Kinetic energy release during CO loss by rearrangement of α-benzoylcarbenium ions

Primary α-benzoylcarbenium ions (a) and tertiary α-benzoyldimethylcarbenium ions (b) are obtained by chemical ionization of ω-hydroxyacetophenone and its dimethyl derivative, respectively. Both α-acylcarbenium ions decompose by a rearrangement reaction and subsequent loss of a CO molecule. The kinetic energy released during this process by metastable ions a and b has been determined under different experimental conditions. The kinetic energy released during the CO elimination from the tertiary α-benzoyldimethylcarbenium ions b is independent of the experimental conditions and gives rise to dish-topped peaks with T₅₀=440±20 meV in the MIKE spectra of b. In contrast to this the kinetic energy releases and the peak-shapes in the MIKE spectra of the primary α-benzoylcarbenium ion a varies with the experimental conditions. The mechanism of this rearrangement reaction is discussed, and it is shown by a MNDO calculation of the heats of formation of the relevant ions that the different characteristics of the kinetic energy release during the fragmentation of primary and tertiary carbenium ions can be attributed to different types of reaction energy profiles.     

Destabilized carbenium ions: α-carbomethoxy-α,α-dimethyl-methyl cations

Tertiary α-carbomethoxy-α,α-dimethyl-methyl cations a have been generated by electron impact induced fragmentation from the appropriately α-substituted methyl isobutyrates 1–4. The destabilized carbenium ions a can be distinguished from their more stable isomers protonated methyl methacrylate c and protonated methyl crotonate d by MIKE and CA spectra. The loss of I^— and Br˙ from the molecular ions of 1 and 2, respectively, predominantly gives rise to the destabilized ions a, whereas loss of Cl˙ from [3]^{+ ˙} results in a mixture of ions a and c. The loss of CH₃˙ from [4]⁺˙ favours skeletal rearrangement leading to ions d. The characteristic reactions of the destabilized ions a are the loss of CO and elimination of methanol. The loss of CO is associated by a very large KER and non-statistical kinetic energy release (T₅₀ = 920 meV). Specific deuterium labelling experiments indicate that the α-carbomethoxy-α,α-dimethyl-methyl cations a rearrange via a 1,4-H shift into the carbonyl protonated methyl methacrylate c and eventually into the alkyl-O protonated methyl methacrylate before the loss of methanol. The hydrogen rearrangements exhibit a deuterium isotope effect indicating substantial energy barriers between the [C₅H₉O₂]⁺ isomers. Thus the destabilized carbenium ion a exists as a kinetically stable species within a potential energy well.     

Destabilized carbenium ions. Secondary and tertiary α-acetylbenzyl cations and α-benzoylbenzyl cations

The secondary α-acetylbenzyl and α-benzoylbenzyl cations, as well as their tertiary analogues, have been generated in a mass spectrometer by electron impact induced fragmentation of the corresponding α-bromoketones. These ions belong to the interesting family of destabilized α-acylcarbenium ions. While primary α-acylcarbenium ions appear to be unstable, the secondary and tertiaiy ions exhibit the usual behaviour of stable entities in a potential energy well. This can be attributed to a ‘push-pull’ substitution at the carbenium ion centre by an electron-releasing phenyl group and an electron-withdrawing acyl substituent. The characteristic unimolecular reaction of the metastaible secondary and tertiary α-acylbenzyl cations is the elimination of CO by a rearrangement reaction involving a 1,2-shift of a methyl group and a phenyl group, respectively. The loss of CO is accompanied by a very large kinetic energy release, which gives rise to broad and dish-topped peaks for this process in the mass-analysed ion kinetic energy spectra of the corresponding ions. This behaviour is attributed to the rigid critical configuration of a corner-protonatei cyclopropanone derivative and a bridged phenonium ion derivative, respectively, for this reaction. For the tertiary α-acetyl-α-methylbenzyl cations, it has been shown by deuterium labelling and by comparison of collisional activation spectra that these ions equilibrate prior to decomposition with their ‘protomer’ derivatives formed by proton migration from the α-methyl substituent to the carbonyl group and to the benzene ring.     

Radiochemical study of gas-phase reactions of diethylstannyl cations Et2SnT+ with oxygen-containing compounds: II. Reaction of diethylstannyl cations with butanol

Gas-phase reaction of nucleogenic diethylstannyl cations Et₂SnT⁺ with butan-1-ol has been studied by the radiochemcal method, and probable reaction mechanisms have been proposed. During the process diethylstannyl cations undergo isomerization into tertiary Me₂EtSn⁺ cations and rearrangement with elimination of ethane. Thermochemical parameters of reactions of diethyl-substituted cations derived from Group 14 elements (Et₂TM⁺; M = C, Si, Ge, Sn) with alcohols have been analyzed by the M06L aug-cc-pVDZ quantum chemical method.     

Zum Mechanismus Massenspektrometrischer Fragmentierungsreaktionen. XVI—hydrid-Wanderungen bei der Fragmentierung von αω-Dimethoxyalkyl-Ionen

The isomerization and fragmentation of α,ω-dimethoxyalkyl ions a (CH₃OCH₂(CH₂)_n- CH⁺OCH₃, n = 1-6) has been investigated by deuterium labelling. It is shown that a isomerizes to ion a' by hydride transfer from the ω-CH₂ group to the positive charge at the α-C-atom before elimination of methanol. Both methoxy groups are lost as methanol. The amount of isomerization can be deduced from alkene elimination from [a ? CH₃OH]+ ions in deuterated derivatives of a. On average at 70 eV three rearrangement steps involving hydride transfer are observed.     

Destabilized carbenium ions: α-imidoyl carbenium ions and the electron impact mass spectra of α-haloaldimines and α-haloketimines

A series of α-chloro- and α-bromoketimines compounds (1-9) with different substituents at the α-position and at the imino group has been investigated by electron impact mass spectrometry as possible precursors of the correspondingly substituted α-imidoyl carbenium ion, an important class of destabilized carbenium ions. The main fragmentation of the molecular ions of compounds, 1-9 in the ion source corresponds to an α-cleavage at the imino group; however, fragment ions are also formed by loss of the α-halo substituent. These fragment ions correspond at least formally to α-imidoyl carbenium ions. Their further reactions in dependence on the type of substituents at the imino group and at the α-C atom, were studied by mass-analysed ion kinetic energy and collisional activation mass spectrometry. The results agree with the initial formation of destabilized α-imidoyl carbenium ions but indicate an easy rearrangement of these ions in the presence of suitable alkyl substituents by 1,2- and 1,4-hydrogen shifts to more stable isomers.     

Isomerization and fragmentation of methyl benzofuran ions and chromene ions in the gas phase

The rearrangement of the molecular ions of the isomeric 2- and 3-methyl benzofurans (1 and 2), 2H-chromene (3) and 4H-chromene (4) has been studied as a further example of the isomerization of oxygen-heteroaromatic radical cations via a ring expansion/ring contraction mechanism well documented for molecular ions of alkyl benzenes. The ions 1⁺˙?4⁺˙ fragment mainly by H loss into identical chromylium ions a. The process exhibits consistently a large kinetic energy release and an isotope effect k_H/k_D, which arise from a rate-determining energy barrier of the last dissociation step. Differences of the kinetic energy releases, the isotope effects and the appearance energies of the methyl benzofuran ions and the chromene ions indicate a large energy barrier also for the initial hydrogen migration during the rearrangement of the methyl benzofuran ions. This is substantiated by an MNDO calculation of the minimum energy reaction path. In contrast to the behaviour of alkyl benzene ions, a unidirectional isomerization of the methyl benzofuran ions by ring expansion takes place but no mutual interconversion of the molecular ions of methyl benzofurans and chromenes.     

Eine Abfangmethode zum Nachweis kurzlebiger Carbenium-Ionen in stark sauren Reaktionsmedien

In a new approach, unstable carbenium ions produced in strong protic media could be trapped by thioethers yielding sulfonium compounds. The method has successfully been applied to the rearrangement reactions of sec-butyl alcohol and pinacol, where carbenium ions could be trapped before their rearrangement. In the case of the pinacol-pinacon rearrangement the carbenium ion 13 has already been discussed but for the first time its existence is proved. Since compound 16 has been found, a new mechanism for the pinacol-pinacon rearrangement is postulated starting with diprotonated pinacol.     

Collison-induced dissociation and photodissociation of nitroaromatic molecular ions: A unique isomerization for p-nitrotoluene and p-ethylnitrobenzene ions

The photodissociation and low-energy collision-induced dissociation of p-nitrotoluene and p-ethylnitrobenzene molecular ions were studied using Fourier transform ion cyclotron resonance mass spectrometry. The dissociation of these ions is highly dependent on the time-scale of the experiment and the pressure of the nitroaromatic compound. Collisions of the ions with nitroaromatic neutral species increase the abundance of fragment ions due to NO elimination, while collisions with inert gases such as sulfur hexafluoride and argon have no effect. Evidence is presented for the occurrence of an ion–molecule reaction between p-alkylnitrobenzene ions and nitroaromatic neutral species that induces isomerization of the ion. This isomerization is proposed to involve a nitro-to-nitrite rearrangement. Although the mechanism of this process is unknown, isotopic labeling experiments have shown that it does not involve nitrogen atom transfer between the two reactants. The dissociations of o-nitrotoluene, m-nitrotoluene and nitrobenzene ions are also discussed. For these ions, no pressure- or time-scale-dependent behavior was observed, indicating that an isomerization did not occur.     

Ion–molecule reactions of fragment ions derived from protonated allyl methyl ether

Unusual behaviour has been noted for allyl methyl ether (1) chemically ionized in a high-pressure ion source. Tandem mass spectrometry indicates the formation of methylcyclopentadienyl and methoxy-1-propenylcarbenium ions (d, m/z 81 and e, m/z 85). The origin of these unexpected ions has been elucidated using conventional and Fourier transform ion cyclotron resonance experiments: primary fragment ions derived from protonated 1 (allyl ions a and methoxymethyl cations b) generate collision complexes with neutral 1, giving rise to the ions d and e, respectively, after methanol elimination.     

The energy surface for isomeric [C2H3O]+ ions: Further experimental evidence

Mass spectra from collisionally activated dissociation (CAD) of [C₂H₃O]⁺ ions, including isotopically labeled analogs, provide further information on the isomers [CH₃C?O⁺] (a), [CH₂?C?O⁺H] (b), [⁺CH₂CH?O] (c) and (d). Our data generally support the recent conclusions from theory by Radom and coworkers and from experiment by Terlouw, Holmes and coworkers. Most acetyl-containing molecular ions form a ions in high purity only at low energies, consistent with isomerization of higher energy molecular ions to form the more stable enol which dissociates to b. Isomer d, prepared from (CICH₂)₂CHOH, undergoes facile hydrogen scrambling, presumably through a degenerate 1,2-hydrogen shift. Theory suggests that c undergoes spontaneous isomerization to a and d; although [C₂H₃O]⁺ ions from BrCH₂CHO appear to consist of a and ～15% d, the latter are formed without substantial hydrogen scrambling.     

Ionic and excited species in irradiated poly(dimethylsiloxane) doped with pyrene

The influence of temperature (77–230 K) on the fate of pyrene (Py) radical ions and Py excited states in irradiated poly(dimethylsiloxane) (PDMS) doped with Py is described. At 77 K, the Py radical ions seem to be stable, whereas the Py excited states [fluorescence (λ = 395 nm) and phosphorescence (λ = 575–650 nm)] are generated via tunneling charge transfer. In the range of the glass‐transition temperature (T_g = 152–153 K), the Py radical ions start to decay, taking part in a recombination process and leading to the Py monomer and Py excimer fluorescence (λ = 475 nm). The wavelength‐selected radiothermoluminescence (WS RTL) observed at approximately 395, 475, and 600 nm has helped us to identify the T_g range (152–153 K). The absorption maximum at approximately 404 nm, found in the temperature range under consideration, is thought to represent PyH^?, cyclohexadienyl‐type radicals produced as a result of the reaction of Py^?? with protonated PDMS macromolecules. With the initial‐rise method of evaluating the activation energy (E_a) with the WS RTL peaks observed in the T_g range, E_a values of 123–151 kJ mol^?1 have been found. Such high E_a values can be explained by the contribution of energy connected to the molecular relaxation of the matrix in the T_g range. The well‐known Williams–Landel–Ferry equation, with universal constants C₁ = 17.4 and C₂ = 12.7, has been successfully applied to the interpretation of old pulse‐radiolysis/viscosity data found for crosslinked PDMS doped with Py. The mechanisms involved in these phenomena are discussed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6125–6133, 2004     

Nucleogenic tricoordinated cations of the 14th group elements

The interaction of tricoordinated cations of the 14th group elements generated by the nuclear chemical method with representatives of π-(aryls, olefins) and n-electron-donating (alcohols, ethers) compounds was studied. A comparative analysis of these interactions was carried out on the basis of the results obtained and revealed the differences in the reaction mechanisms of the carbenium ion compounds and mezoid cations. No drastic differences in the behavior of silylium and germylium cations was observed. The possibility of isomerization of the mezoid cations to donor-acceptor complexes of the [HM⁺·H₂] type was experimentally shown for the first time for dimethylgermylium cations taken as an example. The quantum chemical calculations of the studied cations and adducts of their interactions in terms of the functional density theory prove the reaction mechanisms proposed.     

Quantum chemical calculations of intracell potential profile in superionic transition range in LaF3

The results of quantum chemical calculations of the potential profile in the LaF₃ crystal lattice in the range of superionic phase transition are presented for clusters containing 24 to 1200 ions. It is found that the values of formation energy E _a of vacancy-interstitial fluoride ion defects and potential barriers E _d hindering the movement of fluoride ions and determining the efficiency of charge transport in the lattice grow monotonously from the minimum values E _a = 0.12 eV and E _d = 0.22 eV for a 24-ion cluster to the maximum E _a = 0.16 eV and E _d = 0.26 eV for clusters of 576 and 1200 ions. It is shown that the values of E _a and E _d obtained for the dielectric phase (T < T _c) are several times the values of E _a and E _d for the superionic state (T ≥ T _c) of LaF₃. The values of E _a and E _d obtained by quantum chemical calculations from clusters of 576 and 1200 ions agree well with energies E _a and E _d obtained from the analysis of the data of the Raman and quasielastic light scattering.     

Chemical ionization mass spectrometry studies—VII: Deuterium labeled decanes

CI mass spectra of the five isomeric vicinal d₂-decanes have been recorded using methane and d₄-methane as reagent gases. In contrast to earlier suggestions, we find that a large fraction of the alkyl fragment ions from n-decane are formed by elimination of olefins from the abundant [M – 1] ion. Only the C₉ and C₈ fragment ions are produced completely by a one-step reaction between the decanes and the methane reagent ions. Isotope exchange does not occur between the hydrocarbon and the reagent ions derived from d₄-methane but extensive scrambling of the deuterium label in the d₂-decanes does take place in the [M – 1] ion.     

Electron impact induced water elimination from hydroxyamides. 2—N-acetyl- and N-benzoyl-4a-hydroxydecahydroquinoline and N-Methyl-4a-hydroxy-2-oxodecahydroquinoline

Electron impact induced water elimination from the metastable molecular ions of N-acetyl- and N-benzoyl-4a-hydroxydecahydroquinoline mainly follows a formal [1,2] elimination. The initiating and ratedetermining step in the reaction is the hydrogen rearrangement from C-8a onto the carbonyl group. The transferred hydrogen is subsequently lost, together with the hydroxyl group. The almost complete absence of H₂O loss from both diastereomers of N-methyl-4a-hydroxy-2-oxodecahydroquinoline confirms that the reaction can only proceed when the carbonyl group is able to function as ‘hydrogen carrier’ by occupying positions in the vicinity of both a hydrogen and the hydroxyl function.     

Der Zerfall von Substituierten p- Toluesäuren—eine neuartige Skelettumlagerung

p-Carboxy- and p-carbomethoxybenzyl ions further decompose by CO elimination yielding p-hydroxy- and p-methoxybenzyl ions.     

13C labelling study of the interconversion of ethylbenzene, 7-methylcycloheptatriene and p-xylene ions

The isomerization of the molecular ions of ethylbenzene, 7-methylcycloheptatriene and p-xylene by skeletal rearrangement prior to the formation of [C₇H₇]⁺ ions has been investigated by using ¹³C labelled compounds. The results obtained for ions generated by 70 eV and 12 eV electron impact, and fragmenting in the ion source, the 1st field free region and the 2nd field free region, respectively, are compared with those obtained from D labelled derivatives. It is shown that at long reaction times metastable p-xylene ions lose a methyl radical after scrambling of all C atoms and H atoms, while the unstable molecular ions in the ion source react by specific loss of one of the methyl substituents. Both unstable and metastable ethylbenzene ions fragment by two competing mechanisms, one corresponding to specific loss of the terminal methyl group, and the other involving scrambling of all C and H atoms. These results are discussed by use of a dynamic model developed for the mutual interconversion and fragmentation of the molecular ions of ethylbenzene, methylcyclo-heptatriene and p-xylene. The experimental results can be explained by an equilibrium between metastable methylcycloheptatriene ions and p-xylene ions with sufficient energy for skeletal rearrangement, while about 40% of the metastable ethylbenzene ions fragment after rearrangement to methylcycloheptatriene ions and about 60% of the ethylbenzene ions rearrange further to xylene ions before fragmentation. Metastable methylcycloheptatriene ions, mainly lose a methyl group without a skeletal rearrangement, however, because the rearranged ions are kinetically trapped as ‘stable’ xylene ions or ethylbenzene ions.     

USHY分子筛催化剂上2-甲基戊烷异构化反应机理

根据实验观察, 以C6正碳离子基元反应为基础定量地描述了2-甲基戊烷在USHY分子筛上异构化反应机理, 以及反应温度对其的影响. 实验结果表明, 在400 ℃下, C6正碳离子从反应物分子提取氢离子反应的速率是C6正碳离子释放质子氢给表面Brфnsted碱基反应速率的10倍, 导致反应物的异构体C6烷烃产物的生成大大快于C6烯烃产物的生成. 同时发现, C6正碳离子释放质子反应比从反应物分子提取氢离子反应要求更高的活化能, 因此在高温下, C6烷烃产物的生成量比C6烯烃产物的生成量少. 描述了各种C6正碳离子的反应途径和相互转变机理, 定量地比较了它们的反应活性和选择性, 得出了某些烃催化裂化中异构化反应选择性变化的普遍规律.     

Radiochemical study of gas-phase reactions of diethylstannilium cations Et2SnT+ with oxygen-containing compounds: I. Interaction of diethylstannilium cations with methyl tert-butyl ether

Radiochemical method was applied to study the gas-phase interaction between the nucleogenic diethylstannyl cations Et₂SnT⁺ and methyl tert-butyl ether. The possible reaction mechanisms are considered. Diethylstannylium cations are found to isomerize during the reaction to tertiary cation Me₂EtSn⁺, as well as undergo a rearrangement accompanied by elimination of ethane rater than ethylene, in contrast to the cases of silicon and germilium analogs.