H. E. Audier

Catalyzed keto-enol tautomerism of ionized acetone: a Fourier transform ion cyclotron resonance mass spectrometry study of proton transport isomerization

Abstract The unimolecular isomerization of CH3COCH3+· 1 into its more stable enol counterpart CH3C(OH)CH2+· 2 is known not to occur, as a significant energy barrier separates these ions. However, it is shown in this work that this isomerization can be catalyzed within a 1 : 1 ion-neutral complex. For instance, a Fourier transform ion cyclotron resonance mass spectrometry study shows that one, and only one, molecule of isobutyronitrile catalyzes the isomerization of 1 into 2. The rather low efficiency of the reaction (12%), as well as the strong isotope effect observed when CD3COCD3+· is used as the reactant ion, suggest that the catalyzed isomerization implicates a substantial intermediate energy barrier. This was confirmed by ab initio calculations that allow us to propose an isomerization mechanism in agreement with this experiment. The efficiency of different catalysts was studied. To be efficient, the catalyst must be basic enough to abstract a proton from the methyl group of ionized acetone but not too basic to give back this proton to oxygen. In other words, the proton affinity (PA) of an efficient catalyst must lie, in a first approximation, between the PA of the radical CH3COCH2· at the carbon site (PAC) and its PA at the oxygen site (PAO), which have been determined to be, respectively, 185.5 and 195.0 kcal mol−1. Most of the neutral compounds studied follow this PA rule. The inefficiency of alcohols in the catalytic process, although their PAs lie in the right area, is discussed. Keywords: Catalyzed keto-enol tautomerism; gas-phase proton transport; isomerization kinetics; proton affinity rule; FT-ICR mass spectrometry

Gas‐phase Reactions of CH3OCH2+ with Alcohols

As a result of an extensive delocalization of charge and a unique covalent structure, the CH3OCH2+ cation has, in effect, the character of an ambident electrophile. This cation can, on the one hand, be considered to be a classical electrophile. On the other hand, it may be considered to be a facile methyl cation donor. The former character predominates when this cation reacts with alcohols, as is shown in this work. In both the chemical ionization (CI) source of a conventional mass spectrometer and also via low-pressure bimolecular reactions in a Fourier transform ion cyclotron resonance (ICR) cell, the dominant reaction between alkoxymethyl cations and alcohols is the very exothermic formation of a C—O bond to give a covalent adduct having the structure of a protonated dialkoxymethane. A 1,3-hydrogen transfer is observed for the covalent adducts. In the case of those generated in the ICR cell this process is a slow unimolecular reaction. However, the rapid 1,3-hydrogen transfer observed in the CI source is a bimolecular reaction catalysed by a second molecule of alcohol. This is a new example of catalysed isomerization in the gas phase. In competition with the 1,3-hydrogen transfer, the covalent adducts may either undergo simple bond cleavage or may isomerize to proton-bound dimer adducts of ether and aldehyde (or ketone) via a hydride transfer mechanism. This mechanism either may involve an electrostatic complex intermediate or may be an asynchronous concerted process. Since the proton affinities of the ethers involved in these proton-bound dimer intermediates are greater than those of the aldehydes derived from primary alcohols, such dimers dissociate to yield protonated ether and aldehyde. Conversely, those dimers resulting from secondary alcohols involve ketones whose proton affinities are greater than those of the partner ethers and these dimers dissociate to yield protonated ketone and ether. In summary, the reactions of CH3OCH2+ with alcohols occur via several successive and specific steps.

Complexes ion/molecule intermediaires dans la fragmentation des ethers protones

Abstract The experimental study of metastable ions [iso-C 3 H 7 O + H CH 3 ] 1 shows that the dissociation is preceded by H exchange, either by interconversion between [iso-C 3 H + 7 , CH 3 OH] “ α complex” and [C 3 H 6 , CH 3 O + H 2 “ β complex”, or by reversible isomerisation between [iso-C 3 H 7 O + H CH 3 ] 1 and [ n -C 3 H 7 O + H CH 3 ] 2 . More generally, ions [iso-C 3 H 7 O + HR] have been studied. The interconversion between [iso-C 3 H + 7 , ROH] “ α complexes” and [C 3 H 6 , ROH + 2 ] “ β complexes” is only observed if PA[ROH] — PA[C 3 H 6 ] −1 . Ab initio calculations indicate that the association 1α between the cation iso-C 3 H + 7 and CH 3 OH is stabilized by dipolar effects and by weak bonds. However 1α does not correspond to an energetic minimum and therefore to a stable form. On the contrary, the complex [C 3 H 6 , CH 3 O + H 2 ] 1β is a stable form. The stabilization energy Δ H s [ 1β ] is about 12 kcal mol −1 .

Isomerisation des ions alkyl-furannes et alkyl-pyrannes protones en phase gazeuse

Abstract In the gas phase, protonated 2,5-dimethylfuran and ethyl-2 furan are isomerized through common intermediates before cleavage. The reaction pathway requires ring enlargement and ring contraction steps.

Gas-phase unimolecular reactivity of C3H7O+cations : a combined mass spectrometric-molecular orbital study

The unimolecular dissociations of the two isomeric ions [CH 3 CH 2 CHOH] + (1) and [CH 3 CH 2 BCH 2 ] + (2) were re-examined. Molecular orbital calculations conducted at the MP2/6-31G * //HF/6-31G * + ZPE level were used to characterize the corresponding potential energy profile. The experimental data were completed by a Fourier transform ion cyclotron resonance spectrometric investigation on the system [CH2OH] + + C 2 H 4 and by a study of various metastable [C 3 H 7 O] + ions the isomerization pathway of lowest energy connecting 1 and 2 involves two ion-neutral complexes between protonated formaldehyde and ethene. The isomerization 1=2 is typically a complex mediated reaction since the key step consists simply of the reorientation of the two partners [CH 2 OH] + and C 2 H 4 inside the ion-neutral cage. The model is demonstrated to account for the H-D exchange observed during the dissociation of variously deuterated species.

Reactions of silyl cations with ketones in the gas phase

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.

Ambident reactivity and characterization of small ionized carbenes

Abstract The gas phase reactions of five ionized carbenes, HCOH + 1, HCNH2 + 2, CH3COH + 3, HOCOH + 4 and HOCNH2 + 5 with different molecules are studied by FT-ICR mass spectrometry. Interaction between an ionized carbene and a molecule can yield two kinds of stable adducts, as expected from the electronic structure of the carbene radical cations, explaining the ambident reactivity of these ions. The first kind of adduct corresponds to H-bonded species (hydrogen-bridged radical cations), the second to covalent structures. Since interconversion between these adducts is generally slow, each kind of adduct leads to a particular set of reactions. The H-bonded species can be involved in the protonation of the neutral as well as in the catalyzed interconversion between the carbene and its conventional radical cation counterpart. The covalent adducts, formed by reaction of ionized carbenes with methanal and alkenes, are β-distonic ions. Reactions with labeled propene show that the so formed distonic ions either dissociate by simple cleavage or undergo rearrangements and H-exchange after isomerization into conventional ions by 1,4-H transfers. Cyclopropane gives a characteristic reaction of the carbene structure: addition yields a γ-distonic ion which loses ethylene. Finally, H , I and SCH3 abstraction from appropriate neutrals confirms a radical reactivity of the carbenic carbon.

Deprotonation of α-distonic ions. Proton affinities of the α-radicals

Abstract The proton affinity at the heteroatom PAX of four α-radicals (CH2OH, CH3CHOH, CH2OCH3 and CH2NH2) was measured by studying the deprotonation of the corresponding α-distonic ions in the cell of a FTICR spectrometer. This method can only be used for α-distonic ions which are more stable than their molecular ion counterpart. It was found that the PAX of the CH2OH, CH3CHOH, CH2OCH3 and CH2NH2 α-radicals lies respectively 15.7, 14.5, 10.1 and 17.2 kcal mol−1 under that of CH3OH, CH3CH2OH, CH3OCH3 and CH3NH2. These results are in good agreement with the PA obtained by high level ab initio calculations.

Reactions ion-molecule en phase gazeuse : VIII reactions competitives SNi et SN2 induites par le systeme NH3 /NH4+ sur les isomeres cis et trans de l' indanediol-1,2

Abstract Under Chemical lonisation Conditions,cis and trans 1,2-indanediols react with the NH3/NH4+, system via a nucleophilic substitution process. Competition between the SN2 and SNi mechanisms for this substitution depends on the stereochemistry of the diol.

Transposition of linear alkyl-chains into ramificated chains.

Abstract It is shown by the study of alkyl ethers and benzoates, that formation of [C n H 2n ] + ions is preceded by an isomerisation of the alkyl-chains leading to a substituted ethylenic ion. A mechanism of isomerisation is proposed for the respective intensities of peaks and the values of appearance energies in the spectra of labelled compounds. The fragmentation is initiated by the shift of one H at position 3 leading to an intermediary complex in which a neutral molecule interacts with a [C n H 2n ] + ion.