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.