Reactivity of functional groups toward H+ and SiMe3 + ions

No correlation was observed between the gas-phase basicities of various functional groups toward H⁺ and SiMe₃ ⁺ ions. Differences in the reactivity of functional groups studied toward SiMe₃ ⁺ ions are smaller than those in the reactivity toward protons. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 565–567, March, 2000.     

Interaction of trimethylsilyl ions with nitrates of the estrane series in the gas phase

The interaction of trimethylsilyl ions with nitrates of the estrane series affords the adducts [M + SiMe₃]⁺, whose fragmentation proceeds through the elimination of the functional groups either along with the trimethylsilyl residue, or in the form of molecules containing no SiMe₃.     

Interaction of the trimethylsilyl cation with molecules of aliphatic nitro compounds in the gas phase

Interaction of mononitroalkanes with the trimethylsilyl cation in the gas phase under chemical ionization (CI) conditions results in the formation of [M+SiMe₃]⁺ ions, which are more stable than the corresponding protonated molecular ions. In the case of 2-nitro-2-methylpropane and 2-nitropentane, fragmentation of the [M+SiMe₃]⁺ ions occurs with the formation of C₄H₉ ⁺ and C₅H₁₁ ⁺ carbocations, respectively. In the case of 1,1-dinitroethane and 1-halo-1,1-dinitroethane, fragmentation of the [M+SiMe₃]⁺ ions occurs with splitting off of a NO₂ ^. radical or an XNO₂ molecule (X=H, F, or Cl). Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1232–1234, June, 1997.     

Peculiarities of interaction of H+, Me3C+, and Me3Si+ ions with multifunctional molecules in the gas phase

Peculiarities of interaction of H⁺, Me₃C⁺, and Me₃Si⁺ ions with functional groups of molecules in the gas phase have been studied. Proton tends to form chelates with virtually all of the functional groups studied, whereas Me₃Si⁺ ions exhibit no capacity for chelation. Using isomeric xylenes as examples it was shown that Me₃Si⁺ ions (unlike Me₃C⁺ ions) experience virtually no steric hindrance when they react with nucleophilic centers. Effects of functional groups present in molecules of nitriles on the generation of [M+Me₃C]⁺ adducts in the gas phase and the Ritter reaction in solution were estimated.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1767–1773, September, 1995.This work was carried out with financial support from the International Science Foundation (Grant MA7 000) and the Russian Foundation for Basic Research (Project No. 93-03-18033).     

Gas-phase interaction of Me3Si+ ions withcis- andtrans-cyclohexanediols,their methyl ethers,and acetates

The effect of stereochemistry on the mechanism of gas-phase fragmentation of [M⁺SiMe₃]⁺ ions was studied usingcis- andtrans-1,2- and -1,4-cyclohexanediols, their methyl ethers, and acetates as model compounds. The higher stability of the [M⁺SiMe₃]⁺ions is characteristic of cis-isomers of all the compounds examined, which is associated with chelation in the case ofcis-cyclohexanediols andcis-methoxycyclohexanols and with the higher reactivity oftrans-isomers due to anchimeric assistance of the methoxy and acetoxy groups. Dehydration is characteristic of the [M⁺SiMe₃]⁺ ions formed from cyclohexanediols; both hydrogen atoms of the hydroxyl groups take part in the process, thus providing direct evidence of the chelation.Translated from Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 2025–2029, August, 1996.     

Characteristics and origins of common chemical noise ions in negative ESI LC-MS

Ionic chemical background noise in LC-MS has been one of the major problems encountered in trace analysis. In this study, the typical negative background ions in ESI LC-MS are investigated exemplarily. It was carried out using tandem mass spectrometry to study the products and precursors of the major background ions to examine their structures and structure relationship. Various typical LC eluents with different compositions and additives such as ammonium formate/formic acid and ammonium acetate/acetic acid have been studied. Several types of negative noise ions are concluded, which include the cluster chemical background ions only from mobile phase components and additives. Furthermore, there are also abundant clusters resulting from the solvation of some typical individual contaminants (e.g. additives and degradation products from tubing, impurities in the mobile phase, etc.), accompanied by some minor contribution from contaminants. The elemental composition of some selected ions was confirmed using the FT-ICR accurate mass measurement. This work provides us insight into information about the structures and types of common negative background ions and will help to understand their formation and origins. More importantly, it will guide us to prevent chemical noise interference in practice and also contribute to develop methods for noise reduction based on selective ion-molecule reactions.     

Tetramethylsilane as reagent gas: mass spectra of nitrocarboxylic acid esters and nitroalcohols

Nitrocarboxylic acid esters and nitroalcohols react with trimethylsilyl cation in the gas phase under conditions of chemical ionization to form stable [M+SiMe₃]⁺ ions. The pathways of their fragmentation were established and characteristic distinctions in the mass spectra caused by mutual arrangement of functional groups were found. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1257–1259, June, 1998.     

Substituted 3-phenylpropenoates and related analogs: electron ionization mass spectral fragmentation and density functional theory calculations

Analysis of ethyl 3-(2-chlorophenyl)propenoate by electron ionization mass spectrometry showed the distinct loss of an ortho chlorine. To characterize the structural requisites for the observed mass fragmentation, a series of 30 halogen-substituted 3-phenylpropenoate-related structures were examined. All ester-containing alkene derivatives exhibited loss of the distinctive chlorine from the 2-position of the phenyl ring. Analogous derivatives with the halogen (chlorine or bromine) in the para position did not evidence selective halogen loss. Results demonstrated that substituted 3-phenylpropenoates and their analogs fragment via the formation of a previously reported benzopyrylium intermediate. To understand the correlation between the intramolecular radical substitution and the abundance and selectivity of the chlorine (or other halogen) displacement, density functional theory calculations were performed to determine the charge on the principal cation involved in the chlorine loss (in the ortho, meta, and para positions), the charge for the neutral radical (noncation), the excess alpha-electron density on the relevant atom and the energy to form the cation from the neutral atom (ionization energy). Results showed that the selectivity and extent of halogen displacement correlated highly to the electrophilicity of the radical cation as well as the neutral radical. These data further support the proposed fragmentation mechanism involving intramolecular radical elimination.     

Reactivity of gaseous sodiated ions derived from benzene dicarboxylate salts toward residual water in the collision gas

The sodium adduct of disodium salts of benzene dicarboxylic acids (m/z 233), when subjected to collision‐induced dissociation (CID), undergoes a facile loss of CO₂ to produce an ion of m/z 189, which retains all the three sodium atoms of the precursor. The CID spectrum of this unusual m/z 189 ion shows significant peaks at m/z 167, 63 and 85. The enigmatic m/z 167 ion, which appeared to represent a loss of a 22‐Da neutral fragment from the precursor ion is in fact a fragment produced by the interaction of the m/z 189 ion with traces of water present in the collision gas. The change of the m/z 167 peak to 168, when D₂O vapor was introduced to the collision gas of a Q‐ToF instrument, proved that such an intervention of water could occur even in collision cells of tandem‐in‐space mass spectrometers. The m/z 189 ion has such high affinity for water; it forms an ion/molecule complex even during the brief residence time of ions in collision cells of triple quadrupole instruments. The complex formed in this way then eliminates elements of NaOH to produce the ion observed at m/z 167. In an ion trap, the relative intensity of the m/z 167 peak increases with longer activation time even at the lowest possible collision energy setting. Similarly, the m/z 145 ion (which represents the sodium adduct of phenelenedisodium, formed by two consecutive losses of CO₂ from the m/z 233 ion of meta‐ and para‐isomers) interacts with water to produce a fragment ion at m/z 123 for the sodium adduct of phenylsodium. Other uncommon ions that originate also from water/ion interactions are observed at m/z 85 and 63 for [Na₃O]⁺ and [Na₂OH]⁺, respectively. Tandem mass spectrometric experiments conducted with appropriately deuterium‐labeled compounds confirmed that the proton required for the formation of the [Na₂OH]⁺ ion originates from traces of water present in the collision gas and not from the ring protons of the aromatic moiety. Copyright © 2010 John Wiley & Sons, Ltd.     

Formation and stability of oxocarbenium ions from glycosides

Structural, protecting group and leaving group effects in the formation of oxocarbenium intermediates were studied in the gas phase. It is found that significant stabilization of oxocarbenium cations is achieved by protecting groups that interact with the cationic center via neighboring group participation despite the electron-withdrawing character of these moieties. On the other hand, ethereal protecting groups do not facilitate the formation of oxocarbenium intermediates. The experimental findings are supported by DFT calculations that show the following order of stabilization by the group adjacent to the cationic center: RCO > SiR(3) > R, where R is an alkyl group. This indicates that the SN1-like mechanism that is commonly proposed for this reaction is not always valid. Moderate leaving group effect is also detected in a series of thioaryl glucopyranosides.     

Specific features of the interaction of methyl ethers of methyl (methyl-α-D-galactopyranoside)uronate with trimethylsilyl ions in the gas phase

Specific features of the interaction between trimethylsilyl ions and methyl (methyl-α-d-galactopyranoside)uronate and its methyl ethers were revealed. It was shown that a hydrogen atom is generated when the trimethylsilyl ion is located at hydroxyl group. This atom migrates over the methoxy and hydroxyl groups toward the glycoside methoxy group, resulting in the formation of [Me+SiMe₃−MeOH]⁺ ions. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1317–1319, July, 1997.     

The Reactive Sites of Methane Activation: A Comparison of IrC3+ with PtC3+

The activation reactions of methane mediated by metal carbide ions MC₃⁺ (M = Ir and Pt) were comparatively studied at room temperature using the techniques of mass spectrometry in conjunction with theoretical calculations. MC₃⁺ (M = Ir and Pt) ions reacted with CH₄ at room temperature forming MC₂H₂⁺/C₂H₂ and MC₄H₂⁺/H₂ as the major products for both systems. Besides that, PtC₃⁺ could abstract a hydrogen atom from CH₄ to generate PtC₃H⁺/CH₃, while IrC₃⁺ could not. Quantum chemical calculations showed that the MC₃⁺ (M = Ir and Pt) ions have a linear M-C-C-C structure. The first C–H activation took place on the Ir atom for IrC₃⁺. The terminal carbon atom was the reactive site for the first C–H bond activation of PtC₃⁺, which was beneficial to generate PtC₃H⁺/CH₃. The orbitals of the different metal influence the selection of the reactive sites for methane activation, which results in the different reaction channels. This study investigates the molecular-level mechanisms of the reactive sites of methane activation.     

Study on new typical ionic clusters K+ (KNO3)n and NO3‐(KNO3)m

The sample solution of KNO₃ is ejected into the gas phase and the ionic dusters of K⁺(KNO₃)_n and NO₃ (KNO₃)_m are formed and observed by electrospray ionization mass spectrometry (ESIMS). Hie full mass spectra of both the positive ion and the negative ion show that the differences between each peak nearby are all about 101 (m/z), which correspond to the molecular weight of KNO₃. The general formula of the ionic clusters can be assigned as K⁺(KNO₃)_n and NO₃′‐(KNO₃)_m..     

Studies on interactions of ions with nitromethane and acetonitrile using CNDO/force method

CNDO/force calculations on the complexes of nitromethane and acetonitrile with H⁺, Li⁺, F⁻ and Cl⁻ have been carried out to determine their geometrical parameters and stretching force constants. The results are discussed in terms of specific interactions between the ions and the molecules.     

Gas‐phase reactions of XH3+ (X = C,Si, Ge) with NF3: a comparative investigation on the detailed mechanistic aspects

The gas‐phase reaction of CH₃⁺ with NF₃ was investigated by ion trap mass spectrometry (ITMS). The observed products include NF₂⁺ and CH₂F⁺. Under the same experimental conditions, SiH₃⁺ reacts with NF₃ and forms up to six ionic products, namely (in order of decreasing efficiency) NF₂⁺, SiH₂F⁺, SiHF₂⁺, SiF⁺, SiHF⁺, and NHF⁺. The GeH₃⁺ cation is instead totally unreactive toward NF₃. The different reactivity of XH₃⁺ (X = C, Si, Ge) toward NF₃ has been rationalized by ab initio calculations performed at the MP2 and coupled cluster level of theory. In the reaction of both CH₃⁺ and SiH₃⁺, the kinetically relevant intermediate is the fluorine‐coordinated isomer H₃X‐F‐NF₂⁺ (X = C, Si). This species forms from the exoergic attack of XH₃⁺ to one of the F atoms of NF₃ and undergoes dissociation and isomerization processes which eventually result in the experimentally observed products. The nitrogen‐coordinated isomers H₃X‐NF₃⁺ (X = C, Si) were located as minimum‐energy structures but do not play an active role in the reaction mechanism. The inertness of GeH₃⁺ toward NF₃ is also explained by the endoergic character of the dissociation processes involving the H₃Ge‐F‐NF₂⁺ isomer. Copyright © 2009 John Wiley & Sons, Ltd.     

Comparison of gas chromatographic and gas chromatographic/mass spectrometric techniques for the analysis of TNT and related nitroaromatic compounds

The use of capillary column gas chromatography and gas chromatography/mass spectrometry for the analysis of a series of standard solutions (0.1 to 10 μg/ml) of 2,4,6-trinitrotoluene (TNT) and eight other nitroaromatic components was evaluated. The techniques included gas chromatography with electron capture detection (GC/ECD), full scan and selected ion monitoring gas chromatography/mass spectrometry with electron impact ionization (EI/FS and EI/SIM), full scan and selected ion monitoring gas chromatography/mass spectrometry with positive ion chemical ionization using methane reagent gas (PICI/FS and PICI/SIM), and full scan and selected ion monitoring gas chromatography/mass spectrometry with negative ion chemical ionization using methane reagent gas (NICI/FS and NICI/SIM). The performance of the techniques was comapared by determining the linear response range, precision, and detection limits of the analyses.     

Modification of dipeptides by alkyl chloroformate—alkanol mixtures for analysis by gas chromatography/mass spectrometry with electron and chemical ionization and collisional activation: Differentiation of isomers

A possibility of dipeptide derivatization by alkyl chloroformate—alkanol (Alk = Me, Et, Pr, Bu) mixed reactants for their determination and differentiation of isomers in mixtures by gas chromatography/mass spectrometry with electron and chemical ionization and collisional activation was studied. Prospects for using derivatization by mixtures of methyl chloroformate with 2,2,2-trifluoroethanol, 2,2,3,3,3-pentafluoropropanol, and 2,2,3,3,4,4,4-heptafluorobutanol were shown. Diagnostic ions that make it possible to determine the amino acid sequence in dipeptides and distinguish residues of isomeric leucine and isoleucine were revealed. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2200–2204, December, 2006.     

Determination of the configuration of piperidin-4-ol silyl ethers by mass spectrometry with chemical ionization

Mass spectrometry with chemical ionization (isobutane as reactant gas) can be used for the determination of the configuration of the chiral center at the C(4) atom in molecules of silyl ethers of 2,6-diaryl-substituted piperidin-4-ols. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1625–1627, September, 2006.     

CHR‐Insertion (RH,CH3) into Cyclohexyl‐Substituted Silsesquioxanes: Reactivity and Decomposition Studies

Amorphous silica plays an important role in heterogeneous catalysis as a support and is frequently presumed to be “inert”. The structure of the supported catalyst is key to understanding the stability and reactivity of catalytic systems. To provide vital insights into the surface reactivity of silica, Polyhedral oligomeric silsesquioxanes (POSSs) can act as realistic homogeneous molecular models for silica surfaces. Here, we report novel reactivities associated with the silica surface, derived from our insights obtained by means of such model systems with potentially significant implications in catalysis when employing silica‐supported catalysts. In this work, the gas‐phase reactivities of two cyclohexyl‐substituted POSSs, namely the completely condensed triganol prism [Si₆cy₆O₉] (a6b0), and the incompletely‐condensed partial cube [Si₇cy₇O₉(OH)₃] (a7b3), with cy=c‐C₆H₁₁, were studied by using atmospheric pressure chemical ionisation (APCI) and collision‐induced decomposition (CID) spectroscopies. Silsesquioxane a6b0, containing three‐membered rings, was found to be much more reactive, undergoing novel CH₂‐insertion on reaction with gas phase molecules—a reaction not observed for a7b3, containing only four‐membered rings. Both silsesquioxanes displayed the ability to trap ammonia formed in situ within the mass spectrometer from N₂ in the instrument. This work also demonstrates the applicability of APCI and the role of CID in elucidating reactive POSS structures, highlighting novel gas‐phase reactivities of POSS.