Infrared Spectra of (Z)- and (E)-•CH2C(CH3)CHI Radicals Produced upon Photodissociation of (Z)- and (E)-(CH2I)(CH3)C═CHI in Solid para-Hydrogen.

Abstract

Ozonolysis of isoprene is important in atmospheric chemistry because of the abundant emission of isoprene. This process produces the Criegee intermediates CH2OO, methyl vinyl ketone oxide (MVKO, C2H3C(CH3)OO), and methacrolein oxide (MACRO, CH2C(CH3)CHOO). Gaseous MACRO was recently produced and identified in laboratories after photolysis of a mixture of 1,3-diiodo-2-methyl-prop-1-ene [(CH2I)(CH3)C═CHI] and O2, but the conformation-dependent formation mechanism remains unexplored. We report conformation-distinct IR spectra of (E)- and (Z)-(CH2I)(CH3)C═CHI isolated in solid p-H2. Upon irradiation near 300 nm of (E)- and (Z)-(CH2I)(CH3)C═CHI in solid p-H2 at 3.3 K, 3-iodo-2-methyl-prop-1-en-3-yl [•CH2C(CH3)CHI] radicals were characterized, with intense infrared absorption lines at 2991.3, 1458.7, 1434.7, 1317.4, 1190.4, 786.3, 677.9, and 467.2 cm-1 and additional 11 weaker ones assigned to (E)-•CH2C(CH3)CHI and intense lines at 3108.5, 3076.2, 3028.5, 2970.0, 1174.2, 796.0, 683.6, and 609.5 cm-1 and additional 7 weaker ones to (Z)-•CH2C(CH3)CHI. The assignments were derived according to the behaviors of secondary photolysis at 495 and 460 nm and a comparison of the vibrational wavenumbers and IR intensities of the observed lines with those calculated with the B2PLYP-D3/aug-cc-pVTZ-pp method. These observations confirm that only the allylic C-I bond, not the vinylic one, was photodissociated at 290 nm, and in solid p-H2, the excess energy upon photolysis induced no conformational change. When O2 was present in the matrix, several intense lines at 1147.5, 1025.7, 914.4, and 728.7 cm-1, and 4 weaker ones were tentatively assigned to the adduct CH2C(CH3)CHIOO; the assignments were supported by 18O2 isotopic experiments. Unlike in the gaseous phase, the remaining C-I bond of this adduct could not break to form MACRO because of the efficient quenching in a low-temperature matrix.