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Intermolecular Triplet Energy Transfer. 111. From Benzene to 2-Chlorocyclobutanone and the Trans and Cis Isomers of 2,3- and 2,4-Dimethylcyclobutanones1 John Metcalfe and Edward K. C. Lee*
Contribution from the Department of Chemistry, Unicersity of California, Irvine, California 92664. Received October I O , 1972 Abstract: The triplet energy transfer rates from C6H6 ( 3 B ~ u84.4 , kcal/mol) to cyclobutanone (CB), chlorocyclobutanone (Chloro CB), and the cis and trans isomers of 2,3- and 2,4-dimethylcyclobutanones(DMCB) have been measured in the gas phase. These ketones have their triplet energies (ET)below 84 kcal/mol. The collisional electronic quenching efficiencies range from 0.01 for CB to 0.4 for Chloro CB, and a n explanation for the quenching
based on the exchange mechanism is given. The degree of the substituent effect on the uQ2(triplet) by a-CHa substitution ( 2 X/CH3) is much less than a-C1 substitution on the aQ2(triplet). This positive a-CHa-substituent effect on the aQ2(triplet)cannot be due to “steric hindrance” which seems to operate on the uQ*(singlet).
T
he mechanism a n d rates of the intermolecular triplet electronic energy transfer processes involving the benzene (3B1,, 84.4 kcal/mol) d o n o r a n d a variety of simple 7r-bonded acceptor molecules i n t h e gas phase have been studied extensively i n the past 10 years.?-fi The triplet energy transfer process c a n be used to evaluate t h e yield of the aromatic triplet state either by t h e sensitized phosphorescence emission f r o m biacety12 or b y the sensitized cis-trans isomerization of a 2-butene isomer3 a n d critical reviews o n this subject a r e available i n 1iteratu1-e.~ A review dealing with t h e application of these methods a n d t h e singlet benzene (‘Bsu. 109 kcallmol) sensitization t o the kinetic studies of t h e photochemistry of carbonyl c o m p o u n d s is also available.a The present study is a brief extension of o u r earlier systematic study o n the rates a n d mechanisms of t h e intermolecular triplet energy transfer in the gas phase,jl9 a n d it specifically examines the effects of the a-chlorine substitution 2 a n d t h e a-methyl substitutions 3-6 on
1
2
3
4
5
6
the cyclobutanone ring 1. Furthermore, t h e a-substituent effects observed i n the triplet benzene sensitization of these cyclobutanones here will be compared with t h e steric hindrance (due to a-methyl substitutions) (1) Acknowledgment is made to the donors of the Petroleum Re-
search Fund administered by the American Chemical Society for the financial support of this research. (2) H. Ishikawa and W. A. Noyes, Jr., J . Chem. Phys., 37, 583
observed in the singlet benzene sensitization of aliphatic ketones. lo Experimental Section Cyclobutanone (Aldrich Chemical Co.) was used after degassing in Cacuo at liquid nitrogen temperature, since it was of sufficient purity ( > 9 9 7 3 for this study. Chlorocyclobutanone (2) was prepared by the method used by Durig and Green” and purifed by vpc as described in detail elsewhere.12 The four isomers of dimethylcyclobutanones (3-6) were prepared and purified in the manner described earlier.I3 Zone-refined grade benzene (Litton Chemicals, >99.999 %) and cis-2-butene (Phillips Research grade, > 9 9 . 9 % ) were used after degassing iu cucuo at liquid nitrogen temperature. Samples were handled on a glass-metal vacuum line free from grease and mercury, and photolysis mixtures were contained in a quartz cylindrical cell of 5.0-cm od and 5.0-cm length. Irradiation of the samples was at 253.7 nm, using a low-pressure mercury grid lamp (Mineralight R51, Ultraviolet Products, Inc.) with a Corning CS-7-54 filter and a 1.0 cm thick DJP filter solution as described elsewhere.’‘ The product separation by vpc was carried out on a DMS column (3097,, 0.25-in. 0.d. by 25 ft, 23”) for the hydrocarbone (C2,C3,and Cj’s from 1 and 3-6)13 or on a Carbowax-300 column (20%, 0.25-in. 0.d. by 25 ft, 23”) for the chlorohydrocarbons (C, and C3’s from 2).12 The product yields were determined with a hydrogen flame ionization detector (Perkin-Elmer, F-11)and a 1-mV potentiometer recorder with a Disc integrator. Results T h e following mechanism should be considered for the competitive quenching of the triplet benzene (3C6H6o r 3Blu) by cis-2-butene a n d t h e substituted cyclobutanones (XCB). unimolecular decay: ‘CaH6 +C6H6 self-quenching: 3C6He C6H6 +2C6H6 butene-quenching: 3C6H6 + cis-B +C6H6 + j(cis-B)
+
XCB-quenching:
(1962).
(3) (a) R. B. Cundall, F. J. Fletcher, and D. G. Milne, Trans. Faraday
(8) E. Isensitization of the DMCB isomers (3-6) are cis- and trans-dimethylcyclopropanes (DMCP, 7
+
+
+
(16) (a) C . S. Parmenter and B. L. Ring, J . Chem. PhJ)s.,46, 1998 (1967); ( b ) see footnote 23 ofref 15. (17) C. S. Burton and H. E. Hunziker, Chem. Phj.5. Lett., 6, 352 (1970).
Journal qf
the American Chemical Society
/ 95.6
(8)
(XCB) and 8, respectively), l 3 whereas the corresponding "primary" product from Chloro CB (2)'* is chlorocyclopropane (Chloro CP, 9) which is uibrationally hot.
7
I
1.43 X lo4 sec-I
8
9
Some unimolecular decomposition products from this vibrationally hot Chloro CP are formed at low pressures ( 6 2.55 Torr of total pressure): 3-chloropropene ( 6 15%), cis- and trans-1-chloropropene (
