Mitsuo Takayama

The order of the intensities of alkene fragment ions m+• observed in the field ionization mass spectra of n-heptane, 2-methylhexane, and 2-methylheptane has been examined by quantum chemical calculations for the enthalpies of reactants, products, and transition states. The alkene fragment ions can be detected through a slow time window of 10-6 to 10-5 s for metastable reaction, while a rapid time window of 10-14 to 10-12 s for field dissociation allows alkyl fragment ions m+ to enter an orthogonal time-of-flight mass analyzer. The present study hypothesizes that alkene fragment ions are produced through 1,3-hydrogen atom transfer via metastable reaction. The potential energy profiles for the loss of methane from the terminal branched 2-methylalkanes, including 2-methylpropane, 2-methylbutane, 2-methylhexane, and 2-methylheptane, indicated that the product is lower in energy by 0.10 to 0.24 eV than that of the molecular ion, suggesting an exothermic reaction. The order of the alkene fragment ion intensities for each compound of n-heptane, 2-methylhexane, and 2-methylheptane can be explained by the energy barrier in TS and TS properties such as TS tightness/looseness.

The formation of M+∙, [M−2H]+∙, and [M−nH]+ (n=1, 3, 5) ions has been reported by using atmospheric pressure corona discharge (APCD) of n-heptane. The M+∙ and [M−H]+ ions can be produced by high-electric field tunnel ionization (HEFTI) and hydride abstraction reactions, respectively. It has been proposed that the [M−2H]+∙, [M−3H]+, and [M−5H]+ ions are produced by the loss of hydrogen atoms from the [M−H]+ ion. The geometric and electronic configurations of the M+∙, [M−2H]+∙, and [M−nH]+,(n=1, 3, 5) ions were determined by quantum chemical calculations (QCCs). The product ions resulting from higher-energy collisional dissociation (HCD) of the M+∙ ion can be characterized by the loss of alkyl radicals and alkanes, while those of the [M−2H]+∙ ion can be characterized by the loss of alkyl radicals and alkenes. The HCD product ions of the [M−nH]+ (n=1, 3, 5) ions can be fundamentally characterized by the loss of alkenes. The product ions present in the HCD spectra can be supported by the geometric and electronic configurations determined by the QCCs. The fragment ions observed in the APCD mass spectrum are likepply attributable to the result of collisional interactions between the M+• ion and the air components such as N2, O2, and Ar, or to the result of the HEF-induced cracking reaction leading to the alkyl fragments.

Access full-text academic articles: J-STAGE is an online platform for Japanese academic journals.

The alkyl fragment ions (C_nH_2n+1)⁺, produced by field dissociation of n-heptane, 2-methyl hexane, and 2-methyl heptane, have been examined by using a field ionization time-of-flight mass spectrometer, which enables the detection of fragment ions by the rapid field dissociation (10^-14 to 10^-12 s) and slow (10^-6 to 10^-5 s) time windows. A rigorous analysis of the intensity and stability of the alkyl fragment ions was conducted by employing density functional theory (DFT) calculations. The observed order of intensity for the complementary pair of alkyl fragment ions m₁⁺ and m₂⁺ arising from the C-C σ-bond cleavage of the molecular ion M^+• was found to be consistent with the order of the hyperconjugative stabilization energy, as determined by natural bond orbital analysis. However, the observed order of fragment ion intensity could not be explained by Stevenson's rule, hydride ion affinity, and energy barrier as a kinetic parameter. According to the DFT analysis, it was suggested that the field dissociation of the C-C σ-bond is accompanied by the partitioning of the total charge and spin density of the M^+• ion into fragment ions m₁⁺ and m₂⁺. The singly occupied molecular orbital of the molecular ions predicted the C-C bonds that underwent preferential cleavage, as evidenced by the extension of bond length of the C-C bonds. This is especially evident in the formation of the fragment ion (C₂H₅)⁺ at m/z 29 of n-heptane and the secondary carbocation fragment (CH₃CHCH₃)⁺ at m/z 43 of 2-methyl hexane and 2-methyl heptane.

The McLafferty rearrangement of the methyl valerate molecular ion involving both stepwise and concerted processes, based on the global reaction route mapping (GRRM) program and the strong-field tunnel ionization mass spectrometry (SFTI MS).

The molecular ion M+· was observed when the liquid sample of butyrophenone was supplied using atmospheric pressure corona discharge (APCD). In contrast, the vapor supply resulted in the formation of the protonated molecule [M+H]+. The mass spectrum obtained with the liquid supply showed two distinctive fragment ions at m/z 105 and 120, resulting from α-cleavage and McLafferty rearrangement (McLR), respectively. The APCD spectrum showed peaks of M+· and the characteristic two fragment ions that were the same as the field ionization mass spectra of butyrophenone as reported by Chait et al. and Beckey et al. The formation of the molecular and fragment ions strongly indicated that high-electric field tunnel ionization (HEFTI) occurs by the HEF strength exceeding 108 V/m at the tip of the corona needle in APCD. The charge and spin density distributions of the molecular and fragment ions were analyzed by quantum chemical calculations using time-dependent density functional theory (TDDFT) and natural bond orbital (NBO) analysis.

The formation of a peptide fragment ion [c + 2H]⁺ was examined using ultraviolet matrix-assisted laser desorption/ionization in-source decay mass spectrometry (UV/MALDI-ISD MS). Unusually, an ISD experiment with a hydrogen-abstracting oxidative matrix 4-nitro-1-naphthol (4,1-NNL) resulted in a [c + 2H]⁺ ion when the analyte peptides contained serine (Ser), threonine (Thr), and/or cysteine (Cys) residues, although the ISD with 4,1-NNL merely resulted in [a]⁺ and [d]⁺ ions. The [c + 2H]⁺ ion observed could be rationalized through intramolecular hydrogen atom transfer (HAT), like a Type-II reaction via a seven-membered conformation involving intramolecular hydrogen bonding (HB) between the active hydrogens (–OH and –SH) of the Ser/Thr/Cys residues and the backbone carbonyl oxygen at the adjacent amino (N)-terminal side residue. The ISD of the Cys-containing peptide resulted in the [c + 2H]⁺ ions, which originated from cleavage at the backbone N-Cα bonds far from the Cys residue, suggesting that the peptide molecule formed 16- and 22-membered transient conformations in the gas phase. The time-dependent density functional theory (TDDFT) calculations of the model structures of the Ser and Cys residues indicated that the Cys residue did not show a constructive bond interaction between the donor thiol (-SH) and carbonyl oxygen (=CO), while the Ser residue formed a distinct intramolecular HB.