Gas basicities (GBs) and proton affinities (PAs) of CH3SiHO, CH3CH2SiHO, (CH3)2SiO (CH3CH2)2SiO, were determined by using the kinetic method as well as calculations. From experiment, GB of these molecules were respectively measured to be 203.3, 206.3, 213.4 and 218.0kcalmol−1. The precision is estimated to be 0.5kcalmol−1. Calculations are in excellent agreement with experiment.

In the second field-free region of a double focusing mass spectrometer, the spontaneous fragmentation of the (CH3)(3)C-O-C+ (CD3)(2) ion is not preceded by direct methyl cation migrations. In contrast, a complete exchange of the methyl groups occurs prior to dissociation of the (CH3)(3)Si-O-Si+(CD3)(2) ion. This is in agreement with the calculated energy diagram which shows that two factors explain this behavior.(i) The simple cleavage of (CH3)(3)C-O-C+(CD3)(2) giving (CH3)(3)(C+) only requires 25.8 kcal mol(-1) while the corresponding fragmentation in (CH3)(3)Si-O-Si+(CD3)(2) necessitates 74.2 kcal mol(-1).(ii) The transition state for the 1,3-CH3+ transfer from carbon to carbon lies in energy 59.1 kcal mol(-1) above the covalent structure (CH3)(3)C-O-C+(CD3)(2) and only 12.4 kcal mol(-1) for the 1,3-CH3+ transfer from silicon to silicon.Calculations and experiments show that the situation is intermediate for the 1,3-CH3+ from silicon to carbon. (Int J Mass Spectrom 217 (2002) 245-255) (C) 2002 Published by Elsevier Science B.V.

CH2=Si(CH3)OSi(CH3)2+ ions 1 were formed in the external source of a Fourier-transform ion cyclotron resonance (FT-ICR)mass spectrometer by fragmentation of (CH)3SiOSi(CH3)3. In the cell, their reactions with acetaldehyde were studied by using labeled reactants and compared with those with acetone, studied in previous work, which gives a general view of the reactions of ion 1 with carbonyl compounds. In contrast with the behavior of the corresponding carbon-containing ions, the results confirm facile 1,3-methyl group transfers from silicon to silicon. Otherwise, the silicon-containing adducts often eliminate C nH2 n. This process, not observed in (C,O,H)-containing ions, is studied by using selectively deuterated reactants.

The unimolecular reaction of the metastable tert-butyl vinyl ether radical cation leads mainly to elimination of ethylene. From metastable ion studies as well as Fourier transform ion cyclotron resonance measurements and by using isotopic labeling, it is proposed that the fragmentation begins with the cleavage of the O–C(butyl) bond. This behavior, which contrasts with that of ionized alkylethers, is the consequence of the presence of a vinyl group that allows the formation of a neutral allylic radical. The key intermediate of the fragmentation is likely to be a [CH 2 CHOH, (CH 3 ) 2 CCH 2 ] •+ complex in which a cycloaddition–cycloreversion reaction occurs. This intermediate is the same as that produced by a McLafferty rearrangement in appropriate aldehyde radical cations.

In the gas phase, (CH(3))(3)SiOSi(+)(CH(3))(2) and (CH(3))CH(2)SiOSi(+)(CH(3))(2) ions 1 and 2 were formed in the external source of a Fourier transform ion cyclofrom resonance (FT-ICR) spectrometer by electron impact ionization of (CH(3))(3)SiOSi(CH(3))(3). In the FT-ICR cell, the electrophilic center of these ions reacts with acetone to give product ions whose structures are probed by comparison with those of the products formed by reaction with water. The mechanisms of formation of these products, studied by labeling, involve facile 1,3-methyl transfer from silicon to silicon and cyclic intermediates. Copyright 1999 John Wiley & Sons, Ltd.

In the gas phase, the unimolecular reactions of metastable protonated alkyl arenes lead to alkyl cation formation with arene loss, protonated arene formation with alkene loss, and benzylic cation formation with alkane elimination. The dissociations are often preceded by an exchange between the hydrogen atoms of the chain and those of the ring, which can be rationalized by an interconversion between [arene alkyl cation] and [protonated arene alkene] ion-neutral complexes. By comparing the reactions of the metastable protonated alkyl arenes and those of their isomeric adducts (arene/alkyl cation) generated in the ion source by ion-molecule reaction, we show that ct-complexes [arene alkyl cation] have a significant lifetime. By ab initio calculations, [C6H6 iso-C3H7+] and [C6H7+ C3H6] ion-neutral complexes are shown to be energy minima on the potential energy surface. They correspond therefore to stable structures. Two methods, namely, ab initio and semiempirical calculations, are used to calculate the interaction energies of [arene alkyl cation] complexes. The smallest stabilization energy (49 kJ/mol) corresponds to [C6H6 tert-C4H9+] and the largest (65 kJ/mol) corresponds to [p-CH3C6H4CH(3) iso-C3H7+] The [protonated arene alkene] complexes are less stabilized (ca. 25 kJ/mol). The calculations also indicate that [arene alkyl cation] complexes have pi-structures, but do not correspond to electron donor-acceptor (EDA) systems since, among the components of their interaction energy, the charge exchange term is negligible. Finally, these results are used to propose energy diagrams and fragmentation pathways.

AbstractIn the gas phase, π‐complexes are common intermediates in unimolecular reactions of metastable protonated alkylbenzenes. The transient formation of two π‐complexes generated from protonated dialikylbenzenes has been proposed in the Literature, even when the substitution degree of the two benzylic carbons is different. For metastable tert‐C4H9C6H5R+ cations (R ‐ n‐C4H9, sec‐C4H9, iso‐C4H9), we demonstrate in this paper that the [tert‐C4H, C6H5‐R] π‐complex is produced and does not interconvert into the [tert‐C4H9C6H5, R+] π‐complex prior to fragmentation.

In the gas phase, sc-complexes are common intermediates in unimolecular reactions of metastable protonated alkylbenzenes. The transient formation of two sc-complexes generated from protonated dialkylbenzenes has been proposed in the Literature, even when the substitution degree of the two benzylic carbons is different, For metastable tert-C4H9-C6H5-R(+) cations (R = n-C4H9, sec-C4H9, iso-C4H9), we demonstrate in this paper that the [tert-C4H9+ C6H5-R] pi-complex is produced and does not interconvert into the [tert-C4H9-C6H5, R(+)] pi-complex prior to fragmentation.

The first step in the unimolecular reaction of metastable protonated alkylbenzenes is the stretching of the C(benzene)-C(alpha) bond. Therefore, the intermediacy of a pi-complex [C6H6, alkyl+] has often been proposed. In this work, the kinetic energy releases associated with the [ tert-alkyl ]product were measured for a large number of [C6H6-CnH2n+1+] (n > 3) ions. At least for beta-branched alkylbenzenes, it is shown that the chain isomerization which occurs prior to dissociation involves neither the [C6H6, CnH2n+1+] pi-complex nor a [C6H7+, alkene] ion-neutral complex. The data are explained by a concerted process in which the stretching of the C(benzene)-C(alpha) bond is accompanied by the migration of the tertiary beta-hydrogen from C(beta) to C(alpha).

The addition-fragmentation reactions of phenyl-substituted enol radical cations, C6H5C(OH)CHR+. (R = H, CH3), with alkenes were studied by Fourier transform ion cyclotron resonance spectroscopy. These reactions (involving both labelled and unlabelled reactants) were compared with the unimolecular reactions of aromatic ketone molecular ions and related species that give rise to C6H5C(OH)CHR+. and alkene products. It is shown that the phenyl-substituted enol ions react with alkenes by C-C bond formation leading to an intermediate gamma-distonic ion; the reactions are not accompanied by cyclization to cyclobutanol intermediates.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXT[C6H6iso-C3H7+] and [C6H7+C3H6] ion-molecule complexes: theoretical calculationsD. Berthomieu, V. Brenner, G. Ohanessian, J. P. Denhez, P. Millie, and H. E. AudierCite this: J. Am. Chem. Soc. 1993, 115, 6, 2505–2507Publication Date (Print):March 1, 1993Publication History Published online1 May 2002Published inissue 1 March 1993https://doi.org/10.1021/ja00059a055RIGHTS & PERMISSIONSArticle Views54Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (414 KB) Get e-Alertsclose Get e-Alerts

Alkylation of arenes by propyl iodides in the gas phase has been studied under chemical ionisation conditions by mass spectrometry. Whereas i-C3H7l gives an isopropylated product without preliminary H exchange, n-C3H7l affords either a n-propylated ion or an isopropylated ion, according to the pressure and relative concentrations in the source. The reactant is [n-C3H7l]+. in the first case, and [i-C3H7]+ in the second one. An ICR study confirms that last result. Protonated dipropylbenzenes have been studied to elucidate the mechanisms of these alkylations, and the rate determining step of the unimolecular dissociations of these ions have been established.

Alkylation of arenes by propyl iodides in the gas phase has been studied under chemical ionisation conditions by mass spectrometry. Whereas i-C 3 H 7 I gives an isopropylated product without preliminary H exchange, n-C 3 H 7 I affords either a n-propylated ion or an isopropylated ion, according to the pressure and relative concentrations in the source. The reactant is [n-C 3 H 7 I] +• in the first case, and [i-C 3 H 7 ] + in the second one. An ICR study confirms that last result. Protonated dipropylbenzenes have been studied to elucidate the mechanisms of these alkylations, and the rate determining step of the unimolecular dissociations of these ions have been established

From the mass-analysed ion kinetic energy spectra of labelled ions, kinetic energy releases and thermodynamic data, it is proved that protonated n-propylbenzene (1) isomerizes into protonated isopropylbenzene (2). It is also shown that the dissociation of the less energetic metastable ions of (2), leading to ]iso-C3H7]+ and [C6H7]+ product ions, is preceded by H exchange. This H exchange involves two interconverting ion-neutral complexes [C6H6,iso-C3H7+) (2-pi) and [C6H7+, C3H6] (2-alpha).

AbstractLabelled derivatives of the 5‐methyl‐2‐hexanone radical cation have permitted the elucidation of the origin of the main fragment ion [MC2H5]+ and the characterization of different rearrangement processes.

AbstractNous proposons dans ce travail deux méthodes générales de synthése de composés trideutériés sur le méthyle terminal d'une chaine aliphatique. Dans les deux cas. l'hydrolyse de l'un des groupements esters constitue la premiére étape du schéa réactionnel. L'acide ester obtenu est transformé i) en aldéhyde ester; une réaction de Wittig conduit au vinyl ester. Le groupement ester est réduit d'abord en alcool 1‐2H2, puis par l'intermédiaire du mésylate en ω‐2H3 l‐alcene. L'oxydation de ce composé donne l'acide ω‐2H3. ii) en alcool ester. La fonction alcool est protégée par le dihydropyranne et le groupement ester est traité comme ci‐dessus. On obtient ainsi l'alcool ω‐2H3.

Les cations radicaux [CH3-(CH2)n-1-NH2]+. s'isomérisent en phase gazeuse avant dissociation. La réaction débute par le transfert réversible d'un hydrogène en position 4, 5 ou 6 sur l'azote, se poursuit par le transfert d'un hyrogène en position 2 sur le site radicalaire ainsi créé et conduit à un ion adduit [alcéne-1, NH3]+.. Ce même ion adduit peut être aussi engendré par réaction ion-molécule ; il se dissocie en donnant un ion abondant CH3-C+H-NH2 après des transferts 1–5 et 1–6 d'hydrogène.

AbstractThe metastable fragmentations of the amines (I) have been studied by the MIKE technique.