Infrared Spectra of ()- and ()-CHC(CH)I Radicals Produced upon Photodissociation of ()- and ()-(CHI)HC═C(CH)I in Solid Hydrogen.

Ozonolysis of isoprene to produce Criegee intermediates such as methyl vinyl ketone oxide (MVKO), CHC(CH)OO, is an important process in atmospheric chemistry. MVKO was recently produced and identified in laboratories after photolysis of a gaseous mixture of 1,3-diiodo-but-2-ene, (CHI)HC═C(CH)I, and O, but the mechanism of its formation remains unexplored. We synthesized pure ()- and ()-1,3-diiodo-but-2-ene and measured their distinct IR spectra. Upon irradiation at 280 nm of ()- and ()-1,3-diiodo-but-2-ene in solid -H at 3.3 K, the fission of the terminal C-I bond yields ()- and ()-3-iodo-but-2-en-1-yl [CHC(CH)I] radicals, respectively. These radicals were characterized with infrared absorption lines at 2962.4, 1423.8, 1265.3, 1120.9/1127.0, 921.4/922.3, and 792.5/791.7 cm, and 16 additional weaker lines for ()-CHC(CH)I and 1405.2, 1208.2, 1106.0/1103.9, 934.2/933.4, and 785.1/784.9 cm and five additional weaker ones for ()-CHC(CH)I. The assignments were derived according to behavior on secondary photolysis and comparison of the vibrational wavenumbers and the IR intensities of observed lines with those calculated with the B2PLYP-D3/aug-cc-pVTZ-pp method. These observations confirmed that only the terminal I atom, not the central one, was photodissociated at 280 nm and, in solid -H, the excess energy after photodissociation induced no change in conformation. These new spectra of CHC(CH)I radicals can provide valuable information for the understanding of the mechanism of formation of Criegee intermediate MVKO from the source reaction of photolysis of (CHI)HC═C(CH)I in O in the laboratory.