T H E
J O U R N A L
OF
PHYSICAL CHEMISTRY
Registered in U.S. Patent Ofice @ Copyright, 1972, b y the American Chemical Society
VOLUME 76, NUMBER 10 MAY 11, 1972
Production of Triplet Methylene in the Reaction of Carbon Atoms with Hydrogen by Timothy L. Rose' Tezas A&M University, College Station, Team 778&
(Received November 10, 19'71)
Publication costs assisted by the Petroleum Research Fund
The reactions of free carbon atoms in hydrogen-cyclopentadiene samples were studied using the nuclear recoil technique. A comparison of these hot-atom results with photochemical methylene studies shows that the carbon atoms react with hydrogen to produce methylene which is almost entirely in the triplet state. Recently several papers have appeared concerning the reaction of carbon atoms with h y d r ~ g e n . ~Ground,~ state C(3P) atoms produced by photolysis of GO2 react rapidly with hydrogen.2" It was assumed that vibrationally excited methylene (CHZ*) was formed which is then collisionally stabilized. Carbon atoms produced by the nuclear recoil technique indeed react with hydrogen to give methylene. The spin state of the methylene produced was not identified although the importance of 3CH2 was p o ~ t u l a t e d . ~The ~ present paper presents evidence that the methylene produced in the recoil system is almost entirely in the triplet state. This result is in contrast to the situation for hot carbon atoms reacting in alkane systems where the methylene formed is predominantly in the singlet ~ t a t e . ~The ~ , recoil ~ technique with hydrogen present, therefore, may prove to be a useful source of triplet methylene for studies on the effect of spin state on methylene reactions, a subject of current interest in the The nuclear recoil methodg was used to produce hot radioactive carbon-11 atoms (20.5-min half-life) in a 20 : 1 hydrogen-cyclopentadiene (CPD) sample. Since the reactions of free llC with CPD have been characterized previously,1o changes in the product spectrum were attributed to trapping by the CPD of radicals formed in the reaction of carbon with hydrogen. The products formed were identified and assayed by standard radio gas chromatographic techniques.10-12 Absolute yields of the products were determined using ethane
as an external monitor of the number of carbon atoms p r 0 d ~ c e d . l ~No determination was made of products having more than six carbon atoms. Table I lists the products whose yields increased when hydrogen was added to the CPD. The other products formed which are not listed are attributed to the direct reaction of carbon with CPD.10r12 The effect (1) National Institutes of Health Predoctoral Fellow, 1965-1967. (2) (a) W. Braun, A. M. Bass, D. D. Davis, and J. D. Simmons, Proc. Roy. Soc., Ser. A , 312,417 (1969); (b) F. F. Martinotte, M. J. Welch, and A. P. Wolf, Chem. Commun., 115 (1968). (3) (a) C. MacKay, J. Nicholas, and R. Wolfgang, J . Amer. Chem. Soc., 89, 5758 (1967); (b) R. F. Peterson, Jr., Ph.D. Thesis, Yale University, 1970. (4) M. J. Welch and A. P. Wolf, J. Amer. Chem. SOC.,91, 6584 (1969). (5) T. W. Eder and R. W. Carr, Jr., J. Phys. Chem., 73, 2074 (1969); J . Chem. Phys., 53, 2258 (1970). (6) (a) P. 8.T. Lee, R. L. Russell, and F. S. Rowland, Chem. Commun., 19 (1970): (b) R. L. Ruaaell and F. S. Rowland, J. Amer. Chem. SOC.,90, 1671 (1968). (7) W. G. Clark, D. W. Setser, and E. E. Siefert, J . Phys. Chem., 74, 1670 (1970). (8) D. F. Ring and B. 5. Rabinovitch, Int. J. Chem. Kinet., 1, 11 (1969); Can. J . Chem., 46, 2436 (1968). (9) (a) C.MacKay and R. Wolfgang, Science, 148, 899 (1966); (b) A. P.Wolf, Advan. Phys. Org. Chem., 2 , 210 (1964). (10) T. Rose, C. MacKay, and R. Wolfgang, J . Amer. Chem. Soc., 8 8 , 1064 (1966). (11) R. Wolfgang and C. MacKay, i?uct?eonics, 16 (lo),69 (1968). (12) T . Rose, Ph.D. Thesis, Yale University, 1967. (13) (a) J. Dubrin, C. MacKay, M. L. Pandow, and R. Wolfgang, J . Inorg. Nucl. Chem., 26, 2113 (1964); (b) G.StGcklin and A. P. Wolf, J. Amer. Chem. Soc., 85, 229 (1963).
1389
1390
TIMOTHY L. ROSE
Table I : Yields of W-Labeled Products from Reactions of
1%
with Cyclopentadiene-Hydrogen Mixtures" Yields as % ' total 'IC atoms Sam&
c
Products
Carbon dioxide Carbon monoxide Methane 3-Methylcyclopentene Bicyclo [3.1.0]hex-2-ene trans-1,3,5-Hexatriene cis-l13,5-Hexatriene 1,3-Cyclohexadiene
---
CPD (30 om)
CPD (4 om) HZ(76 om)
C P D ( 0 . 4 om) HZ( 7 . 6 om)
CPD (3.8om) HZ(76 om) 0%(0.2 om)
n.d.b 0.7f0.4
n.d. 0 . 3 f 0.1' 6 . 8 It 0.6 2 . 1 2c 1.3 13.3 f 1 . 0 2.0 f0 . 2 2.4 f0.4 1.4 f0.4
n.d. 3 . 2 & 0.6" 4 . 6 2c 0.9 10.2 2c 1 . 7 3 . 8 =k 0 . 2 3 . 5 i.0 . 7 4.1 f0.2 1 . 6 f 0.1
7.72c2.1 11.03~ 1.6 0 . 7 2c 0 . 1
