Infrared Spectra of (Z)- and (E)‑^•C₂H₃C(CH₃)I Radicals Produced upon Photodissociation of (Z)- and (E)‑(CH₂I)HCC(CH₃)I in Solid para-Hydrogen

Ozonolysis of isoprene to produce Criegee intermediates such as methyl vinyl ketone oxide (MVKO), C₂H₃C­(CH₃)­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, (CH₂I)­HCC­(CH₃)­I, and O₂, but the mechanism of its formation remains unexplored. We synthesized pure (Z)- and (E)-1,3-diiodo-but-2-ene and measured their distinct IR spectra. Upon irradiation at 280 nm of (Z)- and (E)-1,3-diiodo-but-2-ene in solid p-H₂ at 3.3 K, the fission of the terminal CI bond yields (Z)- and (E)-3-iodo-but-2-en-1-yl [^•C₂H₃C­(CH₃)­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^–1, and 16 additional weaker lines for (Z)-^•C₂H₃C­(CH₃)I and 1405.2, 1208.2, 1106.0/1103.9, 934.2/933.4, and 785.1/784.9 cm^–1 and five additional weaker ones for (E)-^•C₂H₃C­(CH₃)­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 p-H₂, the excess energy after photodissociation induced no change in conformation. These new spectra of ^•C₂H₃C­(CH₃)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 (CH₂I)­HCC­(CH₃)I in O₂ in the laboratory.