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A Quantum Chemical Study of Hydrogen Abstraction from Silane by Methyl Radical

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A Quantum Chemical Study of Hydrogen Abstraction from Silane by Methyl Radical Indian Journal of Chemistry . Vol. 32A, July \.993, pp. 557-560 f A quantum chemical study of hydrogen abstraction from silane by methyl radical E J Padma Malar Department of Physical Chemistry, University of Madras, Guindy Campus, Madras 600 025, India Received 23 November 1992; accepted 5 January 1993 The potential profile for the hydrogen abstraction reaction from silane by methyl radical is com• puted quantum chemically. The all-valence electrons semiempirical SCF MO methods SINDOI and MNDo predict comparable activation barriers of 68.5 and 75.2 kJ mol-I respectively. The lower activation barrier of the title reaction as compared to that of the thermoneutral reaction CH) + CH4 -+ CH4 + CH) reflects the greater ease of abstracting a hydrogen atom from silane than that from methane. This is in agreement with the. lower bond dissociation energy in silane (377.5 kJ mol-I) than that in methane (431.4 kJ mol-t). Hydrogen atom abstraction from organic sub• the abstraction of hydrogen by triplet methylene strates by odd electron species such as inorganic and silylene from silane3f. So far, there is no re• and organic radicals, diradicals and excited states port of theoretical studies of hydrogen abstraction is a subject of many studies1•2• These reactions from silane by alkyl radicals. The aim of the pres• play an important role in the propagation of many ent work is to investigate quantum-chemically the chain reactions. A number of theoretical investi• hydrogen abstraction reaction from the parent si• gations of hydrogen abstraction reactions using lane by methyl radical: quantum chemical techniques at varying levels of sophistication have contributed towards the un• CHj + SiH4 -+ CH4 + SiHj ... (2) derstanding of the potential profiles along the reaction paths, activation barriers, structures of Computational procedure the transition states, tunneling, etc.3 However, Hydrogen atom abstraction from silane by the considerably less work has been reported4 on the methyl radical has been studied using the all-val• abstraction of hydrogen from the silicon ana• ence electrons semiempirical SCF MO method logues of the substrates. Chatgilialoglu5 has re• SIND019• Our recent analysis of hydrogen ab• cently shown that tris(trimethylsilyl)silane is a val• straction from methane by methyl radical using, uable reducing agent for a variety of organic sub• the SINDOI method yields activation barrier and strates. The key step in the reduction reactions in• transition state geometry comparable to ab initio volves hydrogen abstraction from the silane deri• results3n. This method includes d-orbitals in the vative: basis set of the second-row atoms9h; earlier stud• ies show good agreement with 6-31 G* and R + (Me3SihSiH -+ RH + (Me3Si)3Si' '" (1) experimental results for compounds containing The trialkyl silyl radicals are found to be highly them 10,11. We have also studied the reaction using reactive species towards various organic function• the widely applied MINDO/312 and MNDO\3 al groupS6. However, the corresponding silanes methods. are rather poor hydrogen atom donors towards The reaction was studied by treating the com• alkyl radicals under normal conditions 7• Substitu• posite system as a supermolecule. Since the com• tion of the silyl hydrogens results in significant posite system contains an odd electron, comput• improvement in the hydrogen donor ability of the ations were performed using the unrestricted Har• organosilane8• A knowledge of the potential pro• tree-Fock formalism in the SINDOI and MIN• files and activation barriers of these reactions will DO/3 methods. The MNDO method treats the be helpful in the design of new radical reactions. open-shell system by the restricted Hartree-Fock Theoretical investigations of hydrogen abstrac• formalism but the half-electron correction is in• tion from silane are limited. Gordon has studied troduced 13. The distance R between the methyl ,: ir 558 INDIAN J CHEM, SEe. A, JULY 1993 R > 3.0 A, ~ E is nearly zero and the geometries oj the methyl and silane units are intact. When R is decreased, the geometries of the constituent un• 'if"',q - - - - - - "/' ---H1---Si its reorganise to achieve minimum energy configu• ration. The total energy of the composite system I ,!~\ increases till the transition state is reached. Struc• H5 H~ HJ tural optimisation along the reaction path reveals Fig. 1-- Definition of the reaction coordinates and the atomic that _the linear approach of the reactants with , labelling used in the study. L CHI Si= 1800 is the energetically preferred path. Computations using the three methods in• dicate that at R > 3.0 A, the total system has a 8 D·D C3v symmetry with the methyl group in a plane perpendicular to the plane containing HISiHz. Both the methyl and silane fragments gradually change into pyramidal structures as they are 41)·0 brought closer (R < 3.0 A). The silyl group main• tains a pyramidal structure throughout the reac• 20·0 tion coordinate. SiHI length (R') increases along the course of the reaction till the bond is broken and the products are formed. The study shows that the silyl radical possess pyramidal geometry around silicon, in agreement with earlier theoreti• cal3e and ESR4b findings. The C3v structure of the transition state of the above reaction is depicted in Fig. 3. The methyl group has reorganised considerably in the trans• ition state as reflected by the bond angles and 1 1 dihedral angles. However, the major change in the 25 1·5 10 silane unit is that the SiH1 bond is stretched. in the transition state as compared to the SiH bond Fig. 2 - Poterjtial profiles of the hydrogen abstraction reaction length in silane. The increase in SiH1 length is re• spectively 0.08, 0.03 and 0.07 A according to from silaneM1NPO/3;~y methyland[---'-'-'- curve curveI, SINDOI;3, MNDOj.---- curve 2, SIND01, MINDO/3 and MNDO calculations. In Table 1, the activation barrier of the title carbon an4 hydrogen atom H I of silane which is reaction is compared with that of the thermoneu• tral reaction. abstracted,j was taken as the reaction coordinate as shown *1 Fig. 1. Calculations were done star• ... (3) ting with jthe reactants separated by large R (3.5 A). T~e energetics of the reaction were exa• The SINDO] and the MNDO methods yield comparable activation barriers of 68.5 and lueminedof R,by a~1~raduallythe remainingdecreasinggeometricalR. For aparametersgiven va• 75.2 kJ mol~ I respectively for the reaction (2). of the comjposite system were optimised. Geome• MINDO/3 predicts consistently lower barriers for try optimi*tion was carried out by the Newton• both the reactions (2) and (3). It is noticed that the activation barrier for the methyl radical towards Raphsonby the FI~tcher-PoweJJPlroccdure in thealgorithmSINDO 1inmethodboth andthe abstraction of hydrogen from silane is much lower MINDO;]' and MNDO methods. The transition than that from methane. An analogous conclusion state of th¢ reaction was located in the potential has been drawn by Gordon3f who obtained a bar• hypersurfade and the activation barrier was deter- rier of 38.0 kJ mol~1 for the abstraction of a hy• mined. ' '. drogen atom from silane by triplet methylene while barrier is 74.8 kJ mol- 1 for the reaction Results an~ Discussion 3CHz + CH4 -> CH) + CH). The smaller barrier in Figure 2 ~hows the potential profiles for the title reaction (2) relative to that in reaction (3) reflects reaction. ~fE is the relative energy of the compo• the greater ease of abstracting a hydrogen atom site system iwith reference to the sum of the ener• from silane than from methane. This observation gies of the i separated reactants. It is seen that at is in accordance with the bond dissociation ener- !'.• lit! I I ;I; I 1'1111'i If Illtlf ,I ·1id ~ I rl Ill' ~ '1'1 PADMA MALAR: HYDROGEN ABSTRACfION FROM SILANE BY METHYL RADICAL 559 1·46!; (1·46!;1 1·!;43 1100 [1.434) 113.5 (1·709) (109·1) (1\3.5) ------H1--------11·6!;0) • 1110·6] 1117.4) 10!;·0 1.!;4!; (10!;·0) (1.!;Q~) I 99·!;] [HOll 1·076 H!; (1.096) '1-\, 11·0a!;) HI HS C H6 ' 114·0 (114.1)[10!;·3) II, HzSI HI' 120·6 (119.!;1l121.3) HI Hs CH, :-114·0 (-114·1)[-10!;·3) HI HZ Si Hit ' -120·6 (-ml.!;)[-121.3) Fig. 3-SINDOl, (MINOO/3) and [MNDO] optimised structural parameters of the transition state. Bond lengths are in angstroms and bond angles and dihedral angles are in degrees .. Table I-Activation energies and heats of formation of the transition states Activation energy Heat of formation (kJ mol-I) (kJ mol-I) Reaction 231.1MINOO/3MINDO/321.0109.0-173.9b28.4b75.2230.0129.3MNOO187.2MNOO-ab initio SINDOI112.0'68.5 CHjCHj+ CH4+ SiH4- CH4- CH4+ CHj+ SiHj (a) ref. 3n; (b) ref. 3m. gies of the substrates: 377.5 kJ mol-1 for SiH4 Acknowledgement and 431.4 kJ mol-1 for methanel5. In thank Prof. A K Chandra, Indian Institute of The strength of the Si-H bond in silane is Science, Bangalore for his keen interest in this found to decrease when the silyl hydrogens are work and djscussions. Computations were per• substituted by alkyl, alkoxy, substituted silyl, al• formed using the DEC 1090 computer at the In• kylthio and phenyl groups5.8.14.Si-H bond weake• dian Institute of Science, Bangalore and a PC/ AT• ning is associated with the increased reactivity of 386. Financial support by the UGC, New Delhi is the silane towards hydrogen abstraction by radi• gratefully acknowledged. cals8. In the case of alkyl substituted hydrocar• bons, experimental16 and theoretical17 studies led References to the conclusion that the reactivity towards hy• 1 For reviews see: drogen atom abstraction by methyl radical follows (a) PlatzMS,Acc Chem Res, 21 (1988) 241.
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