P. Mourgues

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

“Proton-transport” catalysis in the gas phase. Keto-enol isomerization of ionized acetaldehyde

Abstract Fourier transform ion cyclotron resonance experiments show that a variety of molecules catalyze the hydrogen transfer which converts ionized acetaldehyde CH 3 CHO ·+ 1 to its vinyl alcohol counterpart CH 2 CHOH ·+ 2 . Each of these ions has been characterized by its specific bimolecular reactions with selected reactants. Calculations show that two pathways, for which the rate determining barriers have almost the same energy, are feasible. The first transition state involves a direct catalyzed 1,3-H transfer, while the second involves two successive 1,2-H transfers. A detailed experimental study, using methanol as a catalyst as well as labeled reactants, indicates that only the first pathway operates in the isomerization process. The different steps of these two independent pathways were elucidated. The first begins with the formation of a highly stabilized complex 3 , involving a two-center-three-electron interaction between the two oxygen atoms and an interaction between a hydrogen of the methyl group of 1 and the oxygen of methanol. This complex isomerizes into a complex 4 , which in turn gives the complex 5 , via a transition state located 6.3 kcal mol −1 below the energy of the reactants. This complex 5 corresponds to ionized vinyl alcohol hydrogen bonded to the oxygen of methanol, which dissociates to yield ion 2 . The second pathway begins with the interaction between the hydrogen of the CHO group and the oxygen of methanol and gives the complexes 6 and then 7 , which correspond to protonated methanol hydrogen bonded to a CH 3 CO · radical. Dissociation of 7 to give protonated methanol is favoured with respect to further isomerization leading to ionized vinyl alcohol. Compared to the unimolecular conversion between energetic ions 1 and 2 , which can occur either by a direct 1,3-H transfer or by a double 1,2-H transfer, the reaction of 1 with methanol catalyzes the first pathway while inhibiting the second one. In the case studied, catalysis is perhaps better described as a hydrogen atom transport.

Proton Affinity and Heat of Formation of Vinyloxy [CH2CHO]. and Acetonyl [CH2COCH3]. Radicals

Frequently found in hydrocarbon oxidation and in the photochemistry of carbonyl compounds, the β-carbonyl radicals are of interest. The experimental proton affinities of the two title radicals have been determined from proton transfer reactions (as shown) monitored in an FT-ICR mass spectrometer. This led to an estimation of their heats of formation (1: 13±3; 2: -34±3 kJ mol(-1)). Ab initio molecular orbital calculations, up to the G2 level, confirmed these results.

Two different pathways for unimolecular and for catalyzed keto-enol isomerization of ionized acetophenone

Abstract Studies of the unimolecular reactions in the gas phase of the C 6 H 5 COCH 3 +· ( 1 ) and C 6 H 5 C(OH)CH 2 +· ( 2 ) ions have shown (1) that ion 1 does not convert to ion 2 prior to methyl radical loss, (2) that ion 2 isomerizes into ion 1 prior methyl radical loss, and (3) that this keto-enol isomerization does not occur by a direct 1,3-H transfer but by two successive 1,4-H transfers. Fourier transform ion cyclotron resonance experiments show that acetone catalyses the isomerization 1 → 2 . Further, by using labeled reactants, it is demonstrated that this isomerization occurs by a direct catalyzed 1,3-H transfer whereas the less energy demanding pathway connecting bare ions 1 and 2 is a double 1,4-H transfer. This represents the first description of a system for which the pathways connecting two isomeric ions are different for the unimolecular and for the catalyzed isomerizations.

Gas phase catalyzed keto-enol isomerization of cations by proton transport☆

Abstract In the gas phase, the unimolecular isomerization of the H 3 COC(O)CH 2 CO + cation 1 ( m / z 101) into the H 3 CO(HO)CCHCO + enol ion 2 by a 1,3-H shift possesses a high energy barrier and is therefore not observed. In contrast, in the cell of a FT-ICR mass spectrometer, interaction with gaseous methanol catalyzes the isomerization of 1 into its more stable isomer 2 , which can be characterized by low energy collision with argon. This exothermic reaction is irreversible. Reaction with labeled methanol and ligand exchange experiments indicate the existence of two distinct reactions. By formation of a covalent bond, one reaction yields protonated dimethyl malonate while the second one leads to ion 2 by a 1,3-H transfer catalyzed by methanol. Conversely, loss of methanol from collisionally activated long-lived m / z 133 cations formed by protonation of dimethyl malonate yields some m / z 101 ions with structure 2 , which shows that methanol catalyzes the isomerization of ion 1 within a [ 1 , CH 3 OH] complex. The efficiency of different catalysts is studied in order to probe the mechanism of the isomerization processes.

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.

The [+CH2OH, H2O] and [+CH2OH, 2H2O] solvated ions

Abstract The reaction of CH3CH2OH·+ with H182O in the gas phase has been studied by FT-ICR spectroscopy. The main reaction yields the product ion [CH2O⋯H+⋯18OH2]. This ion slowly exchanges its second oxygen with H182O. Two possible channels for this reaction will be successively discussed. (i) In the first, the permutation of both oxygens take place within [CH182O⋯H+⋯18OH2], leading to [CH182O⋯H+⋯OH2]. This last ion can in turn undergo a ligand exchange with H182O, yielding [CH182O⋯H+⋯18OH2]. In this first hypothesis, a second molecule of water intervenes after an unimolecular rearrangement process within the [CH2O⋯H+⋯18OH2] ion. (ii) Conversely, a second possible mechanism involves as a first step a nucleophilic attack of H182O at the carbon atom. In that case the rearrangement takes place within a [+CH2OH, 2H2O] solvated ion. It will be shown that the first process is strongly endothermic and therefore less likely than the second, in which the solvation effect strongly decreases the energy of the intermediate, and where H2O catalyses migrations of H.

Ter-Body Intermediates in the Gas Phase: Reaction of Ionized Enols with tert-Butanol

In the gas phase, the CH2CHOH.+ enol radical cation 1 as well as its higher homologues CH3CHCHOH.+2 and (CH3)2CCHOH.+3, undergo exactly the same sequence of reactions with tert-butanol, leading to the losses of isobutene, water and water plus alkene. Fourier transform ion cyclotron resonance (FT-ICR) experiments using labeled reactants as well as ab initio calculations show that independent pathways can be proposed to explain the observed reactivity. For ion 1, taken as the simplest model, the first step of the reaction is formation of a proton bound complex which gives, by a simple exothermic proton transfer, the ter-body intermediate [CH2CHO., H2O, C(CH3)3+]. This complex, which was shown to possess a significant lifetime, is the key intermediate which undergoes three reactions. First, it can collapse to yield tert-butylvinyl ether with elimination of water. Second, by a regiospecific proton transfer, this complex can isomerize into three different ter-body complexes formed of water, isobutene and ionized enol. Within one of these complexes, which does not interconvert with the others, elimination of isobutene leads to the formation of a solvated enol ion. Within the others, a cycloaddition—cycloreversion process can proceed to yield the ionized enol 3 (loss of water and ethylene channel).

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.