A study of the gas-phase reaction between protonated acetaldehyde and methanol

Protonated acetaldehyde is methylated on the oxygen during interaction with methanol in the gas phase. The ionic product of the ion/molecule reaction between methanol and protonated acetaldehyde is identical with C-protonated methylvinyl ether (high-pressure ionization), and with the (M − C₂H₅)⁺ fragment ion of sec-butyl methyl ether (following electron ionization), and also with the (M − OCH₃)⁺ fragment ion of acetaldehyde dimethylacetal (following electron ionization). The structures of these ions and the mechanism of their formation were established by isotope-labeling experiments and collision-induced dissociation mass spectra of model compounds obtained with three different types of tandem mass spectrometers (BEQQ, triple-quadrupole, and a penta-quadrupole instrument). Gas phase synthesis of the product ion from [²H₃]-methanol or [²H₄]-acetaldehyde provided insight into its mode of formation and collision-induced dissociation.     

Quantitative analysis of fluorimated ethylchloroformate derivatives of non-protein amino acids using positive and negative chemical ionization gas chromatography-mass spectometry

The GC-MS characterization of the ethylchloroformate derivatives of amino acids in an aqueous medium has been applied to non-protein amino acids. Derivatization of non-protein amino acids using ethylchloroformate, trifluoroethanol, and pyridine produced strong [M + 1]⁺ and [M - 1]⁻ ions in positive and negative chemical ionization (CI) modes, respectively. Twenty-one out of the twenty-three non-protein amino acids studied produced detectable ion chromatograms in both ionization modes when methane was used as the CI reagent gas. Mass spectra of these non-protein amino acid derivatives showed characteristic [M - 19]⁺, [M + 1]⁺, [M + 29]⁺, and [M + 41]⁺ peaks in the positive chemical ionization mode, and [M - 1]⁻, and [M + 35]⁻ peaks in the negative chemical ionization mode. The detection limits and the linear dynamic range of trifluorethanol ethylchloroformate derivatives of non-protein amino acids were studied using positive chemical ionization. The detection limits are mostly in the femtomole range.     

Chemical ionization mass spectrometry of hydrofluorocarbons, hydrofluorocarbon ethers and perfluoroalkenes

Positive and negative chemical ionization (CI) mass spectrometry is studied for hydrofluorocarbons (HFCs), hydrofluorocarbon ethers (HFEs), and perfluoroalkenes (PFCs) using various kinds of reagent gas. While no quasi-molecular ion was observed under electron impact ionization for saturated MFCs, [M-F]⁺ is detected under CI conditions using methane as a reagent gas. Mechanisms for the generation of [M-F]⁺ are discussed. Furthermore, nitrogen monoxide can be used as a reagent gas to observe [M + NO]⁺ for many HFCs and HFEs. In negative mode chloroform is also available to generate [M + Cl]⁻ for HFCs and HFEs containing -CHF- groups.     

激光解吸电离飞行时间质谱下的银簇行为

近年来,对金属簇的研究已成为化学与物理学中最活跃的研究领域之一［１］．金属簇被认为是介于单个原子与固体之间的中间相［２］．深入地研究其结构、形成机理及物理与化学行为,对于寻找新的催化剂［３］,重新认识气相化学与凝聚相化学的关系［４］,都有非常重要的意...     

Role of the sulfhydryl group on the gas phase fragmentation reactions of protonated cysteine and cysteine containing peptides

The gas phase fragmentation reactions of protonated cysteine and cysteine-containing peptides have been studied using a combination of collisional activation in a tandem mass spectrometer and ab initio calculations [at the MP2(FC)/6-31G*//HF/6-31G* level of theory]. There are two major competing dissociation pathways for protonated cysteine involving: (i) loss of ammonia, and (ii) loss of the elements of [CH₂O₂]. MS/MS, MS/MS of selected ions formed by collisional activation in the electrospray ionization source as well as ab initio calculations have been carried out to determine the mechanisms of these reactions. The ab initio results reveal that the most stable [M + H − NH₃]⁺ isomer is an episulfonium ion (A), whereas the most stable [M + H − CH₂O₂]⁺ isomer is an immonium ion (B). The effect of the position of the cysteine residue on the fragmentation reactions of the [M + H]⁺ ions of all the possible simple dipeptide and tripeptide methyl esters containing one cysteine (where all other residues are glycine) has also been investigated. When cysteine is at the N-terminal position, NH₃ loss is observed, although the relative abundance of the resultant [M + H − NH₃]⁺ ion decreases with increasing peptide size. In contrast, when cysteine is at any other position, water loss is observed. The proposed mechanism for loss of H₂O is in competition with those channels leading to the formation of structurally relevant sequence ions.     

Electrospray ionization tandem mass spectrometry of 3-[4-bis-N,N-(2- chloroethyl)aminophenyl]acetates of estrane series

Collision-induced fragmentation of the [M + Na](+) and [M + H](+) ions generated from 3-[4-bis-N,N-(2-chloroethyl)aminophenyl]acetates in the estrane series under electrospray/ionization was studied. Some regularities in fragmentation pathways depending on the nature of functional groups were established. Formation of the [(M + Na) NaCl](+) ions along with [(M + Na) HCl](+) ions from the [M + Na](+) ions was explained using quantum chemical calculations for some simplified models.     

Electrospray ionization mass spectrometry of metal complexes. Gas phase formation of a binuclear copper(II)–5-Br-PADAP complex

The complexes formed from copper(II) and 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP or HL) in aqueous methanol solution was studied by electrospray ionization mass spectrometry. The solution of a 1:1 complex of Cu(II) with 5-Br-PADAP showed five peaks assignable to a binuclear complex [Cu₂L₂(AcO)]⁺ and mononuclear complexes [CuL]⁺, [CuL(H₂O)]⁺, [CuL(AcOH)]⁺ and [CuL(HL)]⁺ (AcO=acetate). Collision activated dissociation revealed the relative order of bonding strengths; Cu–L>Cu–HL>CuL–AcOH>CuL–H₂O. The peak intensities of the binuclear complex showed second-order dependency on those of the mono complex. As for the solution of Ni(II)–5-Br-PADAP, no binuclear complex was observed in the mass spectra. Thus, it was suggested that [Cu₂L₂(AcO)]⁺ was formed by the fast gas phase reaction: 2[CuL]⁺+AcO⁻[Cu₂L₂(AcO)]⁺.     

Acetogenic isoquinoline alkaloids: CXII. Separation and identification of dimeric naphthylisoquinoline alkaloids by liquid chromatography coupled to electrospray ionization mass spectrometry

The atropodiastereomeric dimeric naphthylisoquinoline alkaloids, michellamines A (1a), B (1b) and C (1c), together with their monomers, korupensamines A (2a) and B (2b), were investigated using electrospray ionization tandem mass spectrometry coupled to liquid chromatography (LC–ESI-MS–MS). From the spectra obtained, characteristic product ions were chosen to monitor the chromatographic separation achieved on an RP-18 column. Under acidic conditions required for chromatographic analysis, the monomeric alkaloids 2a and 2b yielded protonated molecules [M+H]⁺, while the dimers, the michellamines, exhibited doubly protonated [M+2H]²⁺ molecules. In addition, the coeluting alkaloids 1b and 2b were identified unambiguously by means of tandem mass spectrometry. Thus, together with the retention times of the alkaloids, the product ion spectra allowed us the identification of michellamines in the presence of their presumed biogenetic monomeric precursors. Application of the HPLC–MS–MS method successfully proved the enzymatic formation of michellamine C (1c) by in vitro dimerization of korupensamine B (2b).     

碱金属离子与三肽复合物的气相裂解反应

为了探索侧链R基团对碱金属离子与多肽复合物气相裂解反应的影响, 采用电喷雾电离质谱法研究了碱金属离子Li⁺, Na⁺和K⁺分别与甘氨酸三肽(GGG)、 甘氨酰-苯丙氨酰-甘氨酸三肽(GFG)和甘氨酰-甘氨酰-苯丙氨酸三肽(GGF)形成的复合物的气相裂解反应. 质谱定性实验结果表明, Li⁺, Na⁺和K⁺与GGG, GFG或GGF在气相中可以形成稳定的复合物, 配合比为1∶1或2∶1. 竞争反应质谱图显示, GGG, GFG或GGF与碱金属离子形成的复合物的质谱峰丰度按Li⁺, Na⁺, K⁺顺序依次下降, 表明随着碱金属离子半径的增加, 它们与三肽的结合强度依次减弱. 碰撞诱导解离显示, 母体离子[GGG+Na]⁺, [GGF+Na]⁺和[GFG+Na]⁺ 的质心碰撞能量E(CM)₅₀数值分别为1.94, 1.76和1.63 eV. 通过质谱滴定法测得[GGG+Na]⁺, [GFG+ Na]⁺和[GGF+Na]⁺ 的结合常数lg\begin{array}{c}{K}_{a_1}\end{array}分别为5.30, 5.25和5.17. 质谱法定量结果进一步确认复合物的稳定性顺序为[GGG+Na]⁺>[GGF+Na]⁺>[GFG+Na]⁺, 表明由于空间位阻的影响, 侧链R基团含有苄基的GFG或 GGF与Na⁺的键合强度要小于侧链R全部为H的GGG. 串级质谱分析结果显示, 碱金属化的GGG断裂位点较多, 可解离出丰富的金属化a₂, b和y型碎片离子, 而碱金属化的GGF和GFG解离出的金属化y型离子较多, b型离子其次, 金属化a型离子几乎没有. 此外, 双碱金属化的GGF可解离出较多金属化y型离子. 复合物[GGF+Na]⁺的裂解曲线显示, 当碰撞能量为25 eV时, [y₂+Na-H]⁺ 和[b₂+Na+OH]⁺为主要碎片离子, 当碰撞能量>40 eV时, 只有[b₂+Na+OH]⁺ 碎片离子占有优势数量. 根据质子化三肽裂解机理可以推测, 钠化GFG裂解后生成含噁唑酮的[b₂+Na]⁺离子, 该离子经过一系列过渡态生成[a₂+Na]⁺(2-苄基-4-咪唑酮), 而不是常见的亚胺离子.     

Reactivity of tri- and tetra-nuclear ethylidyne cyclopentadienylcobalt clusters towards protonic acids

The bis(μ₃-ethylidyne) tricobalt cluster [(CpCo)₃(μ₃-CCH₃)₂] (1b) is protonated by trifluoroacetic acid to give the dicobalt edge-protonated cation [H(CpCo)₃(μ₃-CCH₃)₂]⁺ [lb + H]⁺. Protonation of the μ₃-ethylidyne tetracobalt cluster hydride [H(CpCo)₄(μ₃-CCH₃)] (3) takes place in two consecutive steps. At low temperature [H₂(CpCo)₄(μ₃-CCH₃)]⁺ [3 + H]⁺ is formed first, and is then slowly converted into [H₃(CpCo)₄(μ₃-CCH₃)]²⁺ [3 + 2H]²⁺ by an excess of acid. As judged by the ¹H NMR data and the crystal structure of [3 + X]⁺[(CF₃COO)₂X]⁻ (X = H or D) the endo hydrogens in [3 + H]⁺ and [3 + 2H]²⁺ occupy μ₃-(Co₃) face capping hydridic positions. The cations [1b + H]⁺ and [3 + H]⁺ show hydride fluxionality in solution, which in the case of [3 + H]⁺ can be frozen out on the NMR timescale at low temperature (ΔG ^≠ (203 K) = 40.8 kJ/mol). The structure of [3 + X]⁺ [(CF₃COO)₂X]⁻ (X = H or D) was determined by X-ray crystallography. One of the hydrides/deuterides is located on the crystallographic mirror plane, capping a tricobalt face of the cluster cation. The other endo hydrogen atom is believed to be disordered between the other two μ₃-(Co₃) sites, which are related by space group symmetry. Deuteronation of 3 shows a strong normal kinetic deuterium isotope effect. From the temperature independence of the ¹H NMR spectrum of [3 + 2D]²⁺ a non-fluxional solution structure can be inferred. In all the systems studied, hydridic (μ₂- or μ₃-) sites are thermodynamically preferred to possible isomeric agostic CoHC or Co₂HC sites for the endo hydrogens. Agostic interactions cannot, however, be ruled out in transient intermediates during the course of the protonations.     

A ring-fission/C–C bond cleavage reaction with an N-alkyl-N-methyl-N-[(5-phenyl-1,2,4-oxadiazol-3-yl)methyl]amine

The reaction of N-(3,4-dichlorophenethyl)-N-methylamine (1) with 3-chloromethyl-5-phenyl-1,2,4-oxadiazole (2) was investigated. Employment of an equimolar amount of 1 and 2 in the presence of potassium carbonate led to the expected tertiary amine 3 (N-[(3,4-dichlorophenyl)ethyl]-N-methyl-N-[(5-phenyl-1,2,4-oxadiazol-3-yl)methyl]amine), whereas an excess of 1 and prolonged reaction time resulted in ring fission of the oxadiazole system in 3 and finally in the formation of N′-benzoyl-N-[(3,4-dichlorophenyl)ethyl]-N-methylguanidine (4) and N,N′-bis[(3,4-dichlorophenyl)ethyl]-N,N′-dimethylmethanediamine (5). The structures of products 3–5 were determined by means of ¹H and ¹³C NMR-spectroscopy, mass spectrometry and IR-spectroscopy, for 3 (as picrate) and 4 also X-ray structure analysis was employed. A possible mechanism of the reaction pathway leading to compounds 4 and 5 is proposed.     

Matrix-assisted laser desorption/ionisation mass spectrometry of oligosaccharides and glycoconjugates

The technique of matrix-assisted laser desorption/ionization mass spectrometry (MALDI) is described and examples are given of its use for the examination of glycoproteins, glycopeptides, glycolipids and oligosaccharides. Abundant [M + H]⁺ ions are produced by the glycoproteins and glycopeptides, whereas glycolipids and oligosaccharides give mainly [M + Na]⁺ ions. Resolution on time-of-flight (TOF) instruments is poor but improved resolution can be obtained by use of ion cyclotron resonance or magnetic sector instruments. Although the technique gives mainly [M + Na]⁺ ions from neutral, underivatised oligosaccharides, with little fragmentation when implemented on TOF systems, the use of a reflectron enables fragment ions produced by post-source decay to be obtained. Acidic sugars give less satisfactory positive ion spectra with TOF analysers. but generally produce abundant negative ions. Extensive fragmentation is observed with these compounds when the spectra are recorded with magnetic sector instruments. Neutral glycolipids produce strong spectra from several matrices but acidic glycolipids show extensive fragmentation as the result of sialic acid loss.     

Gas-phase C-H and N-H bond activation by a high valent nitrido-iron dication and NH-transfer to activated olefins

A tetrapodal pentadentate nitrogen ligand (2,6-bis(1,1-di(aminomethyl)ethyl)pyridine, 1) is used for the synthesis of the azido-iron(III) complex [(1)Fe(N3)]X2 where X is either Br or PF6. By means of electrospray ionization mass spectrometry, the dication [(1)Fe(N3)]2+ can be transferred into the gas phase as an intact entity. Upon collisional activation, [(1)Fe(N3)]2+ undergoes an expulsion of molecular nitrogen to afford the dicationic nitrido-iron species [(1)FeN]2+ as an intermediate, which upon further activation can intramolecularly activate C-H- and N-H bonds of the chelating ligand 1 or can transfer an NH unit in bimolecular reactions with activated olefins. The precursor dication [(1)Fe(N3)]2+, the resulting nitrido species [(1)FeN]2+, and its possible isomers are investigated by mass spectrometric experiments, isotopic labeling, and complementary computational studies using density functional theory.     

Analysis of trichothecene mycotoxins in human blood by capillary column gas chromatography-ammonia chemical ionization mass spectrometry

Capillary column gas chromatography-ammonia chemical ionization mass spectrometry was found to be an excellent technique for the trace detection and identification of underivatized trichothecene mycotoxins. Abundant (M + H)+ and/or (M + NH4)+ pseudo-molecular ions were observed for T-2 toxin, HT-2 toxin, T-2 triol, diacetoxyscirpenol, deoxynivalenol and verrucarol under the conditions developed. This method was successfully applied to the analysis of human blood samples spiked with mycotoxins in the 0-500 ng/g range during a recent interlaboratory exercise. T-2 toxin and diacetoxyscirpenol were detected in these samples in the 2-180 ng/g range. Detection limits of 0.7 and 3.6 ng/g for T-2 toxin and diacetoxyscirpenol, respectively, were possible owing to the specificity of the method.     

C1～C3正构醛、醇在质子转移反应飞行时间质谱中的产物离子特征分析

为分析C₁～C₃正构醛、 醇化合物在质子转移反应飞行时间质谱(PTR-TOF MS)中的产物离子特征, 考察了不同E/N值(E: 电场强度, N: 气体分子数密度)下C₁～C₃正构醛、 醇的产物离子种类和强度的变化. 结果表明, 低分子量正构醇类(甲醇、 乙醇和丙醇)倾向于形成质子化聚合物[nMH]⁺及其失水离子[nMH-H₂O]⁺, 且随着E/N值升高, 醇类会产生较多裂解碎片和多聚体离子. 低分子量正构醛(甲醛、 乙醛和丙醛)主要产生质子化产物[MH]⁺和一水合质子化产物[M·H₃O]⁺, 高E/N值(>125 Td)会抑制甲醛质子化, 也会抑制其加合产物的生成. 乙醛倾向于形成水加合物, 且随着E/N值增高, 质子化乙醛与一水合质子化乙醛的变化趋势相反. 另外, 丙醛在较高的E/N值下会产生一系列聚合物, 如[MH·C₂H₅]⁺和[2MH]⁺. 通过分析C₁～C₃正构醛、 醇的质子转移反应特征及产物离子形成过程, 获得了C₁～C₃正构醛、 醇的特征离子和对应的最佳E/N设置值, 为低分子量醛、 醇的定性分析提供了重要依据.     

Metallaheterocumulene : III. Ungewöhnliche bildung von (CO)5Cr=C=NCPh2N=C=Cr(CO)5 bei der reaktion von [(CO)5Cr=C=N=CPh2] AlBr4 mit tetrahydrofuran

Pentacarbonyl(3,3-diphenyl-2-azaallenylidene)chromium tetrabromoaluminate, [(CO)₅Cr=C=Nz.dbnd;CPh₂]⁺ AlBr₄⁻, reacts with THF in dichloromethane to give the dinuclear bis(isonitrile) complex [(CO)₅Cr=C=N]₂CPh₂ which formally corresponds to the product of a reductive dimerization of two [(CO)₅Cr=C=N=CPh₂]⁺ moieties with concomitant elimination of one CPh₂ fragment.     

Synthesis and Characterization of Two [2]Catenanes Based on Phenylene-diacetylene Crown Ethers

Two charged donor-acceptor [2]catenanes were synthesized by the cyclization of the π-acceptor 1,1 ＇-[1, 4-phenylenebis-（methylene）]bis（4,4＇-bipyridinium）bis（hexafluorophosphate）（2） and bis（bromomethyl）benzene with the templates of the π-donor phenylene-diacetylene crown ethers la and lb in the yields of 31%, 35%, respectively. The mass spectra, 1H and 13C NMR spectra confirm that the phenylene-diacetylene crown ethers, with a nonaromatic ~r-system, successfully template the cyclization of CBPQT4＋. The X-ray crystallography analysis shows that the CBPQT4＋ ring encircles around the phenylene motif of the crown ethers, leaving the 1,3-butadiyne fragment out of the cavity of CBPQT4＋ ring. The mechanically interlocked structure of [2]catenanes was stabilized by the cooperative effects of π-stacking and hydrogen bonding interactions.     

Generation of arylnitrenium ions by nitro-reduction and gas-phase synthesis of <Emphasis Type="Italic">N</Emphasis>-Heterocycles

Nitro-reduction by the vinyl halide radical cation CH2 = CH-X+* (X = Cl or Br) converts nitroaromatics into arylnitrenium ions, significant intermediates in carcinogenesis, and the present study reports on the scope and regioselectivity of this versatile reaction. The reaction is general for different kinds of substituted nitroaromatics; para/meta substitutents have little effect on the reaction while ortho substitutents result in low yields of arylnitrenium ions. The phenylnitrenium ion PhNH+ can be generated by chemical ionization (CI) of nitrobenzene using 1,2-dichloroethane as the reagent gas or by atmospheric pressure chemical ionization (APCI) of 1,2-dichloroethane solution doped with nitrobenzene. The chemical reactivities of the arylnitrenium ions include one-step ion/molecule reactions with nucleophiles ethyl vinyl ether and 1,3-dioxolanes, respectively, involving the direct formation of new CN bonds and synthesis of indole and benzomorpholine derivatives. The indole formation reaction parallels known condensed phase chemistry, while the concise morpholine-forming reaction remains to be sought in solution. The combination of collision-induced dissociation (CID) with novel ion/molecule reactions should provide a selective method for the detection of explosives such as TNT, RDX and HMX in mixtures using mass spectrometry. In addition to the reduction of the nitro group, reduction of methyl phenyl sulfone PhS(O)2Me to the thioanisole radical cation PhSMe+* occurs using the same chemical ionization reagent 1,2-dichloroethane. This probably involves an analogous reduction reaction by the reagent ion CH2 = CH-Cl+*.     

Chuan-Cai Fan;Li-Jin Xu;Han-Yuan Gong;%T Neutral C–H bond vs. electron pair of N（sp~2）： A binding site effect study of macrocycle anion receptor

To evaluate the effect of neutral C–H bond or electron pair of nitrogen atom with sp2hybridization（N（sp2）） involving into the same chemical environment for anion binding, two analogous tetracationic imidazolium macrocycles, namely cyclo[2]（2,6-bis-（1H-imidazol-1-yl）pyridine） [2]（1,3-dimethylenebenzene）（14＋）, and cyclo[2]（2,6-bis-（1H-imidazol-1-yl）pyridine）[2]（2,6-di methylenepyridine）（24＋）were studied in detail as small inorganic anion receptors. The guest anions with different shapes are Cl,N3, NO3, and H2PO4. The host–guest interactions were characterized via1 H NMR spectroscopy,electrospray ionization mass spectrometry（ESI-MS） and single crystal X-ray crystallography. The results implied that macrocyclic hosts with similar backbone but two distinct binding sites（14＋with neutral C–H vs. 24＋with N（sp2）） vary markedly in their response to anions, including the binding modes and association constants. The finding will serve to the construction of new anion receptors, even improve insights into the anion binding process in biology.     

Vibrational spectrum of [(CD3)3S]I and normal coordinate calculations for [(CH3)3S] and [(CD3)3S] cations

IR (4000-30 cm⁻¹) and Raman (4000-0 cm⁻) spectra of [(CD₃)₃S]I have been observed, together with those of [(CH₃)₃S]I. By assuming a C_3v molecular symmetry for the cations [(CH₃)₃S]⁺ and [(CD₃)₃S]⁺, all the active fundamentals of [(CD₃)₃s]⁺ have been assigned and normal coordinate calculations have been carried out by a symmetry force field for [(CH₃)₃S]⁺ and [(CD₃)₃S]⁺. The strength of the S---C and C---H bonds in the compound has been compared with that in dimethyl sulfide by using their valence stretching force constants.