Ab initio and RRKM study of photodissociation of azulene cation

Abstract

The ab initio/Rice-Ramsperger-Kassel-Marcus (RRKM) approach has been applied to investigate the photodissociation mechanism of the azulene cation at different values of the photon energy. Reaction pathways leading to various decomposition products have been mapped out at the G3(MP2,CC)//B3LYP level and then the RRKM and microcanonical variational transition state theories have been applied to compute rate constants for individual reaction steps. Relative product yields (branching ratios) for the dissociation products have been calculated using the steady-state approach. The results show that a photoexcited azulene cation can readily isomerize to a naphthalene cation. The major dissociation channels are elimination of atomic hydrogen, an H2 molecule, and acetylene. The branching ratio of the H elimination channel decreases with an increase of the photon energy. The branching ratio of the acetylene elimination as well as that of the H2 elimination rise as the photon energy increases. The main C8H6+ fragment at all photon energies considered is a pentalene cation, and its yield decreases slightly with increasing excitation energy, whereas the branching ratios of the other C8H6+ fragments, phenylacetylene and benzocyclobutadiene cations, grow.

A complete map of the ion chemistry of the naphthalene radical cation? DFT and RRKM modeling of a complex potential energy surface

“…13 Schroeder and co-workers have provided some experimental evidence in favor of F2a. 15 The pentalene cation (F2c) has been reported as a possible product, but coming from the isomeric ion 11 (the azulene molecular ion), 13 an ion that should be taken SCHEME 1. Unimolecular decompositions of the naphthalene molecular ion 1.…”

“…13 Similarly, ion [C 10 H 6 ] +• (F3a in Scheme 1) could be generated by H 2 loss from 1, 12-14 with a critical energy of 4.72 eV, 12 15 (F2a) and phenylacetylene 11 (F2b) cations, generated by multistep isomerizations (1 → → 12, 1 → → 10) followed by dissociations (Scheme 1). 13 Schroeder and co-workers have provided some experimental evidence in favor of F2a. 15 The pentalene cation (F2c) has been reported as a possible product, but coming from the isomeric ion 11 (the azulene molecular ion), 13 an ion that should be taken SCHEME 1.…”

“…13 Dyakov and coworkers 13 have obtained an impressively complete collection of potential energy profiles for the photodissociations of the molecular ions of naphthalene and azulene, calculated at the G3(MP2,CC)//B3LYP level of theory. According to this modeling, a photoexcited azulene cation can isomerize to a naphthalene cation (Scheme 1 shows the minimum-energy reaction pathway from 1 to 11).…”

UV action spectroscopy of protonated PAH derivatives

“…The obtained activation energy is in relatively good agreement with the lowest dissociation energy of 4.41 eV ( Table 1). The dissociation rate gives a lifetime of ∼4-5 ms and is of the same order of magnitude as calculated by Dyakov et al (2006). The two factors in the Arrhenius expression cannot be determined exactly as it requires time spectra at more wavelengths over the absorption feature.…”

“…found to be 9.9 eV, whereas C 2 H 2 loss from a homoatomic ring is known to have a barrier of only 4.6 eV (Dyakov et al 2006). Figure 6 shows as example the decay of 4-MeQ after excitation at 230 nm.…”