Issue 7, 2012

Gas-phase reactions of aryl radicals with 2-butyne: experimental and theoretical investigation employing the N-methyl-pyridinium-4-yl radical cation

Abstract

Aromatic radicals form in a variety of reacting gas-phase systems, where their molecular weight growth reactions with unsaturated hydrocarbons are of considerable importance. We have investigated the ion–molecule reaction of the aromatic distonic N-methyl-pyridinium-4-yl (NMP) radical cation with 2-butyne (CH3C[triple bond, length as m-dash]CCH3) using ion trap mass spectrometry. Comparison is made to high-level ab initio energy surfaces for the reaction of NMP and for the neutral phenyl radical system. The NMP radical cation reacts rapidly with 2-butyne at ambient temperature, due to the apparent absence of any barrier. The activated vinyl radical adduct predominantly dissociates via loss of a H atom, with lesser amounts of CH3 loss. High-resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry allows us to identify small quantities of the collisionally deactivated reaction adduct. Statistical reaction rate theory calculations (master equation/RRKM theory) on the NMP+2-butyne system support our experimental findings, and indicate a mechanism that predominantly involves an allylic resonance-stabilized radical formed via H atom shuttling between the aromatic ring and the C4 side-chain, followed by cyclization and/or low-energy H atom β-scission reactions. A similar mechanism is demonstrated for the neutral phenyl radical (Ph˙)+2-butyne reaction, forming products that include 3-methylindene. The collisionally deactivated reaction adduct is predicted to be quenched in the form of a resonance-stabilized methylphenylallyl radical. Experiments using a 2,5-dichloro substituted methyl-pyridiniumyl radical cation revealed that in this case CH3 loss from the 2-butyne adduct is favoured over H atom loss, verifying the key role of ortho H atoms, and the shuttling mechanism, in the reactions of aromatic radicals with alkynes. As well as being useful phenyl radical analogues, pyridiniumyl radical cations may form in the ionosphere of Titan, where they could undergo rapid molecular weight growth reactions to yield polycyclic aromatic nitrogen hydrocarbons (PANHs).

Graphical abstract: Gas-phase reactions of aryl radicals with 2-butyne: experimental and theoretical investigation employing the N-methyl-pyridinium-4-yl radical cation

Supplementary files

Article information

Article type
Paper
Submitted
18 Sep 2011
Accepted
09 Dec 2011
First published
16 Jan 2012

Phys. Chem. Chem. Phys., 2012,14, 2417-2426

Gas-phase reactions of aryl radicals with 2-butyne: experimental and theoretical investigation employing the N-methyl-pyridinium-4-yl radical cation

A. K. Y. Lam, C. Li, G. Khairallah, B. B. Kirk, S. J. Blanksby, A. J. Trevitt, U. Wille, R. A. J. O'Hair and G. da Silva, Phys. Chem. Chem. Phys., 2012, 14, 2417 DOI: 10.1039/C2CP22970F

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