Infrared Spectra of ( Z ) - and ( E )-•CH 2 C(CH 3 )CHI Radicals Produced upon Photodissociation of ( Z ) - and ( E ) - (CH 2 I)(CH 3 )C═CHI in Solid para- Hydrogen.

Ozonolysis of isoprene is important in atmospheric chemistry because of the abundant emission of isoprene. This process produces the Criegee intermediates CH 2 OO, methyl vinyl ketone oxide (MVKO, C 2 H 3 C(CH 3 )OO), and methacrolein oxide (MACRO, CH 2 C(CH 3 )CHOO). Gaseous MACRO was recently produced and identified in laboratories after photolysis of a mixture of 1,3-diiodo-2-methyl-prop-1-ene [(CH 2 I)(CH 3 )C═CHI] and O 2 , but the conformation-dependent formation mechanism remains unexplored. We report conformation-distinct IR spectra of ( E )- and ( Z )-(CH 2 I)(CH 3 )C═CHI isolated in solid p -H 2 . Upon irradiation near 300 nm of ( E )- and ( Z )-(CH 2 I)(CH 3 )C═CHI in solid p -H 2 at 3.3 K, 3-iodo-2-methyl-prop-1-en-3-yl [•CH 2 C(CH 3 )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)- •CH 2 C(CH 3 )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 ) - •CH 2 C(CH 3 )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 -H 2 , the excess energy upon photolysis induced no conformational change. When O 2 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 CH 2 C(CH 3 )CHIOO; the assignments were supported by 18 O 2 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.