J . Am. Chem. Soc. 1989, 111, 4198-4209
4198
Ab Initio Studies of the C3H4Surface. 3. Thermal Isomerization M. Yoshimhe,* J. Pacansky, and N. Honjout Contribution from the IBM Almaden Research Center, San Jose, California 951 20. Received October 3. 1988
Abstract: SCF, MCSCF, and CI calculations have been carried out to study thermal interconversions occurring on the singlet
C3H4 surface. Transition states and reaction paths between the pairs of possible isomers, methylacetylene, allene, cyclopropene, propenylidene, vinylmethylene, and cyclopropylidene were determined. In addition, the zero-point energies were calculated, and the activation energies for pertinent reactions were evaluated. The thermal rearrangement of allene to methylacetylene was found to proceed in four steps via vinylmethylene, cyclopropene, and propenylidene with the activation energy of 65.8 kcal/mol, which is in good agreement with the observed values of 60.5 and 63.8 kcal/mol. The same reaction paths can also apply to pyrolysis of cyclopropene, in which it undergoes conversion to methylacetylene via propenylidene more easily than to allene via vinylmethylene; the calculated activation energies are 38.1 and 43.4 kcal/mol, respectively. These are again in excellent agreement with the observed values of 37.5 and 43.3 kcal/mol. The activation energies for the allene to cyclopropylidene and the reverse conversions were calculated to be 72.2 and 10.2 kcal/mol. This indicates that cyclopropylidene may not be involved in the interconversion of allene, cyclopropene, and methylacetylene. One of the significant findings in this study is the reaction path for the cyclopropene to methylacetylene conversion via propenylidene, which is less energy demanding than that via vinylmethylene. This made the calculated mechanisms in accord with the experimental data. Furthermore, we will present and discuss reaction mechanisms for pyrolysis of singly and doubly substituted cyclopropene in which this particular reaction path is expected to play a dominant role.
I. Introduction
Scheme I
Many of the mechanisms for the thermal interconversion of isomers on the C3H4 surface play a central role in organic chemistry. Thus, they not only serve as models for much larger systems but also provide a fundamental basis for reactions of hydrocarbons. In Scheme I (see also Figure 1) a number of possible thermal reactions are shown that provide the basis for our a b initio study. These mechanisms for thermal interconversions of cyclopropene, allene, and methylacetylene involve ring opening and closure reactions, 1,2- and 1,3-hydrogen shifts, formation of double and triple bonds, and reactive intermediates like the carbene and diradical forms of vinylmethylene. Although many have been conducted to elucidate kinetics and mechanisms for the possible reactions shown in Scheme I, their details have not been e~tablished.~For example, gas-phase pyrolysis experiments indicate that the conversion of allene and methylacetylene goes through cyclopropene and that cyclopropene undergoes bond fission to form an intermediate that produces allene and methylacetylene.2d,e A convenient structure for this intermediate, proposed by Bailey et a1.,2eis the 1,3-diradical form of vinylmethylene. Photochemical interconversion of the C3H4isomers in low-temperature matrices indicated that the stable isomers also interconvert through a common intermediate, for which the bisected diradical form of vinylmethylene has been proposed.3a Consequently, since a number of experimental studies propose a “vinylmethylene” intermediate, then a key to understanding the chemistry on the C3H4surface may be to characterize the structure of vinylmethylene. As Steinmetz et aL4have pointed out, the experimental evidence for the involvement of vinylmethylene is indirect, and its structure and bonding are not well established due to the possibility of equilibration among various structures and electronic states possible for vinylmethylene. O n the other hand, previous theoretical studies5-I0 which employed SCF, GVB, and simple CI wave functions indicated that the stable structures for both triplet and singlet are planar carbene species. However, our extensive M C S C F and multireference C I study on the potential energy surface1IJ2located six local minima which are different from the previous results. The lowest two of these minima belong to the 3A‘’ states of trans and cis planar vinylmethylene and are isoen‘Present address: IBM Tokyo Research Laboratory, Tokyo 102, Japan.
CH2=C=CH2
- C.?
f
\\ % s,
-
[CH3CH=C:]
(Path 3 )
1 2 H-shift
CH3C-CH
(Path 4 A . B )
ergetic lying 46 kcal/mol above the ‘Al state of methylacetylene. Their structures are allylic with the C-C bond lengths of 1.39 (1) (a) Hutton, R. S.; Manion, M. L.; Roth, H. D.; Wasserman, E. J . Am. Chem. SOC.1974,96,4680. (b) Palmer, G. E.; Bolton, J . R.; Arnold, D. R. Ibid. 1974, 96, 3708. (c) Chapman, 0. L.; Chedekel, M.; Pacansky, J.; Rosenquist, N.; Roth, R.; Sheridan, R. S., unpublished. (2) (a) York, E. J.; Dittmar, W.; Stevenson, J. R.; Bergman, R. G. J . Am. Chem. SOC.1973, 95, 5680. (b) Bradley, J. N.; West, K. 0. J . Chem. SOC., Faraday Trans. I 1975, 71,967. (c) Lifshitz, A,; Frenklack, M.; Burcat, A. J . Phys. Chem. 1975,79, 1148. (d) Walsh, R. J . Chem. SOC.,Faraday Trans. 1976, 72, 2137. (e) Bailey, I. M.; Walsh, R. J . Chem. SOC.,Faraday Trans. 1 1978, 74, 1146. (0 Hopf, H.; Priebe, H.; Walsh, R. J . Am. Chem. SOC. 1980, 102, 1210. (3) (a) Chapman, 0. L. Pure Appl. Chem. 1974, 40, 5 1 1.
(b) Arnold, D. R.; Humphreys, R. W.; Leigh, W. J.; Palmer, G. E. J . Am. Chem. SOC.1976, 98, 6225. (4) Steinmetz, M. G.; Srinivasan, R.; Leigh, W. J. Reu. Chem. Inrermed. 1984, 5 , 57. (5) (a) Hoffmann, R.; Zeiss, G. D.; Van Dine, W. J . Am. Chem. SOC. 1968, 90, 1485. (b) Davis, J. H.; Goddard 111, W. A,; Bergman, R. G. Ibid. 1976, 98, 4015; 1977, 99, 2424. (c) Sevin, A,; Arnaud-Danon, L. J . Org. Chem. 1981, 46, 2346. (d) Feller, D.; Borden, W. T.; Davidson, E. R. J . Phys. Chem. 1983, 87,4833. (e) Chang, C. S . C. J . Chem. SOC.,Faraday Trans. III 1976, 72, 456. (6) (a) Binkly, J. S.; Pople, J . A.; Hehre, W. J. Chem. Phys. Lefr. 1975, 36, 1. (b) Nomura, 0.; Iwata, S . J . Chem. Phys. 1981, 74, 6830. (7) (a) Schaad, L. J.; Burnelle, L. A,; Dressler, K. P. Theor. Chim. Acra 1969, 15, 91. (b) Dykstra, C. E. J . Am. Chem. SOC.1977, 99, 2060. (c) Staemmler, V. Theor. Chim. Acra 1977, 45, 89. (d) Rauk, A,; Drake, A. F.; Mason, S. F. J . A m . Chem. SOC.1979, 101, 2284. (e) Lam, B.; Johnson, R. P. J . Am. Chem. SOC.1983, 105, 7479.
0002-7863/89/1511-4198$0l.50/00 1989 American Chemical Society
J. Am. Chem. SOC.,Vol. 111, No. 12, 1989 4199
Ab Initio Studies of the C3H4Surface H1 H2
&C l - C 2 E C 3 - - H 4
H1
\
/
H3
/
H3
c1=c2=c3\\''1'
H4
Hi
1 . methylacetylene
( M A . CQ")
H1
H2
H4
two parts of path 1, the first involves a transition state with a C1 symmetry (path 1A) and the second with a C, symmetry (path 1B). the ring closure reaction paths which are designated as path 2A and 2B initiate a t cis- and tram-vinylmethylenes, respectively. Because propenylidene was found to lie 15 kcal/mol below vinylmethylene," one must consider two possible paths for the cyclopropene to methylacetylene conversion. The most likely one, the lowest energy path, would be via propenylidene path 4
path 3
propenylidene
cyclopropene and the other \ H4
/ H3
3 cyclopropene (CP.C*")
4
cyclopropene
5 . cyclopropylidene (C'J C2")
H3
H3
I
I
H2
H2
vinylmethylene
7. cis vinylmethylene
(TV, Cy)
(CV, C,)
A
H4
Figure 1. Structures of possible C3H4isomers. Abbreviation and symmetry of each isomers are given in parentheses.
A and are global minima.
The other four minima belong to singlet trans- and cis-vinylmethylenes (plus their steroisomers) and are also isoenergetic to each other lying 12 kcal/mol above triplet vinylmethylene. These singlet species also have allylic-like structures with the terminal hydrogen above or below the plane of the three carbon atoms (see 6 and 7 in Figure 1). It is also found that the potential energy barriers for the ring closure reaction to cyclopropene are small (1-5 kcal/mol) implying that singlet vinylmethylenes are thermally unstable. Thus, if these were the intermediates involved in the allene to methylacetylene isomerization, then the ring closure to cyclopropene is also a highly competitive process. It has been conjectured that the possible mechanismZefor the interconversion of allene and methylacetylene would be via vinylmethylene with the ring closure to cyclopropene as a competing reaction. Gas-phase pyrolysis experimentsZeof cyclopropene indicated that the cyclopropene to methylacetylene conversion to be a faster process, as the experimental activation energies in kcal/mol are shown below. Therefore, the overall rate-deterallene
43.3
cyclopropene
37.5
vinylmeth ylene
path 5
methylacetylene
In addition, we also investigated a direct path from allene to methylacetylene (path 6) involving a 1,3-H shift, which was
propenylidene (PD. C,)
6. trans
path 2
/
H3
methylacetylene
methylacetylene
mining process would be the conversion of vinylmethylene to allene or vice versa. W e shall label this path 1 and we have investigated (8) (a) Peyerimhoff, S. D.; Buenker, R. J. Theor. Chim. Acta 1969, 14, 305. (b) Kao, J.; Radom, L. J . Am. Chem. SOC.1978, 100, 379. (c) Wiberg, K. B.; Wendoloski, J. J. Ibid. 1978, 100, 723. (d) Gordon, M. S. Ibid. 1980, 102, 7419. (9) (a) Dillon, P. W.; Underwood, G. R. J . Am. Chem. SOC.1974,96,7'19; 1977, 99, 2435. (b) Baird, N.C.; Taylor, K. F. Ibid. 1978, 100, 1333. (c) Pasto, D. J.; Haley, M.; Chipman, D. M. Ibid. 1978, 100, 5272. (d) Minato, T.; Osamura, Y.; Yamabe, S.;Fukui, K. Ibid. 1980, 102, 581. (10)Honjou, N.;Pacansky, J.; Yoshimine, M. J . Am. Chem. SOC.1985, 107, 5332. (1 1) Honjou, N.;Pacansky, J.; Yoshimine, M. J . Am. Chem. SOC.1984, 106, 5361. (12) Yoshimine, M.; Pacansky, J.; Honjou, N. J . Am. Chem. SOC.1989, 111, 2785-2798.
proposed as the reaction mechanism for this interconversion before the cyclopropene involvement was discovered. The paths are labeled in Scheme I. Another interconversion of interest of the C3H4surface involves cyclopropylidene. This ring-opening reaction to allene has been studied theoretically in connection with the synthesis, structure, and chemical reactivity of allene.9 W e have also studied this path (path 7), and, furthermore, the conversion of cyclopropylidene to cyclopropene (path 8) was investigated in order to gain insight into experimental evidence that photolysis of cyclopropylidene always produces allene but not c y ~ l o p r o p e n e . ~ ~ In this paper we report the details of SCF, MCSCF, and CI calculations on the reaction paths, transition-state structures, potential energy barriers, and activation energies for the interconversions mentioned above and illustrated in Scheme I. The mechanistic details of the various reactions occurring on the C3H4 surface are also discussed and summarized. Finally, generalization of the mechanism for pyrolysis of cyclopropene emerged from the present study to methyl-substituted cyclopropene are presented. Before embarking onto a detailed discussion on the C3H4 surface, a few comments on the notation used are pertinent. Structures of possible isomers and intermediates are sketched in Figure 1, where the abbreviation and symmetry of each isomer are given. In the text and tables, the isomers will be referred by a number or by an abbreviation written in capitalized italic letters. A number of points are computed along reaction paths and are listed in tables and discussed in the text. These are labeled by a capitalized italic letter or letters for a particular reaction path followed by a number indicating the particular point on the path. Thus, the points computed along the path 1A listed in Table I are lA.lfor the first, 1A.2for the second, etc. Transition states are also labeled by the path name prefixed by TS in capitalized italic letters as TSlA,TSlB. In addition, acronyms are used to specify a particular a b initio method (e.g., SCF, MCSCF), and these are also capitalized in nonitalicized letters to distinguish them from the computed structures.
11. Computational Procedure In order to locate transition states, we first selected an appropriate reaction coordinate for each rearrangement, and then a path was determined by optimizing the geometry at selected values of the reaction coordinate with all other geometrical parameters relaxed. When an approximate transition-state structure was located, its structure was refined using the SADDLE POINTprogram in the GAMESS program system.13sL4The force constants were also calculated in order to assure that the structure so determined has at the least the necessary conditions for a transition state; that is, all gradients are zero (less than 0.001) and only one vibrational frequency is imaginary. In the path determinations we used two types of wave functions, MCSCF and SCF. The MCSCF functions were used for those paths that involve vinylmethylene (paths 1, 2,and 5; designated as MCSCF paths). The SCF functions were used for the rest of the paths studied where the transition states were well represented by a single configuration (SCF paths). The 4-31G basis setI5 (431G) was employed for the (13) Dupuis, M.; Wendoloski, J. J.; Spangler, D. Nut. Res. Comput. Chem. Software Cat. 1980, I , QGO1. (14) Dupuis, M.;Rys, J.; King, H. F. J . Chem. Phys. 1976, 65, 1 1 1 . Dupuis, M.;Kmg, F. Ibid. 1978, 68, 3998.
4200 J . Am. Chem. SOC.,Vol. 111, No. 12, 1989
Yoshimine et al.
Table I. Geometrical Parameters" and Energies for Selected Points on the Lowest Energy Path between Allene and Vinylmethylene (Path 1A) point lA.l IA.2 IA.3 1A.4 1A.5 IA.6 IA.7 AL
state c1c2 C2C3 HlCl H2C1 H3C2 H3C3 H4C3 c 1c 2 c 3 HlClC2 H2C 1C2 H3C2C3 H4C3C2 HlClC2C3 H2C 1C2C3 H3C2C3C1 H4C3C2C1 SCF(DZP) SDCI(DZP) C(1) SDQCI(DZP)
'A, 1.302 1.302 1.077 ,077 ,077 ,077 80.0 21.0 21.0 26.2 21.0
0.88887 0.26704 0.93774 0.3 1266
TSlA
'A 1.315 1.345 1.075 1.073 1.231 1.387 1.OS9 169.7 119.9 121.4 65.0 111.4 29.4 150.9 144.2 125.4 0.78056 0.15791 0.93736 0.20370
'A 1.317 1.361 1.074 1.073 1.188 1.469 1.091 165.2 119.4 121.6 70.0 109.9 13.4 166.3 163.0 109.8
TSCT
'A 1.317 1.385 1.075 1.074 1.154 1.598 1.093 157.2 120.0 121.1 77.4 108.1 11.0 169.1 167.5 107.1
'A 1.317 1.393 1.075 1.074 1.142 1.640 1.094 154.4 120.3 121.0 79.9 107.5 10.2 170.0 169.1 106.1
A 1.325 1.425 1.078 1.079 1.114 1.809 1.094 145.0 120.5 121.3 90.0 109.7 7.3 172.9 172.3 103.2
A 1.327 1.454 1.076 1.074 1.087 2.085 1.096 130.9 122.2 120.4 109.5 109.4 2.9 175.0 175.2 85.5
Total Energiesb (hartrees) 0.78116 0.78373 0.15649 0.15621 0.93749 0.93766 0.20195 0.20120
0.78486 0.15639 0.93773 0.20122
0.7 8982 0.15836 0.93788 0.20273
0.79454 0.16065 0.93809 0.20458
1
1
Relative Energies (kcal/mol) with Respect to Methylacetylene SDQCI(DZP) 0.7 69.1 70.2 70.7 70.7 69.7 68.6 fractional parts are given; integer parts are -1 15 and -1 16 for SCF and CI, respectively. "Bond lengths are in 8, and bond angles are in deg. C ( l ) is the dominant CI coefficients. MCSCF paths and the DZP basis setI6 was used in addition to the 431G basis set for the SCF paths. In both cases, few points in the vicinity of the transition state are calculated to study polarization function and correlation energy effect on the transition state and their potential energy barrier heights. Our best estimates on the potential energy profile of the reaction paths thus determined were obtained by performing CI calculations with the DZP basis set on the selected points on the paths. Interpolation between points yielded our estimates for the transition-state structures and potential energy barrier heights. The CI wave function used for the SCF paths includes all singly and doubly excited configurations with respect to an SCF configuration (SDCI(DZP)). Quadruple excitation corrections were estimated by Davidson's formula" (SDQCI(DZP)). For the MCSCF paths multireference CI wave functions (MRCI) were employed, and a description of these and MCSCF functions will be given below. For both cases, the inner atomic shells, the 1s orbitals of three carbons, were frozen, and the corresponding core complement orbitals are excluded from the virtual orbital space. Calculations using MRCI(DZP) functions, which in fact are based on MCSCF(DZP) functions, did yield a very reasonable potential energy surface for vinylmethylene and also for the ring closure reactions to cyclopropene.'* The MCSCF functions include all configurations generated by distributing four electrons in the four active orbitals. The active orbitals selected are a u lone-pair orbital and three ?r orbitals for the planar carbene structure (see Figure I ) . We shall designate this function as MC4 function. For determination of paths 1 and 5 we used an MCSCF function of six electrons in six orbitals (MC6) that is constructed by adding two orbitals to the MC4 function. The added orbitals are bonding and antibonding ones of a C-H bond where a migrating hydrogen is involved. We noticed however that the dominant configurations involve only four orbitals even at the transition-state structures. Therefore, we repeated MC4(DZP) calculations, and the resultant natural orbitals and six CSF's with CI coefficients greater than 0.05 are used in constructing MRCI(DZP) wave functions for paths 1B and 5. We note here that SCF and SDCI functions are used for path 1A because MC6(43 1G) calculations indicated only one configuration would dominate throughout the path, particularly around the transition state. MRCI energies were further improved by the empirical formulaI2 MRCI*(DZP) = MRCI(DZP) - 0.2(1.0 - NORM) where NORM is a sum of the squares of the MC4 CI coefficients of those used for reference configurations in the MRCI wave functions. (15) Ditchfield, R.; Hehre, W. J.; Pople, J. J . Chem. Phys. 1971, 54, 724. (16) Tanaka, K.; Yoshimine, M . J . Am. Chem. SOC.1980, 102, 7655. (17) Langhoff,S.R.; Davidson, E. R. J . In?. Quantum Chem. 1974,8,61.
Thus, the MRCI* energies represent an estimated limit of a MRCI calculation in which all the CSF's in the MC4 function are included as reference configurations. The formula was derived from the results of the investigation, carried out at several points on the vinylmethylene surface, on a convergence pattern for the MRCI energy as a function of NORM by deleting least important configurations one at a time. We found that the formulae most likely gives the limit within 0.2 kcal/mol if NORM is greater than 0.99 (see ref 12 for details). Our best estimates are SDQCI(DZP) and MRCI*(DZP) potential energy profiles for the SCF and MCSCF paths, respectively. Total energies calculated in these two types of profiles are qualitatively different although the relative energies among the profiles of the same type are expected to be consistent and reasonably accurate. Therefore, in order to get a consistent overall picture of the potential energy surface for C3H4,these two potential surfaces are aligned at the trans planar carbene structure determined by the SCF(DZP) wave function.1° This structure was chosen because it is the most stable conformer of vinylmethylenes in the SDQCI(DZP) approximation, and the energy separation between this structure and methylacetylene should be the most reliable one. SDQCI(DZP) and MRCI*(DZP) energies at this structure are -1 16.21787 and -1 16.185 73 hartrees, respectively. The MRCI*(DZP) energy for the 'A, state of methylacetylene resulted from this alignment of the two potential surfaces is -116.281 68 hartrees, with respect to which all the MRCI*(DZP) relative energies are evaluated. The ALCHEM 11 program18 was used for the MCSCF and CI calculations in this study. 111. Reaction Paths In this section results are presented for the reaction paths, 1-8, defined in Scheme I. T h e energy profiles, transition-state structures and potential energy barriers a r e presented separately for each of the paths. Subsequently, the zero-point energies for pertinent structures and resultant activation energies are presented and discussed. A. Allene-Vinylmethylene Interconversion (Path 1). For this interconversion two possible pathways were investigated. Calculated geometrical parameters and energies for selected points on the lowest energy path, path lA, are listed in Table I, and those for path 1B in which the transition state had been constrained to have a C, symmetry a r e given in Table 11. For both cases, LH3C2C3 is taken t o be a reaction coordinate. T h e results for path 1A a r e summarized in Figure 2 where relevant geometrical parameters a r e also shown. W e note first that the transition state ( T S l A or 1A.4 in Table I) still retains a vinylmethylene-like structure, that is, the migrating hydrogen
J . Am. Chem. SOC., Vol. 111. No. 12, 1989 4201
Ab Initio Studies of the C3H4Surface
Table 11. Geometrical Parameters“ and Energies for Selected Points on the Path between Allene and Vinylmethylene Constrained to a C, Symmetry (Path 1B) I B.2 18.3 18.4 IB.5 18.6 point 1B.I TSI B ‘A’ ‘A’ ‘A’ state ‘A’ ‘A‘ ‘A’
c1c2 C2C3 HlCl H2C1 H3C2 H3C3 H4C3 ClC2C3 HlClC2 H2ClC2 H3C2C3 H4C3C2 HlClC2C3
1.393 1.298 1.073 1.073 1.239 1.524 1.066 166.5 119.3 119.3 73.8 142.7 102.1
1.373 1.289 1.073 1.073 1.287 1.514 1.063 170.0 119.9 119.9 72.0 145.1 99.6
MC6(431G) MC6((DZP) MC4((DZP) MRCI(DZP) NORM MRCI*(DZP)
1.433 1.319 1.074 1.074 1.169 1.603 1.070 155.5 118.5 118.5 80.0
140.7 104.8
Total Energies” (hartrees) 0.62233 0.6275 1 0.81997 0.82163 0.79215 0.79699 0.14351 0.14250 0.99974 0.99990 0.14357 0.14252
0.623 17 0.79184 0.14479 0.99964 0.14487
1.456 1.350 1.070 1.070 1.121 1.755 1.076 133.8 120.4 120.4 90.0 136.3 84.4
1.460 1.343 1.073 1.073 1.103 1.946 1.075 120.6 119.5 119.5 105.0 134.0 78.1
1.463 1.333 1.073 1.073 1.068 2.145 1.073 106.9 120.0 120.0 126.1 133.4 79.0
0.64497 0.83588 0.8 1424 0.15331 0.99988 0.15334
0.67 124 0.86232 0.84270 0.18104 0.99983 0.18107
0.68389 0.87642 0.85876 0.19907 0.99976 0.19912
Rr tive Energies (kcal/mol) with Respect to :thylacetylene MRCIIIDZP) 85.9 86.7 87.4 80.6 63.2 51.8 “Bond lengths are in 8, and bond angles are in deg. *Only fractional parts are given; integer parts are -1 15 and -1 16 for MC6 (or MC4) and CI, respectively. NORM is the sum of the squares of the MC4 CI coefficients of those used as the reference configurations. ~
~
a H2
H4
- 70 -
Hl
HI
Hl
0
AL )A,
70 0
__c
TSlA ’A
(65 81 0 7 (0 31
E , -
12 6
1
TV ] A
1’2 11
70 7 166 1 ,
E 58
1
60-
(5401 Y
b 50
I
I
I
60
80
1
1
100
120
L H3C2C3
AL ? A ,
0 7 10 3 )
86 7
TSlB ’A a7 4
29 5 c _
cv
‘A
57 9 (54 5 )
Figure 2. Schematic reaction paths for the allene-vinylmethylene interconversion. The potential energy barriers and relative energies in kcal/mol are given with the corresponding energies with the zero-point energy correction in parentheses: (a) allene-frans-vinylmethylene interconversion (path 1A) and (b) allene-cis-vinylmethyleneinterconversion with a C, symmetry constrain (path 1B).
H 3 is still attached to the central carbon C 2 (the H3-C2 bond length is 1.1 58 A). Consequently, the potential energy barrier is only 12.6 kcal/mol for the vinylmethylene-to-allene conversion. On the other hand, the barrier for the allene-to-vinylmethylene conversion is 70.7 kcal/mol indicating the amount of energy needed not only for the 1,2-H shift over the double bond but also for the internal rotation about the double bond involved. W e note also H 4 which does not participate in the migration is relatively unperturbed. The structure of TSIA indicates that the H4
