Ab Initio and RRKM/Master Equation Analysis of the Photolysis and Thermal Unimolecular Decomposition of Bromoacetaldehyde

Abstract

Bromoacetaldehyde (BrCH 2 CHO) is a major stable brominated organic intermediate of the bromine−ethylene addition reaction during the arctic bromine explosion events. Similar to acetaldehyde, which has been recently identified as a source of organic acids in the troposphere, it may be subjected to photo-tautomerization initially forming brominated vinyl compounds. In this study, we investigate the unimolecular reactions of BrCH 2 CHO under both photolytic and thermal conditions using high-level quantum chemical calculations and Rice−Ramsperger−Kassel−Marcus (RRKM)/master equation analysis. The unimolecular decomposition of BrCH 2 CHO takes place through 14 dissociation and isomerization channels along a potential energy surface involving eight wells. Under the assumption of singlet ground-state potential energy surfacedominated photodynamics, the primary photodissociation yields of BrCH 2 CHO are investigated under both collision-free and collision energy transfer conditions. At atmospheric pressure and under tropospheric actinic flux conditions at ground level, depending on the assumed collisional energy transfer parameter, 150 cm −1 < ⟨ΔE down ⟩ < 450 cm −1 , 78−33% of BrCH 2 CHO undergoes direct photodissociation instead of collisional deactivation at an excitation wavelength of 320 nm. This is significantly higher than the 14% reported for acetaldehyde, hence indicating a strong effect of bromine substitution on the product photolysis yield that is related to additional favorable Br and HBr forming dissociation channels. In contrast to the overall photodissociation quantum yield, the relative branching fractions of the photodissociation products are less dependent on the collisional energy transfer parameter. For a representative value of ⟨ΔE down ⟩ = 300 cm −1 and an excitation wavelength of 320 nm, with 27% for C−C bond fission, 11% for C−Br bond fission, 7% for HBr elimination, and only below 2% each for a consecutive O−Br fission reaction and the photo-tautomerization channel yielding brominated vinyl alcohol, the photodissociation is markedly different from the acetaldehyde case. Finally, as brominated halogenated compounds are of interest for flame inhibition purposes, thermal multichannel unimolecular rate constants were calculated for temperatures in the range from 500 to 2000 K. At a temperature of 2000 K and ambient pressure, the two main reaction channels are the C−Br and C−C bond fissions, contributing 35 and 43% to the total reaction flux, respectively.

UV photolysis of oxalyl chloride: ClCO radical decomposition and direct Cl2${\rm Cl}_2 {\rm }$ formation pathways

“…By limiting ourselves to the ground‐state potential, we implicitly assume that a rapid relaxation process of electronically excited $${\rm {(ClCO)}}_2$$ takes place after photoexcitation, with the $${\rm T}_1$$ and $${\rm S}_1$$ states lying about 294 and 331 kJ/mol above the $${\rm S}_0$$ ground state 43 . Although this approach has already been used successfully many times, 44–46 it is clear that excited‐state photodynamics can become very important, especially at higher excitation energies. Thus, the following discussion offers only basic orientation on possible reaction channels and branching fractions.…”

Virtual Issue on Photodissociation: From Fundamental Dynamics and Spectroscopy to Photochemistry in Planetary Atmospheres and in Space

“…Computational simulations of the dynamics must first describe the energetically accessible electronic states by mapping potential energy surfaces, conical intersections, and regions of intersystem crossing , and then must propagate the nuclear dynamics of the molecules as their structures evolve over these excited-state surfaces, using either classical or quantum mechanical descriptions of the nuclear motions. , Simulations of this type can be used to unravel the electronic states responsible for bond breaking, to characterize nonadiabatic dynamics as molecules switch between different electronic states, and to reveal unusual mechanisms such as the “roaming” dynamics of molecules that explore long-range, weakly bound regions of their potential energy surfaces. More computationally efficient methods based on master equation treatment of reaction kinetics are also finding applications in modeling photodissociation, while statistical theories offer computationally low-cost methods to predict the outcomes of photochemical reactions that occur from long-lived intermediates, which include energized ground-electronic-state molecules.…”

“…25,26 Simulations of this type can be used to unravel the electronic states responsible for bond breaking, to characterize nonadiabatic dynamics as molecules switch between different electronic states, and to reveal unusual mechanisms such as the "roaming" dynamics of molecules that explore long-range, weakly bound regions of their potential energy surfaces. More computationally efficient methods based on master equation treatment of reaction kinetics are also finding applications in modeling photodissociation, 27 while statistical theories offer computationally low-cost methods to predict the outcomes of photochemical reactions that occur from long-lived intermediates, which include energized ground-electronic-state molecules.…”