Hristo Nedev

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.

Silicon vs. carbon containing ions: 1,3-proton transfers within the (CH3)(X)Si(OR)(+OHR′) units

Abstract In the cell of an FT-ICR spectrometer, (CH3)(X)C(OCR)( + OHR ′ ) and (CH3)(X)Si(OCR) + OHR ′ (R and R′=H, CH3 or C2H5; X=H or CH3) covalent ions were generated by reaction of the (CH3)(X)+SiOCR′ cations with water or alcohols. In the so-formed covalent ions, experiment shows that 1,3-H+ transfers from oxygen to oxygen are often easy in silicon containing ions while they are not observed in the corresponding ions containing only carbon. Calculations indicate that the energy required for a 1,3-H+ transfer from oxygen to oxygen is almost identical whether the transition state contains a silicon atom or not. The greater strength of the SiO bond in cations, compared to that of the CO bond or, in other words, the great electrophilic character of cations possessing a Si+, is the main factor explaining the difference in the behavior of the studied silicon containing ions and ions containing exclusively carbon.

Reactions of CH(3)CHO(.+) and of CH(3)COH(.+) with water upon Fourier transform ion cyclotron resonance conditions.

The reactions of CH3CHO + and of CH3COH + with water yield the same products, at almost the same rate. It is shown, by using a characteristic reaction of the carbene structure, that a molecule of water converts CH3COH + into its more stable isomer CH3CHO +, which is a new example of catalyzed 1,2-H transfer. The dominant product is the proton-bound dimer of water which, in fact, comes from the [H2OH+…CH3] and [H2OH+…CO] primary products whose observed abundances are poor. In a related system, ionized formamide/water, a water molecule catalyzes the 1,3-transfer leading from the solvated carbene to the [H2O…H+…H2N–C=O] stable intermediate, which eliminates CO without back energy. In contrast, such a process does not take place in the studied system since the cleavage of the so formed [H2OH+…CH3CO] transient intermediate involves a high back energy; this is explained by the charge repartition within this intermediate. In fact, a different pathway takes place. The solvated acetaldehyde ion isomerizes into a terbody intermediate in which protonated water is bonded to a CO molecule on the one hand and to a methyl radical on the other hand. Simple cleavages of this complex yield the observed products.

The [CH2CHOH.+, CH3CHO] solvated enol radical cation

The bimolecular reaction of the CH2CHOH.+ enol ion (m/z 44) with acetaldehyde gives a strongly dominant product,m/z 45, formed mainly by proton transfer from the ion to the molecule. The abundance of the product coming from a H. abstraction reaction from the neutral, albeit more exothermic, is negligible. In order to explain this result, the long lived [CH2CHOH.+, CH3CHO] solvated ion was generated by reaction of the CH2CHOH.+ enol ion with (CH3CHO)n in the cell of a Fourier transform ion cyclotron resonance mass spectrometer. The structure of this solvated ion was clearly established. Labeling indicates that [CH2CHOH.+, CH3CHO], upon low energy collisions, reacts by H. abstraction more rapidly than by H+ transfer to the neutral moiety. This shows that the entropic factors are determinant when the enol ion reacts directly with acetaldehyde.