Gas-phase ion chemistry of silyl cations obtained from hexamethyldisilazane

The gas-phase ion-molecule reactions of the Me3SiN(H)SiMe2+ ion, obtained by electron ionization from Me3SiN(H)SiMe3, have been studied in a Fourier transform ion cyclotron resonance spectrometer in order to understand the mechanistic details of an important chemical system presently used in film formation. This silyl cation has been observed to undergo addition reactions at electron rich centers to form stable adducts that may undergo further methane elimination in the case of alcohols and amines. The most important feature of these reactions is the fact that a metathesis type reaction can be observed in the presence of H2O, and other hydrogen labile substrates like alcohols, leading to the formation of the corresponding oxygen-containing ion, i.e. Me3SiOSiMe2+. For alcohols (ROH), facile formation of a tertiary product ion, presumably corresponding to an Me3Si-O-Si(Me)=O+-R structure with elimination of an amine reveals the strong tendency of these nitrogen-containing ions to undergo metathesis type reactions with oxygen containing substrates.

Gas-phase reactivity of 2-hydroxy-1,4-naphthoquinones: a computational and mass spectrometry study of lapachol congeners

In order to understand the influence of alkyl side chains on the gas-phase reactivity of 1,4-naphthoquinone derivatives, some 2-hydroxy-1,4-naphthoquinone derivatives have been prepared and studied by electrospray ionization tandem mass spectrometry in combination with computational quantum chemistry calculations. Protonation and deprotonation sites were suggested on the basis of gas-phase basicity, proton affinity, gas-phase acidity (?Gacid), atomic charges and frontier orbital analyses. The nature of the intramolecular interaction as well as of the hydrogen bond in the systems was investigated by the atoms-in-molecules theory and the natural bond orbital analysis. The results were compared with data published for lapachol (2-hydroxy-3-(3-methyl-2-butenyl)-1,4-naphthoquinone). For the protonated molecules, water elimination was verified to occur at lower proportion when compared with side chain elimination, as evidenced in earlier studies on lapachol. The side chain at position C(3) was found to play important roles in the fragmentation mechanisms of these compounds. Copyright (c) 2012 John Wiley & Sons, Ltd.

Sequential IRMPD of XGe(OEt)(4)(-) ions: Gas-phase synthesis of novel oxy-germanium anions

The gas-phase ion/molecule reactions of F(-) and EtO(-) with Ge(OEt)(4) yield readily and exclusively pentacoordinated complexes XGe(OEt)(4)(-) (X = F, EtO) at pressures in the 10(-8) T range as observed by FT-ICR techniques. These hypervalent species are prone to undergo sequential fragmentations induced by infrared multiphoton excitation that lead to a variety of germyl and germanate anions. In the case of FGe(OEt)(4)(-), three primary competitive channels are observed in the IRMPD process that can be identified as (EtO)(3)GeO(-), F(EtO)(2)GeO(-) and (EtO)(3)Ge(-). Ab initio calculations have been carried out to characterize the primary fragmentation paths induced by IRMPD and the most favorable structure of the resulting anions. The gas-phase acidity of a number of these germanium-containing ions have been estimated by bracketing experiments and by theoretical calculations. Germanate anions such as (EtO)(3)GeO(-) undergo some interesting reactions with H(2)S to give rise to anions such as (EtO)(3)GeS(-) and (EtO)(2)Ge(OH)S(-). (C) 2010 Elsevier B.V. All rights reserved.

A theoretical study on the gas phase reactions of the anions NbO3-, NbO5-, and NbO2(0H)(2)(-) with H2O and O-2

The potential energy surfaces at the singlet (s) and the triplet (t) electronic states associated with the gas-phase ion/molecule reactions of NbO3-, NbO5-, and NbO2(OH)(2)(-) with H2O and O-2 have been investigated by means of DFT calculations at the B3LYP level. An analysis of the results points out that the most favorable reactive channel comprises s-NbO3- reacting with H2O to give an ion-molecule complex s-NbO3(H2O)without a barrier. From this minima, an intramolecular hydrogen transfer takes place between the incoming water molecule and an oxygen atom of the NbO3- fragment to render the most stable minimum, s-NbO2(OH)(2)(-). This oxyhydroxide system reacts with O-2 along a barrierless process to obtain the triplet t-NbO4(OH)(2)(-)-A intermediate, and the crossing point, CP1, between s and t electronic states has been characterized. The next step is the hydrogen-transfer process between the oxygen atom of a hydroxyl group and the one adjacent oxygen atom to render a minimum with the two OH groups near each other, t-NbO4(OH)(2)(-)-B. From this point, the last hydrogen migration takes place, to obtain the product complex, t-NbO5(H2O)(-), that can be connected with the singlet separated products, s-NbO5- and H2O. Therefore, a second crossing point, CP2, has been localized. The nature of the chemical bonding of the key minima (NbO3-, NbO2(OH)(2)(-), NbO4(OH)(2)(-)-B, and NbO5-) in both electronic states of the reaction and an interaction with O-2 has been studied by topological analysis of Becke-Edgecombe electron-localization function (ELF) and atoms-in-molecules (AIM) methodology. The niobium-oxygen interactions are characterized as unshared-electron (ionic) interactions and some oxygen-oxygen interactions as protocovalent bonds.

Theoretical study on the reaction mechanism of VO2+ with propyne in gas phase

Possible molecular mechanisms of the gas-phase ion/molecule reaction of VO2+ in its lowest singlet and triplet states ((1)A(1)/(3)A '') with propyne have been investigated theoretically by density functional theory (DFT) methods. The geometries, energetic values, and bonding features of all stationary and intersystem crossing points involved in the five different reaction pathways (paths 1-5), in both high-spin (triplet) and low-spin (singlet) surfaces, are reported and analyzed. The oxidation reaction starts by a hydrogen transfer from propyne molecule to the vanadyl complex, followed by oxygen migration to the hydrocarbon moiety. A hydride transfer process to the vanadium atom opens four different reaction courses, paths 1-4, while path 5 arises from a hydrogen transfer process to the hydroxyl group. Five crossing points between high- and low-spin states are found: one of them takes place before the first branching point, while the others occur along path 1. Four different exit channels are found: elimination of hydrogen molecule to yield propynaldehyde and VO+ ((1)Sigma/(3)Sigma); formation of propynaldehyde and the moiety V-(OH2)(+); and two elimination processes of water molecule to yield cationic products, Prod-fc(+) and Prod-dc(+) where the vanadium atom adopts a four- and di-coordinate structure, respectively.

Generation of Naphthoquinone Radical Anions by Electrospray Ionization: Solution, Gas-Phase, and Computational Chemistry Studies

Radical anions are present in several chemical processes, and understanding the reactivity of these species may be described by their thermodynamic properties. Over the last years, the formation of radical ions in the gas phase has been an important issue concerning electrospray ionization mass spectrometry studies. In this work, we report on the generation of radical anions of quinonoid compounds (Q) by electrospray ionization mass spectrometry. The balance between radical anion formation and the deprotonated molecule is also analyzed by influence of the experimental parameters (gas-phase acidity, electron affinity, and reduction potential) and solvent system employed. The gas-phase parameters for formation of radical species and deprotonated species were achieved on the basis of computational thermochemistry. The solution effects on the formation of radical anion (Q(center dot-)) and dianion (Q(2-)) were evaluated on the basis of cyclic voltammetry analysis and the reduction potentials compared with calculated electron affinities. The occurrence of unexpected ions [Q + 15](-) was described as being a reaction between the solvent system and the radical anion, Q(center dot-).The gas-phase chemistry of the electrosprayed radical anions was obtained by collisional-induced dissociation and compared to the relative energy calculations. These results are important for understanding the formation and reactivity of radical anions and to establish their correlation with the reducing properties by electrospray ionization analyses.

Gas-Phase Acylium Ion Transfer Reactions Mediated by a Proton Shuttle Mechanism

The mechanism and the energy profile of the gas-phase reaction that mimics esterification under acidic conditions have been investigated at different levels of theory. These reactions are known to proceed with rate constants close to the collision limit in the gas-phase and questions have been raised as to whether the typical addition-elimination mechanism via a tetrahedral intermediate can explain the ease of these processes. Because these reactions are common to many organic and biochemical processes it is important to understand the intrinsic reactivity of these systems. Our calculations at different levels of theory reveal that a stepwise mechanism via a tetrahedral species is characterized by energy barriers that are inconsistent with the experimental results. For the thermoneutral exchange between protonated acetic acid and water and the exothermic reaction of protonated acetic acid and methanol our calculations show that these reactions proceed initially by a proton shuttle between the carbonyl oxygen and the hydroxy oxygen of acetic acid mediated by water, or methanol, followed by displacement at the acylium ion center. These findings suggest that the reactions in the gas-phase should be viewed as an acylium ion transfer reaction. (C) 2010 Wiley Periodicals, Inc. Int J Quantum Chem 111: 1596-1606, 2011

Corrole isomers: intrinsic gas-phase shapes via traveling wave ion mobility mass spectrometry and dissociation chemistries via tandem mass spectrometry

Corrole and four of its isomers with subtle structural changes promoted by exchange of nitrogen and carbon atoms in the corrole ring have been studied by traveling wave ion mobility mass spectrometry and collision induced dissociation experiments. Significant differences in shapes and charge distributions for their protonated molecules were found to lead to contrasting gas phase mobilities, most particularly for corrorin, the most "confused" isomer. Accordingly, corrorin was predicted by B3LYP/6-31g(d,p) and collisional cross section calculations to display the most compact tri-dimensional structure, whereas NCC4 and corrole were found to be the most planar isomers. Better resolution between the corrole isomers was achieved using the more polarizable and massive CO2 as the drift gas. Sequential losses of HF molecules were found to dominate the dissociation chemistry of the protonated molecules of these corrole isomers, but their unique structures caused contrasting labilities towards CID, whereas NCC4 showed a peculiar and structurally diagnostic loss of NH3, allowing its prompt differentiation from the other isomers.

Comparative analysis of the gas-phase reactions of cylindrospermopsin and the difference in the alkali metal cation mobility

Cylindrospermopsin (CYN) belongs to a group of toxins produced by several strains of freshwater cyanobacteria. It is a compact zwitterionic molecule composed of a uracil section and a tricyclic guanidinium portion with a primarily hepatotoxic effect. Using low multi-stage and high-resolution mass spectrometry, the gas-phase reactions of this toxin have been investigated. Our data show that collision-induced dissociation (CID) spectra of CYN are dominated by neutral losses, and three major initial fragmentation pathways are clearly distinguishable. Interestingly, comparative analysis of protonated and cationizated molecules showed a significant difference in the balance of the SO(3) and terminal ring elimination. These data indicate that the differential ion mobility of H(+), Li(+), Na(+) and K(+) leads to different fragmentation pathways, giving rise to mass spectra with different profiles. Copyright (C) 2008 John Wiley & Sons, Ltd.

Gas-phase dissociation of 1,4-naphthoquinone derivative anions by electrospray ionization tandem mass spectrometry

Gas-phase dissociation pathways of deprotonated 1,4-naphthoquinone (NQ) derivatives have been investigated by electrospray ionization tandem mass spectrometry (ESI-MS/MS). The major decomposition routes have been elucidated on the basis of quantum chemical calculations at the B3LYP/6-31+G(d,p) level. Deprotonation sites have been indicated by analysis of natural charges and gas-phase acidity. NQ anions underwent an interesting reaction under collision-induced dissociation conditions, which resulted in the radical elimination of the lateral chain, in contrast with the even-electron rule. Possible pathways have been suggested, and their mechanisms have been elucidated on the basis of Gibbs energy and enthalpy values for the anions previously described at each pathway. Copyright (C) 2009 John Wiley & Sons, Ltd.

Selectivity and Mechanisms Driven by Reaction Dynamics: The Case of the Gas-Phase OH-+CH3ONO2 Reaction

Well-established statistical approaches such as transition-state theory based on high-level calculated potential energy profiles are unable to account for the selectivity observed in the gas-phase OH- + CH3ONO2 reaction. This reaction can undergo bimolecular nucleophilic displacement at either the carbon center (S(N)2@C) or the nitrogen center (S(N)2@N) as well as a proton abstraction followed by dissociation (E(CO)2) pathway. Direct dynamics simulations yield an S(N)2:E(CO)2 product ratio in close agreement with experiment and show that the lack of reactivity at the nitrogen atom is due to the highly negative electrostatic potential generated by the oxygen atoms in the ONO2 group that scatters the incoming OH-. In addition to these dynamical effects, the nonstatistical behavior of these reactions is attributed to the absence of equilibrated reactant complexes and to the large number of recrossings, which might be present in several ion-molecule gas-phase reactions.

CCSD(T), W1, and other model chemistry predictions for gas-phase deprotonation reactions

A series of CCSD(T) single-point calculations on MP4(SDQ) geometries and the W1 model chemistry method have been used to calculate ΔH° and ΔG° values for the deprotonation of 17 gas-phase reactions where the experimental values have reported accuracies within 1 kcal/mol. These values have been compared with previous calculations using the G3 and CBS model chemistries and two DFT methods. The most accurate CCSD(T) method uses the aug-cc-pVQZ basis set. Extrapolation of the aug-cc-pVTZ and aug-cc-pVQZ results yields the most accurate agreement with experiment, with a standard deviation of 0.58 kcal/mol for ΔG° and 0.70 kcal/mol for ΔH°. Standard deviations from experiment for ΔG° and ΔH° for the W1 method are 0.95 and 0.83 kcal/mol, respectively. The G3 and CBS-APNO results are competitive with W1 and are much less expensive. Any of the model chemistry methods or the CCSD(T)/aug-cc-pVQZ method can serve as a valuable check on the accuracy of experimental data reported in the National Institutes of Standards and Technology (NIST) database.

Experimental and Theoretical Study of the OH Vibrational Spectra and Overtone Chemistry of Gas-Phase Vinylacetic Acid

In this study we present the gas-phase vibrational spectrum of vinylacetic acid with a focus on the ν = 1−5 vibrational states of the OH stretching transitions. Cross sections for ν = 1, 2, 4 and 5 of the OH stretching vibrational transitions are derived on the basis of the vapor pressure data obtained for vinylacetic acid. Ab initio calculations are used to assist in the band assignments of the experimental spectra, and to determine the threshold for the decarboxylation of vinylacetic acid. When compared to the theoretical energy barrier to decarboxylation, it is found that the νOH = 4 transition with thermal excitation of low frequency modes or rotational motion and νOH = 5 transitions have sufficient energy for the reaction to proceed following overtone excitation.

Gas-phase Dissociation of homo-DNA Oligonucleotides

Synthetic modified oligonucleotides are of interest for diagnostic and therapeutic applications, as their biological stability, pairing selectivity, and binding strength can be considerably increased by the incorporation of unnatural structural elements. Homo-DNA is an oligonucleotide homologue based on dideoxy-hexopyranosyl sugar moieties, which follows the Watson-Crick A-T and G-C base pairing system, but does not hybridize with complementary natural DNA and RNA. Homo-DNA has found application as a bioorthogonal element in templated chemistry applications. The gas-phase dissociation of homo-DNA has been investigated by ESI-MS/MS and MALDI-MS/MS, and mechanistic aspects of its gas-phase dissociation are discussed. Experiments revealed a charge state dependent preference for the loss of nucleobases, which are released either as neutrals or as anions. In contrast to DNA, nucleobase loss from homo-DNA was found to be decoupled from backbone cleavage, thus resulting in stable products. This renders an additional stage of ion activation necessary in order to generate sequence-defining fragment ions. Upon MS(3) of the primary base-loss ion, homo-DNA was found to exhibit unspecific backbone dissociation resulting in a balanced distribution of all fragment ion series.