Reactions of phenylium ions C6(H,D)5+ with D2

TOLUENIUM AND OTHER GASEOUS METHYLBENZENIUM IONS: COMPLEX INTERPLAY OF PROTONATED ARENES AND CYCLO-OLEFINS

The development of the current knowledge of the gas-phase chemistry of protonated methylbenzenes, such as toluenium, xylenium and mesitylenium ions, their higher congeners as well as of their mostly cyclo-olefinic isomers by mass spectrometric methodology is presented. Starting from the observation of the characteristic expulsion of dihydrogen from metastable C₇ H₉ ⁺ ions, which is associated with the release of large amounts of kinetic energy, and the composite C- and H-scrambling prior to the loss of methane, in particular, insights into the isomerization scenario of various isomeric C₇ H₉ ⁺ , C₈ H₁₁ ⁺ , and C₉ H₁₃ ⁺ ions, based on a large variety of independent techniques, are discussed. Besides isotope labeling and metastable ion methodology, these include flowing afterglow mass spectrometry, gas-phase titration and infrared spectroscopy of mass-selected ions. The particularly complex energy hypersurface of isomerizing and fragmenting toluenium ions, which has been elaborated in various reports over the years, is presented in a combined way to assess the role of protonated cycloheptatriene, norbornadiene, and 6-methylfulvene as well as a number of further C₇ H₉ ⁺ isomers. The formation and nature of C₇ H₉ ⁺ ions generated by fragmentation of various hydrocarbon precursors, such as monoterpenes and adamantane, is also addressed. The contribution of infrared multiphoton dissociation spectroscopy (IRMPD) and tagged-ion infrared photodissociation (IRPD) of the gaseous C₇ H₉ ⁺ ions as compared to the wealth of previous understanding of their chemistry is commented on as well. Finally, remarkable parallels of the gas-phase chemistry of methylbenzenium ions and the role of such species within the cavities of acidic zeolite catalysts in the course of the industrially important methanol-to-hydrocarbon reaction are discussed. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

The reaction of N2O with phenylium ions C6(H,D)5(+): an integrated experimental and theoretical mechanistic study

The reaction of N(2)O (known to be an O atom donor under several conditions) with the phenyl cation is studied by experimental and theoretical methods. Phenyl cation (or phenylium), C(6)H(5)(+), and its perdeuterated derivative C(6)D(5)(+) are produced either by electron impact or by atmospheric pressure chemical ionization of adequate neutral precursors, and product mass spectra are measured in a guided ion beam tandem mass spectrometer. The ions C(5)(H,D)(5)(+), C(6)(H,D)(5)O(+), and C(3)(H,D)(3)(+) are experimentally detected as the most relevant reaction products. In addition, the detection of the adduct (C(6)H(5)N(2)O)(+), which is collisionally stabilized in the scattering cell of the mass spectrometer, is reported here for the first time. The reaction pathways, which could bring about the formation of the mentioned ions, are then explored extensively by density functional theory and, for the more promising pathways, by CASPT2/CASSCF calculations. The two reacting species (1) form initially a phenoxydiazonium adduct, C(6)H(5)ON(2)(+) (2a), by involving the empty in-plane hybrid C orbital of phenylium. The alternative attack to the ring pi system to produce an epoxidic adduct 2c is ruled out on the basis of the energetics. Then, 2a loses N(2) quite easily, thus affording the phenoxyl cation 3. This is only the first of several C(6)H(5)O(+) isomers (4-6 and 8-12), which can stem from 3 upon different cleavages and formations of C-C bond and/or H shifts. As regards the formation of C(5)H(5)(+), among several conceivable pathways, a direct CO extrusion from 3 is discarded, while others appear to be viable to different extents, depending on the initial energy of the system. The easiest CO loss is from 4, with formation of the cyclopentadienyl cation 7. Formation of C(3)H(3)(+) is generally hindered and its detection depends again on the availability of some extra initial energy.

Dissociation Routes of Protonated Toluene Probed by Infrared Spectroscopy in the Gas Phase

The structures of C(7)H(9)(+) ions generated by protonation of toluene are investigated by means of gas-phase infrared spectroscopy in conjunction with labeling experiments and complementary mass spectrometric studies. In full consistency with previous studies, the unimolecular as well as the multiphoton-induced dissociation of mass-selected C(7)H(9)(+) ions lead to losses of molecular hydrogen and methane. Labeling data clearly imply the occurrence of skeletal rearrangements of protonated toluene to isomeric structures in the course of fragmentation. Complementary reactivity studies indicate, however, that the C(7)H(7)(+) ions generated upon dehydrogenation of C(7)H(9)(+) bear the benzylium structure, rather than that of the more stable tropylium ion. Combination of labeling data and extensive theoretical studies lead to a scheme for the fragmentation of protonated toluene, which can account for all experimental findings reasonably well. As far as infrared spectroscopy of gaseous ions is concerned, the present results confirm the structural predictions derived from theory and provide evidence for the existence of protonated cycloheptatriene but also pose some questions about the comparability of intensities in multiphoton dissociation and linear absorption spectra.

Doubly Protonated Benzene in the Gas Phase

Structural aspects and the unimolecular fragmentations of doubly protonated benzene are studied by means of tandem-mass spectrometry. The corresponding dications are generated by electron ionization (EI) of 1,3- and 1,4-cyclohexadienes, respectively. It is suggested that EI of 1,3-cyclohexadiene leads to the singlet state of doubly protonated benzene, whereas EI of 1,4-cyclohexadiene yields a mixture of singlet and triplet states. Unimolecular fragmentation of doubly protonated benzene exclusively proceeds via dehydrogenation leading to the benzene dication. The proton affinities (PAs) of protonated benzene amount to PA(C(6)H(7)(+))(meta) = 1.9 +/- 0.3 eV for protonation taking place at the meta-position, PA(C(6)H(7)(+))(ortho) = 1.5 +/- 0.2 eV, and PA(C(6)H(7)(+))(para) = 0.9 +/- 0.2 eV, respectively. Various facets of the experiments are compared with density functional theory calculations and generally good agreement is found.

Reactivity of C10H7+ and C10D7+ with H2 and D2

We have investigated, both theoretically and experimentally, the reactions of naphthylium C10H7+ and d-naphthylium C10D7+ ions with H2 and D2. Cross sections as functions of the collision energy have been measured for a variety of reaction channels. Theoretical calculations have been carried out at the density functional theory level which utilizes the hybrid functional B3LYP and the split-valence 6-31G* basis set. The key features of the potential energy surfaces and the relevant thermochemical parameters have been calculated and they provide insights on the reaction mechanisms. The bimolecular reactivity of C10H7+ with H2 is dominated by the production of naphthalene cation C10H8+. The reaction is not a direct atom-abstraction process, but instead it proceeds via the formation of a stable intermediate complex C10H9+ of sigma type geometry, with a significant mobility of hydrogen along the ring. This mobility allows the scrambling of the hydrogen atoms and causes the successive statistical fragmentation of the complex into a variety of product channels. Elimination of one H(D) atom appears to be favored over elimination of one H2 or HD molecule. Alternatively, the intermediate complex can be stabilized either by collision with a third body or by emission of a photon.