1. Quantum Chemical Characterization of Sulfur Compounds and Their Chemistry for Venus and the Interstellar Medium
David E. Woon
FA08

2. 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 types

4. Published in 1968, when Venera 4 returned first Venus atmospheric data.
“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…

5. P T Characteristics of the Atmosphere of Venus
Temperature-Pressure profile: P T

6. Composition of the Atmosphere of Venus
 observed species 
CO2 96.5%
N2 3.5% noble gases O2 HF
OCS CO
HCl
H2O
SO2 SO
H2SO4 NO
Krasnopolsky 2006 O3
Montmessin 2011
ClO
Sandor 2013
OSSO
Frandzen 2016 S3
Maiorov 2005

7.  plausibly present species 
Composition of the Atmosphere of Venus
 plausibly present species 
H2S SH OH
S O Cl H
<23 ppb
HOOH
HSOH
HSSH
HOO
HOS
HSO
HSS S2 S4
… Sn?
SSO CS
CS2
Cl2
OCCl

8.  plausibly present species 
Composition of the Atmosphere of Venus
 plausibly present species 
HSCl
SCl2
SCl
HOCl
HSOCl
HOSCl
ClSSCl
SSCl
OSCl
SO2Cl
SO2Cl2
HSO2
SO3

9. Models for the Atmosphere of Venus

10. 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.

11. 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.

12. Computational Strategy
Methodology for Determining Properties
1. Optimize structure at RCCSD(T)/AVTZ level  rotational constants
2. Calculate m, a with finite field approach, {Fx,Fy,Fz} = 0.001
3. Properties can be used to calculate an approximate upper rate limit for capture-dominated barrierless reactions.
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

13. Properties of Sulfur and Chlorine Compounds
NEUTRALS
m (D)
a (Å3)
m (D)
a (Å3) OH
1.64
1.08 S2
0.00
5.99
HCl
1.08
2.54
Cl2
0.00
4.54 N2
0.00
1.75
SCl
1.56
3.42 CO
0.10
1.95
CO2
0.00
2.60 O2
0.00
1.54
OCS
0.71
5.17 SH
0.75
3.26
CS2
0.00
8.43 CS
1.95
4.26 SO
1.56
3.42
ClO
1.25
3.03

14. Properties of Sulfur and Chlorine Compounds
NEUTRALS
m (D)
a (Å3)
m (D)
a (Å3)
H2O
1.85
1.41
OCCl
0.47
4.67
HO2
2.13
1.95
SO2
1.66
3.81 O3
0.55
2.81
HS2
1.44
6.47
H2S
0.96
3.60 S3
1.16
5.51
HOS
1.58
3.96
S2O
1.50
6.73
HSO
2.44
3.84
OSCl
1.54
5.95
HOCl
1.50
3.33

15. Properties of Sulfur and Chlorine Compounds
NEUTRALS
m (D)
a (Å3) S3
0.43
10.21
S2Cl
0.71
9.02
SCl2
0.40
7.64
HSOH
1.58
4.21
HSSH
1.10
6.57
HSO2
2.53
4.42

16. Properties of Sulfur and Chlorine Compounds
NEUTRALS
m (D)
a (Å3)
HSOCl
1.37
6.66
HOSCl
1.68
6.29
SO3
0.00
4.27
OSSO
1.40
7.08
OSSO
0.00
7.89

17. Properties of Sulfur and Chlorine Compounds
NEUTRALS
m (D)
a (Å3) S4
1.09
14.14
ClSSCl
0.88
11.39
H2SO4
3.11
5.35

18. Properties of Sulfur and Chlorine Compounds
CATIONS
m (D)
a (Å3)
SH+
1.26
1.95
CS+
0.52
3.00
SO+
2.20
2.30
S2+
0.00
4.24
H2S+
1.60
2.30
HOS+
1.75
2.81
HSO+
3.06
2.74

19. 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.

20. S + SH Reaction S + SH 0.0 H + S2 -13.8
This is a typical radical-radical addition-elimination reaction with no barriers.
HSS
-69.0
2A”

21. HSCl + OH Reactions Abstraction:
0.0
HSCl—OH complex
-1.0 TS
-0.1
H2O + SCl
-35.3
The abstraction reaction is very exothermic. A barrier is present but it is submerged (just barely) below reactants.

22. HSCl + OH Reactions Addition-Elimination:TS
HSOH + Cl
0.0
-1.8
-5.8 TS
-21.6
-22.1
-17.2
HSClOH
isomer 1
HSClOH
isomer 2
The alternative pathway is modestly exoergic, but the entrance channel barrier is even more submerged.

23. Acknowledgments
Funding source: NASA Grant NNX14AK32G from the Planetary Atmospheres program.