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It is well known that chlorine radicals (Cl ) are one of the leading chemical species
responsible for destroying the stratospheric ozone layer. 1,2 It is estimated that one chlorine
atom can remove about a million ozone molecules in the stratosphere. 2 In the upper
stratosphere, the primary source of Cl is the direct photolysis of the dimers of the ClOx
family (Cl 2 , ClO 2 , etc.). 3 While at lower altitudes, Cl are mainly present in the form of
their reservoirs like HOCl, HCl, ClONO2, etc., which release Cl by reacting with a
chemical activator. Among such reactions, the OH + HCl is one of the most important
reactions. Consequently, the thermodynamics and kinetics of this have been investigated
by several experimental groups, which provide rate constant values within a wide
temperature range from 138 K to beyond 1000 K. Besides the atmospheric importance,
this reaction involves only four atoms which makes the system a prototype model for
extensive theoretical studies.
The present work provides a benchmark calculation for the reaction barrier and energetics
of the reaction using a high-level ab initio composite protocol 4 , and using this corrected
reaction energetics, we have performed variational transition state theory (VTST) in
conjunction with small curvature tunneling (SCT) within a temperature range of 138-
1000 K. Afterwards, the effect of a water molecule on this reaction has been investigated
which showed that the rate constant in the presence of water is enhanced by 2-5 times
than that of the uncatalyzed reaction 5 . Some recent studies considered the surface of a
water droplet as a platform to perform a chemical reaction due to the unique properties of
the water surface 6,7 . Although the facts are known for long, slow progress in this field
(both experimental as well as theoretical) can be attributed to the lack of appropriate
tools. Although experimental studies became feasible after the advent of some modern
spectroscopic tools, like X-ray photoelectron spectroscopy, high-resolution mass
spectroscopy, etc. It is worth mentioning that, an experimental research group led by R.
N. Zare enriched this field with their significant contribution. Here we used Born-
Oppenheimer molecular dynamics (BOMD) simulation for studying the reaction
theoretically at the water surface 7 , which reveals that the reaction at the air-water
interface is ~10 times faster compared to the same in the gas phase. The post-reaction
dynamics of the key product Cl- suggest that, at equilibrium, the Cl- formed four
simultaneous hydrogen bonds with four interfacial water molecules, which stabilized the
system. |