Mass-analyzed Ion Kinetic Energy Research Articles

Dissociation dynamics of propargyl chloride molecular ion near the reaction threshold: manifestation of quantum mechanical tunneling

The Cl loss from the propargyl chloride molecular ion has been investigated using mass-analyzed ion kinetic energy spectrometry (MIKES). The kinetic energy release distribution in the unimolecular dissociation has been determined. The potential energy surface for the mechanistic pathway has been calculated at the B3LYP/6-311G** density functional theory level. The calculated potential energy surface suggested that the threshold dissociation of the propargyl chloride molecular ion produces the C(3)H(3)(+) ion, only with the cyclopropenium structure, and with the release of a large amount of kinetic energy. This is in agreement with experimental results. Also, calculation of the rate constants with statistical rate models predicted that the reaction observed on a microsecond time scale occurs via tunneling through the rate-determining isomerization barrier for H-atom transfer. It has been found that a broad lifetime distribution is a manifestation of quantum mechanical tunneling of a precursor prepared under thermal conditions. Reinterpretation of previous photoelectron-photoion coincidence results taking into account the tunneling effect necessitated raising the critical energy to 0.64 eV from the energy of 0.34 eV reported previously. Copyright 2000 John Wiley & Sons, Ltd.

Fragmentation study of mixed-ligand zirconocene and hafnocene alcoholates of the type Cp2M(OR)Cl

The electron ionisation mass spectrometric behaviour of a new series of hafnocene and zirconocene chloroalcoholates is discussed. These data are also compared with those of mass-analysed ion kinetic energy spectrometry. With respect to dialcoholato complexes previously studied by us, the monochloro-alcoholato species show rather high intensities in their molecular ion peaks, which reflects their higher volatility and sufficient stability in the vapour phase. Fragmentation patterns of the complexes in question give relevant information on their gas-phase behaviour and also on the influence of the ligand structure on the decomposition pathways. The complexes in question appear to be promising precursors for metal-organic chemical vapour deposition (MOCVD).

Electron impact mass spectral studies of 2a,4-disubstituted 2-phthalimido-2,2a,3,4-tetrahydro-1H-azeto[2,1-d][1, 5]benzothiazepin-1-ones.

The mass spectrometric behaviour of seven 2a,4-disubstituted 2-phthalimido-2,2a,3,4-tetrahydro-1H-azeto[2,1-d][1, 5]benzothiazepin-1-ones has been studied with the aid of mass-analyzed ion kinetic energy spectrometry and accurate mass measurements under electron impact ionization. All compounds show a tendency to eliminate a CO molecule, a phthalimido (PhthN) radical, or a phthalimide (PhthNH) molecule. All of the resulting fragment ions could further lose a propene or (substituted) styrene molecule. The molecular ions could also undergo a reverse [2 + 2] cycloaddition reaction.

Stereochemical analysis of deuterated alkyl chains by MS/MS

Vicinally deuterated sec-alkyl phenyl ethers, CH(3)(CH(2))(m)()CH(OPh)CHD(CH(2))(n)()CH(3), display significant differences in mass spectra between threo and erythro stereoisomers. MS/MS experiments, in which parent ions of a single mass are selected and their fragmentation patterns subsequently measured, show that alkene expulsion represents virtually the only decomposition pathway. Two types of MS/MS experiment are reported: mass-analyzed ion kinetic energy (MIKE) spectroscopy of metastable ions and collisionally activated decomposition (CAD) of stable ions. The expulsion of a deuterated alkene from a monodeuterated precursor yields ionized phenol, PhOH(*)()(+) (m/z 94). The expulsion of an undeuterated alkene yields PhOD(*)()(+) (m/z 95). Without exception, the ratios (PhOD(*)()(+)/PhOH(*)()(+)) from precursors in the threo series have values greater than their diastereomers in the erythro series. The ratio of ratios, r = PhOD(*)()(+)/PhOH(*)()(+) for the threo divided by PhOD(*)()(+)/PhOH(*)()(+) for the erythro, has a value of 1.2 for the 2-phenoxy-3-deuteriobutanes and larger values for all of the higher homologues up through the monodeuterated phenoxyoctanes (m + n = 4). The highest degree of stereoselectivity, r = 5.8, is measured for 3-phenoxy-4-deuteriohexane. Experiments with multiply deuterated analogues show that alkene elimination is highly regioselective, unlike the corresponding decompositions of ionized sec-alcohols or their acetates. The fact that a large fraction of ionized sec-alkyl phenyl ethers undergo stereospecific syn-elimination means that mass spectrometry has a useful capacity to distinguish one isotopically labeled diastereomer from another.

Kinetic Energy Release Distribution in the Dissociation of Toluene Molecular Ion. The Tropylium vs Benzylium Story Continues

Production of C7H7+ from toluene molecular ion (TOL•+) was studied using mass-analyzed ion kinetic energy spectrometry. The kinetic energy release distribution (KERD) was determined, which showed that both benzylium (Bz+) and tropylium (Tr+) ions were produced from TOL•+. The interconversion between the molecular ions of toluene and cycloheptatriene prior to the dissociations was confirmed. Ab initio calculations at the HF/6-31G** and G2(MP2,SVP) levels were carried out for some important chemical species. The KERD in the Bz+ channel was calculated using the statistical phase space theory, while classical trajectory calculations were done to obtain the theoretical KERD in the Tr+ channel. The Tr+/Bz+ branching ratio determined by analyzing the experimental KERD was much larger than reported previously.

Primary and secondary locations of charge sites in angiotensin II (M + 2H) 2+ ions formed by electrospray ionization

High-energy tandem mass spectrometry and molecular dynamics calculations are used to determine the locations of charge in metastably decomposing (M + 2H) 2+ ions of human angiotensin II. Charge-separation reactions provide critical information regarding charge sites in mutiply charged ions. The most probable kinetic energy released ( T m.p.) from these decompositions are obtained using kinetic energy release distributions (KERDs) in conjunction with MS/MS (MS 2), MS/MS/MS (MS 3), and MS/MS/MS/MS (MS 4) experiments. The most abundant singly and doubly charged product ions arise from precursor ion structures in which one proton is located on the arginine (Arg) side chain and the other proton is located on a distal peptide backbone carbonyl oxygen. The MS 3 KERD experiments show unequivocally that neither the N-terminal amine nor the aspartic acid (Asp) side chain are sites of protonation. In the gas phase, protonation of the less basic peptide backbone instead of the more proximal and basic histidine (His) side chain is favored as a result of reduced coulomb repulsion between the two charge sites. The singly and doubly charged product ions of lesser abundance arise from precursor ion structures in which one proton is located on the Arg side chain and the other on the His side chain. This is demonstrated in the MS 3 and MS 4 mass-analyzed ion kinetic energy spectrometry experiments. Interestingly, (b ″ 7 + OH) 2+ product ions, like the (M + 2H) 2+ ions of angiotensin II, are observed to have at least two different decomposing structures in which charge sites have a primary and secondary location.

Mass spectrometry of 3,5- and 4,5-dicaffeoylquinic acids and selected derivatives

The mass spectral properties of 3,5- and 4,5-dicaffeoylquinic acids (DCQAs) and selected derivatives were examined using electron ionization (EI), fast atom bombardment (FAB) and electrospray ionization (ESI). EI analysis of the trimethylsilyl derivatives provides molecular mass (M(r)) information, but the spectrum is dominated by fragment ions of the caffeic acid group; isomers cannot be differentiated using EI. FAB analysis, in both the positive and negative ion detection modes, provides M(r) information on the free compounds, but little fragmentation is observed using normal scan conditions. The FAB mass-analyzed ion kinetic energy spectroscopic analysis of the free compounds does, however, permit differentiation of the isomers, with 3,5-DCQA showing selective loss of water, a process not observed with the 4,5-isomer. Both EI and FAB provide M(r) and some structural information when applied to the peracetate derivatives of the DCQAs. ESI of the DCQAs provides considerably more structural information, especially in the negative ion detection mode, and is the recommended method of analysis of the quinic acid esters. M(r) information, identity of the ester groups and differentiation of isomers are possible using ESI. Copyright 1999 John Wiley & Sons, Ltd.

Mass spectrometry of 3,5- and 4,5-dicaffeoylquinic acids and selected derivatives

The mass spectral properties of 3,5- and 4,5-dicaffeoylquinic acids (DCQAs) and selected derivatives were examined using electron ionization (EI), fast atom bombardment (FAB) and electrospray ionization (ESI). EI analysis of the trimethylsilyl derivatives provides molecular mass (Mr) information, but the spectrum is dominated by fragment ions of the caffeic acid group; isomers cannot be differentiated using EI. FAB analysis, in both the positive and negative ion detection modes, provides Mr information on the free compounds, but little fragmentation is observed using normal scan conditions. The FAB mass-analyzed ion kinetic energy spectroscopic analysis of the free compounds does, however, permit differentiation of the isomers, with 3,5-DCQA showing selective loss of water, a process not observed with the 4,5-isomer. Both EI and FAB provide Mr and some structural information when applied to the peracetate derivatives of the DCQAs. ESI of the DCQAs provides considerably more structural information, especially in the negative ion detection mode, and is the recommended method of analysis of the quinic acid esters. Mr information, identity of the ester groups and differentiation of isomers are possible using ESI. Copyright © 1999 John Wiley & Sons, Ltd.

The mass spectral fragmentation of 1a,3-disubstituted 4-benzoyl-1a,2,3,4- tetrahydrooxazirino

The mass spectrometric behaviour of four 1a,3-disubstituted 4-benzoyl-1a,2,3,4-tetrahydrooxazirino[2,3-a][1,5]benzodiazepines has been studied with the aid of mass-analyzed ion kinetic energy spectrometry and accurate mass measurements under electron impact ionization. All compounds show a tendency to eliminate a neutral oxygen atom, or oxygen atom plus benzoyl or aryl radicals, or oxygen atom plus CO molecule to yield 2,3-dihydro-1H-1,5-benzodiazepine derivative ions, or 1-benzoyl-2,3-dihydro-1H-1,5-benzodiazepine ions, which could further lose benzoyl to give 2,3-dihydro-1H-1,5-benzodiazepine ions. All compounds also show a tendency to eliminate a benzoyl radical to produce 1a,2,3,4-tetrahydrooxazirino[2,3-a][1,5]benzodiazepine ions. Both oxazirino[2,3-a][1,5]benzodiazepine ions and 2,3-dihydro-1H-1,5-benzodiazepine ions can undergo diazepine ring contraction rearrangement to yield benzimidazole ions by loss of propene or styrenes and other small fragments. The oxazirino[2,3-a][1,5]benzodiazepine ions [M(+)-PhCO] also undergo rearrangement reactions to form benzoxazole ions and benzimidazole ions. Copyright 1999 John Wiley & Sons, Ltd.

Analysis of alkenes by copper ion chemical ionization gas chromatography/mass spectrometry and gas chromatography/tandem mass spectrometry

A novel chemical ionization/fast atom bombardment (CI/FAB) source was used to analyse alkenes by chemical ionization mass spectrometry (CI-MS) using copper ions as the ionizing agent. The Cu(+)-CI mass spectra showed abundant pseudomolecular adduct ions [alkene-Cu](+) and characteristic fragment ions. Mass-analysed ion kinetic energy spectroscopy was used to study the product ions resulting from the decomposition of adduct ions and to eliminate background interferences derived from the copper ions. The major fragmentations permitted the localization of double bonds and minor fragments allowed the differentiation of alkene isomers. The CI/FAB source was coupled to a gas chromatograph and simple and complex mixtures of octene isomers were analysed by gas chromatography (GC)/Cu(+)-CI-MS and GC/Cu(+)-CI-MS/MS. Copyright 1999 John Wiley & Sons, Ltd.

The structure characterization of dinucleoside and nucleoside glucopyranosides lipophilic phosphotriesters by fast-atom bombardment tandem mass spectrometry

Rules for the gas-phase fragmentation mechanism of the negative ions of lipophilic phosphotriester molecules of biological interest have been established by fast-atom bombardment mass spectrometry/mass spectrometry. The mass-analyzed ion kinetic energy spectra of the [M − H] − of dinucleoside ( 1–4) and nucleoside glucopyranoside ( 5–9) phosphotriesters show that in the absence of charges on the phosphate bridge, the availability of acidic protons on the 5′-end nucleobase drives a preferred reaction path which leads to 5′- O-nucleotide or 6- O-glucopyranoside monophosphate anions.

Doubly charged ions in the electron ionization mass spectra of 4,6-di(substituted)amino s-triazines

Electron ionization mass spectra of the title compounds demonstrate intense peaks due to doubly charged ions. These ions have great analytical significance for the screening of these herbicides. The ions [M–2CH 3] 2+ reach 25%–30% of the base peak at 70 eV. The origin of these ions and their modes of fragmentation were studied through high voltage scans and mass analyzed ion kinetic energy spectroscopy on a reverse geometry double focusing mass spectrometer. The doubly charged parent ions of the title compounds behave like diradicals and undergo α cleavage reactions at the two substituted exocyclic amino groups with prevailing methyl loss. The [M–2CH 3] 2+ ions behave like characteristic even electron ions, either eliminating stable neutral molecules or undergoing Coulomb explosion into pairs of even electron ion products. Some of the products are characterized by proton transfer to a neutral molecule having a high proton affinity, notably the formation of CH 3NCH + from [M–2CH 3] 2+ in simazine. Similar reactions have been recently observed for multiply protonated biomolecules. Kinetic energy releases and intercharge separations were determined for the Coulomb explosion reactions of the [M–2CH 3] 2+ ions.

Internal energy content of n-butylbenzene, bromobenzene, iodobenzene and aniline molecular ions generated by two-photon ionization at 266 nm. A photodissociation study

A technique to investigate photodissociation kinetics on a nanosecond time scale has been devised for molecular ions generated by multiphoton ionization (MPI) using mass-analyzed ion kinetic energy spectrometry. The branching ratio or rate constant has been determined for the photodissociation of the n-butylbenzene, bromobenzene, iodobenzene, and aniline molecular ions generated by MPI at 266 nm. The ion internal energies have been estimated by comparing the measured kinetic data with the previous energy dependence data. The analysis has shown that only those molecular ions generated by two-photon ionization contribute to the photodissociation signals. Around half of the available energy has been found to remain as molecular ion internal energy in the two-photon ionization process. Copyright 1999 John Wiley & Sons, Ltd.

Mass spectral fragmentation of cis- and trans-1a,3-disubstituted 1, 1-dichloro-1a,2,3,4-tetrahydro-1H-azirino[1,2-a] [1, 5]benzodiazepines.

The mass spectrometric behaviour of six cis- and trans-1a, 3-disubstituted 1,1-dichloro-1a,2,3,4-tetrahydro-1H-azirino[1,2-a][1, 5]benzodiazepines has been studied with the aid of mass-analysed ion kinetic energy spectrometry and exact mass measurements under electron impact ionization. All compounds show a tendency to eliminate a neutral propene or styrene molecule from the diazepine ring and to simultaneously eliminate a chlorine atom from the aziridine ring to yield azirino[1,2-b][1,3]benzimidazole ions, and to lose hydrogen chloride plus propene or styrene to give quinoxaline ions. All compounds also show a tendency to eliminate N=CCl(2) or HN=CCl(2) to produce dihydro- or tetrahydroquinoline ions, and to eliminate sequentially hydrogen chloride and a chlorine atom to yield pyrrolo[1,2-a][1, 3]benzimidazole ions.

Electron impact mass spectral characteristics and fragmentation pathways of isomeric tetradecadien-1-ols

Characteristics of mass spectra of isomeric tetradecadien-1-ols were investigated by electron impact mass spectrometry, and mass spectral fragmentation pathways were proposed based on collision-induced dissociation experiments and mass-analyzed ion kinetic energy spectrometry. Copyright 1999 John Wiley & Sons, Ltd.

Electron impact mass spectra of 1a,3‐disubstituted 1,1‐dichloro‐1,1a,2,3‐tetrahydroazirino [2,1‐d][1,5]benzothiazepines

The mass spectrometric behaviour of eight 1a,3-disubstituted 1,1-dichloro-1,1a,2,3-tetrahydroazirino[2,1-d][1,5]benzothiazepines has been studied with the aid of mass-analyzed ion kinetic energy spectrometry and exact mass measurements under electron impact ionization. All compounds show a tendency to eliminate a chlorine radical from the aziridine ring and then eliminate a neutral propene or substituted/unsubstituted styrene from the thiazepine ring to yield 1,4-benzothiazine ions. They can also lose hydrogen chloride and a chlorine radical, or lose two chlorine atoms, to undergo a ring enlargement rearrangement to produce 1,6-benzothiazocine ions, which can further eliminate propyne or phenylacetylene, or small molecular fragments, to yield some abundant fragment ions. Copyright © 1999 John Wiley & Sons, Ltd.

Internal energy content of n‐butylbenzene, bromobenzene, iodobenzene and aniline molecular ions generated by two‐photon ionization at 266 nm. A photodissociation study

A technique to investigate photodissociation kinetics on a nanosecond time scale has been devised for molecular ions generated by multiphoton ionization (MPI) using mass-analyzed ion kinetic energy spectrometry. The branching ratio or rate constant has been determined for the photodissociation of the n-butylbenzene, bromobenzene, iodobenzene, and aniline molecular ions generated by MPI at 266 nm. The ion internal energies have been estimated by comparing the measured kinetic data with the previous energy dependence data. The analysis has shown that only those molecular ions generated by two-photon ionization contribute to the photodissociation signals. Around half of the available energy has been found to remain as molecular ion internal energy in the two-photon ionization process. Copyright © 1999 John Wiley & Sons, Ltd.

Mass spectral fragmentation of cis‐ and trans‐1a,3‐Disubstituted 1,1‐Dichloro‐1a,2,3,4‐tetrahydro‐1H‐azirino[1,2-a] [1,5]benzodiazepines

The mass spectrometric behaviour of six cis- and trans-1a,3-disubstituted 1,1-dichloro-1a,2,3,4-tetrahydro-1H-azirino[1,2-a][1,5]benzodiazepines has been studied with the aid of mass-analysed ion kinetic energy spectrometry and exact mass measurements under electron impact ionization. All compounds show a tendency to eliminate a neutral propene or styrene molecule from the diazepine ring and to simultaneously eliminate a chlorine atom from the aziridine ring to yield azirino[1,2-b][1,3]benzimidazole ions, and to lose hydrogen chloride plus propene or styrene to give quinoxaline ions. All compounds also show a tendency to eliminate NCCl2 or HNCCl2 to produce dihydro- or tetrahydroquinoline ions, and to eliminate sequentially hydrogen chloride and a chlorine atom to yield pyrrolo[1,2-a][1,3]benzimidazole ions. Copyright © 1999 John Wiley & Sons, Ltd.

Electron impact mass spectra of 1a,3-disubstituted 1,1-dichloro-1,1a, 2,3-tetrahydroazirino [2,1-d][1,5]benzothiazepines.

The mass spectrometric behaviour of eight 1a,3-disubstituted 1, 1-dichloro-1,1a,2,3-tetrahydroazirino[2,1-d][1,5]benzothiazepines has been studied with the aid of mass-analyzed ion kinetic energy spectrometry and exact mass measurements under electron impact ionization. All compounds show a tendency to eliminate a chlorine radical from the aziridine ring and then eliminate a neutral propene or substituted/unsubstituted styrene from the thiazepine ring to yield 1,4-benzothiazine ions. They can also lose hydrogen chloride and a chlorine radical, or lose two chlorine atoms, to undergo a ring enlargement rearrangement to produce 1,6-benzothiazocine ions, which can further eliminate propyne or phenylacetylene, or small molecular fragments, to yield some abundant fragment ions.

Mass spectral fragmentation patterns of 1a,3-diaryl-1,1,4, 4-tetrachloro-1a,2,3,4-tetrahydro-1H-azirino

The mass spectrometric behaviour of three 1a,3-diaryl-1,1,4, 4-tetrachloro-1a,2,3,4-tetrahydro-1H-azirino-[2,1-e][1, 6]benzothiazocines has been studied with the aid of mass-analyzed ion kinetic energy spectro--metry and exact mass measurements under electron impact ionization. All compounds show a tendency-to eliminate a neutral substituted/unsubstituted 1,1-dichlorostyrene from the eight-membered thiazocine ring to yield azirino[2,1-c][1, 4]benzothiazine ions, which can then lose a chlorine radical and a sulfur-atom to form azirino[2,1-c][1,4]benzothiazine ions and dihydroquinoline ions. All compounds could also eliminate a chlorine radical plus hydrogen chloride, stepwise or consecutively, to undergo a ring enlargement rearrangement to produce nine-membered 1, 7-benzothiazonine ions. The [M(+) - Cl] ions could undergo a four-membered ring rearrangement to yield 1,5-benzothiazepine ions by loss of 1,1-dichlorostyrene or styrene ions and HCl, and undergo alpha, alpha-cleavage to form benzothiazine ions by loss of arylchlorocyclopropane. The [M(+) - Cl - HCl] ions could further eliminate some atoms, small molecules or molecular fragments to form some abundant fragment ions. Copyright 1999 John Wiley & Sons, Ltd.