A high-pressure mass spectrometric study of the binding of trimethylsilylium to oxygen and aromatic bases

Chem. 66, 3099 (1988).Aromatic desilylation and detertbutylation by CH5+, CzH5+, i-C3H7+, and t-C4H9+ ions have been studied in the gas phase by mass spectrometric and radiolytic techniques. The higher rate of desilylation than of dealkylation is traced to the step involving the formation of ipso arenium ions, protonated at the ring carbon bearing the SiMe, or the CMe3 substituent. The latter has been shown by a b initio calculations at the SCF STO-3G level to selectively depress the basicity of the ipsoposition, hence the stability of the corresponding arenium ion relative to its protomers, e.g. the basicity of the ipso position of PhCMe, is lower by ca. 1 1 kcal mol-' than of the para position. The %Me3 group has no comparable effect, very close basicities being computed for the para and the ipso position of PhSiMe,. The different effects of the CMe3 and SiMe3 groups are traced to electronic and steric factors.FULVIO CACACE, M. ELISA CRESTONI, GIULIA DE PETRIS, SIMONETTA FORNARINI et FELICE GRANDINETTI. Can. J. Chem. 66, 3099 (1988).Operant en phase gazeuse et faisant appel 2 des techniques de spectromttrie de masse et de radiolyse, on a ttudiC les rCactions de dt-tert-butylation et de dCsilylation aromatiques par les ions CH5+, C2H5+, i-C3H7+, et tert-C4H9+. Le fait que la vitesse de dCsilylation est plus rapide que celle de ladealkylation est attribue a l'Ctape impliquant la formation des ions ipso arCnium qui sont protones au niveau du cycle carbone portant les substituants SiMe, ou CMe3. Faisant appel a des calculs ab initio, en champ auto-cohdrent, au niveau STO-3G, on a dCmontre que, par rapport A ses protomeres, le dernier abaisse la basicitt de la position ipso et, par constquent, la stabilitC de l'ion arCnium correspondant; par exemple, la basicitt de la position ipso du PhCMe, est environ 11 kcal mol-' plus faible que celle de la position para. Le groupement SiMe3 n'a pas d'effet semblable et les calculs fournissent des valeurs trks comparables pour les positions para et ipso du PhSiMe3. On attribue les effets difftrents des groupements CMe3 et SiMe, a des facteurs Clectronique et sttrique. [Traduit par la revue] IntroductionElectrophilic cleavage of aryl-silicon bonds, observed as early as in 1901 (I), has been characterized by the systematic work of Eaborn (2, 3) as a typical aromatic substitution, whose rate-determining step is the formation of the ipso u complex.We report here extension of the work to the gas phase by an integrated approach based on the coordinated application of mass spectrometric, radiolytic, and computational techniques (4). The same approach has recently allowed us to demonstrate the first example of the inverse process, the direct aromatic substitution by SiMe3+ ( 5 , 6), unprecedented in solution, nor previously characterized in the gas phase.In order to provide a proper perspective, and in view of the close correlations between carbon and silicon chemistry, a parallel investigation has been carried out of gas-phase aromatic detertb~t~lation, the subject of earlier detaile...

Section: The Ionic Reactions and Their Energeticsmentioning

confidence: 99%

A comparative study of gas phase aromatic desilylation and detertbutylation by charged electrophiles

Cacace¹,

Crestoni²,

Petris³

et al. 1988

“…The tendency for transfer of a silicon-containing group rather than the proton has been used to obtain both relative (CH3),Si' affinities using ICR (3) and also absolute binding energies by the equilibrium method in a pulsed electron beam, high pressure ion source mass spectrometer (HPMS) (4). An interesting observation in the latter study was that the binding energies of (CH,)$i+ to aromatic hydrocarbons is considerably lower than that to bases such as aliphatic alcohols, esters, and ethers.…”

Section: Introductionmentioning

confidence: 99%

The gas phase ion chemistry and proton affinity of hexamethyldisiloxane studied by high pressure mass spectrometry

Li¹,

Stone²

1987

Self Cite

XIAOPING Li and JOHN ALFRED STONE. Can. J. Chem. 65, 2454 (1987). The ion chemistry of hexamethyldisiloxane ((CH3)3SiOSi(CH3)3, HMDS) has been studied under chemical ionization conditions at ion source temperatures of 300-600 K and pressures of 2-4Torr. Highly exothermic proton transfer to HMDS from CH5+ and C2H5+ leads mainly to loss of CH3 but with decreasing exothermicity the yield of HMDSH' increases such that transfer from t-C4H9+ (AH' = -7.5 kcal mol-I) yields almost exclusively HMDSH'. Although HMDSH+ transfers (CH3)3Si+ rather than a proton to most reference bases, the proton affinity of HMDS has been determined from van't Hoff plots using the equilibrium method with methylaromatics as reference bases. PA(HMDS) = 203.4 kcal mol-I is in excellent accord with an earlier estimate of 203 kcal mol-' obtained by the bracketing method. The rate constants for these proton transfer reactions show very large negative temperature coefficients in the exothermic directions which are consistent with the reactions of charge delocalized ions and/or reactions in which considerable loss of rotational freedom occurs along the reaction coordinate. The rate constants in the reverse directions have a positive temperature coefficient only when the endothermicity is significant (>2.5 kcal mol-I). OpCrant dans des conditions d'ionisation chimique, A des tempkratures de la source ionique allant de 300 A 600 K et 2 i des pressions de 2 i 4 Torr, on a CtudiC la chimie ionique de l'hexamCthyldisiloxane ((CH3)3SiOSi(CH3)3, HMDS). Les transferts de proton des cations CH5+ et C2H5+ au HMDS sont tres exothermiques et ils conduisent principalement i une perte de CH3; toutefois, lorsque 1'exothermicitC diminue, le rendement en HMDSH' augmente d'une f a~o n telle que le transfert du t-C4H9+ (AHo = -7,5 kcal mol-I) conduit presqu'exclusivement au HMDSH'. Pour la plupart des bases de rtfkrence, le HMDSH+ transfkre du (CH3)&+ plutBt qu'un proton; malgrt ce fait, on a dCterminC l'affinitt protonique du HMDS i partir des courbes de van't Hoff et en utilisant la mtthode des Cquilibres des produits mCthylaromatiques avec les bases de rCfCrence. La valeur de PA(HMDS) = 203,4 kcal mol-' est en excellent accord avec une Cvaluation antkrieure de 203 kcal mol-' obtenue A l'aide de la mtthode du "bracketing". Les constantes de vitesse pour ces reactions de transfert de proton prksentent d'importants coefficients nCgatifs de tempkrature dans les directions exothermiques et ceux-ci sont en accord avec les reactions d'ions possedant des charges dClocalisCes et/ou des rtactions dans lesquelles il se produit une importante perte de libertC rotationnelle le long des coordonn~es de la rCaction. Dans la direction inverse, les constantes de vitesshe possedent un coefficient de tempirature positif que lorsque I'endothermicitC est importante (>2,5 kcal mol-I).[Traduit par la revue]

“…The major unimolecular decomposition leads to regeneration of (CH3),Si+. It could not be determined whether the two silicon atoms in the reactants which form (CH3)7Si2+ lose their identities or whether the reversal of reaction [5] preserves the initial identities. The only other significant unimolecular dissociation leads to m/z 87 which arises by loss of trimethylsilane.…”

Section: Resultsmentioning

confidence: 99%

“…1. The reagent ions from methane ( C H~+ and C2H5+) react with (CH3),Si to give (CH3),Si+ and the subsequent reaction of this ion with (CH3),Si and adventitious water leads to the array of ions shown in the figure (5). It is immediately apparent that the reaction leading to (CH3)7Si2+ is much faster than any other reaction of (CH,),Si' and indeed the build-up of the (CH3)7Si2+ ion intensity is not observed on the time scale shown.…”

Section: Resultsmentioning

confidence: 99%

The formation of (CH₃)₇Si₂⁺ in (CH₃)₄Si/CH₄ mixtures and CH₃⁻ exchange reactions between (CH₃)₄Si, (CH₃)₄Ge, and (CH₃)₄Sn studied by high pressure mass spectrometry

Wojtyniak¹,

Li²,

Stone³

1987

. J. Chem. 65, 2849Chem. 65, (1987. IThe association equilibrium (CH3)4Si + (CH3)3Si+ (CH3)7SiZ+ has been studied in a high pressure mass spectrometer ion source using tetramethylsilane/methane mixtures. Measurement of the equilibrium constant over a range of temperatures yields AH0 = -22.3 2 0.4 kcal mol-I and AS0 = -35.2 ? 0.9 cal mol-I K-'. Collision-assisted dissociation experiments suggest that the methyl groups retain their integrity in (CH3)7Si2+ Mixed ions such as (CH3)7SiGe+ and (CH3)7GeSn+ were not observed in mixtures of (CH3)4X and (CH3)4Y (X + Y = Si, Ge, Sn). Instead CH3-transfer equilibrium reactions were observed viz. (CH3)3Si+ + (CH3)4Ge e (CH3)4Si + (CH3)3Ge+ (AH0 = -10.2 5 1.2 kcal mol-I, AS0 = -3.7 ? 2.4cal K -I mol-I) and (CH3)3Ge+ + (CH3)4Sn = (CH3)4Ge + (CH3)3Sn+ (AH0 = -8.1 2 0.9 kcal mol-l, AS' = -0.9 2 1.6 cal K-' mol-I).These are in excellent agreement with some published differences in appearance potentials for (CH3)3X+ from (CH3)4X (X = Si, Ge, Sn). (X + Y = Si, Ge, Sn). Toutefois, on a observC des riactions d'kquilibre de transfert de CH3-; par exemple, (CH3)3Si+ + (CH3)4Ge (CH3)4Si + (CH3)3Ge+ (AH0 = -10,2 5 1,2 kcal mol-', AS0 = -3,7 ? 2,4 cal K-' mol-') et (CH3)3Ge+ + (CH3)4Sn = (CH3)4Ge + (CH3)3Sn+ (AH0 = -8 , l t 0,9 kcal mol-l, AS0 = -0,9 2 1,6 cal K-' mol-I). Ces valeurs sont en bon accord avec des valeurs publikes pour les diffkrences dans les potentiels d'apparition des ions (CH3)3X+ ii partir des