Carbene and Silylene Insertion Reactions. Ab Initio Calculations on the Effects of Fluorine Substitution

The insertion reactions of CH,, CHF, CF2, SiH2, SiHF, and SiF2 into hydrogen molecule have been investigated by ab initio molecular orbital methods. Reactants, loose clusters, transition structures, and products were optimized at HF/3-21G and HF/6-3 1G*. For each structure, relative energies have been calculated at MP4SDQ/6-3 lG* and vibrational frequencies at HF/3-21G. A dramatic increase in the barrier height is seen with fluorine substitution: 8, 64, and 197 and 51, 130, and 273 kJ mol-I for CH2, CHF, and CF2 and SiH2, SiHF, and SiF2, respectively, at MP4SDQ/6-31G*//HF/6-31G* plus AZPE. An orbital interaction interpretation is given for the trend in barrier heights, and implications for the chemistry of carbenes and silylenes are discussed. Carbenes have long attracted the attention of experimentalists' and theoretician^.^*^ For silylenes, there has recently been a surge of interest, both as second-row analogues of carbenes and as reactive intermediates in organosilicon ~hemistry.~ A wide variety of silylenes can be generated by flash photolysis and vacuum pyrolysis of silanes, halosilanes, and alkyl silane^.^ Silylenes are also throught to be important constituents in the chemical vapor deposition of amorphous silicon films from SiHd5 and possibly also from SiF4. In addition, silylenes can be formed by rearrangement of unsaturated and/or cyclic organosilicon compound$ because of silicon's reluctance to form multiple bonds, such rearrangement are often thermoneutral or ex other mi^.^ The parent silylene, SiH2, and simple substituted silylenes (SiHX, X = OH, F, C1, Br, I, CH3, SiH3; S ix2 , X = F, C1, Br, I) have been studied experimentally in some detail.* The ground states are invariably closed shell singlets with sharply bent geometries (LxSiY ca. 95O); some excited-state singlets and triplets have also been characterized.* Vibrational frequencies are available for ground and excited states of SiH2, SiHF, and SiF2 (among others) from matrix isolation studies and electronic ~ p e c t r a . l ~ ' ~ Schaefer et a1.I6 have recently carried out extensive ab initio calculations on these species and obtained excellent agreement with experiment. Once formed, silylenes can react by abstraction, dimerization, disproportionation, cycloaddition, and in~er t ion .~ Insertion re* Fellow of the Alfred P. Sloan Foundation, 1981-83. actions of SiHz have been observed with H,, Si-H, and Si-Si bonds. In contrast, SiFz is surprisingly inert in the gas phase,9 (1) Moss, R. A,; Jones, M., Jr. In "Reactive Intermediates"; Jones, M., Jr., Moss, R. A., Eds; Wiley-Interscience: New York, 1981; Vol. 2, pp 59-133. (2) For a review, see: Borden, W. T.; Davidson, E. R. Annu. Rev. Phys. Chem. 1979, 30, 125. (3) For examples of carbene insertions, particularly CH2 + H1, see: Cain, S. R.; Hoffmann, R.; Grant, E. R. J . Phys. Chem. 1981,85,4046. Kollmar, H.; Staemmler, V. Theor. Chim. Actu 1979,51,207. Jeziorek, D.; Zurawski, B. Znt. J . Quuntum Chem. 1979, 16, 277. Bauschlicher, C. W., Jr.; Haber, K.; Schaefer, H. F., 111; Bender, C. F. J. Am. Chem. SOC. 1977,99, 3610 and references cited. (4) Gaspar, P. P. In "Reactive Intermediates"; Jones, M., Jr., Moss, R. A,, Eds.; Wiley-Interscience: New York, 1981; Vol. 2, pp 335-385. (5) Scott, B. A,; Plecenik, R. M.; Simonyi, E. E.; Appl. Phys. Lett. 1981, 39, 73. Haller, I. J. VUC. Sci. Technol. 1983, I , 1376. (6) Reisenauer, H. P.; Mihm, G.; Maier, G. Angew. Chem., Znt. Ed. Engl. 1982, 21, 854. Burns, S. A,; Burns, G. T.; Barton, T. J. J . Am. Chem. SOC. 1982, 104, 6140. (7) Walsh, R. Acc. Chem. Res. 1981, 14, 246. (8) For leading references see ref 8-14. (9) Margrave, J. L.; Wilson, P. W. Acc. Chem. Res. 1971, 4, 145. (10) Lee, H. U.; DeNeufville, J. P. Chem. Phys. Letr. 1983, 99, 394. (11) Dubois. I. Cun. J . Phvs. 1968. 46. 2485. (12j Rao, V~. M.; Curl, R.'F.; Tirnms,'P. L.; Margrave, J. L. J . Chem. (13) Milligan, D. E.; Jacox, M. E. J . Chem. Phys. 1970, 52, 2594. (14) Ismail, Z. K.; Fredin, L.; Hauge, R. H.; Margrave, J. L. J . Chem. (15) Caldow, G. L.; Deely, C. M.; Turner, P. H.; Mills, I . M. Chem. Phys. Phys. 1965, 43, 2551. Phys. 1982, 77, 1626. Letf. 1981, 82, 434. 0002-7863/84/ 1506-5847$01.50/0