Cleavage of C−C and C−F Bonds by Xe+• and I+ Ions in Reactions at a Fluorinated Self-Assembled Monolayer Surface: Collision Energy Dependence and Mechanisms

Gas-phase fluorine migration reactions in the radical cations of pentafluorosulfanylbenzene (Aryl-SF₅) and benzenesulfonyl fluoride (Aryl-SO₂F) derivatives and in the 2,5-xylylfluoroiodonium ion

The gas-phase reactions of Aryl-SF5(·+) and Aryl-SO2F(·+) have been studied with the electron ionization tandem mass spectrometry. Such reactions involve F-atom migration from the S-atom to the aryl group affording the product ion Aryl-F(·+) by subsequent expulsion of SF4 or SO2, respectively. Especially, the 4-pentafluorosulfanylphenyl cation 4-SF5C6H4(+) (m/z 203) from 4-NO2C6H4SF5(·+) by loss of ·NO2 could occur multiple F-atom migration reactions to the product ion C6H4F3(+) (m/z 133) by loss of SF2 in the MS/MS process. The gas-phase reactions of 2,5-xylylfluoroiodonium (pXyl-I(+)F, m/z 251) have also been studied using the electrospray tandem mass spectrometry, which involve a similar F-atom migration process from the I-atom to the aryl group giving the radical cation of 2-fluoro-p-xylene (or its isomer 4-fluoro-m-xylene, m/z 124) by reductive elimination of an iodine atom. All these gas-phase F-atom migration reactions from the heteroatom to the aryl group led to the aryl-F coupling product ions with a new formed C(Aryl)-F bond. Density functional theory calculations were performed to shed light on the mechanisms of these reactions.

Mass spectrometric study of the gas-phase difluorocarbene expulsion of polyfluorophenyl cations via F-atom migration

An increasing number of fluorinated drugs, pesticides, and fine chemicals are now produced and applied, especially those containing polyfluorinated aromatic moieties. However, at present, the extent of literature covering the special mass spectrometric behaviors of these compounds remains limited. Herein, we report an unexpected but also general gas-phase dissociation mode of polyfluorinated aromatics in mass spectrometry: expulsion of difluorocarbene (50-Da neutral loss). Results from accurate mass measurements, tandem mass spectrometric experiments, and density functional theory (DFT) calculations support an intramolecular F-atom “ring-walk” migration mechanism for gas-phase CF2 loss. Based on an assessment of the electron ionization-mass spectrometry (EI-MS) data of more than 40 polyfluorinated aromatic compounds from the National Institute of Standards and Technology data bank, we generalized on the substitution group effects on the difluorocarbene dissociation process of polyfluorinated aromatic compounds in EI-MS. These studies have enriched our knowledge of the special gas-phase reactivity of polyfluorinated aromatics and will provide valuable information in further analytical research of these compounds by mass spectrometry.

Review: gas-phase ion chemistry of the noble gases: recent advances and future perspectives

This review article surveys recent experimental and theoretical advances in the gas-phase ion chemistry of the noble gases. Covered issues include the interaction of the noble gases with metal and non-metal cations, the conceivable existence of covalent noble-gas anions, the occurrence of ion-molecule reactions involving singly-charged xenon cations, and the occurrence of bond-forming reactions involving doubly-charged cations. Research themes are also highlighted, that are expected to attract further interest in the future.

Analysis of self-assembled monolayers on gold surfaces using direct analysis in real time mass spectrometry

Mass spectrometry was performed on self-assembled monolayers (SAMs) of dodecanethiol on gold using the new direct analysis in real time (DART) ionization technique. Observed peaks for the SAMs included monomers, dimers, and trimers of the SAM molecules, with the dimer and trimer relative peak heights enhanced as compared to the spectra for neat dodecanethiol. The possibility that the observed peaks were due to residual (noncovalently bonded) material on the surface was tested by attempting to observe residual dodecanol. No peaks corresponding to dodecanol were observed. These results indicate that DART is an excellent ionization method for the direct and unambiguous mass analysis of chemical species in self-assembled monolayers.

Fluorine-substitution induced switching of dissociation patterns of C6H4*+ produced by photoelimination of MgF2 from the complexes of mg*+ (multifluorobenzene)

Complexes of fluorinated benzenes (o-C6H4-nF2+n) and Mg*+ are subjected to ultraviolet photodissociation (260-340 nm), producing efficiently benzyne radical cations (C6H4-nFn*+) besides Mg*+ and MgF+. We show that the consecutive dissociation of C6H4-nFn*+ follows the [C4(+) + C2] pattern exclusively for n < or = 2 after the parent complexes absorb one or two photons. However, the dissociation pattern is switched to [C5(+) + C1] and [C1 + C5] for n > or = 3. In particular, upon two-photon absorption at 340 nm by the complexes of Mg*+ (C6HF5) (1) and Mg*+ (C6F6) (2), photoproducts of CF+, C5H+, and C5HF*+ from C6HF3*+ and CF+, C5F+, C5F2*+, and C5F3+ from C6F4*+ are detected, respectively. Theoretical calculations are used to explain the switching of the dissociation patterns induced by the fluorine substitutions. It was found that the formation of C5+ + C1 is energetically more favorable than that of C4(+) + C2 from C6HF3*+ and C6F4*+ and of C1(+) + C5. Except for C5H2F(+) + CF, all the channels of [C5(+) + C1] and [C1(+) + C5] are energetically less favorable than those of [C4(+) + C2] from C6H3F*+ and C6H2F2*+. In most cases, the calculated results agree well with the experimental observations.

Reactions of BBr(n)(+) (n = 0--2) at fluorinated and hydrocarbon self-assembled monolayer surfaces: observations of chemical selectivity in ion--surface scattering

Ion-surface reactions involving BBr(n)(+) (n = 0--2) with a fluorinated self-assembled monolayer (F-SAM) surface were investigated using a multi-sector scattering mass spectrometer. Collisions of the B(+) ion yield BF(2)(+) at threshold energy with the simpler product ion BF(+)* appearing at higher collision energies and remaining of lower abundance than BF(2)(+) at all energies examined. In addition, the reactively sputtered ion CF(+) accompanies the formation of BF(2)(+) at low collision energies. These results stand in contrast with previous data on the ion-surface reactions of atomic ions with the F-SAM surface in that the threshold and most abundant reaction products in those cases involved the abstraction of a single fluorine atom. Gas-phase enthalpy data are consistent with BF(2)(+) being the thermodynamically favored product. The fact that the abundance of BF(2)(+) is relatively low and relatively insensitive to changes in collision energy suggests that this reaction proceeds through an entropically demanding intermediate at the vacuum--surface interface, one which involves interaction of the B(+) ion simultaneously with two fluorine atoms. By contrast with the reaction of B(+), the odd-electron species BBr(+)* reacts with the F-SAM surface to yield an abundant single-fluorine abstraction product, BBrF(+). Corresponding gas-phase ion--molecule experiments involving B(+) and BBr(+)* with C(6)F(14) also yield the products BF(+)* and BF(2)(+), but only in extremely low abundances and with no preference for double fluorine abstraction. Ion--surface reactions were also investigated for BBr(n)(+) (n = 0-2) with a hydrocarbon self-assembled monolayer (H-SAM) surface. Reaction of the B(+) ion and dissociative reactions of BBr(+)* result in the formation of BH(2)(+), while the thermodynamically less favorable product BH(+)* is not observed. Collisions of BBr(2)(+) with the H-SAM surface yield the dissociative ion-surface reaction products, BBrH(+) and BBrCH(3)(+). Substitution of bromine atoms on the projectile by hydrogen or alkyl radicals suggests that Br atoms may be transferred to the surface in a Br-for-H or Br-for-CH(3) transfer reaction in an analogous fashion to known transhalogenation reactions at the F-SAM surface. The results for the H-SAM surface stand in contrast to those for the F-SAM surface in that enhanced neutralization of the primary ions gives secondary ion signals one to two orders of magnitude smaller than those obtained when using the F-SAM surface, consistent with the relative ionization energies of the two materials.