2087
Photodimerization of 9-Anthroate Esters and 9-Anthramide which probably leads to more adsorption on the walls of the shock tube and the other gas-handling equipment.
Discussion We feel that convincing arguments have been presented384 which imply that these unimolecular reactions are unperturbed by free-radical or heterogeneous contributions, and these facts may be the source of the considerable discrepancy between our Arrhenius parameters for crotononitrile and those reported by Butler and McAlpine ( E , = 51.3 kcal, log A = 11.0). In fact, extrapolation of our results passes nearly through the middle of Butler and McAlpine’s rate constants over their temperature range of 573-630’K. All three conjugated olefin systems studied have isomerization rate parameters in line with those we found for halogenated 01efins,l-~and all of these appear entirely consistent with theoretical estimates based on a biradical intermediate mechanism as proposed by Benson and coworke m 6 Our previously established rate constant for 2-butene isomerization, log k“ = 14.62 - 66,20014.58T can be compared with the present findings and implies resonance stabilization of the biradical by 8 kcal due to the cyano group, 11 kcal by methyl allyl, and 13 kcal by the allyl interaction. These values are in good agreement with the findings of Sarner et al.,7 who derived a value of 6 kcal for a CN resonance from thermolysis of substituted cyclobutanes which
also decompose through biradical intermediates. The studies of Walter@ and FreyQon isopropyl- and isopropenylcyclobutane indicate 11.6 kcal resonance stabilization with methyl allyl, while 12.6 kcal is the “standard” resonance stabilization accepted by Benson and O’Nea16 for allyl. Thus, these isomerization measurements appear to provide a convenient method for determining reliable resonance interaction energies. Acknowledgments. We appreciate continuing support by the State University of New York Research Foundation.
References and Notes (1) P. M. Jeffers and W. Shaub, J. Am. Chem. SOC., 91, 7706 (1969). (2) P. M. Jeffers, J. Phys. Chem., 76, 2829 (1972). (3) P. M. Jeffers, J. Phys. Chem., 78, 1469 (1974). (4) (a) S. M. Bauer, B. P. Yadava, and P. Jeffers, J. Phys. Chem., 78, 770 (1974); (b) P.Jeffers and S. H. Bauer, lnt. J. Chem. Kinet., VI, 763 (1974). (5) J. N. Butler and R. 0. McAipine. Can. J. Chem., 41, 2487 (1963). (6) A biradical intermediate theory has been developed by Benson and coworkers and can be found in several forms: (a) s. w. Benson. “Thermochemical Kinetics”, Wlley, New York, N.Y., 1968; (b) s. W. Benson and H. E. O’Neal, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 21, 21 (1970); (c) S. W. Benson, K. Egger, and D. M. Golden, J. Am. Chem. SOC., 87, 468 (1965). (7) S. F. Sarner. D. M. Gale, N. K. Hall, Jr.. and A. B. Richmond, J. Phys. Chem., 76, 2817 (1972). (8) M. Zupon and W. D. Walters, J. Phys. Chem., 67, 1845 (1963). (9) R. J. Ellis and H. M. Fray, Trans. faraday SOC.,59, 2076 (1963).
Photodimerization of 9-Anthroate Esters and 9-Anthramidel Rita Shao-Lin Shon, Dwaine 0. Cowan;
and Walter W. SchmiegelZ
Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 212 18 (Received March 14, 1975)
The photodimerization and fluorescence quantum yields of methyl, ethyl, n-butyl, tert- butyl, and cyclohexyl 9-anthroates and 9-anthramide were measured as a function of concentration. These efficiencies along with the fluorescence lifetime measurements are used to evaluate rate ratios and individual rate constants for several mechanistic schemes. In this continuing study3 on substituent and solvent effects on the photodimerization of anthracene4 derivatives, we report on the photochemistry and photophysics of 9anthroate esters (methyl, ethyl, n-butyl, tert- butyl, and cyclohexyl) and 9-anthramide. Relative fluorescence quantum yields and low-conversion photodimerization quantum yields were determined as a function of concentration. In addition absolute fluorescence quantum yields in dilute solution and fluorescence lifetimes were determined for each of the six 9-substituted anthracene derivatives (1).
& /
/
1-6 1, R = -0-methyl
4,
2, R = -0-ethyl 3, R = -O-n-butyl
5, R = Qcyclohexyl 6, R = -NH,
R
=
-0-tert-butyl
Experimental Section All melting points were measured on a Thomas Hoover melting point apparatus and were not corrected. Infrared spectra were determined with a Perkin-Elmer Model 337 spectrophotometer, ultraviolet and visible spectra were recorded with a Cary Model 14 spectrophotometer, and emission spectra were measured with a Hitachi-Perkin-Elmer MPF-2A fluorescence spectrometer. At high substrate concentrations, M, fluorescence measurements were determined by the front surface technique previously de~ c r i b e d This . ~ method uses a cell identical with that used for photodimerization quantum yield studies placed in a solid-sample cell holder such that the intersection of the excitation and emission beams is a t the inside front surface of the cell. Typically 99% absorption occurred in less than M solutions the first 0.5 mm of solution when 1 X were excited a t the wavelength corresponding to the lowest vibronic transition. Fluorescence spectra of concentrated solutions were essentially unchanged, except for strong self-absorption of the first vibronic transition of 9-anThe Journal of Physical Chemistry, Vol. 79, No. 20, 1975
2088
R. S.-L. Shon, D. 0.Cowan, and W. W. Schrniegel
TABLE I: Preparative Photolysis of 9-Anthroate Esters and 9-Anthramide Photolysis
Compound
Monomer mp, "C
Dimer mp, "C
Remelting time, mp, "C hr
Amt photolyzedg
Dimer,g%
Material balance, Monomer,g% %
(1)Methyl ester" (2)Ethylb'e ester (3) n-ButylaVeester ( 4 ) lerl-ButylUifester (5) CyclohexylQTf ester ( 6 ) 9-AnthramideaSe
111-112 228 5-229.5 111-1 15 112-113 216-217 -110 30 O.lOOg 0.0629 (62.9) 0.0315 (31.5) 94.4 43.5-44.5 198-199 44-50 12 0.215 0.128 (60.0) 0.065 (30.0) 90.0 157-158 233-235 655-1 57 4.5 1.40 1.33 (95.0) 0.05 (3.6) 98.6 1 26.2-1 2 7.2 208-2 09 126-1 27 5.9 0.497 0.454 (91.4) 0.0146 (2.9) 94.3 219-219.5 254.5-256 195-200 10 0.332 0.0727 (21.9) 0.259 (78.0) 99.gd (decomp) (6) 9-Anthramidec>f 219-219.5 254.5-256 195-200 6.1 1.599 1.415 (88.4) 0.178 (11.1) 99.5d (decomp) a In 95% ethanol. In benzene. In THF. Small amount (
