J . Phys. Chem. 1989, 93, 1584-1585
1584
Heats of Formation of Alkylsilanes Mark S. Cordon,* J. A. Boatz, Department of Chemistry, North Dakota State University, Fargo, North Dakota 581 05
and Robin Walsh* Department of Chemistry, University of Reading, Whiteknights. P.O. Box 224, Reading RG62AD, England (Received: March 1 , 1988; In Final Form: June 28, 1988)
Theoretical heats of formation at 298 K for several alkylsilanes, predicted at the MP2/6-31G(d) level of theory, are compared with recently obtained experimental and additivity values. Excellent agreement is obtained between the ab initio and additivity values and with the more reliable experimental values for acyclic alkylsilanes. The ab initio heats of formation for the silacycloalkanes permit the evaluation of strain energy increments for the additivity scheme. Comparison is made with limited experimental data.
In a recent paper,l the heats of formation for several cyclic and acyclic alkylsilanes were predicted by using computed energy differences for appropriate homodesmic2 reactions. In such reactions each type of chemical group is conserved from reactants to products. For example, for silacyclopropane, one has c-CH2CH2SiH2 3CH3CH3 CH3SiH2CH3+ 2C3H8
+
-+
The heats of formation were obtained by first calculating energy differences for the homodesmic reactions using second-order many-body perturbation theory (MP2)3 and the 6-3 1G(d) basis set4 at the self-consistent field (SCF) geometries. These reaction energy differences were converted to enthalpy differences at 0 K by adding in the SCF/6-31G(d) zero-point vibrational energies for reactants and products. In principle, these 0 K reaction enthalpies can be combined with the experimental heats of formation for the reference compounds to obtain the heats of formation for the cyclic species. In those cases for which the experimental heats of formation for certain reference compounds are unavailable, the reference heats of formation were also obtained by using the appropriate homodesmic reactions. This was found to be necessary for 1-silabutane, 2-silabutane, 1-silahexane, and 2-silahexane. Walsh and Doncasters have extended the group additivity scheme of Benson et aL6v7in order to predict the heats of formation of alkylsilanes. Until very recently, reliable experimental data on heats of formation of alkylsilanes have been very limited. This is largely because the traditional calorimetric method of static bomb calorimetry does not work for organosilanes due to incomplete combustion.* The additivity scheme is based largely on studies of equilibrium amongst the methylsilanes, although it depends crucially on a calorimetric value for AHfo(Me4Si)obtained by Steele.13 That author used the now recommended method8 in which combustion is carried out in the presence of an auxiliary fluorine-containing substance in order to ensure complete reaction. Other reasons for believing this data have been given e l ~ e w h e r e and , ~ the additivity scheme is thought to give AH,' values reliable to within f l kcal mol-l. ( I ) Boatz, J. A.; Gordon, M. S.J . Am. Chem. SOC.1988, 110, 352. (2) (a) George, P.; Trachtman, M.; Bock, C. W.; Brett, A. M. Tetrahedron 1976, 32, 317. (b) George, P.; Trachtman, M.; Brett, A. M.; Bock, C. W. J . Chem. Soc., Perkin Trans. 2 1977, 1036. (3) Pople, J. A,; Binkley, J. S.;Seeger, R. Int. J . Quantum Chem. Symp. 1976, SIO, 1 . (4) (a) Hariharan, P. C.; Pople, J . A. Theor. Chim. Acta 1973, 28, 213. (b) Gordon, M. S.Chem. Phys. Lett 1980, 76, 163. (5) Doncaster, A. M.; Walsh, R. J . Chem. Soc,, Faraday Trans. 2 1986,
82, 707.
(6) Benson, S.W.; Cruickshank, F. R.; Golden, D. M.; Haugen, G. R.; O'Neal, H. E.; Rodgers, A. S.; Shaw, R.; Walsh, R. Chem. Rev. 1969, 69, 279. (7) Benson, S.W. Thermochemical Kinetics, 2nd ed.; Wiley-Interscience: New York, 1976. (E) See, for example: Cox, J. D.; Pilcher, G. Thermochemistry of Organic and Organometallic Compounds; Academic Press: London, 1970.
0022-365418912093-1584$01.50/0
TABLE I: Standard Heats of Formation for Reference Compounds (kcal mol-') compound experiment additivity' ab initiob CH$H3 -20.24 f 0.05' CH3CH2CHj -24.83 f 0.07' SiH, 8.2 f 0.5' SiH3CH3 -6.9 f 1.0' (CH42SiH2 -22.6 f 1.0' (CH,),SiH -39.0 f 1.0' CH3CH2SiH3 -34.2d -1 1 .o -9.0 -16.0 -14.7 CH,CH2CH2SiH3 -26.7 -25.4 CH3CH2SiH2CH3 (C2HS)2SiH2 -43.6 f 1.4' -30.8 -28.0 -25.9 -24.5 CH3(CH2)4SiH3 -36.7 -35.6 CH3(CH2)$iH2CH3 (C2HS)3SiH -48.0 f 3.6' -5 1.2 -52.0 f 1 . d CH3(C2HJ)2SiH -67.8 f 1.2f -47.1
"Reference 5 . bThis work. CReference8. dReference 10; data corrected to room temperature. Uncertainty unknown. 'Reference 11. 'Reference 17. TABLE II: Standard Heats of Formation of Alkylsilanes (kcal mol-')
compound Me4Si Et2SiMe2 Et,SiMe Et4Si Pr2SiMe2 Pr,SiMe Pr2SiEt2 Pr4Si
experiment' -55.0 -55.7 -58.7 -63.2 -67.2 -70.9 -63.4 -72.9 -83.7 -81.7 -90.6
f 1.4
f 0.8' f 2.4' f 1.5 f 1.4 f 1.4 f 3.7' f 1.6 f 1.7 f 1.4 f 1.4
additivityd -55.7 -63.8 -67.9 -71.9 -13.9 -82.7 -82.0 -91.8
"All data from ref 12 unless otherwise specified. bReference 13.
'Reference 11 . Reference 5.
In the present work, the theoretical heats of formation are updated'and converted to 298 K? so that a direct comparison of theoretical predictions with experimental and/or additivity values may be made. In addition, the additivity scheme is compared with new experimental data which have recently become available. (9) Pople, J. A.; Luke, B. T.; Frisch, M. J.; Binkley, J. S. J . Phys. Chem. 1985, 89, 2198.
(10) Wagman, D. D.; Evans, W. H.; Parker, V. B.; Shumm, R. H.; Halow, I.; Bailey, S. M.; Churney, K. L.; Nuttall, R. L. J . Phys. Chem. ReJ Dura Suppl. 1982, 1 1 , 2. ( 1 1) Data of J. B. Pedley, B. S.Iseard, and J. A. Treverton, summarized in: Pedley, J. B.; Rylance, J. Sussex-NPL Computer Analysed Thermochemical Data: Organic and Organometallic Compounds; University of Sussex, England, 1977.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1585
Heats of Formation of Alkylsilanes TABLE 111: Standards Heats of Formation and Strain Energies of Some Silacycloalkanes and Their 1,l-Dimethyl Derivatives (kcal mol-')
compound silacyclopropane silacyclobutane silacyclopentane silacyclohexane
AH:( 1,I-dimethyl compd, 298 K) AHf' strain (298 K)" energyb additivity' exptl 30.2 9.3
40.5 24.5
-2.9 -23.8
-1 5.3 -22.0
4.8 3.1
-48.4 -55.1
-26.0 -33.0 -19.8 -43.5
f 1.4d f 2.6' f 1.d f 2.9'
Ab initio value. Calculated using strain-free additivity estimates. Calculated assuming strain energy unaffected by methyl group. Reference 15. Reference 11. /Reference 16.
For the acyclic alkylsilanes the comparisons are shown in Tables I and 11. Table I shows experimental, additivity, and ab initio values for methyl- and ethylsilanes and higher mono- and dialkylsilanes. The agreement between additivity and ab initio based values is very good and within 2 kcal mol-' in all but one case. Published experimental values for ethylsilane and diethylsilane (as noted earlier]) appear to be in substantial error, and the earlier value for triethylsilane has a high error margin. This is in spite of the fact that the diethyl- and triethylsilanes were investigated by the auxiliary fluoride combustion method. Table I1 shows a comparison of some recent experimental values obtained by the group of Voronkov and KlyuchnikovIz with additivity estimates for tetraalkylsilanes. The experimental results were obtained by a new high-temperature combustion technique employing ultradispersed carbon to obtain temperatures in excess of 2000 O C . They appear to be in excellent agreement with the additivity estimates and provide a confirmation of the latter. In particular, they support the previously proposed increment of -4.1 kcal mol-' (12) Voronkov, M. G.; Klyuchnikov, V. A.; Danilova, T. F.; Korchagina, A. N.; Baryshok, V. P.; Landa, L. M. Bull. Acud. Sci. USSR (Chem. Sci.) 1986, 1790, 1795. (13) Steele, W. V. J . Chem. Thermodyn. 1983, 15, 595. (14) Walsh, R. In The Chemistry of Orgunosilicon Compounds; Patai, S., Rappport, Z., Eds.; Chapter 5 (Thermochemistry), 1988, in press. (15) Steele, W. V., unpublished results (private communication to R. Walsh). (16) Genshel', V. G.; Demidova, N. V.; Nametkin, N. S.; Gusel'nikov, L. E.; Volnina, E. A,; Burdasov, E. N.; Vdovin, V. N. Izu. Akud. Nuuk SSSR, Ser. Khim. 1976, IO, 2337.
for ethyl-for-methyl replacement suggested by us on the basis of a correlation of heat of formation data for organometallics with metal electronegativity. These data are reviewed in more detail e1~ewhere.l~ It should be noted, however, that they support Steele's value13 for AHfo(Me4Si), but not Pedley's data" on Me4Si or Et4Si. A very recent paper by the group of Voronkov and Klyuchnikov" provides further data on trialkylsilanes. Two of these data, those for Et3SiH and MeEt,SiH are included in Table I. The agreement with additivity is reasonable, and for the whole series of 28 compounds the average difference (regardless of sign) between experiment and additivity is only ca. 0.7 kcal mol-l. The ab initio heats of formation for the silacycloalkanes are listed in Table 111. Based on the comparison in Table I and the previous agreement for cycloalkanes,l the theoretical values are expected to be accurate to within 3-4 kcal mol-'. There are no experimental values for silacycloalkanes, but there are values for 1,l -dimethylsilacyclobutane and 1,l -dimethylsilacyclopentane. We have therefore used our additivity scheme to derive heats of formation of the 1,l-dimethylsilacycloalkaneson the assumption of constant strain energies upon methyl substitution. These are also shown in Table 111. The comparison for 1,i-dimethylsilacyclobutane shows that the derived value is in best agreement with the data of Steele,13 thought to be the most reliable experimental value.I4 For 1,l-dimethylsilacyclopentane the agreement is moderate, but the experimental value is that of Pedley," whose other data appear unreliable. Acknowledgment. This work was supported in part by grants (to M.S.G.) from the National Science Foundation (CHE8640771) and the Air Force Office of Scientific Research (87-0049). Registry No. CH3CH3,74-84-0; CH3CH2CH3,74-98-6; SiH4, 780362-5; SiH3CH3,992-94-9; (CH3)2SiH2,1111-74-6; (CH3)$iH, 993-07-7; CH,CH2SiH3,28 14-79- 1; CH3CH2CH2SiH3,13 154-66-0; CH3CH2SiH2CH3, 18230-82-5; (CZH5)2SiH2,542-91-6; CH3(CH2)4SiH3,1017798-7; CH3(CH2)3SiH2CH3,18143-64-1; (C2H5),SiH, 617-86-7; CH3(C2H5)2SiH,760-32-7; Me4%, 75-76-3; Et2SiMe2,756-81-0; Et3SiMe, 757-21-1; Et4Si, 631-36-7; Pr2SiMe2, 995-89-1; Pr3SiMe, 995-24-4; Pr,SiEt2, 994-59-2; Pr4Si, 994-66-1; silacyclopropane, 157-21-1; silacyclobutane, 287-29-6; silacyclopentane, 288-06-2; silacyclohexane, 657679-0; 1 , l -dimethylsilacyclopropane, 70262-75-8; 1, l-dimethylsilacyclobutane, 2295- 12-7; 1,l-dimethylsilacyclopentane, 1072-54-4; 1,l-dimethylsilacyclohexane, 4040-74-8. (17) Voronkov, M. G.; Baryshok, V. P.; Klyuchnikov, V. A,; Danilova, T. F.; Pepekin, V. I.; Korchagina, A. N.; Khudobin, Yu.I. J . Orgunomet. Chem. 1988, 345, 27.
