Photodissociation dynamics of the ortho- and para-xylyl radicals

Abstract

The photodissociation dynamics of the CH isomers ortho- and para-xylyl are investigated in a free jet. The xylyl radicals are generated by flash pyrolysis from 2-(2-methylphenyl)- and 2-(4-methylphenyl) ethyl nitrite and are excited into the D state. REMPI- spectra show vibronic structure and the origin of the transition is identified at 32 291 cm for the para- and at 32 132 cm for the ortho-isomer. Photofragment H-atom action spectra show bands at the same energy and thus confirm H-atom loss from xylyl radicals. To gain further insight into the photodissociation dynamics, velocity map images of the hydrogen atom photofragments are recorded. Their angular distribution is isotropic and the translational energy release is in agreement with a dissociation to products in their electronic ground state. Photodissociation of para-xylyl leads to the formation of para-xylylene (CH), while the data for ortho-xylyl agree much better with the isomer benzocyclobutene as the dominant molecular fragment rather than ortho-xylylene. In computations we identified a new pathway for the reaction ortho-xylyl → benzocyclobutene + H with a barrier of 3.39 eV (27 340 cm), which becomes accessible at the employed excitation energy. It proceeds via a combination of scissoring and rotational motion of the -CH and -CH groups. However, the observed rate constants measured by delaying the excitation and ionization laser with respect to each other are significantly faster than computed ones, indicating intrinsic non-RRKM behaviour. A comparably high value of around 30% of the excess energy is released as translation of the H-atom photofragment.

Photoinduced C–H bond fission in prototypical organic molecules and radicals

“…271,272 Ion imaging studies of the H atoms formed by photolysis of o-and p-xylyl radicals at wavelengths  ~310 nm and ~250 nm are similarly consistent with C-H bond fission after IC to the ground state PES. 273 These data all serve to illustrate the extent of isomerisation (ring opening and H atom transfer) that is required after accessing the ground state PES in order to sample the lowest energy fragmentation pathways. But these studies also return H atom formation rates that, whilst still slow, are considerably (one or more orders of magnitude) faster than predicted by RRKM calculations for the fully thermalized ground state radical at the relevant excitation energy.…”

“…But these studies also return H atom formation rates that, whilst still slow, are considerably (one or more orders of magnitude) faster than predicted by RRKM calculations for the fully thermalized ground state radical at the relevant excitation energy. 271,273 Might this be a hint that, even in these larger radicals and over these longer timescales, the decay of the 'hot' ground state species is influenced by the dynamical process(es) by which they are formed? In terms of energy disposal at least, this photophysical behaviour appears to extend to heteroaryl radicals also.…”

Probing different spin states in xylyl radicals and ions

“…Recently, the hydrogen loss dynamics of o-and p-xylyl were investigated, albeit details on the electronic structure remained unexplored. 16 In addition, the ground states of the xylyl cations have been probed by PE spectroscopy 48 and, more accurately, by threshold photoelectron (TPE) spectroscopy, 15 yielding ionization energies and vibrational frequencies. In order to understand the fast deactivation mechanisms in flames and reactors, a more profound picture of the electronic structure, including high lying excited states, has to be established if laser excitation is used to trigger chemical reactions.…”

Exploring pyrolysis and oxidation chemistry of o-xylene at various pressures with special concerns on PAH formation

“…Special attentions were paid on the decomposition reactions of o-xylyl radical. Based on available theoretical [3,[33][34][35] and experimental [36][37][38][39] progresses and analogy to benzyl reactions, detailed o-xylyl decomposition pathways were incorporated in the present model. The pathways include its decomposition to oxylylene and H, decomposition to fulvenallene and methyl, 2-methylfulvenallene and H, 1,3cyclopentadiene and methylcyclopentadienyl via ring-rearrangement reactions, decomposition to benzocyclobutene via ring-closure reaction, decomposition to benzyl via H-attack reaction and isomerization to m-xylyl or dimethylphenyl.…”

“…Rate constants of these reactions were referred to similar reactions of benzyl. Pachner et al [33] suggested that o-xylylene can isomerize to styrene or benzocyclobutene and the latter is more energetically favoured. Benzocyclobutene can also isomerize to styrene.…”