Energetics and Site Specificity of the Homolytic C−H Bond Cleavage in Benzenoid Hydrocarbons:  An ab Initio Electronic Structure Study

Abstract

Electronic structure calculations carried out at the BLYP/6-311G** level of theory accurately predict the dissociation energy of the C-H bond in benzene. The analogous energies of the homolytic C-H bond cleavage in the other nine polycyclic aromatic hydrocarbons (PAHs) are found to be governed almost entirely by steric factors, the hydrogens from congested regions of the PAHs being removed preferentially. The removal of hydrogens is accompanied by highly regular changes in the molecular geometries, namely a widening of the ipso bond angle by ca. 6.0°and a concomitant shortening of the adjacent C-C bonds by ca. 0.02 Å. These observations suggest an almost complete localization of the unpaired σ electrons on single carbon atoms and the separation of the local σ and π effects in the aryl radicals under study. This localization is confirmed by the computed charges and spin populations of atoms in the phenyl, 1-naphthalenyl, and 2-naphthalenyl radicals. In contrast with their UHF counterparts, the UBLYP electronic wave functions are only mildly spin contaminated.

Structure‐dependent Photostability of Polycyclic Aromatic Hydrocarbon Cations: Laboratory Studies and Astrophysical Implications

“…Indeed, the calculated C-H dissociation energies of PAH monocations (Fujiwara, Harada, & Aihara 1996) are very similar to those of the corresponding neutral PAH (Cioslowski et al 1996), and this similitude is conÐrmed experimentally (Lifshitz 1997). Furthermore, the molecular structures and intramolecular dynamics of PAH monocations are also closely similar to those of the corresponding neutrals (Leach 1987 ;De Frees et al 1993).…”

Isomer-specific vibronic structure of the 9-, 1-, and 2-anthracenyl radicals via slow photoelectron velocity-map imaging

“…Anionic anthracene derivatives in various protonation states are also used in electron transfer dissociation mass spectrometry as an electron source to induce fragmentation of peptide backbones (27). A fair amount of additional theoretical work has been reported detailing the energetics, electronic structure, geometries, vibrational frequencies, and reactivities of the anthracenyl radicals and anions (1,6,(28)(29)(30)(31)(32). The three C−H bond dissociation energies of anthracene are predicted to be very similar, leading to three anthracenyl radical isomers nearly degenerate in energy (1,28,29,31).…”

“…A fair amount of additional theoretical work has been reported detailing the energetics, electronic structure, geometries, vibrational frequencies, and reactivities of the anthracenyl radicals and anions (1,6,(28)(29)(30)(31)(32). The three C−H bond dissociation energies of anthracene are predicted to be very similar, leading to three anthracenyl radical isomers nearly degenerate in energy (1,28,29,31). The anionic isomers are calculated to be more separated in energy, with the 9-anthracenyl anion being the most stable and the 1-and 2-anthracenyl anions lying 0.14 eV and 0.18-0.21 eV higher in energy, respectively (6,29).…”

Structure Based Predictive Model for Coal Char Combustion

“…This is due to the unique stability of the pyrrol-1-yl radical (Figure 3.4.6), as this radical, like cyclopentadienyl radical, has the unpaired electron in the n-system -for a total of 5 electrons in the n-system. 24,35,35 Most carbon-centered aryl radicals have the unpaired electron in the asystem.24…”