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Ion pair particles at the air-water interface
#MMPMID29109293
Kumar M
; Francisco JS
Proc Natl Acad Sci U S A
2017[Nov]; 114
(47
): 12401-12406
PMID29109293
show ga
Although the role of methanesulfonic acid (HMSA) in particle formation in the gas
phase has been extensively studied, the details of the HMSA-induced ion pair
particle formation at the air-water interface are yet to be examined. In this
work, we have performed Born-Oppenheimer molecular dynamics simulations and
density functional theory calculations to investigate the ion pair particle
formation from HMSA and (R(1))(R(2))NH (for NH(3), R(1) = R(2) = H; for
CH(3)NH(2), R(1) = H and R(2) = CH(3); and for CH(3)NH(2), R(1) = R(2) = CH(3))
at the air-water interface. The results show that, at the air-water interface,
HMSA deprotonates within a few picoseconds and results in the formation of
methanesulfonate ion (MSA(-))??H(3)O(+) ion pair. However, this ion pair
decomposes immediately, explaining why HMSA and water alone are not sufficient
for forming stable particles in atmosphere. Interestingly, the particle formation
from the gas-phase hydrogen-bonded complexes of HMSA with (R(1))(R(2))NH on the
water droplet is observed with a few femtoseconds, suggesting a mechanism for the
gas to particle conversion in aqueous environments. The reaction involves a
direct proton transfer between HMSA and (R(1))(R(2))NH, and the resulting
MSA(-)??(R(1))(R(2))NH(2)(+) complex is bound by one to four interfacial water
molecules. The mechanistic insights gained from this study may serve as useful
leads for understanding about the ion pair particle formation from other
precursors in forested and polluted urban environments.
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