J . Am. Chem. SOC.1991, 113, 7115-7121
7115
Electron Photodetachment of Cyclopentadienylidene Anion Radical in a Flowing Afterglow Apparatus: EA and A&" of Cyclopen t adieny lidene Richard N. McDonald,* Edward J. Bianchina Jr., and Cheng C. Tung Contribution from the Department of Chemistry, Kansas State University, Manhattan, Kansas 66502. Received December 3, 1990 Abstract: The thresholds for electron photodetachment (EPD) from the doublet ground electronic state of cyclopentadienylidene anion radical (c-CsH4'-) producing the 3BIground state of cyclopentadienylidene,and the IA2, 3A2,and IA' (latter tentative) excited states of the carbene, have been measured. The assignments of the observed transitions to the excited states of the carbene are based on the good agreement between the measured and theoretical energy gaps for the ground and excited states for this system. The EPD thresholds occur at 708 (40.4 f 1.08 kcal mol-'), 619 (46.2 0.23 kcal mol-'), 598 (47.8 f 0.23 kcal mol-'), and 576 nm (49.6 0.23 kcal mol-I), respectively. From the previously determined A$,2g8(c-C5H4*-)= 71.9 f 3.6 kcal mol-', A$,?,,('Bl c-CSH4)= 112.3 4.7 kcal mol-l is determined. The AHfoof the observed excited states of the carbene are also given.
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Introduction The determination of electron affinities (EA) of atomic and molecular species' is important to a number of disciplines ranging from gas-phase negative ion chemistry to charge transfer in inorganic, organic, and catalytic condensed-phase reactions and to electron transfer in biological systems. To measure this important thermodynamic property, a variety of methods have been used; in a review by Christodoulides et al.,ln 31 methods are discussed. Several of these methods involve photodetachment of the negative ion (eq 1) and are generally agreed to yield the most accurate A- + hu A + e(1) EA datae2 These photon impact methods include photoelectron spectroscopy (PES) and electron photodetachment (EPD). In PES experiments, the output of a fixed-frequency laser crosses the ion beam and the kinetic energy of the detached electrons is measured. The difference in the incident photon and detached electron energies for the C-0 transition (following certain corrections) is the EA of neutral A.2b In EPD experiments, a tunable light source is employed. The negative ion generation methods for EPD measurements include high-pressure drift tubes,'v4 ion cyclotron resonance (ICR) spectrometers,H and crossed-beam9J0 and coaxial ion-laser beam EPD cross section vs wavelength plots are generated from the loss of a percentage of the negative ion signal as a function of photon energy by using conventional or laser photon
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( I ) (a) Christodoulides, A. A.; McCorkle, D. L.; Christophorou, L. G. In Electron-Molecule Interactions and Their Applications; Christophorou, L. G.,Ed.; Academic Press: New York. 1984;Vol. 2, Chapter 6. (b) For a rccent review of additional EAs, see Kebarle, P.; Chowdhury, S.Chem. Rev. 1987, 87, 513. (2) (a) Miller, T.M. Adu. Electron. Electron Phys. 1981,55, 119. (b) Mead, R. D.; Stevens, A. E,.;Lineberger, W. C. In Gas Phase Ion Chemistry; Bowers, M. T., Ed.; Academic Press: New York, 1984;Vol. 3, Chapter 22. (3) Woo, S.B.;Helmy, E. M.; Mauk, P. H.; Paszek, A. P. Phys. Reu. 1981, 64, 1380. (4) Moseley, J. T.In Applied Atomic Collision Physics; Massey, H. S.W., McDaniel, E. W., Bedemon, B., Eds.;Academic Press: New York, 1982;Vol. 5, p 269. (5) Janousek, B. K.; Brauman, J. 1. In Gas Phase Ion Chemistry; Bowers, M. T., Ed.; Academic Press: New York, 1979;Vol. 2, Chapter IO. (6) Blumberg, W. A. M.; Itano, W. M.; Larson, D. J. Phys. Reu. D 1979, 19, 139. (7) Drzaic, P. S.;Marks, J.; Brauman, J. I. In Gas Phase Ion Chemistry; Bowers, M. T., Ed.; Academic Press: New York, 1984;Vol. 3, Chapter 21. (8) Wetzel. D. M.: Brauman. J. 1. Chem. Reo. 1987. 87. 607. (9) Slater, J.; Read, F. H.; Novick, S.E.; Lineberger, W. C. Phys. Reo. A 1978. 17. 201, (IO)'JoneS, P. L.; Mead, R. D.; Kohler, B. E.; Rosner, S.D.; Lineberger, W. C. J . Chem. Phys. 1980,78,4419. ( I I ) Moseley, J. T.; Durup, J. Annu. Reu. Phys. Chem. 1981, 32, 53. (12) Hefter, U.;Mead, R. D.; Schulz, P. A,; Lineberger, W. C. Phys. Reo. A 1983, 28, 1429. ( I 3) Carrington, A.; Kennedy, R. A. In Gas Phase Ion Chemistry; Bowers, M. T.,Ed.; Academic Press: New York, 1984;Vol. 3, Chapter 26.
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sources. The EPD threshold is then determined. In favorable cases, the 0-0 transition is observed yielding the EA(A), and vibrational and excited electronic state spacings in A- and A can be obtained. In general, the lowest energy required to achieve photodetachment will yield an accurate threshold. The Franck-Condon factors strongly influence the photodetachment behavior of Anear threshold. A problem arises with polyatomic molecules if the structures of the ion and the corresponding neutral differ significantly. This results in poor Franck-Condon overlap, and the detachment threshold may involve transitions from the ground state of the anion to upper vibrational levels of the neutral, and overestimates the EA(A). Further, transitions from higher vibrational states of the anion to the neutral ground state (hot bands) will underestimate EA(A) unless properly identified. However, such errors are likely to be ..
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Figure 3. Experimental EPD cross section vs wavelength plot for the cyclopentadienyl anion. The spacing between data points is - 2 A.
Table 1. Summary of the Experimental and Calculated AH/"of the Ground State and Certain Excited Electronic States of Cyclopentadienylidene (c-CqHA) electronic state of c - C ~ H ,
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obsd threshold," nm (kcal mol-') 708 (40.4 i 1.08)' 619 (46.2 i 0.23)' 598 (47.8 i 0.23)e
exptl aAHHp," kcal mo1-I
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of the state24 kcal mo1-I
0.0 0.0 0.0 112.3 f 4.7 5.8 f 1.3 7.3 6.3 118.1 f 4.7 'A, 7.4 i 1.3 8.4 6.8 119.7 i 4.7 3AZ 576 (49.6 f 0.23)' 9.2 1.3 12.2 121.5 i 4 . 7 y ['A' "This study. bReference 39. 'Reference 40. dBased on Afl,298(~-CsH4*-) = 71.9 f 3.6 kcal mol-'; see text and ref 42. 'The rotational correction is only applied to the 708-nm threshold. For the excited-state transitions, the rotational onset will be part of the EPD background preceding the plateau. /Tentative assignment.
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However, with the laser on, a SFC ion signal was observed that accounted for -50% of the loss in the signal intensity for the enolate anions. 11. Electron Photodetachment of Cyclopentadienyl Anion. The next problem was to determine the accuracy of the EPD threshold measurements for anions incapable of yielding a dipole-supported state. The cyclopentadienyl anion (c-C5H
