Probing the CH3SH + N2O3 reaction
by automated microwave double
resonance spectroscopy
N O
2
3
CH SSCH
3
3
CH SNO
3
0
10
20
30
40
50
60
70
t /min
M. J. Nava,1 M. A. Martin-Drumel,2
S. Thorwirth,3 & M. C. McCarthy 2
2
1
Massachusetts Institute of Technology, Cambridge, MA, USA
Harvard-Smithsonian Center for Astrophysics and Harvard University, Cambridge, MA, USA
3
Universität zu Köln, Köln Germany
Two key questions from HSNO study
• Can the reaction H2 S + N2 O3 → HSNO + HONO
be generalized?
RSH + N2 O3 → RSNO + HONO
(1)
• What is the subsequent RSNO reactivity with RSH?
RSNO + RSH → RSSR + HNO
→ RSSH + RNO
(2)
(3)
This work: a study of the next thiol, R=CH3
• Can the reaction H2 S + N2 O3 → HSNO + HONO
be generalized?
CH3 SH + N2 O3 → CH3 SNO + HONO
(1)
• What is the subsequent RSNO reactivity with RSH?
CH3 SNO + CH3 SH → CH3 SSCH3 + HNO
→ CH3 SSH + CH3 NO
(2)
(3)
This work: a study of the next thiol, R=CH3
• Can the reaction H2 S + N2 O3 → HSNO + HONO
be generalized?
CH3 SH + N2 O3 → CH3 SNO + HONO
(1)
• What is the subsequent RSNO reactivity with RSH?
CH3 SNO + CH3 SH → CH3 SSCH3 + HNO
→ CH3 SSH + CH3 NO
AMDOR spectroscopy + kinetic measurements
(2)
(3)
AMDOR spectroscopy of the
CH3SH + NO mixture
1D spectrum
2D spectrum
10
b
8
b
b
b
11
10000
b
b
b
7
b
b
12
100
6
1
b
14
b
b
12
14
16
18
Identify
unknown lines
GHz
18
b
15
17
16
Establish
linkages
0.1
0.01
b
b
10
10
b
b
8
1000
b
bbbbbb
b
13
Analysis
9
0
1000
2000
3000
4000
GHz
Determine
rotational constants
Step 1: Record CP spectrum
8
• CH3 SH + NO mixture
• 1000 shots (∼ 3 min)
• Standard nozzle
configuration
10
12
14
16
Known transitions:
CH3 SH, CH3 SSCH3
∼ 100 new lines
18
GHz
Step 2: Extensive DR tests
9
10
b
11
8
b
b
b
b
b
b
7
b
b
12
b
bbbbbb
6
b
13
b
b
b
14
b
b
b
b
b
15
∼8h
38 DR matches
18
16
17
GHz
4 series, 2 distinct networks
not possible to use existing
python scripts because CH3 SNO
too light
CH3SNO: 3 conformers predicted to be
low-lying and stable
anti
syn II
syn I
1.03 kcal/mol
0.36 kcal/mol
0
from Ruano, Chem.Phys. Lett. (2012)
In combination with theory, two conformers of
CH3SNO identified
b
b
b
b
7
b
b
syn II
A0
B0
C0
11 291.812
5382.575
3726.861
11 438.301
5437.622
3683.954
1.013
1.010
0.988
anti c
A0
B0
C0
19 177.202
3783.713
3219.940
19 133.110
3776.960
3214.384
0.998
0.998
0.998
b
bbbbbb
6
b
13
Exp.
δb
8
b
b
b
12
A0
B0
C0
9
10
11
syn I
Calc.a
11 387.393
5129.127
3613.459
b
b
b
14
18
b
b
b
b
b
15
16
17
GHz
a
c
CCSD(T)/pwCVQZ
b Scaling factor
Assigned on a deep integration spectrum
Some of the stronger lines remain unassigned
Deep integration spectrum
(135,000 shots)
u-lines
CH SNO
3
syn
linked
II
CH SNO
3
8
10
12
14
anti
16
18
GHz
Additional DR measurements are needed to link
remaining lines
8
10
12
14
16
18
GHz
CH3SNO: No definitive evidence for the syn I
conformer
anti
syn II
syn I
1.03 kcal/mol
0.36 kcal/mol
0
from Ruano, Chem.Phys. Lett. (2012)
Extensive isotopic spectroscopy for syn II
34
15
18
13
34
S in natural
abundance
(analyzed)
15
N from
15
NO
(measured)
18
O from
H18
2 O
(measured)
13
C in natural
abundance
(feasible in
cavity)
Time evolution of reactants and products:
Answers and new questions
25
N O
2
3
Signal /a.u.
20
15
CH SSCH
3
3
10
u-line
5
syn-CH SNO
anti-CH SNO
3
3
0
0
10
20
30
40
50
60
Time after CH SH injection /min
3
70
80
An incomplete explanation of kinetic results
CH3 SH + N2 O3 → CH3 SNO + HONO
(1)
CH3 SNO + CH3 SH → CH3 SSCH3 + HNO
→ CH3 SSH + CH3 NO
(2)
(3)
• rate of Eq. (1) slow
• rate of Eq. (2) faster than (1)
• Eq. (3), no evidence on timescale of experiment
Open questions
Identity of remaining u-lines?
• exhibit identical behavior as two
CH3 SNO isomers
25
N O
2
3
Signal /a.u.
20
15
CH SSCH
3
3
10
u-line
5
syn-CH SNO
anti-CH SNO
3
3
0
0
10
20
30
40
50
60
Time after CH SH injection /min
3
70
80
Is CH3 SSCH3 a product or an
intermediate of reaction?!
• observed before CH3 SNO
• signal decreases when CH3 SNO
increases
CH3SSCH3 reactivity with CH3SH
CH SSCH
1.0
3
3
CH SH
3
Normalized signal
0.8
0.6
0.4
0.2
0.0
5
10
15
20
Flow of CH SH /sccm
3
25
30
Summary & Conclusions
• AMDOR spectroscopy has enabled rapid identification of
two CH3 SNO isomers
• Automatic assignment of spectral features can likely be
made more efficient using targeted DR tests and more
robust algorithms
New insight into CH3 SNO formation and reactivity:
• Like HSNO, CH3 SNO is readily formed, but appears more
stable; other thiols might be studied by similar means
• Evidence for HNO remains elusive
Acknowledgements
Prof. K. N. Crabtree (UC Davis)
Prof. C. C. Cummings (MIT)
Prof. J. F. Stanton (Texas → Florida)
Dr. T. L. Nguyen (Texas → Florida)
NSF: CHE-1362118
DFG: TH 1301/3-2; SFB 956