Coupled Reactions of Condensation and Charge Transfer. 1. Formation of Olefin Dimer Ions in Reactions with Ionized Aromatics. Gas-Phase Studies

Meot‐Ner, Michael; Pithawalla, Yezdi B.; Gao, and J.; El‐Shall, M. Samy

doi:10.1021/ja962635x

Cited by 6 publications

(13 citation statements)

References 40 publications

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“…We also observe a secondary product channel corresponding to H 2 transfer from C 6 H 12 •+ to form C 6 H 10 •+ , in analogy with reaction 5, that we observed previously in the toluene •+ / isobutene system. 38 However, in the benzene •+ /propene R2PI experiment, C 6 H 10 •+ was always a minor product, <7%. C 6 H 12…”

Section: R2pi Of the Benzene/propenementioning

confidence: 93%

“…The mass spectrometric studies were performed using the R2PI-HPMS apparatus that was developed recently. 38,60 Briefly, the HPMS ion source is a cubic aluminum block with a volume of about 2 cm 3 , fitted with quartz windows through which the laser beam enters and exits. Gas mixtures are prepared in a 2 L glass flask heated to >100 °C and admitted to the ion source at selected pressures via an adjustable needle valve.…”

Section: Methodsmentioning

confidence: 99%

“…However, we observed recently a novel subclass of these reactions in a system of an ionized aromatic (toluene, C 6 H 5 -CH 3 •+ ) and a neutral olefin (isobutene, i-C 4 H 8 ) that has a higher ionization potential (IP) than the aromatic molecule. 30,38 In this case charge transfer to a single olefin molecule would be endothermic by 0.42 eV. With an energy barrier at least as large, the rate coefficient would be k e k collision exp(-∆H°/RT) ≈ 10 -17 cm 3 s -1 , and the reaction would be unobservable.…”

Section: Introductionmentioning

confidence: 99%

“…The overall reaction consisting of charge transfer and covalent bond formation is exothermic by 17.5 kcal/mol and was observed to proceed faster by a factor of 10 5 than predicted for the simple endothermic charge-transfer reaction. 38 Pressure and concentration effects suggested that the reaction proceeds through a reactive intermediate π complex, (C 6 H 5 CH The partial positive charge transferred to the olefin in the complex activates it for nucleophilic attack by a second olefin molecule, leading to covalent condensation. It is desirable to extend these studies to other systems to confirm and generalize the new mechanism.…”

Section: Introductionmentioning

confidence: 99%

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Gas-Phase Oligomerization of Propene Initiated by Benzene Radical Cation

Pithawalla¹,

Meot‐Ner²,

Gao³

et al. 2001

J. Phys. Chem. A

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Initiation of the radical cation polymerization of propene has been observed following the selective ionization of benzene in the gas phase by resonance two-photon ionization−high-pressure mass spectrometry (R2PI−HPMS) and by selected ion flow tube (SIFT) techniques. In this system, the aromatic initiator (C6H6) has an ionization potential (IP) between those of the reactant's monomer (C3H6) and its covalent dimer (C6H12), i.e, IP(C3H6) > IP(C6H6) > IP(C6H12). Therefore, direct charge transfer from C6H6 •+ to C3H6 is not observed due to the large endothermicity of 0.48 eV, and only the adduct C6H6 •+(C3H6) is formed. However, coupled reactions of charge transfer with covalent condensation are observed according to the overall process C6H6 •+ + 2C3H6 → C6H12 •+ + C6H6, which results in the formation of a hexene product ion, C6H12 •+. The formation of this ion can make the overall process of charge transfer and covalent condensation significantly exothermic. At higher concentrations of propene, the reaction products are the propene oligomers (C3H6) n •+ with n = 2−7 and the adduct series C6H6 •+(C3H6) n with n ≤ 6. The significance of the coupled reactions is that the overall process leads exclusively to the formation of the condensation product (C3H6) n •+ and avoids other competitive channels in the ion/molecule reactions of propene. Gas-phase nominal second-order rate coefficients for the overall reaction into both channels are in the range of (1−3) × 10-12 cm3 s-1. The rate coefficients into both channels, especially for the formation of the C6H12 •+ dimer, have large negative temperature dependencies. Consistent with the gas-phase results, the intracluster reactions of C6H6 •+ produced selectively by R2PI of mixed benzene/propene clusters also do not form the monomer ion C3H6 •+ but form higher propene clusters (C3H6) n •+ that contain at least the C6H12 •+ hexene ion. The similarity of the reaction mechanisms in the gas phase and in preformed clusters suggests that the mechanism may also apply in the condensed phase in common aromatic solvents such as benzene and toluene.

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Section: R2pi Of the Benzene/propenementioning

confidence: 93%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Gas-Phase Oligomerization of Propene Initiated by Benzene Radical Cation

Pithawalla¹,

Meot‐Ner²,

Gao³

et al. 2001

J. Phys. Chem. A

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“…PANH compounds can be ionized or protonated in low pH solutions in biological systems and in ices, in the gas phase in mass spectrometry and combustion, and in interstellar clouds, where emission bands of ionized PANHs at 6.3–6.5 μm have been indicated. , PANHs have high intrinsic basicities (GB) and proton affinities (PA), making them potential proton sinks in gas-phase environments. The large planar ions can then serve as nucleation centers for clusters and ices and provide catalytic molecular surfaces for associative charge (ACT) or associative proton transfer (APT) reactions. − …”

Section: Introductionmentioning

confidence: 99%

Protonated Polycyclic Aromatic Nitrogen Heterocyclics: Proton Affinities, Polarizabilities, and Atomic and Ring Charges of 1–5-Ring Ions

2014

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Calculated proton affinities, polarizabilities, and some ionization energies and atomic and ring NBO charges are reported for 31 polycyclic aromatic nitrogen heterocyclics (PANHs) with 1-5 rings, calculated on the on the M06-2X/6-311+g**//B3LYP/6-31g* level of theory. The calculated proton affinities from 226 to 241 kcal mol(-1) for 3-5-ring compounds, predict well the relative experimental values. The proton affinities increase with increasing molecular size and show a linear correlation with polarizabilities. Linear geometry and nitrogen located in the central ring also favor increased proton affinity. These trends estimate a PA > 241 kcal mol(-1) for an infinite linear chain, end-ring-N PANH molecule, and >261 kcal mol(-1) for an edge-N-doped graphene sheet, making it a superbase. NBO analysis shows that from pyridineH(+) to large 5-ring ions, the N-H nitrogen carries a constant q(N) = -0.46 ± 0.1 charge, and the N-H hydrogen a constant q(H) = 0.43 ± 0.01 positive charge, similar to the q(H) in NH4(+). Overall, the NH group is nearly electrically neutral, and a nearly full positive charge is distributed on the aromatic hydrocarbon rings of the ions. When the nitrogen is in a central ring, that ring is negative, and the positive ionic charge is delocalized toward the end rings. When the nitrogen is in an end ring, the ionic charge is distributed more evenly. Increasing proton affinities with increasing polarizability result not from increasing charge transfer from the proton to the aromatic rings, but from increasing delocalization of the transferred charge in the aromatic hydrocarbon rings of the ions. In two-nitrogen compounds, interactions between the ring nitrogens decrease the proton affinities, but this effect decreases in larger ions.

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Polymerization in the Gas Phase, in Clusters, and on Nanoparticle Surfaces

El‐Shall

2008

Acc. Chem. Res.

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Gas phase and cluster experiments provide unique opportunities to quantitatively study the effects of initiators, solvents, chain transfer agents, and inhibitors on the mechanisms of polymerization. Furthermore, a number of important phenomena, unique structures, and novel properties may exist during gas-phase and cluster polymerization. In this regime, the structure of the growing polymer may change dramatically and the rate coefficient may vary significantly upon the addition of a single molecule of the monomer. These changes would be reflected in the properties of the oligomers deposited from the gas phase. At low pressures, cationic and radical cationic polymerizations may proceed in the gas phase through elimination reactions. In the same systems at high pressure, however, the ionic intermediates may be stabilized, and addition without elimination may occur. In isolated van der Waals clusters of monomer molecules, sequential polymerization with several condensation steps can occur on a time scale of a few microseconds following the ionization of the gas-phase cluster. The cluster reactions, which bridge gas-phase and condensed-phase chemistry, allow examination of the effects of controlled states of aggregation. This Account describes several examples of gas-phase and cluster polymerization studies where the most significant results can be summarized as follows: (1) The carbocation polymerization of isobutene shows slower rates with increasing polymerization steps resulting from entropy barriers, which could explain the need for low temperatures for the efficient propagation of high molecular weight polymers. (2) Radical cation polymerization of propene can be initiated by partial charge transfer from an ionized aromatic molecule such as benzene coupled with covalent condensation of the associated propene molecules. This novel mechanism leads exclusively to the formation of propene oligomer ions and avoids other competitive products. (3) Structural information on the oligomers formed by gas-phase polymerization can be obtained using the mass-selected ion mobility technique where the measured collision cross-sections of the selected oligomer ions and collision-induced dissociation can provide fairly accurate structural identifications. The identification of the structures of the dimers and trimers formed in the gas-phase thermal polymerization of styrene confirms that the polymerization proceeds according to the Mayo mechanism. Similarly, the ion mobility technique has been utilized to confirm the formation of benzene cations by intracluster polymerization following the ionization of acetylene clusters. Finally, it has been shown that polymerization of styrene vapor on the surface of activated nanoparticles can lead to the incorporation of a variety of metal and metal oxide nanoparticles within polystyrene films. The ability to probe the reactivity and structure of the small growing oligomers in the gas phase can provide fundamental insight into mechanisms of polymerization that are difficult to obtain from c...

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Coupled Reactions of Condensation and Charge Transfer. 1. Formation of Olefin Dimer Ions in Reactions with Ionized Aromatics. Gas-Phase Studies

Cited by 6 publications

References 40 publications

Gas-Phase Oligomerization of Propene Initiated by Benzene Radical Cation

Gas-Phase Oligomerization of Propene Initiated by Benzene Radical Cation

Protonated Polycyclic Aromatic Nitrogen Heterocyclics: Proton Affinities, Polarizabilities, and Atomic and Ring Charges of 1–5-Ring Ions

Polymerization in the Gas Phase, in Clusters, and on Nanoparticle Surfaces

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