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Abstract: The fragmentation of gaseous ion/neutral complexes [R+⋯C6H5CH2CH2CH2C6H4–R′] with (i) R=R′=C4H9, (ii) R=C4H9 and R′=CH3 and (iii) R=C6H11 and R′=H has been studied by CI(CH4)-MIKE spectrometry of the corresponding alkyl-substituted 1,3-diphenylpropanes. Different from all other isomers containing two para-alkyl substituents, the [M+H]+ ion generated from the symmetrical ion [(4-tert-C4H9-C6H4)CH2CH2CH2(C6H4-4-tert-C4H9)+H]+ shows the characteristic fragmentation pattern of ion–neutral complexes containing a meta-alkyl-substituted 1,3-diphenylpropane. This indicates a proton-induced 1,2-shift of one or even both of the tert-C4H9 groups and requires the presence of the meta-(tert-C4H9)-substituted diphenylpropane as the neutral constituent of the eventually fragmenting I/N complex. As a consequence, it appears that the reactive complex [C4H9 +⋯C6H5CH2CH2CH2(C6H4-3-tert-C4H9)] is formed prior to the generation of the expected “para-isomer”, [C4H9 +⋯C6H5CH2CH2CH2(C6H4-4-tert-C4H9)]. Isobutyl analogues, such as [(4-iso-C4H9-C6H4)CH2CH2CH2(C6H4-4-iso-C4H9)+H]+, do not show evidence for the intermediacy of “isomerized” I/N complexes containing a tert-C4H9 + ion. The fragmentation of ion–neutral complexes containing C6H11 + ions, formed from the [M+H]+ ions of (4-cyclohexyl)- and of 4-(1-methylcyclopentyl)-substituted 1,3-diphenylpropane, indicate that the C6H11 + ions only partially retain their structural identity: while the secondary isomer, (CH2)5 >CH+, predominantly transfers a proton in competition to hydride abstraction, indicating its stronger Bronsted acidity, the tertiary isomer, (CH2)4 >C+CH3, mainly reacts by hydride abstraction. In spite of the partial isomerization, deuterium labelling experiments corroborate the usual regioselectivity of the hydride abstraction from the benzylic methylene groups in both cases. [Copyright &y& Elsevier]

Abstract: Using MS/MS/MS experiments and quantum chemical calculations, it is demonstrated that oxygen (carbonyl) methylated phthalic anhydride cations are produced when metastable dimethyl phthalate molecular ions expel a methoxyl radical. At higher internal energies, isomeric acyl ions are competitively generated. [Copyright &y& Elsevier]

Abstract: The levels of post-source metastable ion decay (PSD) observed in several peptides and proteins ionized by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI TOF-MS) are measured utilizing both infrared (IR) and ultraviolet (UV) desorption wavelengths. A gridless deceleration ion optic is employed to temporally separate stable analyte ions from analyte metastable neutral and ion fragments. Comparisons of the extent of PSD that is observed in UV-MALDI at 337nm and IR-MALDI at multiple wavelengths between 2.8 and 3.0μm are made using the same matrices and analytes. The amount of PSD observed using IR-MALDI was found to be highly dependent on the specific IR wavelength (2.8–3.0μm) employed for desorption. IR wavelengths shorter than 2.86μm tended to produce higher levels of PSD, while longer IR wavelengths typically produced significantly less PSD when using a number of common MALDI matrices. Relative PSD levels are quantified by determining the percentage of the neutral fragment signal intensity to the intensity of the stable singly protonated molecular species observed in decelerated MALDI spectra. These studies suggest that an analyte ion activation pathway leading to significant PSD in IR-MALDI may proceed by way of vibrational excitation of the analyte molecules during the desorption event. [Copyright &y& Elsevier]

The MS/MS spectrum of the metastable molecular ions of dimethyl isophthalate 1 differs from that of the isomeric dimethyl terephthalate 2 by the observation of, inter alia, a quite intense loss of C,H2,O ascribed to formaldehyde. Results obtained using a combination of mass spectrometry techniques suggest that this process could consist of an isomerization reaction of the molecular ion into an ion–neutral complex (INC) linking a benzoyl radical and neutral formaldehyde to a proton [Ar—C=O⋯H⋯O=CH2]•+. Within the complex, a proton transfer catalyzed by formaldehyde occurs resulting in the production of an ionized cyclohexadienylidene methanone (ketene) structure. [Copyright &y& Elsevier]

Abstract: The decay of metastable negative ions resulted from the resonant electron capture by the molecules of acridanoneacetic acid benzyl ester has been studied using the negative ions’ mass spectrometry technique. The simple model of metastable decay has been developed on the basis of the statistical Rice–Ramsperger–Kassel–Markus theory (RRKM). Based on the comparative analysis of the experimental and calculated effective yield curves of ions the shape of the internal energy distribution of molecular negative ions has been estimated. [Copyright &y& Elsevier]

Abstract: The electron ionization-induced fragmentation paths of various substituted 9,9a-dihydro-3aH-[1]benzopyrano[2,3-c]pyrrole-1,3-diones by use of metastable ions mass spectrometry have been studied. It was found that two main fragmentation pathways are representative for compounds under study. The first starts at the succinimide ring and its pattern mainly depends on the substituents in the benzene ring. The second one corresponds to the retro Diels–Alder reaction (RDA) that takes place at tetrahydropyran-type ring with charge migration. [Copyright &y& Elsevier]

Abstract: The unimolecular dissociation of n-propanol molecular cation (1) has been investigated theoretically. Density functional theory (DFT) molecular orbital calculations have been performed at the UB3LYP/aug-cc-pVQZ//UB3LYP/6-31+G(d) level to obtain pathways for water and hydrogen atom loss from 1. On the basis of the DFT results, the rate-energy dependences have been calculated for water and hydrogen atom loss from various deuterated analogues of 1 by Rice–Ramsperger–Kassel–Marcus modeling. The large kinetic deuterium isotope effects on the metastable water and hydrogen atom loss reported previously are rationalized from the obtained rate-energy dependences. [Copyright &y& Elsevier]

Abstract: The dissociations of methylphenylsilane and dimethylphenylsilane molecular cations have been investigated by obtaining the 70-eV electron ionization mass spectra and the metastable ion decomposition spectra. Benzene loss from these molecular ions was noticeable compared to the dissociation of phenylsilane molecular cation. To understand the overall dissociation mechanisms, density functional theory calculations were performed at the B3LYP/6-311++G(d,p) level. It is proposed that the benzene loss occurs via the ion–molecule complexes formed by 1,2 shift of an α-H atom from the silicon to the ipso-carbon. The experimental observation that the dimethylphenylsilane ion loses benzene more easily than the methylphenylsilane ion can be understood by considering the stabilities of the product ions. [Copyright &y& Elsevier]

Tandem mass spectrometry combined with ab initio calculations was used to explore the chemistry of the water-solvated methanol cation and its isomers, including the distonic methanol ion solvated by water, ⋅CH2OH2+/H2O (I), the methanol molecular ion associated with water, CH3OH⋅+/H2O (II), and protonated methanol bound with a hydroxyl radical, CH3OH2+/HO⋅ (IV). Another ion, [H2O⋯H&z.sbnd;CH2OH⋅+] (III), where the water molecule is attached to a methyl H of the methanol cation, proved inaccessible experimentally although it was predicted to be stable in a shallow potential well by theoretical calculations. The ions were generated by collision-induced loss of a radical or atom from appropriate proton bound molecular pairs. The heats of formation of ions (I) to (IV) were calculated to be 448, 482, 486 and 538 kJ/mol, respectively. Ions (I), (II) and (IV) can be identified by their metastable ion and collision-induced dissociation mass spectra. A partial potential energy surface linking the isomers is used to discuss the experimental data obtained with deuterium-labeled species. [Copyright &y& Elsevier]

Hydration of the adenine and thymine cations was studied in the gas phase. Metastable fragmentation of the cluster ions was used to draw information on the relative strengths of the binding energy of each additional solvent molecule. Both cations of adenine and thymine exhibited a well-defined hydration shell structure, with the first hydration shell complete with four water molecules. On the other hand, the hydration shell structure for the cation dimer of adenine was less evident, but appeared to require seven or eight water molecules for the first hydration shell. An ab initio calculation was carried out at the Hartree–Fock level to address some of these issues. [Copyright &y& Elsevier]

The mechanism of the elimination of CH2O and C2H4O from the molecular ions of benzaldoxime-O-n-propyl ether (1) was studied using specifically deuterated derivatives, methods of tandem mass spectrometry, and ab initio calculation of the minimum energy reaction path (MERP) at the B3LYP/6-311G*//UHF/3-21G* level of theory. The results show that the eliminated CH2O (major) and C2H4O (minor) contain specifically the α- and β-CH2 groups, respectively, of the propoxy chain of 1, requiring a rearrangement of the molecular ion of 1 by a 1,5-shift of the benzaldimine moiety along the propoxy chain. This rearrangement reaction follows the general Longevialle mechanism of functional group transposition along an aliphatic chain by rearrangement of distonic ions. In the case of 1⋅+ the initial δ-distonic ion 1dist is generated by a 1,5-H shift from the terminal CH3 group to the N-atom of the oxime group. The compound 1dist is converted to the α-distonic isomer 5dist by cyclization of the molecular ion of N-benzyl-1,2-oxazolidine (5) and by subsequent ring opening into the δ-distonic isomer 2dist of N-(3-hydroxypropyl)benzaldimine (2) (see Scheme 4). Eventually the distonic ion 2dist fragments by loss of CH2O and C2H4O. The possibility that the distonic isomers of 1⋅+ and 2⋅+ interconvert with their cyclic isomers 2H-3,4,5,6-tetrahydro-3-phenyl-1,2-oxazine radical cation (3⋅+) and 2H-3,4,5,6-tetrahydro-3-phenyl-1,3-oxazine radical cation (4⋅+) was also examined by comparing the EI- and MIKE-spectra and the CID of the molecular ions of the isomers and by ab initio calculation. [Copyright &y& Elsevier]

The characterization of nanomaterials is critical to understand the size/structure-dependent properties of these particles. In this report, a form of heavy ion mass spectrometry, namely, superconducting tunnel junction (STJ) cryodetection mass spectrometry (MS) is used to characterize quantum dot semiconductor nanocrystals and gold nanoparticles. The nanoparticles studied ranged in mass from 200 kDa to >1.5 MDa and included lead sulfide quantum dots, various cadmium selenide and/or telluride-based core-shell quantum dots coated with different ligands, and gold nanoparticles. Nanoparticles were ionized by both matrix-assisted laser desorption ionization (MALDI) and laser desorption ionization (LDI), shot with an aimed ion gun into a flight tube, mass separated by time-of-flight (TOF), and detected by an energy-sensitive STJ cryodetector. STJ cryodetection MS can be used to analyze intact heterogeneous nanoparticles, allowing determination of average particle mass, dispersity, and ligand loading. Some nanoparticles, however, do undergo fragmentation during the MALDI or LDI-TOF mass analyses. The measurement of the energy deposited into the detector was found to be different for different types of particles. Metastable fragments from these nanoparticles were observed at lower energies. The lower energies deposited for metastable fragments can provide insight into the stability and surface compositions of these materials. Cadmium selenide core-shell quantum dots (655 nm emission) conjugated to biomacromolecules, such as cholera toxin B and human serum transferrin, were also analyzed. When compared to unconjugated particles by mass, it was determined that ∼96 cholera toxin B and ∼14 transferrin proteins were attached to the surface of these nanoparticles.