J. Phys. Chem. 1992, 96,8820-8821
8820
cm-I/A2 and a reduced mass value of pT = 30.38 amu. Table V presents the calculated frequencies for this potential function and compares these to the observed values. This potential function has a barrier of 3000 cm-I, a value, which because of the onedimensional approximation, is most likely somewhat of an overestimation.' For the undeuterated species, the twisting barrier calculated in this manner was 2720 cm-I. In order to determine the barrier to planarity more accurately, we will carry out a two-dimensional potential energy surface calculation which simultaneously considers the ring-twisting and ring-bending motions (which can be transformed to radial and twisting motions).
Conclusion The barrier of 577 f 20 cm-' determined for the d2 species compares favorably with the 541 f 20 cm-' value we reported for the undeuterated 1,3-oxathiolane, especially in view of the fact that both calculations assume the validity of the one-dimensional approximation. Some coupling with other vibrations is expected to result in at least a small perturbation on the calculation. It was very satisfying that we were once again able to observe pseudorotational transitions both below and above the pseudorotational barrier and to accurately calculate the frequencies in a region where the energy level diagram is very complex and sensitive to even minor changes in the potential function. The twisting barrier (barrier to planarity) for the d2 molecule was calculated to be 3000 cm-l using the one-dimensional approximation. A similar calculation resulted in a value of 2720 cm-' for the undeuterated molecule. A somewhat lower value is
expected when an improved two-dimensional potential energy surface is calculated for both isotopic species.
Acknowledgment. The authors thank the National Science Foundation, the Robert A. Welch Foundation, and the Texas Advanced Research Program for financial assistance. R.V. and J.V. acknowledge assistance from the NIH-NIGMS. References rad Notes (1) Leibowitz, S.J.; Laane, J.; Verastegui, Jr.; Villarreal, J. R. J . Chem. Phys. 1992, 96, 7298. (2) Durig, J. R.; Wertz, D. W. J . Chem. Phys. 1968, 49, 2118. (3) Bauman, L. E.; Laane, J. J . Phys. Chem. 1988,92, 1040. (4) Carreira, L. A.; Jiang, G. J.; Person, W. B.; Willis, J. N . J . Chem. Phys. 1972,56, 1440. (5) Lafferty, W. J.; Robinson, D. W.; St. Louis, R. V.; Russell, J. W.; Strauss, H. L. J . Chem. Phys. 1965,42, 2915. (6) Engerholm, B. G.; Luntz, A. C.; Gwinn, W. D.; Hams, D. 0.J. Chem. Phys. 1969, 50, 2446. (7) Greenhouse, J. A.; Strauss. H. L. J. Chem. Phys. 1969, 50, 124. (8) Durig, J. R.; Wertz, D. W. J. Chem. Phys. 1968, 49, 679. (9) Wertz, D. W. J . Chem. Phys. 1969, 51, 2133. (10) Laane, J. J . Chem. Phys. 1969, 50. 1946. (11) Colegrove, L. F.;Wells, J. C.; Laane, J. J. Chem. Phys. 1990, 93, 6291. (12) Green, W. H.; Harvey, A. B.;Greenhouse. J. A. J . Chem. Phys. 1971, 54, 850. (13) Durig, J. R.; Willis, J. N. J . Chem. Phys. 1970, 52, 6108. (14) Schmude, R. W.. Jr.; Harthcock, M.A.; Kelly, M.B.; Laane, J. J . Mol. Spectrosc. 1987, 124, 369. (15) Gokel, G. N.; Gerdes, H. M.;Dishong, D. M.J . Org. Chem. 1980, 45, 18.
Fourier Transform Electron Paramagnetic Resonance Study of the Photoreduction of Anthraquinone with 4-Methyl-2,6-dEtert-butylphenol In Alcoholic Solutions M. Pliiscbau, G.Kroll, Institut fur Physik, Universitdt Dortmund, Otto-Hahn-Strasse, W-4600 Dortmund, Germany
K.-P. Dinse,* Physikalische Chemie III, TH Darmstadt, Petersenstrasse 20, W-6100 Darmstadt, Germany
and D. Beckert Max- Planck- Arbeitsgruppe 'Zeitaufgeloste Spektroskopie" an der Universitdt Leipzig, Permoserstrasse 15, 0-7050 Leipzig, Germany (Received: June 16, 1992)
Using FT-EPRfollowing laser excitation, the primary photochemical process in the photoreduction of anthraquinone with 4-methyl-2,6-di-tert-butylphenoi was investigated. Highly-resolved spin-polarized EPR spectra taken with nanosecond time resolution gave unambiguous evidence for a two-step hydrogen abstraction reaction, consisting of a primary electron transfer followed by proton abstraction with a time delay, which allows for a noticeable escape probability of the initially generated anthrasemiquinoneradical anion (AQ-). The time dependence of the EPR intensities of the neutral 10-hydroxyanthroxyl-9 radical (AQH') as well as of AQ'- could be simulated for the full experimentally accessible time interval of 10 ns to 100 ps. The kinetic model used invokes optical spin polarization, spin-lattice relaxation, radical generation, and AQH'/AQ'interconversion. In addition, from an analysis of the highly-resolved FT-EPR spectra a complete set of AQH' hyperfine splitting (hfs) constants could be measured in two different alcohols for the first time.
1. Introduction The photoreduction of aromatic ketones and quinones with different donors has been investigated extensively,'-1othe primary electron-transfer involved being one of the most important processes in photochemistry and in photosynthesis reaction centers. Several reaction intermediates, like exiplexes and/or complexes of triplet ketones (quinones) with ground-state donors (amines, phenols) have been proposed. However, it should be expected that the mechanism for the formation of ionic or neutral radical species 0022-3654/92/2096-8820$03.00/0
and their interconversion will generally be dependent on the specific combination of donor and acceptor molecules as well as on the nature of the solvent. The distinction of the different intermediates like contact ion pairs, solvent-separated ion pairs, exciplexes, and/or free radicals by optical methods is Micult and not always unambiguous, although optical methods excel in time resolution. The recent development of timeresolved EPR, resulting in the improvement of the time resolution of Fourier transform EPR (FT-EPR) into the range of 10 ns, renders it possible, however, 0 1992 American Chemical Society
Photoreduction of Anthraquinone with DTBP to combine time resolution with unsurpassed selectivity for paramagnetic species like coupled radical pairs,11-13radical ions, and neutral free r a d i ~ a l s . ’ ~ J ~ In this paper, we report on FT-EPR laser photolysis experiments on the anthraquinone/4-methyl-2,6-di-tert-butylphenol system in 2-propanol and ethanol. It was the aim of this investigation to find unambiguous evidence for a two-step hydrogen abstraction reaction via a primary electron transfer followed by subsequent proton abstraction. Furthermore, a quantitative description of the time dependence of the electron spin polarization of the various radicals involved in terms of the kinetic constants of the photoreaction should be given. 2. Experimental Section Laser photolysis was performed with 308-nm radiation of an excimer laser (Lambda Physik, EMG 103MSC). Laser pulse energy incident on the sample was 8-10 mJ. From consideration of a quantum yield of the anthraquinone triplet (AQT) of approximately 0.8,16the initial radical concentration can be estimated as 10-4-10-3 M. Even the Boltzmann-equilibrated radical spectra could be observed after 100 ps, indicating a rather slow secondorder radical decomposition. We have not attempted to determine this reaction rate. FT-EPR spectra were taken with a home-built spectrometer described elsewhere.”J4 Its time resolution is determined from the laser pulse width (15 ns), the width of the r / 2 microwave pulse (16 ns), and the time jitter between these pulses. It is well-known that the jitter between the W-light pulse and the electric discharge pulse in the laser head can be as large as 30-50 ns, depending on the laser repetition rate and the condition of the gas in the discharge cell. For short delay times, where time synchronization between laser and microwave pulses necessitates the use of the electronic laser trigger, the time resolution of the apparatus would therefore be completely dominated by this contribution. In order to exclude this effect, we implemented a coincidence circuit, phase-synchronizedto the master pulse generator, allowing the accumulation of data only under the condition that the time delay between laser and microwave pulses was within a preset time window of 6 13s.’’ Using a photoreaction with a predicted rise time of electron spin polarization of
