posted on 2020-07-15, 21:44authored byJacopo Lupi, Cristina Puzzarini, Carlo Cavallotti, Vincenzo Barone
The atmospheric reaction
of H2S with Cl has been reinvestigated
to check if, as previously suggested, only explicit dynamical computations
can lead to an accurate evaluation of the reaction rate because of
strong recrossing effects and the breakdown of the variational extension
of transition state theory. For this reason, the corresponding potential
energy surface has been thoroughly investigated, thus leading to an
accurate characterization of all stationary points, whose energetics
has been computed at the state of the art. To this end, coupled-cluster
theory including up to quadruple excitations has been employed, together
with the extrapolation to the complete basis set limit and also incorporating
core–valence correlation, spin–orbit, and scalar relativistic
effects as well as diagonal Born–Oppenheimer corrections. This
highly accurate composite scheme has also been paralleled by less
expensive yet promising computational approaches. Moving to kinetics,
variational transition state theory and its variable reaction coordinate
extension for barrierless steps have been exploited, thus obtaining
a reaction rate constant (8.16 × 10–11 cm3 molecule–1 s–1 at 300
K and 1 atm) in remarkable agreement with the experimental counterpart.
Therefore, contrary to previous claims, there is no need to invoke
any failure of the transition state theory, provided that sufficiently
accurate quantum-chemical computations are performed. The investigation
of the puzzling case of the H2S + Cl system allowed us
to present a robust approach for disclosing the thermochemistry and
kinetics of reactions of atmospheric and astrophysical interest.