Hydrogen-atom attack on phenol and toluene is ortho-directed

Abstract

The reaction of H + phenol and H/D + toluene has been studied in a supersonic expansion after electric discharge. The (1 + 1') resonance-enhanced multiphoton ionization (REMPI) spectra of the reaction products, at m/z = parent + 1, or parent + 2 amu, were measured by scanning the first (resonance) laser. The resulting spectra are highly structured. Ionization energies were measured by scanning the second (ionization) laser, while the first laser was tuned to a specific transition. Theoretical calculations, benchmarked to the well-studied H + benzene → cyclohexadienyl radical reaction, were performed. The spectrum arising from the reaction of H + phenol is attributed solely to the ortho-hydroxy-cyclohexadienyl radical, which was found in two conformers (syn and anti). Similarly, the reaction of H/D + toluene formed solely the ortho isomer. The preference for the ortho isomer at 100-200 K in the molecular beam is attributed to kinetic, not thermodynamic effects, caused by an entrance channel barrier that is ∼5 kJ mol(-1) lower for ortho than for other isomers. Based on these results, we predict that the reaction of H + phenol and H + toluene should still favour the ortho isomer under elevated temperature conditions in the early stages of combustion (200-400 °C).

Hydrogen-adduction to open-shell graphene fragments: spectroscopy, thermochemistry and astrochemistry

“…As with our previous studies on radicals, B3-LYP/6-311++G(3df,3pd) and G3X(MP2)-RAD indeed bracket the experimental result, with the highest-level W1X-2 IE of 7.49 eV close to the experimental value of 7.449(17) eV. The related W1X-1 method, when applied to the cyclohexadienyl radical, predicts an IE which is 20 meV too high, a similar discrepancy to the present case, 66–68 noting that the experimental IE could lie anywhere within the stated range.…”

Jet-Cooled Spectroscopy ofortho-Hydroxycyclohexadienyl Radicals

“…Intramolecular hydrogen migration from 1i to form the 1o radical was ruled out due to its high barrier. 22 Since none of the intermediates were directly observed in this study, however, this mechanism remains speculative. It does nonetheless well illustrate the importance of the o-chdOH radical in the chemistry of the benzene + OH/phenol + H system.…”

“…21 The dotted line emphasizes that the hydrogen migration pathway from 1i to 1o is energetically unfavorable. 22 The favored matrix mechanism suggested by Mardyukov is indicated by the blue arrows.…”

“…A recent paper by our group described how hydrogen atom addition to phenol is ortho directed. 22 An absorption spectrum taken of the first excited state revealed a complicated spectrum, containing numerous peaks over an extended range. An unexpectedly small spacing between the first two peaks of only 18 cm −1 led us to conclude that the spectrum contained more than a single compound.…”

Low-temperature reaction dynamics of paramagnetic species in the gas phase

“…This was attributed to the approximately 5 kJ mol −1 lower entrance channel barrier for the formation of ortho isomers when compared to meta or para isomers (and even lower when compared to the barrier to formation of ipso products). 84 …”