Gas-Phase Oligomerization of Propene Initiated by Benzene Radical Cation

Abstract

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.

Associative Charge Transfer Reactions. Temperature Effects and Mechanism of the Gas-Phase Polymerization of Propene Initiated by a Benzene Radical Cation

“…In our previous study, at lower [Pr] values of 10 13 -10 14 molecules/cm 3 , the ACT/ association product ratio increased proportionally with [Pr]. 2 In the present work, we noted a similar trend at 370 K, and Table 1 shows a slight effect at 271 K but not at 245 K. It appears that the (Bz +• Pr δ +) * ACT complex may rearrange slowly to the (Bz‚Pr) +• * association complex, especially at higher temperatures, as indicated by the down arrow in Scheme 1. However, competition by collision with Pr eliminates this rearrangement, even at low [Pr] values of about 10 15 molecules/cm 3 .…”

9 Organic gas phase ion chemistry

“…125 The gas phase radical cation polymerization of propene initiated by the radical cation of benzene has been studied by resonance two-photon ionization-high-pressure mass spectrometry (R2PI-HPMS) and selected ion flow tube (SIFT) techniques. 126 At higher concentrations of propene, reaction products are the propene oligomers (C 3 H 6 ) n ϩ (n = 2-7) and the adduct series C 6 H 6 ϩ (C 3 H 6 ) n (n ≤ 6). Several ring closing reactions of radical cations have been examined.…”

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

“…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. − …”