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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 124; no. 28; pp. 5887 - 5895
• Main Authors: Haupa, Karolina Anna, Chen, Kuang-Po, Li, Yaw-Kuen, Lee, Yuan-Pern
• Format: Journal Article
• Language: English
• Published: American Chemical Society 16-07-2020
• Subjects: A: Spectroscopy, Molecular Structure, and Quantum Chemistry
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/acs.jpca.0c03987

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.