Sulfur Compounds and Their Chemistry for Venus and the Interstellar Medium David E Woon FA08 Overview of Talk Sulfur and Chlorine in Venus Atmospheric Chemistry Conditions and composition of Venuss atmosphere ID: 632270
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Slide1
Quantum Chemical Characterization of
Sulfur Compounds and Their Chemistry for Venus and the Interstellar MediumDavid E. Woon
FA08Slide2
Overview of Talk
Sulfur (and Chlorine) in Venus Atmospheric Chemistry
Conditions and composition of Venus’s atmosphere
Status of Venus atmospheric modeling
Computational Approach
Goals and strategy for properties and reactions
m
,
a
Properties
S + SH
Reactions
OH
+ HSCl
Reaction typesSlide3Slide4
“Brian Aldiss would like to point out that some early travel guides, such as those published by Ray Bradbury, overestimated the need for jungle survival gear.”
Conditions on the surface of Venus are far more extreme than any of the early SF writers imagined…Published in 1968, when Venera 4 returned first Venus atmospheric data.Slide5
Characteristics of
the Atmosphere of Venus
Temperature-Pressure profile:
P
TSlide6
Composition of
the Atmosphere of Venus
CO
2
96.5%
N
2
3.5
% + noble gases
O
2
HF
OCS
CO
HCl
H
2
O
SO2
SO
H
2
SO
4
observed species
NO
Krasnopolsky
2006
O
3
Montmessin
2011
ClO
Sandor 2013
OSSO
Frandzen
2016
S
3
Maiorov
2005Slide7
Composition of
the Atmosphere of Venus
H
2
S
S O Cl H
SH
<23 ppb
plausibly present species
OH
HOOH
HSOH
HSSH
HOO
HOS
HSO
HSS
SSO
CS
CS
2
Cl
2
OCCl
S
2
S
4
… S
n
?Slide8
Composition of
the Atmosphere of Venus
HSCl
SCl
2
SCl
HOCl
HSOCl
HOSCl
ClSSCl
SSCl
OSCl
SO
2
Cl
SO
2
Cl
2
HSO
2
SO
3
plausibly present species
Slide9
Models
for the Atmosphere of VenusSlide10
Models
for the Atmosphere of Venus
1. Much of the chemistry in the atmosphere of Venus is driven by photolysis of stable molecules into reactive radicals.
2. The vast majority of the reactions included in Venus modeling studies involve compounds containing S and/or Cl.Slide11
Models
for the Atmosphere of Venus: Status
PRIMARY:
The rate coefficients of many of the reactions included in the models have not been measured or treated with high level theory.
Viable pathways are missing.
Compounds that may plausibly be present are missing.
SECONDARY:
Spectroscopic characterization of many exotic compounds containing S and/or Cl is incomplete, making it difficult to identify them with probes or remote observations.Slide12
Computational Strategy
Methodology for Characterizing Reaction Surfaces
1. Survey with B3LYP/AVTZ (often problematic)
2. Refine as necessary with (full opts/
freqs
):
a. RCCSD(T)/AVTZ
b. MRCI+Q/AVTZ
check basis set convergence with single point AVQZ calcs
Methodology for Determining Properties
1. Optimize structure at RCCSD(T)/AVTZ level rotational constants2. Calculate m, a with finite field approach, {Fx,F
y,Fz} = 0.0013. Properties can be used to calculate an approximate upper rate limit for capture-dominated barrierless reactions.Slide13
Properties of Sulfur and Chlorine Compounds
O
2
CO
HCl
ClO
N
2
SH
OH
CS
SO
0.00
0.10
1.08
1.25
0.00
0.75
1.64
1.95
1.56
1.54
1.95
2.54
3.03
1.75
3.26
1.08
4.26
3.42
m
(D)
a
(Å
3
)
m
(D)
a
(Å
3
)
Cl
2
SCl
S
2
0.00
1.56
0.00
4.54
3.42
5.99
OCS
CS
2
CO
2
0.71
0.00
0.00
5.17
8.43
2.60
NEUTRALSSlide14
Properties of Sulfur and Chlorine Compounds
m
(D)
a
(Å
3
)
0.96
2.13
0.55
1.85
3.60
1.95
2.81
1.41
H
2
S
HO
2
O
3
H
2
O
1.58
3.96
HOS
2.44
3.84
HSO
1.50
3.33
HOCl
m
(D)
a
(Å
3
)
0.47
4.67
OCCl
1.66
3.81
SO
2
1.44
6.47
HS
2
1.50
6.73
S
2
O
1.16
5.51
S
3
1.54
5.95
OSCl
NEUTRALSSlide15
Properties of Sulfur and Chlorine Compounds
m
(D)
a
(Å
3
)
0.43
10.21
S
3
0.71
9.02
S
2
Cl
0.40
7.64
SCl2
1.58
4.21
HSOH
1.10
6.57
HSSH
2.53
4.42
HSO
2
NEUTRALSSlide16
Properties of Sulfur and Chlorine Compounds
m
(D)
a
(Å
3
)
1.37
6.66
HSOCl
1.68
6.29
HOSCl
0.00
4.27
SO
3
1.40
7.08
OSSO
0.00
7.89
OSSO
NEUTRALSSlide17
Properties of Sulfur and Chlorine Compounds
1.09
14.14
S
4
3.11
5.35
H
2
SO
4
0.88
11.39
ClSSCl
m
(D)
a
(Å
3)
NEUTRALSSlide18
1.26
1.95
0.52
3.00
2.20
2.30
0.00
4.24
Properties of Sulfur and Chlorine Compounds
m
(D)
a
(Å
3
)
SH
+
CS
+
SO
+
S
2
+
1.60
2.30
H
2
S
+
1.75
2.81
HOS
+
3.06
2.74
HSO
+
CATIONSSlide19
Post-Photolysis Reaction Types for Venus
Three types of reactions involving S and Cl compounds can occur in the atmosphere of Venus:
(1) Abstraction:
HX
+ HY
H2X + Y
(2) Addition of two radicals followed by elimination:
X + Y XY (intermediate) products
(3) Addition of radical to closed shell molecule followed by elimination: X + Z XZ (intermediate) products
This type occurs because S can form hypervalent species.Slide20
S + SH Reaction
S + SH
0.0
This is a typical radical-radical addition-elimination reaction with no barriers.
HSS
-69.0
H + S
2
-13.8
2
A”Slide21
HSCl
+ OH Reactions
Abstraction:
The abstraction reaction is very exothermic. A barrier is present but it is submerged
(just
barely) below reactants.
HSCl
+ OH
0.0
HSCl
—OH complex
-1.0
TS
-0.1
H
2
O +
SCl
-35.3Slide22
HSCl
+ OH Reactions
Addition-Elimination:
The alternative pathway is modestly exoergic, but the entrance channel barrier is even more submerged.
HSCl
+ OH
0.0
TS
-1.8
HSClOH
isomer 1
-21.6
HSClOH
isomer 2
-22.1
-17.2
HSOH + Cl
-5.8
TSSlide23
Acknowledgments
Funding source: NASA Grant NNX14AK32G from the Planetary Atmospheres program.