Direct Observation of Photodissociation Products from Phenylperoxyl Radicals Isolated in the Gas Phase

Abstract

Gas phase peroxyl radicals are central to our chemical understanding of combustion and atmospheric processes and are typically characterized by strong absorption in the UV (λ(max) ≈ 240 nm). The analogous maximum absorption feature for arylperoxyl radicals is predicted to shift to the visible but has not previously been characterized nor have any photoproducts arising from this transition been identified. Here we describe the controlled synthesis and isolation in vacuo of an array of charge-substituted phenylperoxyl radicals at room temperature, including the 4-(N,N,N-trimethylammonium)methyl phenylperoxyl radical cation (4-Me3N([+])CH2-C6H4OO(•)), using linear ion-trap mass spectrometry. Photodissociation mass spectra obtained at wavelengths ranging from 310 to 500 nm reveal two major photoproduct channels corresponding to homolysis of aryl-OO and arylO-O bonds resulting in loss of O2 and O, respectively. Combining the photodissociation yields across this spectral window produces a broad (FWHM ≈ 60 nm) but clearly resolved feature centered at λ(max) = 403 nm (3.08 eV). The influence of the charge-tag identity and its proximity to the radical site are investigated and demonstrate no effect on the identity of the two dominant photoproduct channels. Electronic structure calculations have located the vertical B ← X transition of these substituted phenylperoxyl radicals within the experimental uncertainty and further predict the analogous transition for unsubstituted phenylperoxyl radical (C6H5OO(•)) to be 457 nm (2.71 eV), nearly 45 nm shorter than previous estimates and in good agreement with recent computational values.

Tropospheric ozone and its precursors from the urban to the global scale from air quality to short-lived climate forcer

“…Kline and Miller, 2014), to date there has been almost no laboratory evidence of HO x formation following absorption by RO 2 in the near IR. However, Maccarone et al (2013) recently showed that arylperoxy radicals (RO 2 derived from aromatic hydrocarbons) are able to photodissociate in the visible spectrum to yield O( 3 P) and hence produce O 3 in the troposphere, without the need for NO x . As highlighted by the work of Lelieveld et al (2008) and Frost et al (1999), modelling experiments are increasingly being used to help direct laboratory studies in the search for radical propagating reactions.…”

Further studies into the photodissociation pathways of phenylperoxyl radicals

“…An imaginary level shift of 0.3 has been used in order to avoid the incorporation of intruder states. [24] Based on the ground state geometries, combined with CASPT2 energy calculations and CAS state interaction (CASSI) method, [25,26] we obtained the vertical excitation energies [7] and oscillator strengths (ƒ) for the four low-lying doublet states.…”

“…[6] The charge-substituted C 6 H 5 OO has been studied by utilizing linear ion-trap mass spectrometry to explore their photodissociation products of wave lengths ranging from 310 to 500 nm. [7] The results indicated that the photoproducts of dissociative ‫ܤ‬ ෨ ← ܺ ෨ state should remain dominated by O and O 2 loss. In order to interpret the experimental results, the experimental workers also carried out theoretical calculations using TDDFT computation C 6 H 5 OO radical performed in M06 functional to get the ‫ܤ‬ ෨ ← ܺ ෨ transition lies ca.…”

“…CASSCF a 1.412 1.294 1.422 1.432 1.401 1.384 1.431 1.428 115.6This work. b Calculated at B3LYP/aug-cc-pVTZ level taken from ref[8].Calculated at B3LYP/6-311+G(d) taken from ref[7].…”

Hydroxyl radical formation in the gas phase oxidation of distonic 2-methylphenyl radical cations

“…[24][25][26][27] A distonic radical ion approach has previously been employed to probe the fate of peroxyl radical intermediates; including, pyridinium-2-ethylperoxyl radical cation, 28,29 N-methylpyridinium-4-peroxyl 27,30 and 4-(N,N,N-trimethylammonium)phenylperoxyl radical cation, 27,31,32 in addition to 4-(N,N,N-trimethylammoniummethyl)phenylperoxyl, 3-(N,N,Ntrimethylammonium)phenylperoxyl, and 4-ammoniumphenylperoxyl radical cations. 32 In each case, these distonic studies provided insight into the reactivity of the neutral radical analogue. In this report we investigate the gas-phase synthesis and oxidation of distonic 2-methylphenyl radical cations.…”