Formation reaction mechanism and infrared spectra of anti-trans-methacrolein oxide and its associated precursor and adduct radicals

Jia-Rong Cai,Jung-Hsuan Su,Yuan-Pern Lee

doi:10.1038/s42004-022-00644-0

Abstract

Methacrolein oxide (MACRO) is an important carbonyl oxide produced in ozonolysis of isoprene, the most abundantly-emitted non-methane hydrocarbon in the atmosphere. We employed a step-scan Fourier-transform infrared spectrometer to investigate the source reaction of MACRO in laboratories. Upon UV irradiation of precursor CH2IC(CH3)CHI (1), the CH2C(CH3)CHI radical (2) was detected, confirming the fission of the allylic C‒I bond rather than the vinylic C‒I bond. Upon UV irradiation of (1) and O2 near 21 Torr, anti-trans-MACRO (3a) was observed to have an intense OO-stretching band near 917 cm−1, much greater than those of syn-CH3CHOO and (CH3)2COO, supporting a stronger O‒O bond in MACRO because of resonance stabilization. At increased pressure (86‒346 Torr), both reaction adducts CH2C(CH3)CHIOO (4) and (CHI)C(CH3)CH2OO (5) radicals were observed, indicating that O2 can add to either carbon of the delocalized propenyl radical moiety of (2). The yield of MACRO is significantly smaller than other carbonyl oxides.

Highlights

Methacrolein oxide (MACRO) is an important carbonyl oxide produced in ozonolysis of isoprene, the most abundantly-emitted non-methane hydrocarbon in the atmosphere
We extended our focus to MACRO and investigated the UV photodissociation of the precursor CH2IC(CH3) CHIOO (4) and (CHI) (1) to observe the CH2C(CH3)CHI radical (2), confirming that only the allylic C–I bond was broken
Relative energies of conformers are listed in these figures and those of MACRO are compared with high-level calculations by Vansco et al.[15]

Summary

Introduction

Methacrolein oxide (MACRO) is an important carbonyl oxide produced in ozonolysis of isoprene, the most abundantly-emitted non-methane hydrocarbon in the atmosphere. Vansco et al reported a unique method to produce MACRO on photolysis at 248 nm of a gaseous mixture of 1,3-diiodo-2-methyl-prop1-ene [CH2IC(CH3)CHI] (1) and O2 that was pulsed into a quartz capillary reactor tube[15]. These authors assumed that photolysis of CH2IC(CH3)CHI (1) at 248 nm resulted in a preferential dissociation of the allylic, rather than the vinylic, C–I bond, to form the iodoalkenyl radical 3-iodo-2-methyl-prop-1en-3-yl [CH2C(CH3)CHI] (2). Subsequent addition of O2 with this resonance-stabilized radical (2) to form adduct 3-hydroperoxy-3-iodo-2-methyl-prop-1-ene CH2C(CH3)CHIOO (4) that readily breaks the remaining C–I bond to produce the carbonyl oxide MACRO (3). The mid-infrared spectrum of MACRO and other related intermediates will provide a clue to the conformation of these species and a detailed mechanism for the production of MACRO from UV photolysis of (1) in O2

Methods

Results

Conclusion