Metastable Ion Mass Research Articles

N‐Methylimidazolidin‐4‐one organocatalysts: gas‐phase fragmentations of radical cations by experiment and theory

Electron ionisation mass spectra of N-methylimidazolidin-4-one organocatalysts were studied by experimental and theoretical means. The molecular ions mostly undergo alpha cleavages of exocyclic substituents that leave the five-membered ring intact. The type of substituent strongly dominates the appearance of the spectra. Fragmentation cascades are corroborated by metastable ion mass spectra. Quantum Chemistry Electron Ionisation Mass Spectra calculations correlate reasonably well with the experimental electron ionisation spectra and reveal mechanistic details of fragmentation pathways. The drawbacks and benefits of such calculations are discussed. Copyright © 2017 John Wiley & Sons, Ltd.

Fragmentation reactions of Si2Cl6+[rad] in the gas phase—A quantum-chemical and mass-spectrometric assessment

The gas-phase ion chemistry of the hexachlorodisilane radical cation Si2Cl6+ has been reinvestigated by a combined electron ionization (EI) mass spectrometry (MS) and high-level quantum chemistry approach. The computational investigation of isomerization and fragmentation pathways of [Si2Cln]+() (n=1–6) species reveals (a) the coexistence of structures of the type Cl3Si–SiClm+1+() and chlorobridged SiCl4–SiClm+() isomers for m=1,2, and (b) a wealth of nearly isoenthalpic structures[Si2Cln]+() for each n with n=2–4, which can result from degenerate rearrangements. This structural diversity appears to be a general feature of the disilicon (radical) cations with low chlorine content. The ionization energy of Si2Cl6 determined by EI-MS (10.1±0.2stat±0.2systeV) with argon (15.759±0.001eV) used as reference, is slightly lower than previously reported experimental values, but in good agreement with the CBS-QB3 (9.87eV) and W1U (9.97eV) adiabatic ionization energy. Measured appearance energies for the lowest two fragmentation channels, which afford SiCl2+ and SiCl3+, are well in line with CBS-QB3 values and with the literature value determined for SiCl3+ in threshold photoelectron–photoion coincidence experiments. Using the experimental heats of formation (ΔfH°298) for SiCl4 and Cl· as anchor points, W1U theory was employed to derive ΔfH°298 values for Si2Cln+() with n=0–6 and for SiClm+() with m=1–4 based on the lowest energy isomer identified in our quantum chemical investigation of reaction pathways for all species investigated. W1U heats of formations obtained for select neutrals lead to a more consistent description of kinetic aspects previously inferred from metastable ion mass spectra.

The acrylonitrile dimer ion: A study of its dissociation via self-catalysis, self-protonation and cyclization into the pyrimidine radical cation

Large energy barriers prohibit the rearrangement of solitary acrylonitrile ions, CH 2 CHC N +, into their more stable hydrogen-shift isomers CH 2 C C NH + or CH CH–C NH +. This prompted us to examine if these isomerizations occur by self-catalysis in acrylonitrile dimer ions. Such ions, generated by chemical ionization experiments of acrylonitrile with an excess of carbon dioxide, undergo five dissociations in the μs time frame, as witnessed by peaks at m/ z 53, 54, 79, 80 and 105 in their metastable ion mass spectrum. Collision experiments on these product ions, deuterium labeling, and a detailed computational analysis using the CBS-QB3 model chemistry lead to the following conclusions: (i) the m/ z 54 ions are ions CH 2 CHC NH + generated by self-protonation in ion–dipole stabilized hydrogen-bridged dimer ions [CH 2 CHC N⋯H–C(C N)CH 2] + and [CH 2 CHC N⋯H–C(H)C(H)C N] +; the proton shifts in these ions are associated with a small reverse barrier; (ii) dissociation of the H-bridged ions into CH 2 C C NH + or CH CH–C NH + by self-catalysis is energetically feasible but kinetically improbable: experiment shows that the m/ z 53 ions are CH 2 CHC N + ions, generated by back dissociation; (iii) the peaks at m/ z 79, 80 and 105 correspond with the losses of HCN, C 2H 2 and H , respectively. The calculations indicate that these ions are generated from dimer ions that have adopted the (much more stable) covalently bound “head-to-tail” structure [CH 2 CHC N–C(H 2)C(H)C N] +; experiments indicate that the m/ z 79 (C 5H 5N) and m/ z 105 (C 6H 6N 2) ions have linear structures but the m/ z 80 (C 4H 4N 2) ions consist of ionized pyrimidine in admixture with its stable pyrimidine-2-ylidene isomer. Acrylonitrile is a confirmed species in interstellar space and our study provides experimental and computational evidence that its dimer radical cation yields the ionized prebiotic pyrimidine molecule.

Dissociation of protonated oxalic acid [HOOC-C(OH) 2] + into H 3O + + CO + CO 2: An experimental and CBS-QB3 computational study

The predominant dissociation process observed for metastable protonated oxalic acid ions HOOC-C(OH) 2 + (generated by self-protonation) leads to H 3O ++ CO + CO 2. We have traced the mechanism of this intriguing reaction using the CBS-QB3 model chemistry. Our calculations show that a unique ter-body complex, O C O⋯H 3O +⋯CO, plays a key role in the rearrangement process. This complex can also dissociate to the proton bound dimers [H 2O⋯H⋯O C O] + and [H 2O⋯H⋯CO] + which are minor processes observed in the metastable ion mass spectrum. A further minor process leads to the proton bound dimer O C O⋯H +⋯CO which is formed by water extrusion from the ter-body complex. Arguments are provided that the ter-body complex is also generated in the ion source by the collision encounter between neutral and ionized oxalic acid.

Letter: On the occurrence of (M – •SH)+ ions in the electron impact mass spectra and metastable ion mass spectra of thiophene-2-carboxamides

Letter: On the occurrence of (M – •SH)+ ions in the electron impact mass spectra and metastable ion mass spectra of thiophene-2-carboxamides

A tandem mass spectrometry study of the gas phase RSC 6H 5Cr +L (R = H, C 6H 5; L = arene) and C 6H 5SSC 6H 5Cr + ions

Ion-molecule reactions of chromium containing ions with arylsulfides have been studied in the gas phase and their products have been characterized by tandem mass spectrometry. C 6H 5SH and (C 6H 5) 2S react as typical aromatic compounds and give rise to Cr +C 6H 5SR′ and RC 6H 5Cr +C 6H 5SR′ [R = H, CH 3, CH(CH 3) 2; R′ = H, C 6H 5] ions. Metastable ion mass spectra of the latter species show that the metal is more strongly bound to diphenylsulfide than to alkylbenzenes. C 6H 5SSC 6H 5 reacts with chromium-containing ions to form only Cr +(C 6H 5SSC 6H 5). The decomposition characteristics of this ion and, in particular, the presence of a recovery signal in the neutralization-reionization mass spectrum are in keeping with the formation of a 1,2-dithia[2]cyclophane complex ion, which rearranges into a structure(s) that contains CrS bond(s). No evidence was found for metal atom insertion

Subtle interplay of functional groups and chain length effects in the Fe +-mediated gas-phase chemistry of alkyl halides containing double and triple bonds

The chemistry of Fe + complexes of alkyl halides of the general structures CH 3(CH 2) m CC(CH 2) n X and CH 3(CH 2) m CHCH(CH 2) n X ( m = 2–4; n = 2–4; X = Cl, Br) in the gas phase has been studied. The chain length (CH 2) n is found to play a decisive role for the dissociation characteristics of metastable ions. For n = 3, irrespective of the nature of X and of the carbon-carbon multiple bond, the only process occurring corresponds to loss of C 2H 4. Based on labeling experiments, ⪢ 93% of ethylene originates from the alkynes and 100% of ethylene originates from the olefins from the C(1) and C(2) positions. For n = 4, the decomposition modes are determined by the nature of the CC multiple bond. For the acetylene derivatives, most reactions seem to originate from initial complexation of the triple bond; in contrast, for the substrates with CC double bonds, the metastable ion mass spectra are indicative of preferential reactions involving the halogen site. For complexes with n = 2 no clear-cut behaviour exists.

Low‐energy fragmentations of five isomeric [H3, C, N, O2] +· ions

AbstractThe low‐energy fragmentation characteristics of the [H3,C,N,O2]+· isomers [H3CNO2]+· (a), [H2CN(O)OH]+· (b), [H3CONO]+· (c), [HC(O)NHOH]+· (d) and [HC(OH)NOH]+· (e) were studied in detail by metastable ion mass spectrometry. In agreement with most earlier observations, appearance energy measurements established the potential energy surface of the isomers a, b and c, showing the intricate interrelations between them. It was concluded that a isomerizes into b prior to fragmentation by loss of ·OH and H2O and into c before loss of ·H and H3CO· moreover, the reverse reactions do not take place on the metastable time‐frame. The dominant metastable process for isomers d and e (obtained via HCN loss from glyoxime) was generation of [H2NOH]+·. For isomer e this process was proposed to involved a rate‐determining isomerization into d. It was concluded that isomers d and e do not intercommunicate with ions a, b and c prior to fragmentation. Neutralization‐reionization mass spectrometry indicated that the enol form of formohydroxamic acid as well as the keto counterpart are stable in the gas phase.

Mass spectra of aliphatic dicarboxylic acids and their dimethyl esters: Cyclic structures for the [M − H2O]+˙ ions from the diacids and [M − MeOH]+˙ ions from the dimethyl esters

AbstractMethyl 2‐oxocycIoalkane carboxylate structures are proposed lor the [M − MeOH]+˙ ions from dimethyl adipate, pimelate, suberate and azelate. This proposal is based on a comparison of the metastable ion mass spectra and the kinetic energy releases for the major fragmentation reaction of these species with the same data for the molecular ions of authentic cyclic β‐keto esters. The mass spectra of α,α,α′,α′‐d4‐pimelic acid and its dimethyl ester indicate that the α‐hydrogens are involved only to a minor extent in the formation of [M − ROH]+˙ and [M − 2ROH]+˙ ions, while these α‐hydrogens are involved almost exclusively in the loss of ROH from the [M − RO˙]+ ions (R = H or CH3). The molecules XCO(CH2)7COOMe (X = OH, Cl) form abundant ions in their mass spectra with the same structure as the [M − 2MeOH]+˙ ions from dimethyl azelate.

ChemInform Abstract: Tautomerism in Aromatic Hydroxy N‐Heterocyclics in the Gas Phase by Metastable Ion Mass Spectrometry

ChemInformVolume 19, Issue 24 Physical Organic Chemistry ChemInform Abstract: Tautomerism in Aromatic Hydroxy N-Heterocyclics in the Gas Phase by Metastable Ion Mass Spectrometry M. A. BALDWIN, M. A. BALDWIN Sch. Pharm., Univ., London WC1N 1AX, UKSearch for more papers by this authorG. J. LANGLEY, G. J. LANGLEY Sch. Pharm., Univ., London WC1N 1AX, UKSearch for more papers by this author M. A. BALDWIN, M. A. BALDWIN Sch. Pharm., Univ., London WC1N 1AX, UKSearch for more papers by this authorG. J. LANGLEY, G. J. LANGLEY Sch. Pharm., Univ., London WC1N 1AX, UKSearch for more papers by this author First published: June 14, 1988 https://doi.org/10.1002/chin.198824056Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume19, Issue24June 14, 1988 RelatedInformation

Tautomerism in aromatic hydroxy N-heterocyclics in the gas phase by metastable ion mass spectrometry

Download options Please wait... Article information DOI https://doi.org/10.1039/P29880000347 Article type Paper Download Citation J. Chem. Soc., Perkin Trans. 2, 1988, 347-350 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Tautomerism in aromatic hydroxy N-heterocyclics in the gas phase by metastable ion mass spectrometry M. A. Baldwin and G. J. Langley, J. Chem. Soc., Perkin Trans. 2, 1988, 347 DOI: 10.1039/P29880000347 To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Search articles by author Michael A. Baldwin G. John Langley

Mass spectral behavior of n‐alkynes

AbstractThe 70 e V‐electron impact mass spectra of the C7–C10 n‐alkynes have been determined as well as the metastable ion spectra of the molecular ions and the [CS2]+ and [N2O]+ charge exchange mass spectra of the C7‐C9 n‐alkynes. The metastable ion mass spectra provide only a limited opportunity to distinguish between isomers; however, the 70‐eV EI mass spectra of isomeric compounds permit a ready distinction between isomers. The [CS2]+ charge exchange mass spectra of isomeric compounds also show substantial differences. The [N2O]+ charge exchange mass spectra do not show the enhancement of β‐fission fragments observed in field ionization experiments, despite representing ions of similar internal energy, and it is concluded that field dissociation is responsible for the β‐fission fragments in the field ionization experiments.

Sequence analysis for an unknown peptide by molecular secondary ion mass spectrometry

AbstractPositive and negative molecular secondary ion as well as metastable ion mass spectra for various peptides are investigated to clarify their fragmentation regularity. The fragmentation regularity is derived by considering all the possible bond cleavages at the peptide bond or at the adjacent bonds. It is demonstrated that the amino acid sequence for an unknown peptide can be determined by interpreting the positive and negative secondary ion and metastable ion mass spectra using this fragmentation regularity.

Molecular species analysis of lipids by metastable ion mass spectrometry

A convenient universal and fast mass spectrometrical method designed for the molecular species analysis of natural lipids is described. In contrast to the commonly employed procedures the methods does not require chemical or enzymatic treatment and does not include chromatographic steps. The method relies on the recognition of ions characteristic of individual molecular species in the mass spectrum of a particular lipid fraction, that is accomplished on the basis of metastable ion spectra. The efficiency of this approach is demonstrated with a variety of natural lipids: triglycerides, glycerophospholipids, sphingomyelin and ornithinolipids. The advantages and limitations of the method as well as possible further developments are discussed.

A mass spectral study of 1‐(aryldimethylsilyl)‐2‐propanones and their isomeric 2‐(aryldimethylsiloxy)‐1‐propenes

AbstractThe mass spectra of a series of β‐ketosilanes, p‐YC6H4Me2SiCH2C(O)Me and their isomeric silyl enol ethers, p‐YC6H4Me2SiOC(CH3)CH2, where Y = H, Me, MeO, Cl, F and CF3, have been recorded. The fragmentation patterns for the β‐ketosilanes are very similar to those of their silyl enol ether counterparts. The seven major primary fragment ions are [MMe·]+, [MC6H4Y·]+, [MMe2SiO]+˙, [MC3H4]+˙, [MHCCCF3]+˙, [Me2SiOH]+˙ and [C3H6O]+˙ Apparently, upon electron bombardment the β‐ketosilanes must undergo rearrangement to an ion structure very similar to that of the ionized silyl enol ethers followed by unimolecular ion decompositions. Substitutions on the benzene ring show a significant effect on the formation of the ions [MMe2SiO]+˙ and [Me2SiOH]+˙, electron donating groups favoring the former and electron withdrawing groups favoring the latter. The mass spectral fragmentation pathways were identified by observing metastable peaks, metastable ion mass spectra and ion kinetic energy spectra.