Predicted Enthalpies of Formation for MethyCSubstltuted

7244

J . Phys. Chem. 1991, 95,7244-1247

arguments and DSW calculations on the staggered conformer allow us to reassign the two lowest absorption bands seen in the visible spectrum as being primarily metal-metal based. Acknowledgment. I thank Professor Fernando Zuloaga for

useful discussions. This work has been supported by FONDECYT (Grant No. 90/0800), DIUC (Grant No. 89010E), and the Organization of American States (OAS). Reglshy NO. OS~CI,*-,97523-23-4.

Predicted Enthalpies of Formation for MethyCSubstltuted Sllaethylenes and Msllenes Jerry A. Boatz**tand Mark S. Gordon*

Downloaded by UNIV OF NEBRASKA-LINCOLN on September 5, 2015 | http://pubs.acs.org Publication Date: September 1, 1991 | doi: 10.1021/j100172a028

Department of Chemistry, North Dakota State University, Fargo, North Dakota 58105 (Received: March 26, 1991)

Enthalpies of formation of the methyl-substituted silaethylenes (CH3),H2-,,Si=CH2 ( n = 1, 2) and disilenes (CH3),,H&3i=Si(CH3),HEm (n, m = 0-2) are predicted by using isodesmic reactions which relate the compound of interest to the parent silaethylene (H2Si=CH2)or disilene (H$i=SiH2), for which accurate enthalpies of formation have recently been determined. In turn, the enthalpies of formation of these methyl-substituted compounds are used in conjunction with homodesmic reactions to reevaluate the enthalpies of formation of the silicon-substituted cyclobutenes C,,Si+,H6 ( n = 0-4).

I. Introduction Accurate enthalpies of formation are essential in understanding the thermodynamics of any given class of reactions. Furthermore, a knowledge of enthalpies of formation for a reaction or a group of related reactions is a necessary first step in the deduction of the mechanisms for these reactions. Unfortunately, the thermodynamic properties of many silicon compounds are not wellknown experimentally. In particular, the enthalpies of formation of many unsaturated silicon compounds, such as silaethylene (SiH2=CH2),disilene (SiH,==SiH,), and their derivatives, have not been well characterized.' Fortunately, theory is currently capable of providing quantitative information regarding thermodynamic properties. Several techniques for the theoretical prediction of enthalpies of formation of molecules have recently been developed." ~ referred to as G1) Among these are the G A U S S I A N - ~(hereafter and GAUSSIAN-2' methodologies of Pople and co-workers, which use large basis sets and quadratically convergent configuration interaction (QCI)9 to predict enthalpies of formation and other properties of small molecules to within 1-2 kcal/mol of the experimental values. Binkley and Melius*apply empirical correction factors to fourth-order perturbation theory correlation energies to obtain enthalpies of formation for small compounds. For larger systems, the semiempirical methods of Dewar and Stewart et aL5 are useful for the prediction of enthalpies of formation. When enthalpies of formation are known (from either experiment or theory) for a subset of reference compounds, isodesmic (homodesmic) reactionst0 may be used to predict enthalpies of formation of moderately sized compounds to within 3-5 (2-3) kcal/mol, using split-valence plus polarization basis sets and a modest level of theory (i.e., second-order perturbation theory) to determine correlation corrections.6 Such reactions have been utilized in calculating enthalpies of formation of several highly strained organic molecules and their silicon analogues" as well as several alkylsilanes.'2 Furthermore, recent studies have employed isodesmic reactions') to predict the enthalpy of formation of methyl- and dimethylsilylene." Therefore, once the enthatpies of formation for selected reference compounds have been established from experiment or with a reliable level of theory (e.g., Gl), the analogous properties for larger related compounds may be predicted with the aid of isodesmic or homodesmic reactions. This combination of G 1 theory and isodesmic/homodesmic reactions is therefore a powerful tool for determining thermodynamic properties. ' h n t addmu: Department of Chemistry, University of Utah, Salt Lake City, UT 841 12.

0022-3654191/2095-7244S02.50/0

In the present work, MP2/6-3 lG(d)//RHF/3-21G* energies plus zero-point vibrational energies are used to calculate enthalpy differences for isodesmic reactions which relate the compound of interest to its unsubstituted analogue (either silaethylene or disilene). These computed reaction enthalpies combined with the known enthalpies of formation for the reference compound^'^ provide predicted enthalpies of formation for the compound of interest. 11. Computational Methodology

The geometries for all compounds were optimized at the selfconsistent-field (SCF) level, using the 3-21G* basis set'6 and the analytical gradient routines in G A U S S I A N ~ ~ . ~ All ' structures (1) Walsh, R. In The Chemistry of OrganosiliconGunpounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1988; Chapter 5. (2) Pople, J. A.; Luke, B. T.; Frisch, M. J.; Binkley, J. S. 1.Phys. Chi" 1985.89.2198-2203. ( 3 ) Pople, J. A.; Head-Gordon, M.; Fox, D. J.; Raghavachari, K.; Curtiss, L.A. 1. Chem. Phys. 1989, 90,5622-5629. (4) Curtiss, L.A.; Pople, J. A. To be published. (5) (a) Dewar, M.J. S.;Thiel, W. J. Am. Chem. Soc. 1977,99,4899. (b) Stewart, J. J. P. MOPAC; Quantum Chemistry Program Exchange No. 455; Department of Chemistry, Indiana University: Bloomington, IN 47405. (6) Disch, R. L.;Schulman, J. M.; Sabio, M.L. J . Am. Chem. Soc. 1985, 107, 1904. (7) Ho, P.; Coltrin, M. E.; Binkley, J. S.;Melius, C. F. J. Phys. Chem. 1986,90,3399. (8) (a) Melius, C. F.; Binkley, J. S. ACS Symp. Ser. 1984,219, 103. (b) Ho, P.; Coltrin, M.E.; Binkley, J. S.; Melius, C. F. J . Phys. Chem. 1 W , 89, 4647-4654. (9) POPIC,J. A.; Head-Gordon, M.; Raghavachari. K. J . Chem. Phys. 198'1; 87,-5968. (10) . , (a) . , GCOrRe. P.: Trachtman. M.: Bock. C. W.: Brett. A. M. Tetrahedron 1976,32,30-323. (b) George, P.; Trachtman, M.; Brett, A. M.; Bock, C. W. J . Chem. Soc., Perkin Trans. 2 1977, 1036-1047. ( I 1) (a) Boatz, J. A.; Gordon, M.S.;Hilderbrant, R. L. J. Am. Chem. Soc. 1988,110, 352. (b) Boatz, J. A.; Gordon, M. S. J . Phys. Chem. 1988. 92, 3037. (c) Boatz, J. A.; Gordon, M. S. J . Phys. Chem. 1989, 93, 3025. (12) (a) Gordon,M.S.; Boa@ J. A.; Waleh, R. J . Phys. Chem. 1989,93, 1584-1585. (b) Boatz, J. A.; Gordon, M. S. J . Phys. Chem. 1990, 91, 3874-3876. (13) Hchre, W. J.; Ditchfield, R.; Radom. L.; Pople, J. A. J . Am. Chem. SOC.1970, 92, 4796. (14) Gordon, M. S.;Boatz, J. A. Organomerallics 1989,8, 1978-1980. (15) Boatz, J. A.; Gordon, M.S. J. Phys. Chem. 1990, 94, 7331. (16) (a) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54,724-728. (b) Hehrc, W. J.; Ditchfield, R.; Pople. J. A. J . Chem. Phys. 1972,56,2257-2261. (c) Hariharan, P. C.; Pople, J. A. Theor. Chlm. Acta 1973,28,213-222. (d) Gordon, M.S. Chem. Phys. Lrtr. 1980,76,163-168. (17) Frisch, M. J.; Binkley, J. S.;Schlegel, H. B.; Raghavachari, K.; Melius, C. F.; Martin, R. L.; Stewart, J. J. P.; Bobrowicz, F. W.; Rohling, C. M.;Kahn, L. R.; DeFrces, D. J.; Seeger, R.; Whiteside, R. A.; Fox, D. J.; Fleuder, E. M.;Pople. J. A. GACJSSIANB6; Carnegie-Mellon Quantum Chemistry Publishing Unit: Pittsburgh, PA, 1984.

Q 1991 American Chemical Society

The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 7245

Methyl-Substituted Silaethylenes and Disilenes thus obtained were verified as local minima by demonstrating that their matrices of energy second derivatives (Le., hessians) are positive definite. The energetics of the isodesmic reactions were calculated with the larger 6-31G(d) basis set'* and second-order many-body perturbation theoryt9(denoted as MP2/6-3 lG(d)// RHF/3-21G*), as formulated by Pople and co-workers.zO Reaction energies were converted to enthalpies by incorporation of zero-point energy (ZPE)differences, scaled by 0.89 to correct for the overestimation of S C F vibrational frequencies.z' The following isodesmic reactionsI3 were used in predicting the enthalpies of formation of the methyl derivatives of silaethylene and disilene: (CH3)nH2-$i=CH2 nSiH, SiHz=CH2 + nSiH3CH3 (n = 1, 2) (1)

+

-

(CH3),Hz-,,Si-Si(CH3),,,H2-,,, + ( n + m)SiH, SiH2=SiH2

+ (n + m)SiH3CH3

-

(n,m = 0-2) (2)


111. Results and Discussion

Structures and Vibrational Frequencies. The complete RHF/3-21 G* geometries and scaled harmonic frequencies (including normal-mode descriptions) of the methyl derivatives are available as supplementary material (see Tables SI and SII). The discussion here will be limited to a few general observations concerning the structures of these systems and the harmonic vibrational frequencies of the Si=C and Si-Si bonds. The predicted S i 4 bond lengths for both the 1-methyl- and 1,l-dimethylsilaethylene molecules are 1.691 A (see Table SI). This is in very good agreement with the generally accepted value of 1.70 A for the S i 4 bond length.z2 For the substituted disilenes, the calculated Si=Si bond lengths are in the range 2.109-2.1 11 A. This is approximately 0.04 A shorter than indicated by prior theoretical predictions.22 However, inclusion of correlation corrections is expected to lengthen the Si-Si bond and hence bring the present computed bond length into closer agreement with these earlier values. Only 1 -methylsilaethylene, methyldisilene, and trans- 1,2-dimethyldisilene are planar with respect to the double-bond framework at this level of theory. The remaining compounds, all of which have a pair of geminal methyl groups bonded to silicon, are nonplanar. The nature of the distortions leading to the nonplanar structures is of interest and merits further analysis. For example, consider the C, geometry of 1 ,I-dimethylsilaethylene.The hessian matrix has one negative eigenvalue, which indicates that the planar conformation is not a local minimum on the potential energy surface. There are several types of distortion that the planar conformation can undergo to lower the point group symmetry, including trans bending, twisting about the Si=C bond, and/or internal rotation about the Si-C bonds. In order to assess the relative contributions of these specific distortions to the description of the imaginary normal mode, the vibrational energy density matrixz3 was calculated for this normal mode. This analysis indicates that the normal mode is described almost entirely by an internal rotation of one of the Si-C bonds, with little contributions from trans bending or double-bond twisting motions. ~~

(18) (a) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J . Chem. Phys. 1971, 54, 724-728. (b) Hehre, W.J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972,56, 2257-2261. (c) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acra IW3,28,213-222. (d) Gordon, M.S.Chem. Phys. Lett. 1980,76, 163-168. (19) M~ller,C.; Plesset, M. S.Phys. Rev. 1934,46, 618. (20) Pople, J. A.; Binkley, J. S.;Seeger, R. Inr. J . Quanrum Chem., Quantum Chem. Symp. 1976,SIO, 1-19. (21) Pople, J . A.; Schlegel, H. B.; Raghavachari, K.; DeFrees, D. J.; Binkley, J. S.;Frisch, M. J.; Whiteside, R. A.; Hout, R. J.; Hehre, W. J. Inr. J . Quantum Chem., Quantum Chem. Symp. 3981, SI5, 269-278. (22) (a) Raabe, G.;Michl, J. Chem. Reo. 1985,85,419-509. (b) Gordon, M.S. Molecular Srrucrure and Energetics; VCH Publishers: Cambridge, 1986; Vol. 1, Chapter 4. (c) Apebig, Y. In The Chemistry of Organic Silicon Compounds: Patai. S.,Rappoport, Z., Eds.;John Wiley and Sons: New York, 1988; Chapter 2. (23) (a) Boatz, J . A.; Gordon, M. S.J . Phys. Chem. 1989,93, 1819. (b) Boatz, J. A.; Gordon, M . S.J . Phys. Chem. 1989,93, 5774. (c) Pulay, P.; Tcirak, F. Acta Chim. Acad. Sci. Hung. 1966,47, 273-279.

Therefore, at the SCF level, the distortion from planarity appears to be driven by steric repulsions between the methyl groups. Similar analyses of the other nonplanar molecules also show that the nonplanarity is caused by internal rotations about the Si-C bonds. However, in all cases the magnitude of distortion is small. (The angle between the double bond and the plane containing an unsaturated center and its two bonded hydrogen and/or carbon atoms is consistently less than I ".) On the basis of previous calculations on disilene,u it is likely that introduction of correlation will introduce some trans bending distortion into the substituted disilenes. This is not expected to significantly alter the enthalpies of formation predicted here.22 The predicted harmonic vibrational frequencies (scaled by 0.89,') for the Si==Cbonds in CH3SiH=CH2 and (CH3),Si= CH, are 991 and 1005 cm-I, respectively (see Table SII). These values are in good agreement with the experimentally determinedza Si=C vibrational frequency in CH3SiH==CHz (988 cm-I.) The scaled computed SieSi harmonic stretching frequency in methyldisilene is 539 cm-'. This is in good agreement with the observed Si=Si vibrational frequencies of 539 cm-' in tetramesityldisileneZa (hereafter referred to compound A) and 525 cm-' in (E)-1,2-di-tert-butyl-l,2-dimesityldisiIene2%(hereafter referred to as compound B.) There are two normal modes in 1,l-dimethyldisilene that have significant contributions from the Si-Si stretching motion. The corresponding computed frequencies of 489 and 675 cm-' bracket the experimentally observed values in A and B. The calculated Si=Si stretching frequency of 508 cm-' in (E)-1,Zdimethyldisilene is slightly lower than the experimental values for compounds A and B. The computed Si=Si frequency in 1,1,2-trimethyldisilene is lower still, with a value of 467 cm-'. As in the case of ( E ) -1,2-dimethyldisiIene, tetramethyldisilene has two normal modes with significant Si-Si stretching character, with frequencies (438 and 701 cm-I) that bracket those observed for compounds A and B. In spite of the somewhat wide spread of normal-mode frequencies which have a large degree of Si=S stretching character, (which represent the the scaled i n t r i n s i ~Si=Si ~ ~ ~ , frequencies ~ sum of the contributions from all normal modes to the vibration of a given internal coordinate) of all the methyldisilenes lie between 590 and 594 cm-I. Enthalpies of Formation. The MP2/6-31G(d)//RHF/3-21G* total energies and scaled RHF/3-21G* zero-point energies for all compounds considered in this paper are given in Table I. The experimental (where available) and predicted values for the enthalpies of formation are also listed here, as are the calculated entropies and free energies of formation. The enthalpies of formation of the reference compounds in Table I are taken from two sources. The values for silane and methylsilane are obtained from experimental measurements.' Because of the uncertainty in the experimental enthalpies of formation for silaethylene and di~ilene,'?'~ the predicted enthalpies15 obtained by using the G1 method of Pople et aL3 have been used instead. The predicted enthalpies of formation of CH3SiH=CH2 and (CH3),Si=CH2 are 31 and 15 kcal/mol, respectively. Each value is approximately 10 kcal/mol higher than the corresponding experimental value. In an earlier study,I5 the G1 enthalpy of formation of silaethylene was also found to be 10 kcal/mol higher than the experimental value. Thus, the experimental relatiue enthalpies of formation of silaethylene and its methyl derivatives are consistent with the predicted values, although the absolute values are not. Apparently, there is no experimental thermochemical data for the methyl-substituted disilenes. However, the enthalpies of formation of these compounds have been predicted by using the MOB1 method in a study by Bell, Kieran, Perkins, and PerkinsZ4 (see Table I). The enthalpies of formation obtained in this earlier (24) Bell, T. N.;Kieran, A. F.; Perkins, K. A.; Perkins, P. G. J . Phys. Chem. 1984,88, 1334-1338.

7246 The Journal of Physical Chemistry, Vol. 95. No. 19, I991

Boatz and Gordon

TABLE I: MP2/a31C(d)//RHF/3-2lG* Total Energies, RHF/IZIG* Zero-Point Energies, and Entbdpies, Free Energies, a d Eabopies of Fornution0


moleculeb

total energy -29 I .307 57 -330.485 11 -329.25043 -580.240 38

zero-point energy Reference Compounds 21.0 41.0 27.1 21.1

CH3SiH=CH2 (C,)

-368.428 85

Silaethylenes 46.8

(CH3),Si=CH2 (C,)

-407.607 22

CH3SiH==SiH2 (C,)

-619.417 69

(CH3)2Si=SiH2 (C,)

-658.595 49

60.6

CH3SiH=SiHCH3 (C2,,)

-658.594 12

60.8

(CH&3i=SiHCH3 (C,)

-697.770 84

80.5

(CHMi=Si(CH3)2 (CO

-736.946 77

100.1

60.6 Disilenes 41.0

AGP.298

s?298

(8.2) (-6.96) 46.Y 64.9c [53.5]

13.6 4.6 49.4c 669

-18.2 -38.9 -9.7c -5.4c

30.9 (21 5 ) 15.0 (5 5 )

* *

40.5

-32.1

31.6

-55.6

50.0 [24.7] 34.7 [-0.61 35.7 [-4.21 21.1 [-25.01 7.0 [-46.71

58.6

-28.9

50.2

-51.9

51.8

-53.9

43.8

-76.1

36.9

-100.2

AH?

298

a Total energies in hartrces; zero-point energies, enthalpies of formation, and free energies of formation in kcal/mol; entropies of formation in cal/ (mol K). Experimental values (from ref 1) are given in parentheses. Enthalpies of formation from ref 24 are enclosed in brackets. bThe molecular point gioup is given in parentheses. ‘Taken from re? 15.

TABLE 11: M P Z / I 3 I G ( d ~ / / R H F / I 2 1 G 1 EntbPldes.Free Enerniea and Enbodes of Formation0

55.2 (51.9)

66.5

39.0 (37.5)

50.0

44.9

54.9

-37.8

-32.8

71.9

81.9

-32.5

51.1

61.8

-34.6

51.0

61.4

-35.0

84.8

94.8

-33.7

76.0

86.0

-33.5

71.1

80.8

-32.4

72.1

81.9

-32.9

-33.5

74.2

-35.6

65.5

75.9

-35.0

37.3

47.7

-34.9

87.0

85.1

-37.0

63.3

76.1

75.3

-36.4

63.0

72.7

-32.6

92.9

102.9

-33.6

69.8

80.5

-35.8

85.2

94.7

-3 1.9

.Enthalpies and free energies of formation arc given in kcal/mol and entropics of formation are in cal/(mol K). Experimental values (from Cox,

R. D.; Pilcher, G . Thermochemistry of Organk and Organomerallic Compounds;Academic: New York, 1970) are given in parentheses. work are consistently lower than those computed in the present study. T h e differences between our predicted enthalpies and the

MOB1 values range from approximately 1 1 kcal/mol for the parent disilene to over 50 kcal/mol for the tetramethyl derivative!

J. Phys. Chem. 1991, 95. 7247-7253


The discrepancy of about 10 kcallmol between the observed and computed enthalpies of formation for both of the methylsubstituted silaethylenes considered in this work is not surprising. Since isodesmic reactions generally predict enthalpies of formation with an accuracy of 4-5 kcallmol, it seems plausible that the experimental enthalpies of formation of these compounds may be in error. That the experimental thermochemistry of the methyl-substituted silaethylenes is not yet well understood is exemplified by the wide variation in the observed enthalpies of formation for some of the silaethylenes. For example, recent experimental measurements of the enthalpy of formation of 1,l-dimethylsilaethyleneyielded values of 20.5, 18.2, C-0.5, and 7 kcal/mol.22. Thus, it appears at the present time that theory is able to provide more accurate and consistent enthalpies of formation for unsaturated silicon compounds. Further, in view of the large differences between the enthalpies of formation predicted by a b initio isodesmic reactions versus the MOBI method, it appears that the latter values should be viewed with some degree of caution.

IV. Enthalpies of Formation of Silacyclobutenes In a previous paper from this laboratory,IIb the enthalpies of formation of the silicon-substituted bicyclobutanes and cyclobutenes Si,C+,,H6 (n = 0-4) were predicted by using homodesmic reactions and the experimental enthalpies of formation of some key reference compounds. In particular, the experimental value of 37 kcal/mol for the enthalpy of formation of silaethylene' was used as a basis for predicting the enthalpies of formation for several reference compounds appearing in the homodesmic reactions for the silacyclobutenes. However, as shown in a recent paper,15 this value for the enthalpy of formation of silaethylene appears to be

7247

in substantial error, and the predicted enthalpies in Table I for silaethylene, disilene, and their methyl derivatives are believed to be more accurate. Therefore, the predicted enthalpies of formation of the silacyclobutenes have been updated by using the enthalpies of formation in Table I for the reference compounds which appear in reaction 2 in our earlier paper.'Ib The improved silacyclobutene enthalpies of formation so obtained are summarized in Table 11, along with the free energies and entropies of formation. Although the predicted enthalpies of the silabicyclobutanes are not altered by the new values for the methylsilaethylenes and methyldisilenes, they are also given in Table I1 for completeness. (Note that all values in Table I1 are a t 298 K, whereas those in our previous paper are at 0 K.)

V. Conclusions The enthalpies of formation calculated in the present work suggest that the current experimental values for the methylsilaethylenes need to be reevaluated. Further, the computed enthalpies of formation of the methyldisilenes indicate that the MOBI method may predict inaccurate enthalpies of formation. Acknowledgment. This work was supported in part by grants from the National Science Foundation (CHE89-11911) and the Air Force Office of Scientific Research (90-0052). The computations were performed on the NDSU Quantum Chemistry Group VAXstation 3200 and Celerity C1260D (purchased with the aid of N S F Grant CHE85-11697). Supplementary Material Available: Complete RHF/3-2 1G* geometries (Table SI) and RHF/3-21GZ scaled harmonic frequencies (Table SII) of methyl-substituted silenes (8 pages). Ordering information is given on any current masthead page.

Theoretical Study of the Interaction of Aluminum Atoms with 1,3-Butadiene and Benzene Micbael L. McKee Department of Chemistry, Auburn University, Auburn, Alabama 36849 (Received: April 22, 1991) Theoretical methods are used to study the complexes formed between a neutral aluminum atom and the hydrocarbons, butadiene and benzene. Aluminocyclopentene is predicted to be formed by the addition of aluminum to cis-1J-butadiene, while addition to trans-1,3-butadiene is predicted to form aluminomethylallyl. The results agree well with an ESR study at 77 K,in which both complexes are observed. A planar aluminocyclopenteneis predicted to be bound by 27.2 kcal/mol and aluminomethylallyl is bound by 17.4 kcal/mol with respect to AI + trans-1,3-butadiene (PMP4/6-31G*) when geometries are optimized at the UHF/3-21G* level. When geometries are reoptimized at the UMP2/3-21GS level, an envelope aluminocyclopentene structure is found which is about 4.0 kcal/mol more stable than the planar five-membered ring. Three addition complexes (1.4, 1,2, and 1,3) were considered for the interaction of AI with benzene. In agreement with experiment, the 1P-complex is predicted to be lowest in energy, giving a binding energy at the [PMP4/6-31+G**] level of 7.4 kcal/mol, compared to an experimental binding energy of 11.7 kcal/mol.

Introduction Aluminum atoms are known to interact with such hydrocarbons as acetylene,'S2 ethylene,'+ butadiene,' and b e n ~ e n e . ~ *Since *+~ ( I ) Kasai, P. H.; M c W , D.; Watanabe, T. J. Am. Chem. Soc. 1977,99, 3522. (2) Kasai, P. H. J . Am. Chem. Soc. 1982, 104, 1165. (3) Kasai, P. H.; McLeod, D., Jr. J . Am. Chem. Soc. 1975, 97, 5609. (4) (a) Howard, J. A.; Mile, E.; TSC,J. S.; Morris, H. J . Chem. Soc., Faraday Trans. I 1987,83,3701. (b) Chenier, J. H. E.; Howard, J. A,; Mile, E. J . Am. Chem. Soc. 1987, 109.4109. (5) Mitchell, S. A.; Shard, E.; Raynor, D. M.; Hackett, P. A. J . fhys. Chcm. 1989, 92, 1655. (6) Srinivas, R.; SUlzle, D.; Schwarz, H. J . Am. Chem. Soc. 1990, 112,

___

8334. ..

(7) Chenier, J. H. E.; Howard, J. A,; Tse, J. S.; Mile, SOC.1985, 107, 7290.

B. J . Am. Chem.

0022-365419 112095-7247302.50/0

aluminum has an odd number of electrons, the complexes can be studied by ESR at low temperature. However, there is controversy concerning the structure of the complex with even the simplest hydrocarbon, acetylene. The experimental spectrum of this complex, formed in a matrix at 4 K, has been interpreted in terms of a cis a-bonded complex that can rearrange to a trans u-bonded complex upon photolysis.'V2 However, high-level calculations by several indicate that the a-bonded complex is lower in energy than either a-bonded complex and that a u-bonded (8) Kasai, P. H.; McLeod, D., Jr. J . Am. Chem. Soc. 1979, 101, 5860. (9) Howard, J. A.; Joly, H. A,; Mile, B. J . Am. Chem. Soc. 1989, 111, 8094. (IO) (a) Xie, Y.; Yates, E. F.; Schaefer, H. F. J . Am. Chcm. Soc. 1990, 112,517. (b) Trenary, M.; Casida, M. E.; Brooks,B. R.; Schaefer, H. F. J. Am. Chem. Soc. 1979, 101, 1638. (11) Tse. J. S. J . Am. Chem. Soc. 1990, 112, 5060.

0 1991 American Chemical Society

Predicted Enthalpies of Formation for MethyCSubstltuted

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