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

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 CH(2)IC(CH(3))CHI (1), the CH(2)C(CH(3))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 O(2) 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-CH(3)CHOO and (CH(3))(2)COO, supporting a stronger O‒O bond in MACRO because of resonance stabilization. At increased pressure (86‒346 Torr), both reaction adducts CH(2)C(CH(3))CHIOO (4) and (CHI)C(CH(3))CH(2)OO (5) radicals were observed, indicating that O(2) can add to either carbon of the delocalized propenyl radical moiety of (2). The yield of MACRO is significantly smaller than other carbonyl oxides.