1 research outputs found
Reaction of OH with Aliphatic and Aromatic Isocyanates
Isocyanates are highly relevant industrial intermediates
for polyurethane
production. In this work, we used quantum chemistry and transition
state theory (TST) to investigate the gas-phase reaction of isocyanates
with the OH radical, which is likely one of the most significant chemical
sinks for these compounds in the troposphere. para-Tolyl-isocyanate (p-tolyl-NCO) was chosen as a proxy substance for
the large-volume aromatic diisocyanate species toluene diisocyanate
and methylene diphenyl diisocyanate. Besides p-tolyl-NCO + OH, the
model reactions CH3NCO + OH, H2CCHNCO
+ OH, C6H5-NCO + OH, C6H5-CH3 + OH, and C6H6 + OH have been
studied as well to analyze various substituent effects and to allow
for comparison with literature. Quantum chemical computations at the
CCSD(T)/cc-pV(T,Q → ∞)Z//M06-2X/def2-TZVP level were
used as the basis for tunneling-corrected canonical TST calculations.
For CH3NCO + OH, H abstraction by OH at the methyl group
is the main reaction channel according to our calculations and predicted
to be four orders of magnitude faster than OH addition at the NCO
group. The calculated rate coefficient (8.8 × 10–14 cm3 molecule–1 s–1) at 298 K is in good agreement with experimental data from the literature.
Likewise, for aromatic isocyanates, OH attack at the isocyanate group
was found to be only a minor pathway compared to addition to the aromatic
ring. In the OH + p-tolyl-NCO reaction, OH addition at the ortho-position relative to the NCO group has been identified
as the main initial reaction channel (branching fraction: 53.2%),
with smaller but significant branching fractions for the H abstraction
at the methyl group (9.6%) as well as the other ring addition reactions
(ipso: 2.3%, meta: 24.5%, para: 10.5%, all relative to the NCO group).
By comparing OH addition to the aromatic ring in p-tolyl-NCO with
the respective ring addition reactions of phenyl isocyanate and toluene,
the site-selective reactivity trends observed for ring addition in
the OH + p-tolyl-NCO could be rationalized by a dominating positive
mesomeric effect of the NCO group and a positive electron-donating
(inductive) effect of the CH3 group. Except for the OH
ring adduct formed from OH addition in ipso-position to the NCO group,
we estimate the first-generation radical intermediates in the OH +
p-tolyl-NCO reaction to have sufficiently long lifetimes to react
with O2 under atmospheric conditions and undergo typical
oxidative reaction cascades like those known for benzene or toluene