Phenylium Ion Research Articles

IR photofragmentation of the phenyl cation: spectroscopy and fragmentation pathways.

We present the gas-phase infrared spectra of the phenyl cation, phenylium, in its perprotio (C6H5+) and perdeutero (C6D5+) forms, in the 260-1925 cm-1 (5.2-38 μm) spectral range, and investigate the observed photofragmentation. The spectral and fragmentation data were obtained using Infrared Multiple Photon Dissociation (IRMPD) spectroscopy within a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (FTICR MS) located inside the cavity of the free electron laser FELICE (Free Electron Laser for Intra-Cavity Experiments). The 1A1 singlet nature of the phenylium ion is ascertained by comparison of the observed IR spectrum with DFT calculations, using both harmonic and anharmonic frequency calculations. To investigate the observed loss of predominantly [2C,nH] (n = 2-4) fragments, we explored the potential energy surface (PES) to unravel possible isomerization and fragmentation reaction pathways. The lowest energy pathways toward fragmentation include direct H elimination, and a combination of facile ring-opening mechanisms (≤2.4 eV), followed by elimination of H or CCH2. Energetically, all H-loss channels found are more easily accessible than CCH2-loss. Calculations of the vibrational density of states for the various intermediates show that at high internal energies, ring opening is thermodynamically the most advantageous, eliminating direct H-loss as a competing process. The observed loss of primarily [2C,2H] can be explained through entropy calculations that show favored loss of [2C,2H] at higher internal energies.

Formation of the oxonium phenol ion in the stepwise hydration of the phenyl cation in the gas phase

This study provides the first evidence for the formation of the oxonium phenol ion and its hydrated clusters by the sequential addition of water molecules onto the phenylium ion in the gas phase. The oxonium phenol ion exhibits thermal stability at higher temperatures up to nearly 575 K while the hydrated phenyl cation dissociates by the loss of water at temperatures below 400 K. A second water molecule is attached reversibly to each of the oxonium phenol and the hydrated phenyl ions and equilibrium thermochemical measurements in the low temperature range yield an average −ΔH° of 15.5 kcal/mol reflecting 35% contribution from the hydrated oxonium phenol C6H5OH2+(H2O) and 65% contribution from the second hydration step of the phenyl cation C6H5+(H2O)2. In the high temperature range (373–423 K), the measured average −ΔH° of 18.7 kcal/mol reflects 45% and 55% contributions from the hydrated oxonium phenol and the second hydration step of the phenyl cation, respectively. DFT calculations at the B3LYP/6-311++G** level show that the sequential hydration of the oxonium phenol ion results in the formation of externally hydrated clusters C6H5OH2+(H2O)n where the incoming water molecules form a hydrogen bonding network attached to the oxonium site, and the sequential hydration energy decreases from 25.9 kcal/mol for n = 1 to 11.5 kcal/mol for n = 5. For the hydrated phenyl cation clusters C6H5+(H2O)n, the water molecules are attached at two CHδ+ sites of the phenyl cation, and the sequential hydration energy decreases slowly from 11.3 kcal/mol for n = 1 to 8.1 kcal/mol for n = 5.

Formation of Complex Organics in the Gas Phase by Sequential Reactions of Acetylene with the Phenylium Ion

In this paper, we report a study on the reactivity of the phenylium ion with acetylene, by measuring product yield as a function of pressure and temperature using mass-selected ion mobility mass spectrometry. The reactivity is dominated by a rapid sequential addition of acetylene to form covalently bonded C8H7(+) and C10H9(+) ions with an overall rate coefficient of 7-5 × 10(-10) cm(3) s(-1), indicating a reaction efficiency of nearly 50% at room temperature. The covalent bonding nature of the product ions is confirmed by high temperature studies where enhanced production of these ions is observed at temperatures as high as 660 K. DFT calculations at the UPBEPBE/6-31++G** level identify the C8H7(+) adduct as 2-phenyl-ethenylium ion, the most stable C8H7(+) isomer that maintains the phenylium ion structure. A small barrier of 1.6 kcal/mol is measured and attributed to the formation of the second adduct C10H9(+) containing a four-membered ring connected to the phenylium ion. Evidence for rearrangement of the C10H9(+) adduct to the protonated naphthalene structure at temperatures higher than 600 K is provided and suggests further reactions with acetylene with the elimination of an H atom and an H2 molecule to generate 1-naphthylacetylene or acenaphthylene cations. The high reactivity of the phenylium ion toward acetylene is in sharp contrast to the low reactivity of the benzene radical cation with a reaction efficiency of 10(-4)-10(-5), confirming that the first step in the cation ring growth mechanism is the loss of an aromatic H atom. The observed reactions can explain the formation of complex organics by gas phase ion-molecule reactions involving the phenylium ion and acetylene under a wide range of temperatures and pressures in astrochemical environments.

ChemInform Abstract: Gas-Phase Heteroaromatic Substitution. Part 10. Reaction of Free Phenylium Ion with Simple Five-Membered Heteroarenes in the Gaseous and Liquid Phase.

ChemInform Abstract: Gas-Phase Heteroaromatic Substitution. Part 10. Reaction of Free Phenylium Ion with Simple Five-Membered Heteroarenes in the Gaseous and Liquid Phase.

Ion chemistry in gaseous discharges at atmospheric pressure

The role of ions in plasma chemistry is briefly reviewed on the basis of recent literature reports. In addition, different chemical processes induced by gaseous discharges at atmospheric pressure are discussed here, with particular emphasis on the elucidation of some aspects of the ion chemistry occurring in benzene/air dielectric barrier and corona discharges. Through a multi-technique approach relying on gas chromatographic, mass spectrometric, spectroscopic and computational analyses, we have been able to reveal subtle mechanistic aspects involved in the ionic processes leading to the synthesis of covalent adducts. In particular, we report here the experimental conditions and the thermochemical reasons whereby we can access the synthesis (i) of benzenediazonium ion from the reaction of phenylium ion with N2, (ii) of protonated biphenyl ions from electrophilic aromatic attack of phenylium ion on C6H6, (iii) of phenol radical cations C6H5OH+. via O radical addition on the benzene ring followed by ionization of the neutral product and finally (iv) of biphenyl-oxonium ions C12H11O+ and hydroxylbiphenyl-oxonium ions as end-products of the electrophilic attack of phenol ions on benzene and phenol, respectively. Finally, intriguing chemical paths involved in the corona discharge ionization of acetonitrile are also discussed.

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.

Electrophilic aromatic substitution and single‐electron transfer (SET) by the phenylium ion in the gas phase: characterization of a long‐lived SET intermediate

Gas-phase mass spectrometric studies and calculations were performed for the reaction of naked phenylium ion with several benzene halides. From these reactions, the molecular ion for biphenyl as the predominant product was obtained only from the reaction of phenylium ions with iodobenzene and bromobenzene. Furthermore, through the collision-induced dissociation (CID) of the ion at m/z 281, the only dissociation observed is the loss of a phenyl radical, which indicates that a single-electron transfer (SET) mechanism might have occurred within the reaction. Additionally, according to the comparison between the CID experiments of those isomeric compounds of the sigma-complexes and the CID experiment of the ion at m/z 281 captured in the ion trap, we have also defined the captured ion at m/z 281 as an SET-intimate ion pair rather than those of sigma-complexes or the diphenyliodonium.

Revealing Phenylium, Phenonium, Vinylenephenonium, and Benzenium Ions in Solution

The 4-methoxyphenylium ion has been generated in the triplet state ((3)An(+)) by photolysis of 4-chloroanisole in polar media and detected by flash photolysis (lambda(max)=400 nm). This is the first detection of a phenylium ion in solution by flash photolysis and the assignment is supported by time-dependent density functional theory (TD-DFT) calculations. In neat solvents, the cation was reduced to anisole, a process initiated by electron transfer from the starting compound ((3)An(+)+AnCl-->An(*)+AnCl(*+), with the radical cation detected at 470 nm, then An(*)-->AnH). Addition of pi nucleophiles to the (3)An(+) cation offers a novel access to a number of other cationic intermediates under mild, nonacidic conditions. Two intermediates are successively formed with alkenes, a diradical cation and the phenonium ion, which are detected at 440 and 320 nm, respectively, by flash photolysis and are in accordance with calculations. Allylanisoles or beta-alkoxyalkylanisoles are the end products, with a small amount of alpha-alkoxyalkylanisoles that arises from a Wagner-Meerwein rearrangement to form benzyl cations. Further intermediates that have been predicted and detected are the phenylvinylium ion, possibly in equilibrium with the vinylenephenonium ion, with 1-hexyne (lambda(max)=340 nm) and the benzenium ion with benzene (lambda(max)=380 nm). The final products were anisylhexyne and methoxybiphenyl (an analogous product and intermediate were detected with thiophene).

New Insights into the Reaction Mechanisms of Phenylium Ions with Benzene

The chemistry of phenylium (benzen-1-ylium) cations with benzene is investigated by using a guided ion beam tandem mass spectrometer. The main ionic products from the reaction of C6H5+ with C6H6 are observed at m/z 155 (covalent adduct C12H11+), 154 (C12H10+), 153 (C12H9+), 129 (C10H9+), and 115 (C9H7+). We propose a mechanism according to which channels at m/z 154-115 are formed by elimination of stable neutral molecules (such as H2, C2H2, C3H4) from the collision complex C12H11+, for which the most plausible structure is protonated biphenyl. The proposed mechanism is demonstrated by using partial isotopical labeling of reagents to look for possible H/D atom scrambling. Almost the same ions are produced when benzene is chemically ionized at atmospheric pressure in an APCI source from which oxygen is excluded. Because an ion trap analyzer is coupled to this source, tandem MS experiments can be performed, allowing structural details to be established. Moreover, the use of partially deuterated reagents has allowed the detection of minor reactive channels resulting from charge exchange and H-/D- hydride-transfer processes. Theoretical calculations show that the most stable structure for ions at m/z 129 C10H9+ is that of protonated naphthalene, resulting from the loss of an acetylene molecule by the condensation product, with a reaction exothermicity of 1.27 eV. We have found a possible barrierless pathway for such a channel that might be viable for the synthesis of naphthalene, the smallest PAH, even at low collision energies and therefore would be of particular astrochemical relevance.

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

The reaction of phenylium and d-phenylium ions with molecular deuterium has been investigated both experimentally and theoretically. Theoretical calculations have been carried out at the B3LYP/6-31G* level of theory. The key features of the potential energy surfaces and all the relevant thermochemical parameters have been calculated. These results are in good agreement with previous theoretical studies concerning the unimolecular dissociation of benzenium ions. Theoretical calculations provide a clear insight on the reaction mechanisms for the system under study. Experimental studies have been carried out by means of a guided beam tandem mass spectrometer. We have measured the reaction cross section for four different reaction channels, as a function of the collision energy, in the range from thermal energies up to about 1 eV. The bimolecular reactivity of C6H5+ is dominated by the substitution of one or two hydrogen atoms by deuterium, leading to the formation of C6H4D+ and C6H3D2+ products. The reaction cross section is nearly the same for the two reaction channels. This fact is interpreted as evidence of a statistical behavior, as expected by assuming a fast shift of hydrogen (deuterium) within the aromatic ring. Another very weak reaction channel leading to the formation of the benzene ion has been measured. The cross section for such a process is lower than about 10−2 Å2 over the entire energy range explored in our experiment. A structure observed at collision energies around about 0.1 eV has been attributed to the reaction of phenylium ions in their triplet state. When the density of the gas in the scattering cell is increased, the formation of collisionally stabilized benzenium ions is observed.

Reply to Comment on “Reactions of Phenyl Cations with Methanol and Methyl Fluoride”

Phenylium ion automerization was revealed in the radiochromatographic study of reactions between nucleogenic cations and CH3X (X ) OH, F, Cl, Br).1-3 However, the possibility of automerization was called in question by the results of theoretical studies4-6 which predicted large barriers to 1,2-hydrogen shift. Nucleogenic cations formed by tritium decay may possess excess energy due to the fact that their nascent geometry is that of their neutral precursors.7 Relaxation of their geometry to the equilibrium may release energy which makes the ion vibrationally excited. This excess energy (Edef) may allow the phenyl cation to overcome the 1,2-hydrogen shift barrier (E isomer / ). Thus, the possibility of the phenylium ion automerization depends on the sign of the E isomer / Edef difference. All theoretical estimates predict this value positive. The state-ofthe-art calculation of the 1980s by Schleyer, Kos, and Raghavachari6 (MP2/6-31G(d,p)//3-21G) gave 52.2 kcal/mol for E isomer / and 32.3 kcal/mol for Edef. However, they predicted that at higher theory levels E isomer / should be 40 kcal/mol or more. This allowed researchers to claim that further improvement of the theory level may reduce or even invert the E isomer /

Ion-molecular reactions of free phenylium ions, generated by tritium β-decay with group V–VII elements

Generation of free phenylium ions, obtained by tritium β-decay and their ion-molecular reactions with Group V–VII elements are described. The elaborated nuclear–chemical method has been used for the synthesis of tritium-labeled organic derivatives containing these Group V–VII elements.

Ion-molecular reactions of free phenylium ions, generated by tritium β-decay with bidentate arenes

Generation of free phenylium ions, obtained by tritium β-decay and their ion-molecular reactions with bidentate arenes are described. The elaborated nuclear-chemical method has been used for the synthesis of tritium labeled organic derivatives using halobenzenes. Two alternative pathways for interaction of the phenylium ion with the bidentate arene, addition to the lone pair of the halogen or substitution on the aromatic ring, were revealed.

Gas-Phase Ion Chemistry of Borazine, an Inorganic Analogue of Benzene

The gas-phase reactivity of selected ionic species with borazine and of borazine-derived ions with selected neutrals has been studied by FT-ICR and ab initio computations and related to the corresponding ion chemistry of benzene. The most basic site of borazine is at nitrogen, and its conjugate acid, H3B3N3H4+, is similar in structure to the benzenium ion, as shown by ab initio calculations. H3B3N3H4+ ions undergo H/D exchange of up to four hydrogens with CD3OD and do not isomerize by stepwise 1,2-hydrogen shifts. Protonation at boron is calculated to be unfavored by 28 kcal/mol with respect to protonation at nitrogen. The H4B3N3H3+ ions show the features of a [B3N3H5···H2]+ complex, prone to dissociation at room temperature. The experimental gas-phase basicity of borazine is equal to 185.0 ± 1 kcal/mol. The Lewis basicity toward Me3Si+ places borazine into a linear correlation pertaining to model aromatic compounds. The experimental gas-phase acidity is 365.4 ± 1.5 kcal/mol. The reactions of neutral borazine with protonating, alkylating, and nitrating ions have been characterized and compared with the corresponding reactions of benzene. B3N3H5+ ions, retaining a cyclic structure, react similarly, in some respects, as phenylium ions, C6H5+.

Reaction from isomeric parent ions in the dissociation of dimethylpyrroles

The major dissociation pathways of the [M-H](+) (loss of NH3 or CH4) and the [M+H](+) (loss of NH3 or CH3) ions from dimethylpyrroles have been determined to occur from isomeric parent ions. For the [M-H](+) ion (formed by loss of a methyl hydrogen), loss of NH3 leads to the formation of the phenylium ion and is preceded by consecutive carbon ring expansions followed by a ring contraction to form protonated aniline. Loss of CH4 occurs after the first carbon ring expansion, which forms protonated picoline. The relative partitioning between the two dissociation paths depends upon the internal energy content of the parent ion; the highest point on the potential energy surface is the second ring expansion step. The [M+H](+) ion reacts through a similar pathway via dihydro analogs of picoline and aniline. The proposed reaction pathways are supported by results of semiempirical molecular orbital calculations.

Ion—molecule chemistry of pyrene in an ion‐trap mass spectrometer

AbstractThe quadrupole ion‐trap mass spectrometer is used to investigate aspects of the ion‐molecule chemistry of the pyretic molecular radical cation (M+), the [M  H]+ ion and the [M  H2]+˙ ion. Cycloaddition and alkyl halide addition reactions with the reagents pyrene, methyl iodide, ethyl iodide, isopreue and 2‐iodopropane are investigated. The pyrene molecular radical cation is remarkably unreactive while the closed shell [M  H]+ ion is highly reactive and the [M  H2]+˙ ion shows intermediate reactivity. Analogies are found between the behaviour of the pyrene molecular radical cation and the corresponding [M  H]+ ion on the one hand, and the benzene radical cation and the phenylium ion on the other. The total lack of reactivity of the M+˙ ion is attributed to π‐delocalization of the radical and the cation over the large aromatic system. Addition of isoprene to the [M  H]+ and [M  H2]+˙ ions of pyrene occurs via cycloaddition reactions. A characteristic fragmentation of each adduct ion is methyl radical loss and evidence obtained using pyrene‐d10 indicates that the product in the case of [M  H]+ is an ionized indene. Addition of methyl iodide, ethyl iodide and 2‐iodopropane to the [M  H]+ ion of pyrene occurs to give an iodonium product (C16H9–I+–R) which fragments by loss of I˙ or CH3˙ to give the methylpyrene or iodopyreue product ions. Reactions of ethyl iodide and 2‐iodopropane with the dehydropyrene ion, [M  H2]+˙, give primarily an HI addition product, together with small contributions from the intact adducts. Evidence is provided suggesting that in this case the HI and alkyl halide molecules add across a CC bond rather than forming an iodine‐bound adduct. An experiment in which nine consecutive isolation and activation steps (MS10) are used to couvert the [2M  H]+ adduct, C32H, to C32H˙ exemplifies the intriguing ion chemistry and the stability of the carbon skeleton of the adduct ions.

Gas-phase heteroaromatic substitution. 10. Reaction of free phenylium ion with simple five-membered heteroarenes in the gaseous and liquid phase

Phenylium ion, obtained from the spontaneous β decay of 1,4-ditritiobenzene, has been allowed to react with pyrrole, N-methylpyrrole, furan, and thiophene, in both the gaseous and liquid phases. The differences between the reactivity pattern of phenylium ion in the two environments can be essentially reduced to significant ion-neutral electrostatic interaction in the gas phase and to the much greater efficiency of collisional stabilization in the condensed phase, allowing a larger fraction of the excited ionic intermediates, from the highly exothermic attack of phenylium ion on the aromatic substrate, to survive dissociation and isomerization. The mechanism of the phenylation process and of the subsequent isomerization of the relevant ionic intermediates is discussed and the intrinsic substrate and positional selectivity of the phenylium ion evaluated. While the limited substrate discrimination of phenylium ion fully agrees with its well-known exceedingly high reactivity, its pronounced affinity toward the α carbons of the selected heteroarenes does not conform with the relatively "hard" character of the reactant, expected on the grounds of its STO-3G calculated LUMO energy. The conceivable occurrence of an intimate entropy-favored two-step addition mechanism, involving a preliminary single-electron transfer (SET) from the heteroaromatic substrate to the ionic electrophile, which is thermochemically allowed only for phenylium and methyl cations and prevented for other alkylating electrophiles, is discussed. Key words: gas-phase ion chemistry, electrophilic heteroaromatic substitution, nuclear decay chemistry, phenylium ion, electron transfer.

Concerning the formation and the kinetics of phenylium ions

Kinetic measurements carried out in a pulsed ion cyclotron spectrometer show that C 6H + 5 ions formed in unimolecular and bimolecular processes consist of two populations of different reactivities. As expected, ground-state singlet phenylium ions are predominantly formed in low-energy processes, whereas higher-energy isomeric (mainly acyclic) ions appear in the high-energy electron impact fragmentation of C 6H 6 and halobenzenes, as well as products in highly exothermic ion/molecule reactions. The energy barrier for ring opening was estimated to be 2.0±0.3 eV, in reasonably good agreement with theory. Phenylium ions react readily with hydrocarbons and polar molecules. By means of isotopic labelling it was shown that alkanes transfer H 2 via a two-stage process consisting of the transfer of H − to the phenylium ion followed by a transfer in the complex of H + to the resulting benzene moiety. Cyclopropane was found to transfer CH 2 to the phenylium ion in a direct concerted process. In contrast, reaction with unsaturated hydrocarbons involves extensive hydrogen and carbon atom scrambling. Contrary to earlier findings, phenylium ions were found to react at least ten times faster with acetylene than did the other C 6H + 5 isomer(s).

ChemInform Abstract: Arylation of Cyclic Ethers by Gaseous Phenylium Ions. Formation and Behavior of Phenoxenium Ions in the Gas Phase.

AbstractFree, unsolvated phenylium ions formed by the spontaneous decay of 1,4‐ditritiobenzene are allowed to react with gaseous oxirane, methyloxirane, oxetane, and acetaldehyde in the pressure range 30‐250 torr and in the presence of O2 (4 torr) as thermal radical scavenger.

ChemInform Abstract: Competitive Concerted and Stepwise Addition of Free Arylium Ions to Propane in the Gas Phase.

Labelled tolylium ions from the decay of ring-multitritiated toluene have been allowed to react with propane in the gas phase, at pressures ranging from 20 to 744 Torr, yielding isomeric propyltoluenes as the major addition products. The relative composition of the n-versus iso-propyltoluenes (σp :σs 3.2–4.7) is found to depend appreciably upon the total pressure of the system and the presence of added bases (NH3 or CH3OH). Pressure and base effects on the isomeric distribution of both n- and iso-propyltoluenes are also investigated. The results are consistent with a reaction pattern involving preliminary formation of an electrostatic adduct, wherein fast hydride-ion transfer from a secondary C–H bond of C3H8 to the arylium ion takes place. The same mechanism does not seem operative when the first interaction occurs between the tolylium ions and a primary C–H bond of the substrate. The behaviour of isomeric tolylium ions towards propane is discussed and compared with related studies involving unsubstituted phenylium ions. A mechanistic model is proposed for both reactions, which accounts for the apparent discrepancy between the indiscriminate affinity of arylium ions for any kind of substrate and their ‘abnormally’ high site selectivities.