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MIKE spectra of protonated, substituted phenylacetophenones, show that their fragmentations occur after the interconversion of phenyl groups.

Collision-induced dissociations-mass analyzed ion kinetic energy (CID-MIKE) spectrometry has been used to characterize [M-CH3]+ ions from ionized 2-heptanone. Acylium [CH3(CH2)5 C=0] ions are produced at short times (within the source), while protonated cyclohexanone is formed exclusively in the field-free regions.

International audience; 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 ß-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 ?-distonic ion which loses ethylene. Finally, H*, I* and *SCH3 abstraction from appropriate neutrals confirms a radical reactivity of the carbenic carbon. © 2003 Published by Elsevier Science B.V.

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.

International audience; 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. Copyright © 2000 Elsevier

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 [CH2CHOH, (CH3)2CCH2]•+ 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.

Protonated alkyl phenyl ethers possess more than one stable tautomer. A debate has arisen over whether only one of them gives rise to the principal dissociation pathway observed in their mass-analyzed ion kinetic energy (MIKE) spectra. Alkene loss constitutes the major (often the exclusive) metastable ion decomposition, yielding protonated phenol ions. A hydron deposited by chemical ionization exchanges with some of the alkyl hydrogens (but none of the ring hydrogens) prior to fragmentation. Previously published MIKE spectra have shown that [(CD3)2CHOPh]D+ gives only m/z 97 (C6H5D2O+), but that [(CD3)2CHOPh]H+ gives a mixture of m/z 96 (C6H6DO+) and m/z 97. Exchange must arise via ion-neutral complexes that result from O-protonated ions, (CD3)2CHO(H)Ph+. Current controversy centers around the contribution of ring-protonated ions to the production of unexchanged fragment ions. Here we determine the mole fractions of ring-protonated (X) and O-protonated (1 − X) parent ions using m/z 95:m/z 96:m/z 97 MIKE ion abundance ratios from H2O and D2O CI of (CH3)2CHOPh, CH3(CD3)CHOPh, and (CD3)2CHOPh. Data from the first two compounds give unbiased assessments of X and four other relative rate constants that are obtained using a steady-state kinetic model that gives a set of five equations in five unknowns. The values calculated from the data predict an m/z 96:m/z 97 ratio of 4.7 for [(CD3)2CHOPh]H+ that turns out to be the same ratio as is measured experimentally. This validation of the data analysis corroborates the value of X ≤ 0.01 extracted from the experimental results. The contribution of ring-protonated parent ions to the MIKE spectra of chemically ionized isopropyl phenyl ether is therefore negligible.

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.

Molecules containing a benzene ring and an oxygen atom typically have two types of protonation sites: on the ring (where facile intramolecular hydrogen transposition from carbon to carbon probably takes place) or on an oxygen lone pair. Four aryl ethers are compared: the isomers phthalan (1, a cyclic benzylic ether) and coumaran (2, a cyclic phenyl ether), as well as isochroman (3) and isopropyl phenyl ether (iPrOPh). The proton affinities of 1–3 have been measured using FT-ICR techniques as 830, 855 and 838 kJ/mol, respectively. Comparison with model compounds and Hartree-Fock-based SCF calculations indicate that protonated phthalan (1H) and protonated isochroman (3H) have O-protonated structures. By contrast, the conjugate acids of coumaran and iPrOPh prefer ring-protonated structures. Acidification/neutralization experiments in the ICR, as well as MIKE spectra, demonstrate that chemical ionization of iPrOPh produces noninterconverting O- and ring-protonated forms. Metastable ion decompositions of protonated phthalan and protonated isochroman give evidence of separate decomposition pathways for both types of tautomers. Protonated coumaran exhibits complete randomization of hydrogen between oxygen and the ring, which is attributed to high barriers for expulsion of neutral fragments.

In the gas phase, the (CH 3 )) 3 SSiOSi(CH 3 )) 2 + and (CH 3 )) 2 SSi + OOSi(CH 3 ))CH 2 ions were formed in the external source of a FT-ICR mass spectrometer by fragmentation of (CH 3 )) 3 SSiOSi(CH 3 )) 3 + . In the c. In the cell, the electrophilic center of these ions reacts with water to give product-ions which undergo, spontaneously or upon collision, sequential losses of methane and water. The reactions of D 2 OO and H 2 8 OO indicate that a permutation of the oxygen atom of the reactant and that of water takes place prior to the dissociation of the productions. This means that both oxygens play a symmetric role in an intermediate step of the reaction pathway.

Resume En phase gaseuze, les reactions de C 6 H 5 CH 2 X et C 6 H 5 CH 2 CH 2 X (X = OH ou halogene) avec le benzene, au milieu protonant, conduisent a des diphenylalcanes protones. Les produits formes par reaction avec C 6 D 6 donnent des echanges H/D avant dissociation qui caracterisent leurs structures. Ainsi, les reactions de C 6 H 6 CH 2 CH 2 X donnent selectivement le diphenyl-1,2-ethane protone. Cette selectivite apparente resulte du tres faible temps de vie du 1,1-diphenyl ethane protone issu de la reaction SN1 du cation C 6 H 5 CHCH 3 + .

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.

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. © 1997 by John Wiley & Sons, Ltd.

The loss of alkane molecules from metastable aliphatic alcohol and ether radical cations in the gas phase can be used to probe the properties of intermediate weakly bonded ion-neutral complexes. Very little translational energy is released when the final products form a stable complex prior to dissociation, whereas the energy released can directly reflect the heat of reaction when dissociation takes place subsequent to an exothermic intra-complex reaction. The heats of formation of the intermediate oxonium ions and the product radical cations of enols and enol ethers were determined using the composite G2(MP2) and CBS-Q high-level ab initio methods.

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.

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.

A variety of reactions take place when ethanol, diethyl ether, and ethylamine radical cations react with basic, neutral molecules: isomerization within the encounter complex of the radical cation to its α-distonic isomer, proton transfer to the neutral, and loss of an alkyl radical. The initial step is formation, directly when possible or via isomerization, of a proton-bound adduct. This adduct can act as a proton donor or dissociate by loss of an alkyl radical, giving rise to a second proton-bound adduct. The course of these reactions is determined by the exceptional stability of the adducts.

The reaction of the ˙CH2CH2OHCH3+ distonic ion with CH2O was studied by Fourier transform ion cyclotron resonance spectrometry. The process is induced by an initial regiospecific attack of the carbon CH2O at the radical site of the distonic ion, leading to an intermediate adduct ion which loses water. The mechanism was demonstrated by labelling, by studying the structure of the product ion and by examining independently the behaviour of the putative intermediate radical cation.

Unimolecular decompositions of protonated methyl (MTBE), ethyl (ETBE), n-propyl, isopropyl, and isobutyl tert-butyl ethers have been examined experimentally and the results compared with the outcome of ab initio calculations. Extensive hydrogen transposition between the hydrogen on oxygen and the nine hydrogens of the tert-butyl is revealed by mass-resolved ion kinetic energy spectroscopy (MIKES) experiments on deuterated ions from MTBE and ETBE. Mechanistic possibilities are probed with the help of FT-ICR, and isotope effects are interpreted by comparison with the MIKES of protonated ethyl tert-amyl ether and its deuterated analogues. Protonated MTBE displays a single unimolecular decomposition product, tert-butyl cation. Of the protonated MTBE ions that decompose, 30% do so without any hydrogen transposition, while 70% completely randomize the non-methoxy hydrogens. This calls for the intervention of at least two noncovalent intermediates. In the case of MTBE, Hartree-Fock-based SCF computations exhibit only one plausible candidate that corresponds to an energy minimum, the hydrogen-bonded complex between protonated methanol and isobutene. The other intermediate is inferred to be the ion-neutral complex [MeOH tBu+l, even though that does not correspond to a well on the SCF potential energy surface. Protonated ETBE yields a pair of unimolecular decomposition products, tert-butyl cation and protonated acetaldehyde. Here the ab initio results display two noncovalent potential energy minima, but the observed ion intensities cannot be accounted for without at least three intermediates. Again, the conclusion is that the additional intermediate corresponds to an ionneutral complex, [EtOH tBu+l, which does not correspond to a potential energy minimum. The transient ion-neutral complexes have non-zero lifetimes because their collapse is prevented by

From metastable ion studies, Fourier transform ion cyclotron resonance experiments and semi-empirical calculations, the unimolecular reaction of the (H3CO)3C+ cation 1 is discussed. The first step of the reaction is an irreversible, 1,3-CH transfer giving the H3COC(O)O(CH3) ion, 2. This ion either cleaves or isomerizes into the ion-neutral complex [H3COCO+, CH3OCH3], 3. Since the H3COCO+ cation is a very good CH donor, a CH transfgr then occurs within the complex, 3, to yield the predominant fragment (CH3)3O+.

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 ionmolecule reaction, we show that n-complexes [arene alkyl cation] have a significant lifetime. By ab initio calculations, [C6H6 iso-C3H7+] and [C6H7+ C3H61 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-CJ-b+] and the largest (65 kJ/mol) corresponds to [P-CH3Cd-I&H3 iso-C3H7+]. The [protonated arene alkene] complexes are less stabilized (ca. 25 kJ/mol). The calculations also indicate that [arene alkyl cation] complexes have n-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.

The metastable dissociation of the methoxymethyl cation and a number of its deuterium and 13C variants was examined using a reverse-geometry double-focusing mass spectrometer. The loss of methane from the methoxymethyl cation clearly showed a composite peak shape which, when deconvoluted, revealed a bimodal kinetic energy release distribution in the resulting formyl cations. Labelling experiments revealed that the two carbon atoms and all hydrogens become equivalent on the time-scale of the unimolecular dissociation lifetime of the decomposing ion. A small deuterium isotope effect was found which can be rationalized on the basis of zero point energy effects. The bimodal kinetic energy release distribution was shown, with the aid of a four-sector instrument, to be due to the production of both formyl cation (with a large kinetic energy release) and isoformyl cation (with a much smaller kinetic energy release). The methoxymethyl cation was also prepared with a precisely defined amount of internal energy in a Fourier transform ion cyclotron resonance (FTICR) spectrometer by the reaction of methyl cation with formaldehyde. Experiments with 13C and deuterium labelling revealed that the dissociation to formyl cation of the methoxymethyl cations formed in the low-pressure FTICR cell by reaction of methyl cation with formaldehyde is accompanied by complete scrambling of the carbons and incomplete scrambling of the hydrogens. Ab initio calculations were carried out which identified and characterized each of the stable minima and transition states for the appropriate reactions. The calculations were fully consistent with the mechanism deduced on the basis of the experimental data.

The β-distonic ion . CH 2 CH 2 OHCH 2 CH 2 CH 3 has been shown previously to be an intermediate in the fragmentation of ionized ethyl propyl ether. The reactions of this ion (generated by fragmentation in the ion source of alkyl propyl ethers of glycol) were studied in this work. Labelling showed that this ion undergoes competing hydrogen transfers leading to a series of isometric distonic ions. Each of them was submitted to an isotope effect

Decomposition of nPrNH3+ to NH4+ does not occur via vicinal elimination. Hydrogen transfer takes place from all positions of the alkyl chain. Intermediacy of either an ion–neutral complex. [iPr+ NH3], or rearrangement to iPrNH3+ is required. SCF calculations were used to probe the potential energy surface. There is a minimum corresponding to [iPr+ NH3], in which the nitrogen lone pair points towards the machine hydrogen and is 2.1 A away from it, and another, less stable, minimum corresponding to a hydrogen-bonded structure, H3N … H3CCHCH3+, in which one methyl hydrogen is 2.0 A from the nitrogen. Density of states estimates suggest that avibrationally excited complex can live for the order of 100 ps before either collapsing to iPrNH3+ or yielding NH4+ and propene.

The addition-fragmentation reactions of phenyl-substituted enol radical cations, C 6 H 5 C(OH)CHR +. (R = H, CH 3 ), 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 C 6 H 5 C(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 γ-distonic ion; the reactions are not accompanied by cyclization to cyclobutanol intermediates

The ions corresponding to metastable formyl loss from ionized ethylene glycol and 2-methoxyethanol are the conjugate acids of methanol and dimethyl ether, respectively. The structures of the fragments of deuterated analogues reveal that a hydroxylic hydrogen transfers to carbon to create the new methyl group, while a hydrogen originally attached to carbon migrates to oxygen in the course of this double hydrogen transfer. This is the only metastable decomposition of ethylene glycol radical cation, while 2-methoxyethanol radical cation also loses water to form oxetane radical cation

Unimolecular decompositions of protonated methyl (MTBE), ethyl (ETBE), n-propyl, isopropyl, and isobutyl tert-butyl ethers have been examined experimentally and the results compared with the outcome of ab initio calculations. Extensive hydrogen transposition between the hydrogen on oxygen and the nine hydrogens of the tert-butyl is revealed by mass-resolved ion kinetic energy spectroscopy (MIKES) experiments on deuterated ions from MTBE and ETBE. Mechanistic possibilities are probed with the help of FT-ICR, and isotope effects are interpreted by comparison with the MIKES of protonated ethyl tert-amyl ether and its deuterated analogues. Protonated MTBE displays a single unimolecular decomposition product, tert-butyl cation. Of the protonated MTBE ions that decompose, 30% do so without any hydrogen transposition, while 70% completely randomize the non-methoxy hydrogens. This calls for the intervention of at least two noncovalent intermediates. In the case of MTBE, Hartree-Fock-based SCF computations exhibit only one plausible candidate that corresponds to an energy minimum, the hydrogen-bonded complex between protonated methanol and isobutene. The other intermediate is inferred to be the ion-neutral complex [MeOH tBu+l, even though that does not correspond to a well on the SCF potential energy surface. Protonated ETBE yields a pair of unimolecular decomposition products, tert-butyl cation and protonated acetaldehyde. Here the ab initio results display two noncovalent potential energy minima, but the observed ion intensities cannot be accounted for without at least three intermediates. Again, the conclusion is that the additional intermediate corresponds to an ionneutral complex, [EtOH tBu+l, which does not correspond to a potential energy minimum. The transient ion-neutral complexes have non-zero lifetimes because their collapse is prevented by

The tert -butyl cation ( t Bu + ) is widely used as a chemical ionization reagent ion, for which it is often assumed that simple proton transfer to neutral substrates is the dominant reaction. Mass-analyzed ion kinetic energy (MIKE) spectrometry and the collisionally activated decomposition (CAD) of ions corresponding to the adducts R′O(H)R + formed from mixtures of ROH and R′OH (R = t Bu, R′ = t Bu or 1-methylcyclopentyl, and their deuterated analogues) show a much more complicated pattern of reactivity, which is mirrored in the bimolecular reactions of t Bu + with R′OH in the Fourier transform ion cyclotron resonance (FT-ICR) technique. Production of R′ + in the CAD and FT-ICR experiments is accompanied by isotopic exchange between the alkyl groups, which implicates ion-neutral complexes as accessible intermediates in addition to the covalent adduct ions. A unified pathway for MIKE spectrometry, CAD, and FT-ICR regimes is presented, involving terbody complexes of the form [cation water alkene] as well as two-body complexes of the form [cation alcohol].

Radical cations derived from the ethers ROCH2CH2OR′ (R, R′ = H, CH3, C25) were studied, since β-distonic oxonium ions are often prepared from ionized ethers of glycol. The first step in the fragmentation is a 1,5-transfer of an α-hydrogen to oxygen of a terminal alkoxy group leading to a δ-distonic oxonium ion. This step is thermo-neutral and reversible in the ROCH2CH2OH radical cations and exothermic and irreversible in the dialkyl ether radical cations. Depending on R and R,′ these δ-distonic oxonium ions fragment by three reactions: the loss of an alcohol or a water molecule, the formation of a β-distonic oxonium ion ˙CH2CH2O(H)+R and a 1,4-H migration between carbon atoms. Competition between these processes is discussed.

The rates of proton transfer and hydrogen atom abstraction between ionized enols [CH 2 =CHOHi +. and [CH 2 =C(CH 3 )OH] +. were measured by Fourier transform ion cyclotron resonance spectrometry. It was observeb that proton transfer is favoured by its high rate even when it does not reach the limiting collision rate. When a competition is in principe possible between H + , H . and H − transfers, only the former process is observed. This phenomenon and the low efficiency of hydrogen atom abstraction are discussed in terms of the potential energy profile, where the stability of the intermediate complexes plays a critical role

A simple model is developed to examine the internal energy distribution for metastable ions and the probability of reaction in the field-free regions of sector mass spectrometers in systems where the reactant ions in the ion source react by dissociation in competition with reversible isomerization to non-dissociating species. The internal energy distribution for the reacting ions is strongly dependent on whether dissociation is preceded by isomerization and back-isomerization, and it is shown that a range of reasonable combinations of rate constants for the forward and reverse isomerization result in a bimodal distribution of the internal energy for the ions that dissociate in the field-free regions. The isomerization effectively provides a kinetic reservoir of higher energy reactant ions. The reaction probability of the metastable ions varies strongly with internal energy, and the back-isomerized ions may in some instances be responsible for most of the reactions observed. A bimodal energy distribution will give rise to composite metastable peaks even though neither non-statistical behavior nor multiple reaction channels are involved.

The ions corresponding to metastable formyl loss from ionized ethylene glycol and 2-methoxyethanol are the conjugate acids of methanol and dimethyl ether, respectively. The structures of the fragments of deuterated analogues reveal that a hydroxylic hydrogen transfers to carbon to create the new methyl group, while a hydrogen originally attached to carbon migrates to oxygen in the course of this double hydrogen transfer. This is the only metastable decomposition of ethylene glycol radical cation, while 2-methoxyethanol radical cation also loses water to form oxetane radical cation

Propene radical cations undergo proton transfer reactions with water and methanol at low pressure (Fourier transform ion cyclotron resonance), whereas cyclopropane radical cations react with methanol by addition—fragmentation and with water by collision-induced isomerization to form propene. The isomerization of cyclopropane into propene is slower than the hydrogen exchange between propene and water, making the reaction between C 3 H + 6 and H 2 O a sensitive probe for the structure of the hydrocarbon ion. The structural distinction between the two C 3 H + 6 isomers can also be based on the reactions with methanol.

Metastable protonated butylbenzenes have been studied by extensive labelling and kinetic energy release measurements. Protonated s-butylbenzene isomerizes by 1–2 hydrogen shift to the π complex [C6H6SC4H], and the SC4H cation rearranges to t-C4H. The interconversion between [C6H6SC4H] and [C6H C4H8] ion/neutral complexes is demonstrated. Protonated t-butylbenzene reacts without hydrogen exchange to give t-C4H, while protonated n-butylbenzene isomerizes to s-butylbenzene prior to dissociation. Protonated iso-butylbenzene isomerizes to protonated t-butylbenzene and to protonated s-butylbenzene. In the 2nd field-free region, the dissociation of this last form predominates.

The reactions of metastable decomposing methyl acetoacetate (a mixture of keto a ad enol tautomers) are reported and discussed. The unimolecular fragmentations of the tautomers are different. The metastable decomposing radical cation of the keto form displays four specific ions: [M –CO]+˙, [M – CH2O]+˙, [M – CH2CO]+˙ and m/z 43. The results derived from D-, 13C- and 18O-labelled precursors together with thermochemical data have been used to study the mechanisms. Experimental results indicate that an unexpected isomerization occurs before dissociation. It formally corresponds to oxygen atom permutation of the two carbonyl groups without participation of the carbon atoms. This remarkable process is interpreted in terms of a mechanism involving ion-molecule complexes.

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-C 3 H 7 ] + and [C 6 H 7 ] + product ions, is preceded by H exchange. This H exchange involves two interconverting ion-neutral complexes [C 6 H 6 , iso-C 3 H 7 + (2π) and [C 6 H 7 +, C 3 H 6 ] (2α)

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 .

The reactions of propyl ether radical cations close to threshold are initiated by (reversible) formation of γ-distonic isomers, RO + (H)CH 2 CH 2 CH2: the three methylene groups in these ions lose their positional identity by ring closure/ring opening via [cyclopropane + alcohol] +. intermediates. Extensive hydrogen exchange occurs with the C 3 -chain. When R is not methyl the γ-distonic isomer undergoes further intramolecular hydrogen atom transfer reactions that lead to formation of α- and β-distonic ions. The α-distonic isomers expel ethyl and propyl radicals by C-O bond cleavage

The key intermediates to the fragmentation of metastable methyl and ethyl benzoate radical cations are α- and β-distonic isomers of the molecular ions. The α-distocic isomers are also formed by fragmentation of longer chain alkyl benzoates, but may not be long-lived, stable species. Rearrangement of the α-distonic ions prior to fragmentation can take place, but (re)formation of the benzoate molecular ions does not occur.

International audience; The reactions of the water solvated ammonia radical cation [NH(3)(+*), H(2)O] with a variety of aldehydes and ketones were investigated. The reactions observed differ from those of low energy aldehydes and ketones radical cations, although electron transfer from the keto compound to ionized ammonia is thermodynamically allowed within the terbody complexes initially formed. The main process yields an ammonia solvated enol with loss of water and an alkene. This process corresponds formally to a McLafferty fragmentation within a complex. With aldehydes, another reaction can take place, namely the transfer of the hydrogen from the CHO group to ammonia, leading to the proton bound dimer of ammonia and water, and to the NH(4)(+) cation. Comparison between the available experimental results leads to the conclusion that the McLafferty fragmentation occurs within the terbody complex initially formed, with no prior ligand exchange, the water molecule acting as a spectator partner.

International audience; Catalyzed conversion of HCOOH*+ into HOCOH* + was studied in the cell of a FT-ICR in the presence of different molecules. The reaction of HCOOH*+ with SO2, whose proton affinity (PA) lies between that of the HOCO* radical at the carbon and at the oxygen sites, yields the HOCOH*+ carbene isomer as proved by its characteristic reaction with cyclopropane. When the PA of the catalyst lies above the highest PA of both sites of the HOCO* radical, formation of HOCOH*+ cannot be observed since its final state lies above that corresponding to protonation of the catalyst. However, reactions of DCO2H*+ and of HCO2D*+, which protonate several catalysts in an identical ratio which is very near of 1/1 at the beginning of the reaction, indicates that both ions, DCO2H*+ and of HCO 2D*+, convert into ion DO-C-OH*+ within a complex prior to protonation. The reactions of HCOOH*+ and HOCOH*+ with water were also more particularly studied by using theoretical calculations. Both reactions lead to protonated water and to the ionized water dimer which has been shown to possess the [H 2OH+?OH*] structure. The first step of the process is the conversion of the [HO(O)C*? H+?OH2] complex into [HOCO*?H +?OH2]. This latter complex undergoes two main pathways: on the one hand, it leads to protonation of water; on the other hand, it isomerizes to the [O*COH?H+?OH2] intermediate which dissociates to form [H2 OH+?OH*] with CO loss. Formation of H3O+ and [H2OH+?OH*] being rapid, the [HOCO*?H+?OH2] complex does not dissociate to yield the ionized carbene product which was not detected. Since the catalyzed isomerization of the 1,2-H transfer, converting HCOOH* + into HOCOH*+, is only observed within the corresponding complexes, this is a typical case of hidden isomerization. Finally, the differences in the unimolecular fragmentations of ionized formic acid and of its water solvated ion were explained. © 2003 Elsevier B.V. All rights reserved.

International audience; In the gas phase and within ion-neutral complexes, H. abstraction by the ion from the neutral moiety was studied by using FT-ICR experiments and molecular orbital calculations. Ionized methanol abstracts rapidly H. from methane and other alkanes while its a-distonic counterpart, .CH2OH2+, is completely unreactive. On the other hand, ß-distonic ions, such as .CH2CH2XH+ (X=OCH3, NH2), are also unreactive towards methane but can abstract H . from ketones and ethers. Finally, ionized carbenes, such as HO-C-NH2.+, react with methane by a slow H. abstraction. Ab initio molecular orbital calculations at the G3(MP2) level were performed in order to understand these behaviors. For ionized methanol and its a-distonic counterpart, the reacting structure that could lead to H . abstraction is the highly stabilized complex between protonated methanol and a methyl radical, which yields the final state (CH 3OH2++.CH3) by simple cleavage. In the case of methanol the encounter complex with methane leads easily by rotation of the methane molecule to this reacting structure. In contrast, in the case of the a-distonic ion, the almost linear structure of the encounter complex [.CH2OH2+?CH4] and the high energy required for its isomerization into the reacting structure prevent the reaction. Two factors are required to observe H. abstraction in ß-distonic ions: the interaction energy of the encounter complex and the distance between the hydrogen to be transferred and the carbon radical. Reaction of the HO-C-NH 2.+ carbene with methane lies between these two extreme cases. The encounter complex is poorly stabilized (-8.7kcalmol-1) and the transition state for H. abstraction is very close to the reactants energy. © 2003 Elsevier B.V. All rights reserved.

In 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-C4H9C6H5R+ 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-C4H9C6H5, R+] π-complex prior to fragmentation.