D. C. Tardy, A. S.Gordon, and W. P. Norris
1398
however, show that multistep deactivation yields values of ( E ) which are energetically allowable for each of the four frequency models, and hence reveals another aspect of the inadequacy of the strong collision hypothesis in methylene chemical activation systems. Taken alone, the calculations done here allow no basis for discrimination among the three published values of the preexponential factor. The values of ( E ) obtained as a "best fit" t o the data for each of the models and collision numbers can be related to the sum of the heat of formation of CHp(IA1) plus the excess photon energy carried by methylene a t the time of addition through Afff0o(CHd1Ai))+ E,, = ( E ) - E t h - ufoo(C-C4H8)-k Nf"o(MCB) (9) The results are shown in Table V. Although E,, at 334 nm is unknown, it must be small, probably no more than 1-2 kcal/ mol a t most. Thus the data are consistent with a range of values of AHfo0(CH2(lA1)),and it is quite apparent that nothing very precise can be learned about its value from this type of experiment. The data are not inconsistent with either the low value previously put forward from this laboratory, or the somewhat higher value reported in a previous RRKM study.18
Acknowledgment. This work was supported by the U.S. Energy Research and Development Administration under Contract No. E ( l l ) - l 2026.
References and Notes USERDA Document No. COO-2026-25, (a) D. W. Setser, Phys. Chem. Ser. One, 1972-1973, 9, 11 (1972); (b)R. W. Carr and M. G. Topor. J. Chem. Phys., 58, 757 (1973). J. D. Rynbrandt and B. S.Rabinovitch, J. Phys. Chem., 74, 1679 (1970). H. M. Frey, G. R. Jackson, M. Thompson, and R. Waish, Trans. Faraday SOC., 69. .., -2054 - - . (19731 J. W. Simons, W. L. Hase, R. J. Phillips, E. J. Porter, and F. B.Growcock, lnt. J. Chem. Kinet., to be published. A. F. Pataracchia and W. D. Walters, J. Phys. Chem., 68, 3894 (1964). T. F. Thomas, P. J. Conn, and D. F. Swinehart, J. Am. Chem. SOC., 91,761 1 (1969). M. N. Das and W. D. Walters, Z.Phys. Chem., 15, 22 (1958). A. D. Jenkins, J. Chem. SOC.,2563 (1952). R. L. Russell and F. S.Rowland, J. Am. Chem. Soc., 90, 1671 (1968). R. L. Russell, Ph.D. Thesis, University of California, Irvine, Calif.. 1971. P. J. Robinson and K. A. Holbrook, "Unimolecular Reactions", Wiley, New York, N.Y., 1972. W. Forst, "Theory of Unimolecuiar Reactions", Academic Press, New York, N.Y., 1973. T. Beyer and D. F. Swinehart, Commun. ACM, 18,379 (1973); S.E. Stein and B. S.Rabinovitch, J. Chem. Phys., 58, 2438 (1973). G. Z. Whitten and B. S.Rabinovitch, J. Chem. Phys., 38, 2466 (1963). M. Hoare, J. Chem. Phys., 38, 1630 (1963). Reference 13, p 214. W. L. Hase, R. J. Phillips, and J. W. Simons, Chem. Phys. Lett., 12, 161 (1971). I
-I
Transition State Characterization for the Hydrogen Elimination from 1,4=Cyclohexadiene D. C. Tardy,*' A. S. Gordon, and W. P. Norris Michelson Laboratory, Naval Weapons Center, China Lake, California 93555 (Received November 2 1, 1975) Publication costs assisted by the Naval Weapons Center
The gas phase thermal decomposition of various isotopically substituted 1,4-cyclohexadienes was studied in the 275-375 "C temperature region to obtain transition state information for the Woodward-Hoffman symmetry allowed hydrogen elimination reaction. Competitive decompositions with (i) a 1:l mix of 1,4-cyclohexproduced (i) adiene and perdeuterio-1,4-cyclohexadieneor (ii) 1,2,3,4,5,6-hexadeuterio-1,4-cyclohexadiene H2 and D2 or (ii) H z , Dz, and HD. The difference in E , for H2 and D2 elimination was 2.1 f 0.4 and 2.4 f 0.8 kcal/mol for systems (i) and (ii),respectively; the difference in E , for H2 and H D was 1.5 & 0.4 kcal/mol. The results indicate a tight transition state which resembles benzene in its vibrational frequency assignment while the reaction coordinate is predominately the concerted breaking of two carbon-hydrogen bonds.
The decomposition of l,4-cy~lohexadiene~ has previously been reported. The only detectable reaction is the homoge-
neous first-order intramolecular stereospecific elimination of molecular hydrogen3
The reaction has been studied in the unimolecular high pressure region and the temperature dependence of the rate constant between 330 and 390 "C is best described by the equation The Journal of Physical Chemistry, Vol. 80,No. 13, 1976
log h = 12.4
43 so0
- ~. 2303 RT
where R is 1.98 cal/K mol. The activation energy ( E , = 43.8 kcal/mol) for this elimination reaction is approximately 18 f 3 kcal/mol lower than the reported Ea's of analogous Woodward-Hoffman allowed hydrogen elimination reactions, Le., c y ~ l o h e x e n e(~E, ,~= 62 f 3 kcal/mol) and cis-butene-26 ( E , = 65 f 2 kcal/mol).6s7 The present work was undertaken to obtain information of the A and E factors which are involved in the reaction process and to further test the concerted mechanism for the reaction.
Hydrogen Elimination from 1,4-Cyclohexadiene
6,01
7.0
I
I
I
I
I
1399 I
I
I
--
c
A
5.0
4.0
t
3.0
t -II
I
4
I 2.0
2ol Q
I
t
k
0
o
I-
-201
I
REACTION COORDINATE
Figure 2. Potential energy versus reaction coordinate for the reactions:
! O- - H 2 t
I I
115
I
I
I
1.7
1.6
iiT
I
I
1.8
103
Figure 1. Yield plots of H2/D2for intermolecular (11) and intramolecular (I) comparisons and H2/HD for intramolecular (111) comparison as a function of temperature. The least-squares analysis (solid lines) yielded slopes (kcal/mol) and intercepts of: (I) -2.38 f 0.8,0.67 f 0.4: (11) -2.06 f 0.4, 0.67 f 0.4;(Ill)-1.46 f 0.4, 0.33 f 0.2.
These particular objectives can be obtained through the studies of isotopically substituted cyclohexadienes. Two studies will be reported in this note: (i) The intermolecular competitive eliminations of various hydrogens from a 1:l mix of 1,4-cyclohexadiene (CH)s and perdeuterio-1,4-cyclohexadiene(CD).8 No HD was detected in conformity with the intramolecular molecular hydrogen elimination mechanism of Alfassi et al.7 (ii) The intramolecular competitive elimination of Hz, HD, and D2 from 1,2,3,4,5,6- hexadeuterio - 1,4 -cyclohexadiene (CHD).8,9 Most of the pyrolyses were done between 275 and 375 "C a t pressures of approximately 600 Torr in a seasoned vessel with a surface-to-volume ratio of 1.3 cm-l. A few pyrolyses were carried out with a surface-to-volume ratio of 6.1 cm-l and showed the reaction to be homogeneous. The hydrogen isotopes were quantitatively analyzed with amass spectrometer calibrated with known compositions of H2, HD, and Dz. The hydrogen ratios were found to be independent of reaction time and pressure. Assuming Arrhenius behavior, a two-parameter (the activation energy difference AE and A factor ratio AA) leastsquares analysis of the data in Figure 1 was performed. For the H2 and D2 elimination A E is -2.06 f 0.41° and -2.38 f 0.8 kcal/mol1° while AA is 0.67 f 0.41° and 0.67 f 0.41° for the intermolecular and intramolecular comparisons, respectively. No quantitative conclusions on the fact that the intramolec-
0
(-)
For comparatlve purposes the zero point energies of a and b in the boat forms are taken as equal, while the zero point energy of a in the half boat form is arbitrarily taken as zero.
ular comparison exhibits the largest isotope effect will be made since within experimental error there is no difference in AA or AE for the two comparisons. However, the magnitude of aE suggests a mechanism where the C-H stretch participates in the reaction coordinate since the difference in zero point energies (ZPE) of a C-H and C-D stretch is -1.2 kcal/mol as compared to that of -0.6 kcal/mol for the respective bends. For the CHD system AA(Hz/HD) is experimentally determined to be one-half of AA(H2IDz). This is in agreement with the fact that the CHD was not formed stereospecifically and thus there are twice as many isomers producing HD as compared to Hz or Dz. AE(Hz/HD) is approximately half of AE(H&) as to be expected by a simple difference in ZPE. An analogous isotope effect in the thermal decomposition of cyclopentene has been brought to our attention by one of the referees. Knechtll reported AE(Hz/Dz) and AE(Hs/HD) of -2.50 and -1.25 kcal/mol while AA(Hz/D2) and AA(Hz/ HD) were 0.63 and 0.99, respectively. These values are remarkably similar to our results even though the activation energy in the cyclopentene system is 57.8 kcal/mol. Knecht's results for AA(Hz/HD) may be in error since his value was obtained from a 23.3% cyclopentene-d, impurity in the cyclopentene-ds. The previously reported A factors4s5suggest a tight transition state (TS). A TS theory calculation in which the cyclohexadiene ring modes were tightened to simulate those of benzene and the reaction coordinate was taken as the carbon hydrogen stretch, gives a H2/D2 yield ratio which is greater for the intramolecular comparison, in accordance with the experimental data. This was not true for other models. Hence, the data support a tight T S which resembles benzene in its vibrational frequency assignments while the reaction coordinate is predominately the breaking of carbon hydrogen bonds in the 3 and 6 positions of the cyclohexadiene ring. The allowed cis elimination requires that the hydrogen atoms in the 3 and 6 positions must be either sufficiently close for a bonding interaction or sufficiently extended for the o electrons associated with the C-H bond t o be delocalized in The Journal of Physical Chemistry, Vol. BO, No. 13, 1976
1400
A. S. Gordon, D. C. Tardy, and R. lreton
the K electron system of the nascent benzene. In either case the ring conformation of the TS must have boatlike characteristics. From x-ray datal2 the stable conformation of CH has the nonvinylic carbon atoms approximately 156" from the plane formed by the vinylic carbon atoms, thus supporting a boatlike ground state. With this geometry the hydrogens are approximately 3 from one another, too large for sufficient bonding overlap to give T S stabilization. A discussion of the energetics and T S of the elimination in the CH, cyclohexene, and cyclopentene systems is presented in ref 4b. Since strain energy builds very rapidly with displacement, the observed E , cannot bring these H's close enough to interact. Hence, the relatively low E, for the reaction must reflect the influence of the K bonds on partly decoupling the u bonds of the allylic hydrogens. The difference of 18 kcal/mol between the Ea's for the analogous eliminations of H2 from CH and cyclohexene can be understood with an energy profile as depicted in Figure 2. A difference of 6 kcal/mol13 in E , can be accounted for by the conformation difference of the reactants since cyclohexene is predominately in a half-boat configuration. The remaining difference of 12 f 3 kcal/mol must be due to stabilization of the TS by about one-third of the benzene resonance energy
of 35.95 kcal/mol.14 It is interesting that this resonance energy is present when the TS is in a boat configuration.
References and Notes (1) Assoclate Professor, University of Iowa, Iowa City, Iowa 52240. (2)(a) R. J. Ellis and H. M. Frey, J. Chem. SOC.A, 553 (1966);(b) S.W. Benson and R. Shaw, Trans. Faraday SOC.,63,985(1967). (3) I. Fleming and E. Wildsmith, Chem. Commun., 223 (1970). (4)(a) S.R . Smith and A. S. Gordon, J. Phys. Chem., 65,1125 (1961).(b) The study in (a) has been pursued in greater detail by Ireton, Gordon, and Tardy (manuscript in preparation) and gives E, = 61.6kcal/moi and not 71.2 kcal/moi as reported in (a). (5)S.W. Benson and H. E. O'Neai, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand.,
No. 21 (1970). (6)A. Lifshitz, S. H. Bauer, and E. L. Resler, J. Chem. Phys., 38,2056 (1963). (7) 2 . E. Alfassi, D. M. Golden, and S. W. Benson, lnt. J. Chem. Klnet. 5,991 (1973). (8) Purified by gas chromatography. (9)Synthesized by the Birch reduction of perdeuteriobenzene. Labeling purity was checked by I3C and IH NMR and found to be more than 99 mol % D. (IO)The uncertainties stated correspond to a 95% confidence limit. (11) D. A. Knecht, J. Am. Chem. SOC.,95,7933 (1973). (12)H. Oberhammer and S. H. Bauer. J. Am. Chem SOC.,91, 10 (1969). (13)(a) R. Bucourt and D. Hanaut, Bull. SOC.Chem. Fr., 4563 (1967);(b) R. Bucourt in "Topics in Stereochemistry." Vol. 8, E. L. Eliel and N. L. Aliinger, Ed., W h y , New York, N.Y., 1974. (14)J. D. Cox and G. Pilcher, "Thermochemistry of Organic and Organometallic Compounds", Academic Press, New York, N.Y., 1970.
Ethyl Radical Isomerization. A 1,2=Hydrogen(Deuterium) Shift in the Pyrolysis of l,l,l-Trideuterioethane A.
S.Gordon, D. C. Tardy,"
and R. Ireton'
Research Department, Chemistry Division, Naval Weapons Center, China Lake, California 93555 (Received January 14, 1976) Publication costs assisted by the Naval Weapons Center
l,l,l-Trideuterioethane has been pyrolyzed in the temperature range 500-600 "C. The reaction products are formed via chains initiated by C-C bond cleavage of ethane, but the hydrogen and deuterium content of the ethylene products and the Hz, HD, and Dz yields are not predicted by the conventional mechanism. The results may be understood in terms of a mechanism involving isomerization of ethyl radical by a 1,2-H (or -D) shift along the carbon skeleton competing with abstraction from the parent and decomposition. The kinetic evidence suggests that the E , for decomposition of ethyl radical and the 1,2 isomerization are within 1.5 kcal/ mol, the difference in rate constants reflecting the difference in preexponential factors.
Introduction The intramolecular shift of hydrogen in free radicals has been well documented2 in the liquid and gas phases for transfers involving cyclic transition states larger than five members; i.e., 1,4- and 1,5-hydrogen shifts via five- and sixmembered transition states, respectively, have been observed. These studies have been performed using both thermal and chemical activation techniques. Due to geometrical considerations (i.e., strain energy) 1,2and 1,3-hydrogen shifts should have high energy barriers as compared to 1,4 and 1,5 shifts; barriers between 18 and 1 2 kcal/mol have been measured for 1,4and 1,5 shifts. The small
* Address correspondence to this author at the Department of Chemistry, University of Iowa, Iowa City, Iowa 52242. The Journal of Physical Chemistry, Vol. 80, No. 13, 1976
rate constant associated with these high barriers have large experimental uncertainties and hence, the evidence is somewhat controversial. Jackson and McNesby3 have studied the thermal isomerization of labeled isopropyl radicals and concluded that the 1,2 isomerization rate constant is approximately 9 f 2%of the rate constant for breaking a carbon-hydrogen bond to give a hydrogen atom and propylene between 472 and 553 "C. However, Heller and Gordon4 have also studied the thermal decomposition of isopropyl radicals generated by the photolysis of diisopropyl ketone, and their results suggest that approximately 50% of the isopropyl radicals isomerize. Both studies revealed that the fraction isomerized was independent of temperature. Kerr and Trotman-Dickensonj have studied the isomerization of the isobutyl radical, and their results
