Factors affecting reactivity in ammonia chemical ionization mass spectrometry

Several factors affecting reactivity in ammonia chemical ionization mass spectrometry (NH₃ CI) have been examined. These include the sample proton affinity, the preferred site of protonation and [NH₄]⁺ attachment, and substituent effects. In general, compounds having proton affinities ?787 kJ mol^?1 do not yield analytically useful intensities of the [M·NH₄]⁺ adduct ion. Substituted aromatic compounds in which the ring is the most basic site yield little (if any) [M·NH₄]⁺ ion even if the proton affinity of the compound is greater than 787 kJ mol^?1. On the other hand, some aromatic compounds in which the substituent is the most basic site yield relatively abundant adduct ions. The spectra of compounds possessing a good leaving group (X) exhibit only weak [M·NH₄]⁺ ions, but intense [M·NH₄ ? HX]⁺ and [M ? X]⁺ ions formed by substitution and elimination reactions. Electronic effects strongly influence these processes. Several examples are presented in which isomers are readily differentiated because of different reactivities under ammonia chemical ionization conditions.     

The site of gas phase cation attachment. The protonation,methylation and ethylation of morpholine,thiomorpholine and 1,4-thioxane

The attachment of gaseous positive ions ([H]⁺, [CH₃]⁺ and [C₂H₅]⁺) to morpholine, thiomorpholine and 1,4-thioxane, through chemical ionization, has been studied by collision spectroscopy. The daughter ion spectra of the ion/molecule reaction products were compared to those of model ions, generated by fast-atom bombardment of corresponding quaternary ammonium salts, in order to determine the preferred site of reaction for the protonation and alkylation of these multifunctional nucleophilic compounds. For novel entities with no model precursors, the site of cation attachment was postulated on the basis of characteristic fragmentations and trends established by the study of other bifunctional heterocycles. The site of protonation followed predicted trends in proton affinity differences for the various heteroatoms (N>S>O), and the alkyl ion reactivities followed differences in electronegativity or nucleophilicity (S>N>O).     

DFT investigation of sites of protonation of antitumor of 2‐(4‐aminophenyl)benzazoles in the gas phase and in solution

The proton affinity on each of the possible sites in the antitumor 2‐(4‐aminophenyl)benzazoles has been calculated at the B3LYP/6‐311G** level of theory in the gas phase and in solution. The N₃‐site of protonation is found to be strongly favored over the NH₂‐site for the studied compounds both in gas phase and in solution. The stability of N₃‐protonated species is explained by the resonance interaction of the NH₂‐group with the heterocyclic ring. The potential energy surface (PES) for the protonation process was studied at the density functional theory (DFT)/B3LYP/6‐311++G** level of theory. Solvent effects on the PES were also examined using two models: Onsager self‐consistent field and polarizable continuum model (PCM). © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Clusters of Ammonium Cation—Hydrogen Bond versus σ‐Hole Bond

MP2/6‐311++G(d,p) calculations were performed on the NH₄⁺ ??? (HCN)_n and NH₄⁺ ??? (N₂)_n clusters (n=1–8), and interactions within them were analyzed. It was found that for molecules of N₂ and HCN, the N centers play the role of the Lewis bases, whereas the ammonium cation acts as the Lewis acid, as it is characterized by sites of positive electrostatic potential, that is, H atoms and the sites located at the N atom in the extension of the H?N bonds. Hence, the coordination number for the ammonium cation is eight, and two types of interactions of this cation with the Lewis base centers are possible: N?H ??? N hydrogen bonds and H?N ??? N interactions that are classified as σ‐hole bonds. Redistribution of the electronic charge resulting from complexation of the ammonium cation was analyzed. On the one hand, the interactions are similar, as they lead to electronic charge transfer from the Lewis base (HCN or N₂ in this study) to NH₄⁺. On the other hand, the hydrogen bond results in the accumulation of electronic charge on the N atom of the NH₄⁺ ion, whereas the σ‐hole bond results in the depletion of the electronic charge on this atom. Quantum theory of “atoms in molecules” and the natural bond orbital method were applied to deepen the understanding of the nature of the interactions analyzed. Density functional theory/natural energy decomposition analysis was used to analyze the interactions of the ammonium ion with various types of Lewis bases. Different correlations between the geometrical, energetic, and topological parameters were found and discussed.     

Protonated primary amines induced thymine quintets studied by electrospray ionization mass spectrometry and density functional theory calculations

As the novel magic number clusters of nucleobases, the thymine quintets induced by ammonium ion (NH₄⁺), and particularly by its derivatives such as protonated alkyl amines and protonated aryl amines, have been studied by electrospray ionization mass spectrometry (ESI‐MS) and density functional theory (DFT) calculations. The DFT‐optimized geometry of NH₄⁺ induced thymine quintet ([T₅ + NH₄]⁺) reveals some new features including three additional hydrogen bonds between NH₄⁺ and its surrounding thymine molecules when compared with that of the alkali metal ions induced thymine quintets. In addition, the fourth hydrogen atom of NH₄⁺ is sticking out the assembly, and, thus, it might be replaced by an organic group R to form the protonated primary amine induced thymine quintet ([T₅ + R ? NH₃]⁺), a hypothesis that has been confirmed by both DFT calculations and ESI‐MS experiments. Furthermore, the relative abilities of the different protonated primary amines for inducing the thymine quintets are investigated by ESI‐MS competition experiments, and the results have shown a clear trend of stronger ability as the alkyl chain gets longer or as the aryl ring gets larger for the alkyl amines or the aryl amines. Two basic influence factors are consequently identified: one is the ability of the alkyl amine to accept proton, another is the π–π stacking interaction between the aryl ring and the π‐surface of the thymine molecule(s), whose explanations are strongly supported by multiple types of thermochemical data, various control experiments and DFT calculations. Copyright © 2014 John Wiley & Sons, Ltd.     

Investigation of lacidipine,a new 1,4-dihydropyridine antihypertensive agent and three chemically related compounds by means of chemical ionization,mass spectrometry and collision-induced dissociation

Chemical ionization of two 1,4-dihydropyridines, lacidipine and its Z-isomer, and their corresponding pyridines in three different reagent gases and the collision-induced dissociation (CID) of their respective mass-selected protonated molecular ions in the collision energy range 10–200 eV were performed on a multiple quadrupole instrument. The weakness of the Breasted acid NH₄⁺ as a protonating agent is clearly manifested in one of the ammonia positive-ion chemical ionization (CI⁺) mass spectra which displays the addition ion, [M + NH₄]⁺, as the favoured reaction channel. The stereochemistry of the precursor molecules, the exothermicity of the protonation process and the threshold of certain dissociation channels as a function of the collision energy are among the arguments invoked to explain some of the observed differences between the CI⁺ mass spectra and the CID data of the different isomers investigated. In an attempt to present a more comprehensive study, some high-performance liquid chromatographic retention times and resolutions are also given.     

Vibrational properties and DFT calculations of formamidine-templated Co and Fe formates

Experimental Raman and IR spectra of [NH₂-CH-NH₂][M(HCOO)₃] (M = Co, Fe), containing formamidinium cations [NH₂-CH-NH₂]⁺ (FMD⁺) were recorded at room temperature. In order to assign the vibrational modes corresponding to the FMD⁺ cation, the three-parameter hybrid B3LYP density functional method has been used with the 6-311G(2d,2p) basis to derive the vibrational wavenumbers (harmonic and anharmonic), infrared intensities and Raman scattering activities of formamidine molecule and FMD⁺ cation. The performed calculations revealed that protonation should affect most significantly the ν(CH), ρ(NH₂), ω(NH₂) and τ(NH₂) modes, which are expected to shift towards higher wavenumbers after protonation.     

Strong effect of methyl group on the strength of ionic hydrogen bond between C2H2 and H3O+

The effect of methyl group on the strength of the ionic hydrogen bond between C₂H₂ and H₃O⁺ has been studied with quantum chemical calculations at the UMP2/6‐311++G(d,p) level. The presence of a methyl group in the proton acceptor results in an energetic increase of 6.02 kcal/mol, increased by about 39%, whereas that in the proton donor leads to an energetic decrease of 2.18 kcal/mol, decreased by 14%. The charge analyses indicate that the methyl group in the proton acceptor is electron‐donating and that in the proton donor is electron‐withdrawing. The former plays a positive contribution to the formation of ionic hydrogen bond and the latter plays a negative contribution to the formation of ionic hydrogen bond. The weakening effect of solvent on the role of methyl group in the ionic hydrogen bond has also been studied at the UB3LYP/6‐311++G(d,p) level. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

An experimental and computational investigation on the fragmentation behavior of enaminones in electrospray ionization mass spectrometry

The dissociation pathways of protonated enaminones with different substituents were investigated by electrospray ionization tandem mass spectrometry (ESI‐MS/MS) in positive ion mode. In mass spectrometry of the enaminones, Ar? CO? CH?CH? N(CH₃)₂, the proton transfers from the thermodynamically favored site at the carbonyl oxygen to the dissociative protonation site at ipso‐position of the phenyl ring or the double bond carbon atom adjacent to the carbonyl leading to the loss of a benzene or elimination of C₄H₉N, respectively. And the hydrogen? deuterium (H/D) exchange between the added proton and the proton of the phenyl ring via a 1,4‐H shift followed by hydrogen ring‐walk was witnessed by the D‐labeling experiments. The elemental compositions of all the ions were confirmed by ultrahigh resolution Fourier transform ion cyclotron resonance tandem mass spectrometry (FTICR‐MS/MS). The enaminones studied here were para‐monosubstituted on the phenyl ring and the electron‐donating groups were in favor of losing the benzene, whereas the electron‐attracting groups strongly favored the competing proton transfer reaction leading to the loss of C₄H₉N to form a benzoyl cation, Ar‐CO⁺. The abundance ratios of the two competitive product ions were relatively well‐correlated with the σ_p⁺ substituent constants. The mechanisms of these reactions were further investigated by density functional theory (DFT) calculations. Copyright © 2010 John Wiley & Sons, Ltd.     

The phonon-assisted proton transfer between hydrazinium cations in (N2H5)2HMF6·2H2O (M∈{Ga, Al, Fe}) type compounds. FT IR and quantum theoretical study

FT IR spectra of a series of compounds with a general formula (N₂H₅)₂HMF₆·2H₂O (where M∈{Ga, Al, Fe}) were recorded at variable temperatures (from ∼100 to 300 K, at 10 K intervals). The appearance of the spectral region of ν(N-N) modes due to hydrazinium cations further supports the conclusions regarding the N₂H₅⁺?H⁺?N₂H₅⁺ hydrogen bond potential well based on Raman spectroscopic data [J. Raman Spectrosc. 28 (1997) 315]. The appearance of two bands corresponding to the ν(N-N) modes in the low temperature FT IR spectra that merge into one upon heating is a clear evidence of a symmetric potential well through which a phonon-assisted proton transfer (PAPT) occurs at higher temperatures. Ab initio MP2/6-311++G(2d,p) quantum chemical study of the proton transfer potential within the N₂H₅⁺?H⁺?N₂H₅⁺ cluster confirmed its double-minimum character. The first-order saddle point found on the MP2/6-311++G(2d,p) potential energy hypersurface corresponds to a centrosymmetric structure (C_2h symmetry), with the proton placed at the inversion center. The potential energy curve along the tunnelling coordinate was calculated by the intrinsic reaction coordinate (IRC) methodology, leading to an adiabatic PT barrier height of 3.94 kcal mol⁻¹ and a tunneling rate of 1.98 s⁻¹. The corresponding MP4(SDTQ)/6-311++G(2d,p)//MP2/6-311++G(2d,p) value of the adiabatic PT barrier height is 4.26 kcal mol⁻¹.     

Quantum chemical calculations on the interaction of diazomethane with proton acids

Diazoalkanes may form H-bonded associates with weak proton acids, whereas with strong acids proton transfer leads to the diazonium ion. In order to get information about protonation and association at and N, the corresponding energy balances, electron charge distributions and bond strengths have been calculated by means of quantum chemical ab initio methods with diazomethane as substrate and HF,NH₄⁺, OH₃⁺ as acids.     

A kinetic study of proton transfer reactions of NH+4, CH3NH+3, and PH+4 with calcium atoms

Cross section measurements for the proton transfer reactions of NH⁺₄, CH₃NH⁺₃, and PH⁺₄ with Ca(g) have been obtained over a range of low ion kinetic energies. For all reactions studied the cross sections drop sharply with increase in ion kinetic energy, indicating exothermic behavior. The results show that Ca(g) is an unusually strong base with a proton affinity in excess of 9.2 eV. Cross sections for the PH⁺₄Ca reaction are an order to magnitude higher than those for the NH⁺₄Ca reaction for ion energies between one and three eV. This effect is not explained by simple theories of ion-induced dipole interactions. It is suggested that the enhanced rate of the PH⁺₄Ca reaction may be due to d-orbital participation.     

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.     

Rocking‐Chair Ammonium‐Ion Battery: A Highly Reversible Aqueous Energy Storage System

Aqueous rechargeable batteries are promising solutions for large‐scale energy storage. Such batteries have the merit of low cost, innate safety, and environmental friendliness. To date, most known aqueous ion batteries employ metal cation charge carriers. Here, we report the first “rocking‐chair” NH₄‐ion battery of the full‐cell configuration by employing an ammonium Prussian white analogue, (NH₄)_1.47Ni[Fe(CN)₆]_0.88, as the cathode, an organic solid, 3,4,9,10‐perylenetetracarboxylic diimide (PTCDI), as the anode, and 1.0 m aqueous (NH₄)₂SO₄ as the electrolyte. This novel aqueous ammonium‐ion battery demonstrates encouraging electrochemical performance: an average operation voltage of ca. 1.0 V, an attractive energy density of ca. 43 Wh kg⁻¹ based on both electrodes’ active mass, and excellent cycle life over 1000 cycles with 67 % capacity retention. Importantly, the topochemistry results of NH₄⁺ in these electrodes point to a new paradigm of NH₄⁺‐based energy storage.     

Effect of Different Solvent Characteristics on the Proton‐Ligand and Sm+3‐Ligand Formation Constant of Butanamide‐3‐[2‐[(2‐Hydroxyphenyl Methylene) Amino]Ethylimino]N‐Phenyl(BHPAP) in Mixed Solvents

Complexes equilibrium of (BHPAP) with proton and Sm⁺³ ion has been measured in various mixed aqueous solvents, viz.; methanol‐water, ethanol‐water, acetone‐water and dioxane‐water. Based on potentiometric equilibrium measurements of hydrogen ion concentration at 30 °C, ionic strength 0.1 M KNO₃ and in the above various mixed solvents, the values of protonation constant of BHPAP‐Sm⁺³ complex have been evaluated. The variation of protonation and stability constants with the inverse of dielectric constant or mole fraction of solvent was studied. Application of Fuoss expression and consideration of electrostatic and non‐electrostatic effects are made to explain the above constants. The solid complexes were isolated for each Pr⁺³ and Nd⁺³‐BHPAP. Elemental analysis, conductance, infrared spectra, and electronic spectra for these solutions and TG, DTG and DTA measurements characterized these solids. The ligand behaves towards the metal ion as a dibasic tetradentate ligand.     

Dihydrogen Generation from Amine/Boranes: Synthesis,FT‐ICR,and Computational Studies

A Fourier transform ion cyclotron resonance spectrometry (FT‐ICR) study of the gas‐phase protonation of ammonia‐borane and sixteen amine/boranes R¹R²R³N? BH₃ (including six compounds synthesized for the first time) has shown that, without exception, the protonation of amine/boranes leads to the formation of dihydrogen. The structural effects on the experimental energetic thresholds of this reaction were determined experimentally. The most likely intermediate and the observed final species (besides H₂) are R¹R²R³N? BH₄⁺ and R¹R²R³N? BH₂⁺, respectively. Isotopic substitution allowed the reaction mechanism to be ascertained. Computational analyses ([MP2/6‐311+G(d,p)] level) of the thermodynamic stabilities of the R¹R²R³N? BH₃ adducts, the acidities of the proton sources required for dihydrogen formation, and the structural effects on these processes were performed. It was further found that the family of R¹R²R³N? BH₄⁺ ions is characterized by a three‐center, two‐electron bond between B and a loosely bound H₂ molecule. Unexpected features of some R¹R²R³N? BH₄⁺ ions were found. This information allowed the properties of amine/boranes most suitable for dihydrogen generation and storage to be determined.     

Hypervalent radical formation probed by electron transfer dissociation of zwitterionic tryptophan and tryptophan‐containing dipeptides complexed with Ca2+ and 18‐crown‐6 in the gas phase

The relationship between peptide structure and electron transfer dissociation (ETD) is important for structural analysis by mass spectrometry. In the present study, the formation, structure and reactivity of the reaction intermediate in the ETD process were examined using a quadrupole ion trap mass spectrometer equipped with an electrospray ionization source. ETD product ions of zwitterionic tryptophan (Trp) and Trp‐containing dipeptides (Trp‐Gly and Gly‐Trp) were detected without reionization using non‐covalent analyte complexes with Ca²⁺ and 18‐crown‐6 (18C6). In the collision‐induced dissociation, NH₃ loss was the main dissociation pathway, and loss related to the dissociation of the carboxyl group was not observed. This indicated that Trp and its dipeptides on Ca²⁺(18C6) adopted a zwitterionic structure with an NH₃⁺ group and bonded to Ca²⁺(18C6) through the COO^? group. Hydrogen atom loss observed in the ETD spectra indicated that intermolecular electron transfer from a molecular anion to the NH₃⁺ group formed a hypervalent ammonium radical, R‐NH₃, as a reaction intermediate, which was unstable and dissociated rapidly through N–H bond cleavage. In addition, N–C_α bond cleavage forming the z₁ ion was observed in the ETD spectra of Trp‐GlyCa²⁺(18C6) and Gly‐TrpCa²⁺(18C6). This dissociation was induced by transfer of a hydrogen atom in the cluster formed via an N–H bond cleavage of the hypervalent ammonium radical and was in competition with the hydrogen atom loss. The results showed that a hypervalent radical intermediate, forming a delocalized hydrogen atom, contributes to the backbone cleavages of peptides in ETD. Copyright © 2015 John Wiley & Sons, Ltd.     

Differentiation of the isomeric 1,2-cyclopentanediols by ion-molecule reactions in a triple-quadrupole mass spectrometer

Collisionally activated decompositions and ion-molecule reactions in a triple-quadrupole mass spectrometer are used to distinguish between cis- and trans-1,2-cyclopentanediol isomers. For ion kinetic energies varying from 5 eV to 15 eV (laboratory frame of reference), qualitative differences in the daughter ion spectra of [MH]⁺ are seen when N₂ is employed as an inert collision gas. The cis ?1,2-cyclopentanediol isomer favors H₂O elimination to give predominantly [MH- H₂O]⁺. In the trans isomer, where H₂O elimination is less likely to occur, the rearrangement ion [HOCH₂CHOH]⁺ exists in significantly greater abundance. Ion-molecule reactions with NH₃ under single-collision conditions and low ion kinetic energies can provide thermochemical as well as stereochemical information. For trans ?1,2-cyclopentanediol, the formation of [NH₄]⁺ by proton transfer is an exothermic reaction with the maximum product ion intensity at ion kinetic energies approaching 0 eV. The ammonium adduct ion [M + NH₄]⁺ is of greater intensity for the trans isomer. In the proton transfer reaction with the cis isomer, the formation of [NH₄]⁺ is an endothermic process with a definite translational energy onset. From this measured threshold ion kinetic energy, the proton affinity of cis ?1,2-cyclopentanedioi was estimated to be 886 ± 10 kJ mol^?1.     

N2O…HOCl复合物结构和性质的理论研究

分别在DFT-B3LYP和MP2/6-311++G**水平上求得HOCl + N2O复合物势能面上的六种(S1, S2, S3, S4, S5和S6)和四种(S1, S2, S4和S5)构型. 频率分析表明,其中的S1和S3为过渡态,其它为稳定构型. 在复合物S3, S5 和S6中,HOCl 单体的σ*(5O-6H)作为质子供体,与N2O单体中作为质子受体的3O原子相互作用,形成氢键结构,而在氢键复合物S2中, 质子受体为N2O单体中的端1N原子;复合物S1中,HOCl分子的σ*(5O-4Cl)作为质子供体与N2O分子中的端1N原子(质子受体)相互作用,形成卤键结构,而复合物S4中的卤键结构的质子受体为N2O分子中的端3O原子. 经B3LYP/6-311++G**水平上的计算,考虑了基组重叠误差(BSSE)校正的单体间相互作用能在-1.56 ~ -8.73 kJ·mol-1之间. 采用自然键轨道理论(NBO)对两种单体间相互作用的本质进行了考查,并通过分子中原子理论(AIM)分析了复合物中氢键和卤键键鞍点处的电子密度拓扑性质.     

Chemically modified porous silicon for laser desorption/ionization mass spectrometry of ionic dyes

Desorption/ionization on silicon (DIOS) mass spectra of model ionic dyes methylene blue (MB⁺Cl^?) and methyl orange (Na⁺MO^?) were studied using p⁺ type‐derived porous silicon (PS) free layers. As‐prepared PS (PS‐H), the PS thermally oxidized at 300 °C (PS‐OX), PS with chemically grafted cation‐exchanging alkylsulfonic acid (PS‐SO₃H) and anion‐exchanging propyl‐octadecyldimethylammonium chloride (PS‐ODMA⁺Cl^?) groups was tested as ionization platforms. Two mechanisms of the methylene blue desorption/ionization were found: (1) the formation of [MB + H]^+? ion due to the reduction/protonation of MB⁺, which is predominant for PS‐H and PS‐OX platforms and (2) direct thermal desorption of the MB⁺ cation, prevailing for PS‐SO₃H. The fragmentation of the cation is significantly suppressed in the latter case. The samples of PS‐SO₃H and PS‐ODMA⁺ Cl^? efficiently adsorb the dyes of the opposite charge from their solutions via the ion‐exchange. Consequent DIOS MS studies allow to detect only low fragmented ions (MB⁺ and MO^?, respectively), demonstrating the potential of the ion‐exchange adsorption combined with DIOS MS for the analysis of ionic organic compounds in solutions. Copyright © 2009 John Wiley & Sons, Ltd.