Site of gas-phase methylation of l-phenyl-2-aminopropane

The regioselectivity of methyl cation transfer from (CH₃)₂F⁺, (CH₃)₂Cl⁺ and (CH₃)₃O⁺ to 1-phenyl-2-aminopropane was studied by Fourier transform ion cyclotron resonance in combination with collision-induced dissociation and neutralization-reionization mass spectrometry of the stable [M + CH₃]⁺ ions formed in a chemical ionization source. The (CH₃)₂F⁺ ion transfers a methyl cation to the NH₂ group and the phenyl ring with almost equal probability. Predominant CH₃⁺ transfer to the NH₂ group is observed for the (CH₃)₂Cl⁺ ion whereas the (CH₃)₃O⁺ ion reacts almost exclusively at the amino group. The preference for methylation at NH₂ is discussed in terms of a lower methyl cation affinity of the phenyl ring than of the amino group and the existence of an energy barrier for methylation of the phenyl moiety.     

The role of ion–neutral complexes in the reactions of onium ions and related species

An account is given of the development of the proposal that ion–neutral complexes are involved in the unimolecular reactions of onium ions (R¹R²C?Z⁺R³; Z = O, S, NR⁴; R¹, R², R³, R⁴ = H, C_nH_{2n + 1}), with particular emphasis on the informative C₄H₉O⁺ oxonium ion system (Z = O; R¹, R² = H; R³ = C₃H₇). Current ideas on the role of ion-neutral complexes in cation rearrangements, hydrogen transfer processes and more complex isomerizations are illustrated by considering the behaviour of isomeric CH₃CH₂CH₂X⁺ and (CH₃)₂CHX⁺ species [X = CH₂O, CH₃CHO, H₂O, CH₃OH, NH₃, NH₂CH₃, NH(CH₃)₂, CH₂?NH, CH₂?NCH₃, CO, CH₃˙, Br˙ and I˙]. Attention is focused on the importance of four energetic factors (the stabilization energy of the ion–neutral complex, the energy released by rearrangement of the cationic component, the enthalpy change for proton transfer between the partners of the ion neutral complex and the ergicity of recombination of the components) which influence the reactivity of the complexes. The nature and extent of the chemistry involving ion-neutral complexes depend on the relative magnitudes of these parameters. Thus, when the magnitude of the stabilization energy exceeds the energy released by cation rearrangement, the ergicity of proton transfer is small, and recombination of the components in a new way is energetically favourable, extensive complex-mediated isomerizations tend to occur. Loss of H₂O from metastable CH₂?O⁺C₃H₇ ions is an example of such a reaction. Conversely, if the stabilization energy is small compared with the magnitude of the energy released by eation rearrangement, the opportunities for complex-mediated processes to become manifest are decreased, especially if proton transfer is endoergic. Thus, CH₃CH₂CH₂CO⁺ expels CO, with an increased kinetic energy release, after rate-limiting isomerization of CH₃CH₂CH₂⁺? CO to (CH₃)₂CH⁺? CO has taken place. When proton transfer between the components of the complex is strongly exoergic, fragmentation corresponding to single hydrogen transfer occurs readily. The proton-transfer step is often preceded by cation rearrangement for CH₃CH₂CH₂X⁺ species. In such circumstances, the involvement of ion–neutral complexes can be detected by the observation of unusual site selectivity in the hydrogen-transfer step. Thus, C₃H₆ loss from CH₂?N⁺(R¹)CH₂CH₂CH₃ (R¹ = H, CH₃, C₃H₇) immonium ions is found by ²H-labelling experiments to proceed via preferential α-and γ-hydrogen transfer; this finding is explained if the incipient ⁺CH₂CH₂CH₃ ion isomerizes to CH₃CH⁺CH₃ prior to proton abstraction. In contrast, the isomeric CH₂?N⁺(R¹)CH(CH₃)₂ species undergo specific β-hydrogen transfer because the developing CH₃CH⁺CH₃ cation is stable with respect to rearrangements involving a 1,2-H shift.     

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.     

Reactions within and between adjacent dimethylamino groups: Collision-induced dissociation tandem mass spectrometric study of the 1,9-bis(dimethylamino)phenalenium cation

The electron impact (EI) ionization-induced fragmentation pathways of the new 1,9-bis(dimethylamino) phenalenium cation [1]⁺ were investigated. The peri-dimethylamino substituents of [1]⁺ are incorporated in a trimethine cyanine substructure and show strong steric interactions. A mechanism is proposed for the unusual elimination of CH₃N?CH₂, HN(CH₃)₂ and (CH₃)₃N from [1]⁺ and for the accompanying cyclizations to heterocyclic ions: prior to fragmentation, the intact cation [1]⁺ rearranges, by reciprocal CH₃ and H transfers, to new isomeric cations which decompose subsequently in a characteristic way. A wealth of consistent information on dissociation pathways and fragment structures is provided by collision-induced dissociation tandem mass spectra, collision-induced dissociation mass-analysed ion kinetic energy spectra and exact mass measurements of the salt cation and of its primary fragment ions. The liquid secondary ion mass spectrum of [1]⁺ is very similar to its EI mass spectrum.     

Reactions of the trimethylsilyl ion with 1,2-cyclopentanediol isomers in the collision region of a triple quadrupole instrument

Ion-molecule reactions with the trimethylsilyl ion were used to distinguish between cis- and trans-1,2-cyclopentanediol isomers. The ion kinetic energy of [Si(CH₃)₃]⁺ was varied from 0 eV to 15 eV (center of mass frame of reference). At low ion kinetic energies (<4 eV), there are significant differences in the relative stabilities and decomposition behavior of the adduct ions [M + Si(CH₃)₃]⁺. The cis-1,2-cyclopentanediol isomer favors decomposition of [M + Si(CH₃)₃]⁺ to yield the hydrated trimethylsilyl ion [Si(CH₃)₃OH₂]⁺ at m/z 91. For the trans isomer, the formation of the hydrated trimethylsilyl ion is an endothermic process with a definite threshold ion kinetic energy.     

Comparison of collisional activation spectra of some positive ions produced both by charge reversal of negative ions and by decomposition of positive ions

The charge reversal collision induced decomposition mass analyzed ion kinetic energy spectrum of allyl anion has been compared with the collision induced dissociation mass analyzed ion kinetic energy spectrum of allyl cation and found to be identical except for the presence of +2 ions formed by charge stripping in the spectrum of the [C₃H₅]⁺ ion. Likewise, the collision induced dissociation mass analyzed ion kinetic energy charge reversal spectrum of [CH₃Se]^? has been compared with the collision induced dissociation mass analyzed ion kinetic energy spectrum of [CH₃Se]⁺ and found to be identical. A study of the pressure dependence of the collision induced dissociation mass analyzed ion kinetic energy spectrum of [C₃H₅]⁺ and [C₃H₅]^? showed increasing fragmentation with increasing collision gas pressure, and suggests that a greater mean number of collisions converts more energy to internal modes in the collision induced dissociation mass analyzed ion kinetic energy experiment even at low pressures.     

Fragmentation of the metastable molecular ion of methyl lactate: The formation of oxygen-protonated methanol [CH3OH2]+ involving double hydrogen atom transfers

The spontaneous unimolecular dissociation reaction of methyl lactate (1) ionized by electron impact was investigated by a combination of mass-analyzed ion kinetic energy spectrometry and deuterium labeling. The metastable ions 1^+· decompose in a variety of ways: four fragment peaks are observed at m/z 89, 76, 61, and 45, which correspond to the losses of ?H₃, CO, CH₃?O, and ?OOCH₃, respectively. Double hydrogen atom transfer occurs in the third reaction. The source-generated m/z 61 ions decompose into oxygen-protonated methanols at m/z 33 ([CH₃OH ₂ ⁺ ]) by the loss of CO with double hydrogen atom migration. Both hydroxyl and methyne hydrogen atoms in 1 ^+· are present in the resultant protonated methanols.     

A neutralization-reionization and reactivity mass spectrometry study of the generation of neutral hydroxymethylene

Neutral hydroxymethylene HCOH is an important intermediate in several chemical reactions; however, it is difficult to observe due to its high reactivity. In this work, neutral hydroxymethylene and formaldehyde were generated by charge exchange neutralization of their respective ionic counterparts and then were reionized and detected as positive‐ion recovery signals in neutralization–reionization mass spectrometry in a magnetic sector instrument of BEE geometry. The reionized species were characterized by their subsequent collision‐induced dissociation mass spectra. The transient hydroxymethylene neutral was observed to isomerize to formaldehyde with an experimental time span exceeding 13.9 µs. The vertical neutralization energy of the HCOH^+? ion has also been assayed using charge transfer reactions between the fast ions and stationary target gases of differing ionization energy. The measured values match the result of ab initio calculations at the QCISD/6‐311 + G(d,p) and CCSD(T)/6‐311 + + G(3df,2p) levels of theory. Neutral hydroxymethylene was also produced by proton transfer from CH₂OH⁺ to a strong base such as pyridine, confirmed by appropriate isotopic labeling. There is a kinetic isotope effect (KIE) for H⁺ versus D⁺ transfer from the C atom of the hydroxymethyl cation of ～3, consistent with a primary KIE of a nearly thermoneutral reaction. Copyright © 2011 John Wiley & Sons, Ltd.     

Multiple-stage mass spectrometry in structural characterization of organophosphorus compounds

Multiple-stage mass spectrometry involving consecutive collision-activated dissociation reactions was used to examine the structures of fragment ions commonly formed on electron ionization of organophosphorus esters. The compounds studied include several aryl thiophosphates, some of which are analogs of common pesticides. Energy-resolved collisionactivated dissociation experiments allow the dissociation of the molecular ions of these compounds in such a manner that only a few fragment ions dominate the spectrum. An abundant fragment ion of m/z 109, formed from all of the compounds studied, can have at least four different stable structures: (CH₃O)₂PO⁺, CH₃CH₂OP(O)OH⁺, CH₂ =CHOP(H)(OH)₂ ⁺, and (CH₂O)₂P(H)OH⁺. The structure of the fragment ion of m/z 109 was found to reflect the phosphorus-containing part of the compounds studied. Another abundant fragment ion obtained for all the aryl esters studied is structurally characteristic of the aromatic moiety of the molecule. This fragment ion is the result of a complex rearrangement involving transfer of an alkylene group to the aromatic ring from the phosphoruscontaining part of the molecular ion. The utility of these fragment ions in the structural characterization of unknown organophosphorus esters is discussed.     

Theoretical study on the gas-phase reaction mechanism between rhodium monoxide cation and methane

The gas-phase reaction mechanism between rhodium monoxide cation and methane has been investigated on the singlet and triplet state potential energy surfaces at the CCSD(T)/6-311+G(2d,2p), SDD//B3LYP/6-311+G(2d,2p), SDD level. Over the 300–1100 K temperature range, the branching ratios of Rh⁺ + CH₃OH and RhCH₂ ⁺ + H₂O are 83.8–52.6% and 16.2–47.4%, respectively, whereas the branching ratio of CH₂ORh⁺ + H₂ is so small to be negligible. For the main products (Rh⁺ + CH₃OH and RhCH₂ ⁺ + H₂O) formation, the minimum energy reaction pathway involves singlet–triplet spin inversion, and both b-RhCH₃OH⁺ and H₂ORhCH₂ ⁺ are the energetically preferred intermediates. Alternatively, in the CH₂ORh⁺ + H₂ reaction, both b-RhCH₃OH⁺ and H₂RhOCH₂ ⁺ are the energetically favorable intermediates, and the main products are Rh⁺ + CH₃OH. In the RhCH₂ ⁺ + H₂O reaction, the main products are Rh⁺ + CH₃OH with the energetically predominant intermediate b-RhCH₃OH⁺. In the reaction of Rh⁺ + CH₃OH, both b-RhCH₃OH⁺ and H₂RhOCH₂ ⁺ are the energetically preferable intermediates, and the main products are CH₂ORh⁺ + H₂. Besides, toward methane activation, the cation RhO⁺ exhibits higher reaction efficiency than the cation Rh⁺, the neutral RhO, and its first-row congener CoO⁺, and it exhibits lower methanol branching ratio and higher water branching ratio than RhO and CoO⁺.     

Oxygen rearrangement of molecular ions of 3-phenylpropionates

The oxygen rearrangement in molecular ions of 3-phenylpropionates has been investigated with the aid of mass analyzed ion kinetic energy spectra. Elimination of an allyl radical followed by expulsion of ketene from the molecular ion of allyl 3-phenylpropionate is shown to result in formation of protonated benzaldehyde. The oxygen rearrangement has been found to be inoperative in ionized methyl 3-methyl-3-phenylbutyrate. [M ? CH₃ ? CH₂CO]⁺ ions in the spectrum of the latter compound are formed by elimination of the 3-methyl substituent and subsequent methoxy migration.     

Mass spectral fragmentation pattern of 3-(2′-hydroxyethyl)quinolin-2(1H)-one and its derivatives

Fragmentation patterns resulting from electron impact ionization of 3-(2′-hydroxyethyl)quinolin-2(1H)-one, three of its monosubstituted derivatives and four of its disubstituted derivatives were studied. The molecular ion of quinolinone-2-etbanol undergoes initial fragmentation with the loss of OH^·, H₂O, CO, ^·CHO, CH₂O, CH₂OH^·, CH₂?CHOH and HCNO species. The [M – CHO]⁺ ion is tentatively suggested to have been formed by the expulsion of H^· from the [M – CO]^+· ion and the [M - CHO]⁺ peak may be considered as diagnostic of a 2-quinolone-3-ethanol.     

Isomeric ion-neutral complexes generated from ionized 2-Methylpropanol and n-Butanol: the effect of the polarity of the neutral partner on complex-mediated reactions

Photoionization was used to characterize the energy dependence of C₃H ₇ ⁺ , C₃H ₆ ⁺ , CH₃OH ₂ ⁺ and CH₂=OH⁺ formation from (CH₃)₂)CHCH₂OH^+? (1) and CH₃CH₂CH₂CH₂OH^+? (2). Decomposition patterns of labeled ions demonstrate that close to threshold these products are primarily formed through [CH ₃ ⁺ CHCH₃ ^?CH₂OH] (bd3) from 1 and through [CH₃CH₂CH₂ ^?CH₂=OH⁺] (9) from 2. The onset energies for forming the above products from 1 are spread over 85 kJ mol^?1, and are all near thermochemical threshold. The corresponding onsets from 2 are in a 19 kJ mol^?1 range, and all except that of CH₂=OH⁺ are well above their thermochemical thresholds. Each decomposition of 3 occurs over a broad energy range (> 214 kJ mol^?1), This demonstrates that ion-permanent dipole complexes can be significant intermediates over a much wider energy range than ion-induced dipole complexes can be. H-exchange between partners in the complexes appears to be much faster than exchange by conventional interconversions of the alcohol molecular ions with their distonic isomers. The onsets for water elimination from 1 and 2 are below the onsets for the complex-mediated processes, demonstrating that the latter are not necessarily the lowest energy decompositions of a given ion when the neutral partner in the complex is polar.     

Fragmentation of acylium ions from methyl levulinate. Isomerization processes involving carbon and oxygen atoms of both carbonyl groups

The fragmentations of the acylium ions O?C⁺? CH₂? CH₂? CO₂CH₃ and O?C⁺? CH₂? CH₂? COCH₃ generated from methyl levulinate are governed extensively by the interaction of the two carbonyl groups. Both species eliminate a molecule of CO unimolecularly and under CID conditions. The results derived from measurements of ¹³C and ¹⁸O labelled precursors, together with kinetic energy release values, have been used to study the mechanisms. In the first of these acylium ions, both carbonyl groups are equivalent; this phenomenon can be the result of a 1,4 methoxy shift. In the second acylium ion, only the oxygen atoms change their positions; this isomerization occurs via the [M? H]⁺ of γ-valerolactone. Some other fragmentation processes also discussed in relation to ²H labelling are the formation of the [M ? COOCH₃] ⁺ ion and the loss of HCOOCH₃ in the collision-induced dissociation mass spectra of the first acylium ion, and the formation of the [CH₃CO]⁺ ion and the loss of H₂O for the second one.     

The structure and dissociation pathways of protonated methanol: An ab initio molecular orbital study

Ab initio molecular orbital calculations with moderately large polarization basis sets and including valence-electron correlation have been used to examine the structure and dissociation mechanisms of protonated methanol [CH₃OH₂]⁺. Stable isomers and transition structures have been characterized using gradient techniques. Protonated methanol is found to be the only stable isomer in the [CH₅O]⁺ potential surface. There is no evidence for a tightly-bound complex, [HOCH₂]⁺…?H₂, analogous to the preferred structure [CH₃]⁺…?H₂ of [CH₅]⁺. Protonated methanol is found to possess a pyramidal arrangement of bonds at the oxygen atom with a barrier to inversion of 8kJ mol^?1. The lowest energy fragmentation pathways are dissociation into methyl cation and water (predicted to require 284 kJ mol^?1 with zero reverse activation energy) and loss of molecular hydrogen (endothermic by 138 kJ mol^?1 but with a reverse activation barrier of 149 kJ mol^?1). The results offer a possible explanation as to why production of [CH₂OH]⁺ from the reaction of methyl cation with water is not observed. Other dissociation processes examined include loss of a hydrogen atom to yield the methylenoxonium radical cation or methanol radical cation (requiring 441 and 490 kJ mol^?1, respectively) and loss of a proton to yield neutral methanol (requiring 784 kJ mol^?1).     

Loss of methyl from [H2CC(OH)?CH3]+˙ ions prepared by electron impact ionization of unstable 2-hydroxypropene

Unstable 2-hydroxpropene was prepared by retro-Diels-Alder decomposition of 5-exo-methyl-5-norbornenol at 800°C/2 × 10^?6 Torr. The ionization energy of 2-hydroxypropene was measured as 8.67±0.05 eV. Formation of [C₂H₃O]⁺ and [CH₃]⁺ ions originating from different parts of the parent ion was examined by means of ¹³C and deuterium labelling. Threshold-energy [H₂C?C(OH)? CH₃]^+˙ ions decompose to CH₃CO⁺+CH₃^˙ with appearance energy AE(CH₃CO⁺) = 11.03 ± 0.03 eV. Higher energy ions also form CH₂?C?OH⁺ + CH₃ with appearance energy AE(CH₂?C?OH⁺) = 12.2–12.3 eV. The fragmentation competes with hydrogen migration between C(1) and C(3) in the parent ion. [C₂H₃O]⁺ ions containing the original methyl group and [CH₃]⁺ ions incorporating the former methylene and the hydroxyl hydrogen atom are formed preferentially, compared with their corresponding counterparts. This behaviour is due to rate-determining isomerization [H₂C?C(OH)? CH₃]^+˙ →[CH₃COCH₃]^+˙, followed by asymmetrical fragmentation of the latter ions. Effects of internal energy and isotope substitution are discussed.     

Gas-phase ion–molecule reactions in dilute mixtures of dimethyl ether in krypton. Bimolecular and clustering reactions

The results of high-pressure variable-temperature and variable ionizing electron energy studies of gas-phase ion-molecule reactions of dimethyl ether in krypton are presented. Near the ionization threshold a series of peaks corresponding to (CH₃OCH₃)_nH⁺ (n = 1-4) clusters are observed. At higher ionizing electron energies, two new series of peaks appear, corresponding to [CH₃OCH₂]⁺(CH₃OCH₃)_n and [(CH₃)₃O]⁺ (CH₃OCH₃)_n clusters. The onium ion, [(CH₃)₃O]⁺, has been previously reported at elevated temperatures under methane chemical ionization conditions. It was suggested that the onium ion is formed by reaction of (CH₃)₂OH⁺ with CH₃OCH₃ with subsequent elimination of methanel, i.e. by fragmentation of an adduct ion. The present results strongly suggest that, under our conditions, [CH₃OCH₂]⁺ rather than thermal (CH₃)₃OH⁺, is the precursor to [(CH₃)₃O]⁺.     

Study of ions with excess kinetic energy. III—Mass spectra of excess kinetic energy ions of aliphatic amines

The mass spectra obtained from ions having excess kinetic energy (named the excess kinetic energy mass spectra) formed by electron impact of several aliphatic amines have been studied. Comparison with the excess kinetic energy mass spectra of the analogous alcohols shows many similarities. For example, the intensity of the [CH₂OH]⁺ ion of an aminoalcohol is about the same as the intensity of the [CH₂NH₂]⁺ ion. The excess kinetic energy mass spectra give useful information about the presence or absence of various functional groups, such as CH₂NH₂, CH₂OH, CH₃CHNH₂ and alkyl groups, and also about rearrangements. The fragmentation mechanisms are straightforward and applicable to a variety of classes of compounds. The molecular weight dependence of excess kinetic energy mass spectra is described.     

Protonation sites in methyl nitrate and the formation of transient CH4NO3 radicals. A neutralization—reionization mass spectrometric and computational study

Protonation sites in methyl nitrate (1) were evaluated computationally at the Gaussian 2(MP2) level of ab initio theory. The methoxy oxygen was the most basic site that had a calculated proton affinity of PA = 728–738 kJ mol⁻¹ depending on the optimization method used to calculate the equilibrium geometry of the CH₃O(H)-NO₂⁺ ion (2⁺). Protonation at the terminal oxygen atoms in methyl nitrate was less exothermic; the calculated proton affinities were 725, 722, and 712 kJ mol⁻¹ for the formation of the syn-syn, anti-syn, and syn-anti ion rotamers 3a⁺, 3b⁺, and 3c⁺, respectively. Ion 2⁺ was prepared by an ion-molecule reaction of NO₂⁺ with methanol and used to generate the transient CH₃O(H)-NO₂^. radical (2) by femtosecond collisional electron transfer. Exothermic protonation of 1 produced a mixture of 3a⁺–3c⁺ with 2⁺ that was used to generate transient radicals 3a–3c. Radical 2 was found to be unbound and dissociated without barrier to methanol and NO₂. Radicals 3a–3c were calculated to be weakly bound. When formed by vertical neutralization, 3a–3c dissociated completely on the 4.2 μs time scale of the experiment. The main dissociations of 3a–3c were formations of CH₃O^. + HONO and CH₃ONO + OH^.. The gas-phase chemistry of radicals 3a–3c and their dissociation products, as studied by neutralization—reionization mass spectrometry, was dominated by Franck—Condon effects on collisional neutralization and reionization. The adiabatic ionization energies of 3a–3c were calculated as 7.54, 7.57, and 7.66 eV, respectively.