Photolysis of CH3CHO at 248 nm: Evidence of triple fragmentation from primary quantum yield of CH3 and HCO radicals and H atoms

Abstract

Radical quantum yields have been measured following the 248 nm photolysis of acetaldehyde, CH3CHO. HCO radical and H atom yields have been quantified by time resolved continuous wave Cavity Ring Down Spectroscopy in the near infrared following their conversion to HO2 radicals by reaction with O2. The CH3 radical yield has been determined using the same technique following their conversion into CH3O2. Absolute yields have been deduced for HCO radicals and H atoms through fitting of time resolved HO2 profiles, obtained under various O2 concentrations, to a complex model, while the CH3 yield has been determined relative to the CH3 yield from 248 nm photolysis of CH3I. Time resolved HO2 profiles under very low O2 concentrations suggest that another unknown HO2 forming reaction path exists in this reaction system besides the conversion of HCO radicals and H atoms by reaction with O2. HO2 profiles can be well reproduced under a large range of experimental conditions with the following quantum yields: CH3CHO + hν(248nm) → CH3CHO*, CH3CHO* → CH3 + HCO ϕ(1a) = 0.125 ± 0.03, CH3CHO* → CH3 + H + CO ϕ(1e) = 0.205 ± 0.04, CH3CHO*[Formula: see text]CH3CO + HO2 ϕ(1f) = 0.07 ± 0.01. The CH3O2 quantum yield has been determined in separate experiments as ϕ(CH₃) = 0.33 ± 0.03 and is in excellent agreement with the CH3 yields derived from the HO2 measurements considering that the triple fragmentation (R1e) is an important reaction path in the 248 nm photolysis of CH3CHO. From arithmetic considerations taking into account the HO2 and CH3 measurements we deduce a remaining quantum yield for the molecular pathway: CH3CHO* → CH4 + CO ϕ(1b) = 0.6. All experiments can be consistently explained with absence of the formerly considered pathway: CH3CHO* → CH3CO + H ϕ(1c) = 0.

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

“…Acetaldehyde and other aldehydes: Ion imaging and IR emission studies of the CO products formed when exciting CH3CHO at  > 300 nm imply pathways to forming CH4 + CO products involving both roaming H atoms and CH3 radicals. 106,107 Contributions from three different C-C bond fission mechanisms have also been identified as the photolysis wavelength is reduced across the range 328    265 nm, 108 while recent experiments (cavity ring down absorption measurements of product quantum yields 109 ) and theory (quasi-classical trajectory calculations 110 ) both provide evidence for some C-H bond fission, associated with triple fragmentation to H + CH3 + CO products, following excitation at  = 248 nm. H atom photoproducts have also been reported following excitation at  = 248 nm 111 and 205 nm.…”

Roaming as the dominant mechanism for molecular products in the photodissociation of large aliphatic aldehydes

“…When the excitation energy further increases, a triple fragmentation (alkyl+CO+H) might open to hinder roaming identification. 6,12,13 The branching fractions of roaming/TS pathway to molecular channel in formaldehyde and acetaldehyde at 308 nm were determined to be 10/90% and 84/16%, respectively, 1,2 while the roaming branching in propionaldehyde was reported to be even larger. 10 This work further demonstrates the roaming mechanism as the predominant route for photodissociation of larger size of aliphatic aldehydes.…”

Photodissociation dynamics of acetone studied by time-resolved ion imaging and photofragment excitation spectroscopy

“…46 Experimental evidence for triple fragmentation at 248 nm comes from time-resolved FTIR emission spectroscopy and also cavity ring-down spectroscopy measurements that indirectly determined radical product yields following conversion to HO2. 47,48 Curiously, both studies conclude that the radical channel IIa is inactive at 248 nm. This observation is in contrast to earlier estimates, 49 which suggest a quantum yield >10% at this wavelength.…”

“…We note that other groups have argued that acetyl products are not formed at the nearby photolysis wavelength of 248 nm. 47,48 The barrier on the T1 surface for formation of channel IIb products is ~4000 cm -1 higher than for channel IIa, 76 while unimolecular decomposition rates on S0 also strongly favor channel IIa. 44 For the short-wavelength S0 pathway, there are two steps that are likely rate limiting for product formation: the rate of passage over the S1 barrier, TS S 1 , leading to the conical intersection or the rate of dissociation to products once on the S0 surface, D S 0 .…”