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Author(s) |
Alwe, H. D.; Sharma, A.; Walavalkar, M. P.; Dhanya, S.; Naik, P. D. (RPCD)
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Source |
Journal of Physical Chemistry-A, 2014. Vol. 118 (36): pp. 7695-7706 |
ABSTRACT
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The
reactivities of the Cl atom with triple-bonded molecules were examined
by determining the rate coefficients of reactions of four triple-bonded
alcohols (TA), namely, 2-propyn-1-ol, 3-butyn-1-ol, 3-butyn-2-ol, and
2-methyl-3-butyn-2-ol, using the relative rate method, at 298 K. The
rate coefficients (k) of reaction of the four alcohols with Cl vary in
the range (3.5−4.3) × 10−10 cm3 molecule−1 s−1.
These values imply significant contribution of the Cl reaction in the
tropospheric degradation of TAs in the conditions of the marine boundary
layer. A striking difference is observed in the reactivity trend of Cl
from that of OH/O3. Although the reactivity of OH/O3
is lower with triplebonded molecules, as compared to the double-bonded
analogues, the reactivity of the Cl atom is similar for both. For a
deeper insight, the reactions of Cl and OH with the simplest TA,
2-propyn-1-ol, are investigated theoretically. Conventional transition
state theory is applied to compute the values of k, using the calculated
energies at QCISD and QCISD(T) levels of theory of the optimized
geometries of the reactants, transition states (TS), and the product
radicals of all the possible reaction pathways at the MP2/6-311++G(d,p)
level. The k values calculated at the QCISD level for Cl and the QCIS
(T) level for OH reactions are found to be very close to the
experimental values at 298 K. In the case of the Cl reaction, the
abstraction of α-H atoms as well as the addition at the terminal and
middle carbon atoms have submerged TS and the contribution of the
abstraction reaction is found to be significant at room temperature, at
all levels of calculations. Addition at the terminal carbon atom is
prominent compared to that at the middle carbon. In contrast to the Cl
reaction, only addition at the middle carbon is associated with such low
lying TS in the case of OH. The individual rate coefficients of
addition and abstraction of OH are lower than that of Cl. The negative
temperature dependence of the computed rate coefficients in the
temperature range 200−400 K shows that the difference in the TS energy
of Cl and OH affects the preexponential factor more than the activation
energy. |
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