---) Au-[PMeT (170 nm)-Ag (80 wt %)-Pt(50 wt %)I, Au-[PP (170 nm)-Ag (80 wt %)-Pt (50 wt%)]. (e-.-)
peak can be attributed to the adsorption of the electrosynthesized H2 onto the platinum. Then j undergoes a slow decrease with time, which can be explained by the aggregation of the platinum. On the other hand, the [PMeT-Ag-Pt] electrode (170-nm PMeT, 80% Ag and 50% Pt polymer weight) shows a remarkable stability, the large current density value, j = 220 mA cm-2, remaining constant for more than 170 h. Thus, the [PMeT-Ag-Pt] catalytic system appears to be affected neither by intense native H2 production nor by Pt aggregation. This stability must also be related
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to the intrinsic electrochemical property of PMeT, which we recently showed to be very stable and reversible in contrast to the case of polypyrrole, PP. Thus, with the [PP-Ag-Pt] catalytic system (170-nm PP, 80% Ag and 50% Pt polymer weight), a continuous decrease of the current density is observed with time, which is accompanied by a change of the polymer color, passing from light brown to green and to yellow. After about 25 min, j suddenly falls, as due to the peeling of the polymer film from the Au electrode, which may be due to chemical reactions of the native hydrogen with the polymer or to its porous nature, which allows hydrogen production at the electrode-polymer interface.I6 In conclusion, we have shown that Pto and Ago aggregates can be electrochemically incorporated into an organic conducting polymer like PMeT. The Ag aggregates, which are located along the polymer fibers, enhance the electronic conductivity to the polymer, even at cathodic potential values corresponding to the neutral undoped state of the polymer. The Pt aggregates, which are mainly coating the polymer surface, show an enhanced catalytic activity. The [PMeT-Ag-Pt] catalytic system is thus able to catalyze H+ reduction with a current density of 200 mA cm-2, which is 50% higher than that observed with the corresponding metallic electrode [Ag-Pt]. Furthermore, a remarkable stability with time is observed under polarization, which reveals that PMeT is not affected by native H2 and that the Pt aggregates do not migrate and aggregate on the polymer film surface. The implication in the field of solar energy conversion appears very promising and experiments on the properties of such catalytic films grafted onto semiconductors are under way. Registry No. Ht, 12408-02-5.
Electron Photodetachment from Phenylnitrene, Anilide,‘ and Benzyl Anions. Electron Affinities of the Anilino and Benzyl Radicals and Phenylnitrene Paul S. Drzaict and John 1. Braurnan* Department of Chemistry, Stanford University, Stanford, California 94025 (Received: March 19, 1984)
The photodetachment spectra of the phenylnitrene, anilide, and benzyl anions are presented. Theoretical photodetachment cross sections for each species were calculated and fit to the experimental data. Accurate determinations of the photodetachment onsets in the three spectra are made. From these, the electron affinities of phenylnitrene (EA = 33.7 f 0.3 kcal/mol), anilide radical (EA = 39.3 f 0.7 kcal/mol), and benzyl radical (EA = 19.9 f 0.3 kcal/mol) are determined. These electron affinities, in conjunction with literature values for the proton affinities of these species, lead to the determination of C-H and N-H bond strengths in toluene, aniline, the anilide anion, and the anilide radical. From the phenylnitrene spectrum, the splittings between the ground triplet state and the first and second singlet states of the nitrene are determined as 4.3 f 0.04 and 8.8 f 0.05 kcal/mol, respectively. The photodetachment spectra presented also show a structure indicative of transitions to excited states of the respective anions. These features are assigned as B2 A2 and B2 B2 transitions in the anions. An unusually large blue shift is observed in the lowest-energy excited-state transition in the benzyl anion solution absorption spectrum, relative to the same feature in the photodetachment spectrum. This is reminiscent of a large blue shift also observed in the comparison of solution and gas-phase spectra of the phenoxide anion and may be explained on the basis of differential solvation.
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Introduction Photodetachment spectroscopy of gas-phase molecular anions offers the possibility of observing spectroscopic transitions between different electronic, vibrational, and rotational levels of both the anion and its neutral photoproduct. By determining the energies of channel openings in a photodetachment spectrum, it has been possible to measure the energies of electronically excited states of both anions and neutrals, different rovibrational levels of anions and neutrals, and the adiabatic electron affinity of the neutraL2 The spectroscopic and thermodynamic data obtained from these spectra are often very difficult to obtain by other means. Current Address: Taliq Corporation, Mountain View,
CA.
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In this paper we report our photodetachment studies on three phenyl-substituted anions. The photodetachment spectra of phenylnitrene anion, anilide anion, and benzyl anion share many similarities and contain a good deal of important and previously unreported information. The adiabatic photodetachment threshold (1) The species C6HSNH-is currently referred to in Chemical Absfracfs as benzenarnine, ion I-. The corresponding radical has been referred to as anilino and phenylamidogen. The C6H5NH-anion in this paper is referred to as the anilide anion, in analogy to the methide (CH3-) anion. In order to have a consistent usage throughout the paper, the corresponding radical is referred to the anilide radical. (2) Drzaic, P. S.; Marks, J.; Brauman, J. I. In “Gas Phase Ion Chemistry”; Bowers, M. T., Ed.; Academic Press: New York, 1984; Vol. 111, piJ 167-21 1. ~~
0022-3654/84/2088-5285$01 .50/0 0 1984 American Chemical Society
5286 The Journal of Physical Chemistry, Vol. 88, No. 22, 1984
for each species is determined, and the corresponding electron affinity is reported. These data are related via thermochemical cycles to a number of important neutral N-H and C-H bond strength values. Additionally, each spectrum shows features indicative of autodetaching electronically excited states of the anions. These transitions are assigned on the basis of a simple molecular orbital model. For the benzyl anion, comparison of its gas-phase photodetachment spectrum with its solution-phase absorption spectrum shows an unusually large blue shift for the solvated anion. This difference can be explained on the basis of differential solvation of the ground and excited states of the anion. Finally, the photodetachment spectrum of phenylnitrene anion also shows a number of channel openings near threshold indicative of transitions to low-lying singlet excited states of the neutral. From these data, tripletsinglet splittings for phenylnitrene neutral are determined, the first such values for a substituted nitrene.,
Experimental Section Photodetachment experiments were performed with a modified Varian V-5900 ion cyclotron resonance spectrometer to generate, trap, and detect anions. A home-built capacitance bridge circuit4 was used for ion detection. A Digital Equipment Corporation MINC-11/23 computer was used to collect ion signal level data from the ICR, as well as to control the monochromator micrometer drive through the use of a stepping motor. The light source used in these experiments was a Hanovia 1000-W xenon arc lamp. Wavelength selection was made by passing the lamp output through a Schoeffel '/.,-m grating monochromator. Spectral bandwidth was determined by a pair of matched slits in the monochromator; typically, the narrowest bandwidth that resulted in a reproducible photodetachment cross section spectrum was used. A more complete description of the data collection and analysis procedures used in these experiments is given e l ~ e w h e r e . ~ Phenylnitrene anion was formed by low-energy (1.O-1.5 V above the trapping plate potential) electron impact on phenyl azide, at phenyl azide pressures of
