Structure and fragmentation of [C3H7O]+ ions formed by chemical ionization

The unimolecular metastable and collision-induced fragmentation reactions of [C₃H₇O]⁺ ions produced by gas-phase protonation of acetone, propanal, propylene oxide, oxetan and allyl alcohol have been studied. The CID studies show that protonation of acetone and allyl alcohol yield different stable ions with distinct structures while protonation of propanal or propylene oxide yield [C₃H₇O]⁺ ions of the same structure. Protonated oxetan rearranges less readily to give the same structure(s) as protonated propanal and propylene oxide. The [C₃H₇O]⁺ ions fragmenting as metastable ions after formation by CI have a higher internal energy than the same ions fragmenting after formation by EI. Deuteronation of the C₃H₆O isomers using CD₄ reagent gas shows that loss of C₂H₃D proceeds by a different mechanism than loss of C₂H₄. The results are discussed in terms of potential energy profile for the [C₃H₇O]⁺˙ system proposed earlier.     

Chemical ionization mass spectra of selected C3H6O compounds

The chemical ionization mass spectra of five isomers of C₃H₆O (acetone, propionaldehyde, oxetane, propylene oxide and allyl alcohol) have been determined using a variety of reagent gases (H₂, D₂, N₂/H₂, CO₂/H₂ and CO/H₂). The [C₃H₇O]⁺ ions produced by protonation of these isomers undergo very similar reactions to those reported for analogous [C₃H₇O]⁺ metastable ions; however, decomposing ions generated by chemical ionization appear to have somewhat higher internal energies. The results of ²H labelling studies (D₂ reagent gas or labelled analogues of C₃H₆O) indicate that protonation occurs mainly on oxygen and are consistent with previous investigations of metastable oxonium ions. The protonated acetone ion is particularly stable, in agreement with the higher activation energies for fragmentation of this isomer than for other [C₃H₇O]⁺ structures. As the calculated heat of protonation of C₃H₆O is reduced by changing the reagent gas, so the extent to which fragmentation occurs decreases. This is discussed in the context of competition between fragmentation and collisional stabilization of the excited [C₃H₇O]⁺* ion. It is concluded that on average a large fraction (approaching 1) of the exothermicity of the protonation reaction resides in the [C₃H₇O]⁺* ions produced initially.     

On the affinity of cytosine towards electrophiles

Ab initio SCF computations on the intrinsic preferences of the H⁺, CH ₃ ⁺ and C₂H ₅ ⁺ cations towards the two principal sites of protonation or alkylation on cytosine, N₃ or O₂, show that this preference undergoes a continuous modification with the increase in size and complexity of the cation. N₃ is the preferred site of fixation of H⁺, O₂ the preferred site of C₂H ₅ ⁺ , while CH ₃ ⁺ has no marked preference. The exchange repulsion term of the binding energy appears responsible for the preference of C₂H ₅ ⁺ for O₂.This work was supported by the Ligue Francaise contre le Cancer and the National Foundation for Cancer Research (USA)     

Oxirane as a chemical ionization gas: Reactivity towards different classes of compounds

Oxirane chemical ionization (CI) gives numerous ions, including C₂H₃O⁺ and C₂H₅O⁺. These ions react with organic molecules through various specific ion–molecule reactions such as hydride abstraction, protonation, additions or cycloadditions. Oxirane CI allows discrimination between unsaturated compounds with [M + 43]⁺ and [M + 57]⁺ adduct ions and heteroatom functions with [M + 45]⁺ adduct ion. All are diagnostic ions. Oxirane CI permits selectivity during the ionization process of a mixture and discrimination of isomers.     

DFT Study on the Gold(I)-Catalyzed Dehydrogenative Heterocyclization of 2-(1-Alkynyl)-2-alken-1-ones to form 2,3-Furan-Fused Carbocycles: Effects of Additives C5H5NO vs. PhNO

A computational study with the M06/B3LYP density functional is carried out to explore the effects of additives C₅H₅NO vs. PhNO on the gold-catalyzed dehydrogenative heterocyclization of 2-(1-alkynyl)-2-alken-1-ones to form 2,3-furan-fused carbocycles. The following three conclusions are obtained based on our theoretical calculations. (a) The Au(I) catalyst plays a crucial role on the intramolecular cyclization reaction. (b) Both additives C₅H₅NO and PhNO as the proton shuttle can assist proton-transfer through a two-step proton-transfer mechanism including the protonation of additive and the deprotonation of additive-H⁺, whereas the catalytic capability of PhNO is weaker than that of C₅H₅NO (energy barrier: 90.6 vs. 33.2 kJ/mol). (c) C₅H₅NO-H⁺ has stronger stability comparing with PhNO-H⁺ because the basicity of C₅H₅NO is stronger than that of PhNO, which cause that the energy barrier of ts3 + PhNO-H⁺ (131.5 kJ/mol) is higher than that of ts3 + C₅H₅NO-H⁺ (60.5 kJ/mol) in the intermolecular addition. Therefore, the base strength is the primary factor that controls the catalytic capability of additives C₅H₅NO vs. PhNO. These studies are expected to improve our understanding of Au(I)-catalyzed reactions involving additive as the cocatalyst and to provide guidance for the future design of new catalysts and new reactions.     

Molecular fragmentations: Part VII. Structures and energies of mass spectral fragments derived from benzene

Molecular structures and energies have been calculated, using MINDO/3, of the mass spectral ions arising from benzene: (C₆H₆)⁺ (three non-valence isomers); (C₆H₅⁺); (C₅H₃⁺) (four isomers); (C₄H₄)⁺ (three isomers); (C₄H₃)⁺ (two isomers); (C₄H₂)⁺ (four isomers); (C₃H₃)⁺; and (C₂H₂)⁺. Calculations have been made for the conjugate neutral fragments, allowing calculation of appearance potentials, and also for the ion (C₆H₇)⁺.     

Structure and mechanism of fragmentation of [C2H5O]+

The decomposition reactions of [C₂H₅O]⁺ ions produced by dissociative electron-impact ionization of 2-propanol have been studied, using ¹³C and deuterium labeling coupled with metastable intensity studies. In addition, the fragmentation reactions following protonation of appropriately labeled acetaldehydes and ethylene oxides with [H₃]⁺ or [D₃]⁺ have been investigated. In both studies particular attention has been paid to the reactions leading to [CHO]⁺, [C₂H₃]⁺ and [H₃O]⁺. In both the electron-impact-induced reactions and the chemical ionization systems the fragmentation of [C₂H₅O]⁺ to both [H₃O]⁺ and [C₂H₃]⁺ proceeds by a single mechanism. For each case the reaction involves a mechanism in which the hydrogen originally bonded to oxygen is retained in the oxygen containing fragment while the four hydrogens originally bonded to carbon become indistinguishable. The fragmentation of [C₂H₅O]⁺ to produce [CHO]⁺ proceeds by a number of mechanisms. The lowest energy route involves complete retention of the α carbon and hydrogen while a higher energy route proceeds by a mechanism in which the carbons and the attached hydrogens become indistinguishable. A third distinct mechanism, observed in the electron-impact spectra only, proceeds with retention of the hydroxylic hydrogen in the product ion. Detailed fragmentation mechanisms are proposed to explain the results. It is suggested that the [C₂H₅O]⁺ ions formed by protonation of acetaldehyde or ionization of 2-propanol are produced initially with the structure [CH₃CH?\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm O}\limits^ + $\end{document}H] (a), but isomerize to [CH₂?CH? \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm O}\limits^ + $\end{document}H₂] (e) prior to decomposition to [C₂H₃]⁺ or [H₃O]⁺. The results indicate that the isomerization a → e does not proceed directly, possibly because it is symmetry forbidden, but by two consecutive [1,2] hydrogen shifts. A more general study of the electron-impact mass spectrum of 2-propanol has been made and the fragmentation reactions proceeding from the molecular ion have been identified.     

A theoretical study of protonation of triatomic silicon–carbon compounds

Ab initio molecular orbital methods are employed to study the low-lying states of C₃H⁺, SiC₂H⁺, Si₂CH⁺, and Si₃H⁺. Special attention is paid to a comparative study between C₃H⁺ and Si₃H⁺. In both cases a ³B₂ state is found to lie the lowest at the HF level, although inclusion of correlation effects favor a linear structure (¹Σ⁺ state) for C₃H⁺, which lies 25 kcal/mol below the ³B₂ state at the MP 4 level, and a bent structure (¹A′ state) for Si₃H⁺, which lies just 2 kcal/mol below the ³B₂ state. The proton affinities of C₃, SiC₂, Si₂C, and Si₃ are estimated at different levels of theory. Both protonation at carbon and silicon atoms are considered for SiC₂ and Si₂C. It is found that C₃ comparatively has a low proton affinity. On the other hand, Si₃ has a relatively high proton affinity compared with the protonation at silicon atom for both SiC₂ and Si₂C. These results are discussed on the basis of electronic structure arguments.     

Electrophilic aromatic substitution under chemical ionization conditions. Methylamine and ammonia as reagent gases

For compounds C₆H₅X (X?Cl, Br, I) under chemical ionization conditions, methylamine causes ipso substitution of X by [NH₂CH₃]⁺ and by [NH₂]⁺˙. C₆H₅F is less reactive; it gives some [C₆H₅NH₂]⁺˙. Nitrobenzene gives an adduct ion [M+CH₃NH₃]⁺, a reduction product ion [C₆H₅NO₂]⁺˙, and an ion at m/z93, probably a substitution product [C₆H₅NH₂]⁺˙, but no [C₆H₅NH₂CH₃]⁺. It is also shown that the ion m/z94, formed from nitrobenzene with ammonia as reagent gas, is a substitution product rather than a reduction product ion. Carbonyl compounds C₆H₅. CO. X give adduct ions and some substitution, mainly [C₆H₅NH₂]⁺˙.     

Metastable ions in chemical ionization

The ion [C₃H₅]⁺ generated in a chemical ionization source by a variety of methods, including protonation and charge exchange, exhibits a metastable peak for H₂ loss which is two orders of magnitude weaker than that formed in an electron impact source. The stable [C₃H₅]⁺ ions generated by electron impact and chemical ionization undergo collision-induced dissociation to a comparable extent, both losing H₂ by only one of the two competitive mechanisms observed for metastable ions. In contrast to the behavior of [C₃H₅]⁺, the molecular ions of p-substituted nitrobenzene, generated by charge exchange at high source pressure, yield composite metastable peaks for NO loss which are very similar in shape and intensity to those generated by electron impact. The contrasting behavior of the metastable ions extracted from high pressure ion sources in the two systems may be due to differences in the efficiencies of quenching of the ionic states responsible for fragmentation as metastable ions. It is noteworthy that the NO loss reactions require considerably lower activation energies than does the H₂ loss reaction.     

Studies in chemical ionization mass spectrometry: 17-hydroxy steroids

The chemical ionization mass spectra of several hydroxy steroids were obtained using methane as the reactant gas. The spectra are much less complex than the electron ionization spectra and little fragmentation of the steroid nucleus is observed. The major fragment ions involve the loss of water from [M + H]⁺. A 3-keto group in the steroids was characterized by an abundant [M + C₂H₅]⁺ ion. 5α- and 5β-Dihydrotestosterone could be distinguished by their spectra, with H₂ as the reactant gas by marked differences in amounts of [M + H]⁺, [M + H ? H₂O]⁺ and [M + H ? 2H₂O]⁺. Substituted 3α-X-, 17 β-ol compounds, (X = Cl, Br) were also studied to obtain relative amounts of protonation at these sites.     

Organometallic chemistry : V. Protonation of acylferrocenes under stable ion conditions in fso3h-so2c1f solution

Protonation of acylferrocenes (FeCOR) in FSO₃H-SO₂CIF(SO₂) solution was studied by PMR spectroscopy. The site of protonation is found to be at the carbonyl oxygen atom. Temperature dependent PMR spectra of protonated acylferrocenes FeCROH⁺ (R = CH₃, C₂H₅, C₆H₅, OCH₃) were observed indicating intermolecular hydrogen exchange with the acid solvent system. In addition the PMR spectra of acylferrocenes in FSO₃ H-SO₂ CIF(SO₂) were found to be dependent upon the acid concentration.     

Study of ion structures produced by the reaction of 2-propyl cations with water and methanol; covalently bound v. Hydrogen-bridged adducts

In contrast to an earlier report,¹ the collisonally induced dissociation of protonated 2-propanol and t-butyl alcohol yields spectra that are indistinguishable from those of the corresponding [C₃H₇/H₂O]⁺ and [C₄H₉/H₂O]⁺ ions generated by the (formal) gas phase addition reactions in a high pressure ion source of [s-C₃H₇]⁺ and [t-C₄H₉]⁺ ions with the n-donor H₂O. Similarly, [s-C₃H₇/CH₃OH]⁺ ions generated by both gas phase protonation of n- and s-propyl methyl ethers and addition reactions of [C₃H₇]⁺ to CH₃OH display mode-of-generation-independent collisionally induced dissociation characteristics. However, analysis of the unimolecular dissociation (loss of propene) of the [C₃H₇/CH₃OH]⁺ system, including a number of its deuterium, ¹³C- and ¹⁸O-labelled isotopomers, supports the idea that prior to unimolecular dissociation, covalently bound [C₃H₇- O(H)CH₃]⁺ ions intercovert with hydrogen-bridged adduct ions, analogous to the behaviour of the distonic ethene-, propene- and ketene-H₂O radical cations.     

Mechanism of CO loss for protonated 1-indanone and isomeric [C9H9O]+ ions

In order to establish the mechanism of CO loss occurring during metastable decomposition of protonated 1-indanone, fragmentations of monocyclic [C₉H₉O]⁺ isomers have been studied. These ions of known structure were prepared by CI protonation and fragmentation of the corresponding acids chlorides. It is demonstrated that the wide component of the [MH? CO]⁺ metastable peak induced by protonated 1-indanone fragmentation is the result of fragmentation of the [C₆H₅CH₂CH₂CO]⁺ isomer ion.     

Effect of protonation exothermicity on the reaction of gaseous Brønsted acids with halobenzene derivatives

The protonation of haloaromatics by [N₂H]⁺ and [CO₂H]⁺ has been studied by chemical ionization mass spectrometry. In general, the fragmentation reactions following protonation by [CO₂H]⁺ are similar to those observed following protonation by [CH₅]⁺, while the fragmentation reactions induced by protonation by [N₂H]⁺ are intermediate between those observed on reaction with [CH₅]⁺ and with [H₃]⁺. These results are consistent with the conclusion that the fragmentation mode is determined by the protonation exothermicity since the proton affinity of CO₂ is the same as that of CH₄ while the proton affinity of N₂ is intermediate between that of CH₄ and H₂.     

Positive chemical ionization mass spectra of (η6-arene) tricarbonylchromium complexes with ammonia and isobutane

Summary The positive chemical ionization mass spectra, with ammonia and isobutane at 0.5 torr of (⁶-arene)Cr(CO)₃, where arene is C₆H₅COMe, C₆H₅COEt, C₆H₅COC₃H₇, C₆H₅COCMe₃, 2-MeC₆H₄COC₃H₇, C₆H₅COOMe, C₆H₅CH₂ COEt, C₆H₅Me and 1,3,5-Me₃C₆H₃ are reported. All these compounds exhibit [M + H]⁺ as base peak, with isobutane. In the presence of ammonia, [M + NH₄]⁺ is the most abundant ion, when the arene is a ketone or an ester and the addition site is suggested to be the ligand. Conversely [M + H]⁺ is the base peak when arene is toluene and mesitylene and the protonation site is likely to be the metal atom. In the presence of ammonia, ions such as [M — 3 CO + NH₃]⁺ and [M — 3 CO + 2 NH₃]⁺ are also observed and their abundances strongly depend on the pressure of the reagent gas.     

Thermodynamic and mechanistic studies of initial stages in propane aromatisation over Ga-modified H-ZSM-5 catalysts

¹³C MAS NMR has been performed in situ to investigate the early stages in the conversion of propane to aromatics on Ga-containing ZSM-5 catalysts. Propane 2-¹³C was used as labelled reactant. The scrambling of the ¹³C label in the very early stages of the propane conversion, even at 573 K, indicates that the first reaction intermediate is a protonated pseudocyclopropane (PPCP) species formed by activation of propane on a (Ga³⁺,O²⁻) ion pair and its protonation by a nearby Brønsted acidic site. This PPCP species can decompose in several ways leading to H₂, CH₄, C₂H₄, C₂H₆, and C₃H₆ as primary products. The very same molecules can also be produced as secondary products by cracking and hydrogen transfer at high conversion. CH⁺₃, C₂H⁺₅ and C₃H⁺₇ carbenium ions which are formed by decomposition of PPCP can react further with alkane or olefinic species. Reaction of CH⁺₃ (stabilised by a basic anionic framework oxygen) with propane (activated on a Ga site) may yield n-butane as indicated by the increase in the n-butane/i-butane ratio when the catalyst contains gallium.     

Theoretical Investigation on Photoionization and Dissociative Photoionization of Toluene

The photoionization and dissociation photoionization of toluene have been studied using quantum chemistry methods.The geometries and frequencies of the reactants,transition states and products have been performed at B3LYP/6-311++G (d,p) level,and single-point energy calculations for all the stationary points were carried out at DFT calculations of the optimized structures with the G3B3 level.The ionization energies of toluene and the appearance energies for major fragment ions,C₇H₇⁺,C₆H₅⁺,C₅H₆⁺,C₅H₅⁺,are determined to be 8.90,11.15 or 11.03,12.72,13.69,16.28 eV,respectively,which are all in good agreement with published experimental data.With the help of available published experimental data and theoretical results,four dissociative photoionization channels have been proposed:C₇H₇⁺+H,C₆H₅⁺+CH₃,C₅H₆⁺+C₂H₂,C₅H₅⁺+C₂H₂+H.Transition structures and intermediates for those isomerization processes are determined in this work.Especially,the structures of C₅H₆⁺ and C₅H₅⁺ produced by dissociative photoionization of toluene have been defined as chain structure in this work with theoretical calculations.     

Uniform quality gaussian basis sets for molecular calculations. II. C2 hydrocarbons

This investigation is a continuation of a study on the optimality of MO basis sets of Gaussian functions, when constructed from AO basis sets optimized for the neutral atom or for ions. A formal charge parameter Q is used to adjust AO basis sets to the molecular environment, by virtue of a simple quadratic equation. Calculations are performed on a series of seven C₂ hydrocarbons (C₂H₂, C₂H₄, C₂H₆, C₂H₃⁺ (open), C₂H₃⁺ (bridged), C₂H₅⁺ (bridged), and C₂H₄^? radical anion). A simple rule is formulated to give approximate values of the charge parameter Q.     

Metastable ion studies. IV—thermochemistry and ion structures among fragmenting [C3H6]+· ions

Metastable ion peak shapes, dimensions and relative abundances have been measured for the three fragmentations [C₃H₆]⁺· → [C₃H₄]⁺· + H₂, [C₃H₆]⁺· → [C₃H₅]⁺ + H· and [C₃H₆]⁺· → [C₃H₃]⁺ + H₂ + H·. [C₃H₆]⁺· ions were derived from propene, cyclopropane, tetrahydrofuran, cyclohexanone, 2-methyl but-1-ene and cis-pent-2-ene. Activation energies for these fragmentations have been evaluated. Three daughter ion dissociations ([C₃H₅]⁺ → [C₃H₃]⁺ + H₂, [C₃H₅]⁺ → [C₃H₄]⁺· + H· and [C₃H₄]⁺· → [C₃H₃]⁺ + H·) have been similarly examined. Ion structures have been determined and the metastable energy releases have been correlated with the thermochemical data. It is concluded that the molecular ions of propene and cyclopropane become structurally indistinguishable prior to fragmentation and that differences in their metastable ion characteristics can be ascribed wholly to internal energy differences; the latter can be correlated with the photoelectron spectra of the isomers. The pathway for the consecutive fragmentation which generates the metastable ion peak (m/e 42 → m/e.39) has been shown to be It is likewise concluded that fragmentating [C₃H₆]⁺· ions generated from the various precursor molecules are also structurally indistinguishable and cannot be classified with either molecular ion of the isomeric C₃H₆ hydrocarbons.