The atmospheric reaction between dimethyl sulfide and chlorine
atoms was studied theoretically at the UQCISD(T)/DZP//UMP2/DZP level of calculation. The molecular structure and
relative stability of several possible
adducts between these two species were investigated. We have
obtained four additional adducts bound through
the carbon and hydrogen atoms, besides the one already known, where the
intermolecular bond occurs between
the chlorine atom and the lone pair of the sulfur atom. These
complexes are very weakly bound, and only
one of them can lead to reaction. Four possible channels for the
reaction were investigated, and we have
found that the (CH3)2SCl adduct and the
products of hydrogen abstraction, CH3SCH2
and HCl, are the most
important ones. The reaction ΔG° values for these
two channels are negative, −5.63 −5.33 kcal/mol,
respectively, and the rate constants very large, because these
reactions proceed without energy barrier. However,
under atmospheric conditions, the estimate of the equilibrium constants
indicates that the first channel will
reach the equilibrium faster than the abstraction channel, and the
concentration of the (CH3)2SCl adduct
will
be very small. The formation of the CH3S and
CH3Cl products is considerably hindered. Despite
the fact
that this pathway is spontaneous (ΔG° = −12.13
kcal/mol), it has a high activation free energy barrier
(ΔG
⧧
= 31.45 kcal/mol), and the rate constant was estimated as 2.1 ×
10-30 cm3
molecule-1 s-1.
The channel that
leads to the CH3SCl and CH3 products is
conditional to the formation of the
(CH3)2SCl adduct. However,
its
high activation free energy (ΔG
⧧ = 29.25
kcal/mol) and instability in relation to reactants (ΔG°
= 9.23
kcal/mol) makes this pathway not feasible to the atmospheric fate of
the (CH3)2SCl adduct. The rate
constant
for this channel was evaluated to be 2.2 ×
10-9 cm3
molecule-1 s-1.
These results show that the principal
product of this reaction in the atmosphere will be
CH3SCH2 + HCl.
Doublet and quartet states of the HS radical correlating with H(2S)+S(3P,1D,1S) were investigated by ab initio calculations, at the CASSCF-MRCI/aug-cc-pV5Z level of theory. Molecular parameters and spectroscopic constants obtained for both the ground (X 2Π) and the first excited (A 2Σ+) states represent the best overall theoretical description of this system to date. Transition moments, transition probabilities, and radiative and predissociative lifetimes were also determined for the X 2Π–A 2Σ+ system. The values calculated for the radiative lifetimes of the A state show that previous results were too large. Theoretical predissociative lifetimes, although quite sensitive to the region of crossing of the potential energy curves, reproduce the experimental trends.
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