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)HCC(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 CI 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)HCC(CH3)I in O2 in the laboratory.