1. Facile Formation of Acetic Sulfuric Anhydride in a Supersonic Jet
Characterization by Microwave Spectroscopy and Computational Chemistry
Anna Huff, Rebecca Mackenzie, CJ Smith, Ken Leopold
Department of Chemistry, University of Minnesota
June 19, 2017

2. Gas Phase Hydrolysis of SO3
Formation of atmospheric sulfuric acid
~ 20 kcal/mol
I’m going to begin with talking about where the motivation for this work came from, which starts with considering the production of atmospheric sulfuric acid. As we know, sulfuric acid is an important molecule in aerosol nucleation. a computational study by Hazra and Sinha who were investigating SO3 hydrolysis
This information will be familiar if you were here for CJ’s talk before me, but for those of you who just came in…
Hydrolysis of SO3 through this trimeric intermediate was established as the primary route to form H2SO4
SO3-H2O-H2O ~6 kcal/mol, more stable than SO3-H2O than by at least 20 kcal/mol
Morokuma, K.; Muguruma, C. J. Am. Chem. Soc. 1994, 116, (Adapted by Hazra, M.K; Sinha, A.)
Kolb, C.E. et al. J. Am. Chem. Soc. 1994, 116,

3. Formic-acid catalyzed hydrolysis of SO3
And what they found was compared to the pathway from SO3-H2O-H2O that substituting a formic acid molecule in place of the second water molecule resulted in a nearly barrierless reaction to form SO3. This pathway with formic acid is over 6 kcal/mol lower than with SO3 and two waters. With this finding they suggested that involving formic acid rather than a second water molecule could be a competitive pathway to form atmospheric sulfuric acid
SO3-H2O-H2O ~6 kcal/mol, more stable than SO3-H2O than by at least 20 kcal/mol
Hazra, M.K; Sinha, A. J. Am. Chem. Soc. 2011, 133,

4. Discovering Formic Sulfuric Anhydride (FSA)
Looking for complexes containing HCOOH, SO3, H2O
0.2 kcal/mol
SO3···H2O···HCOOH
HCOOH···SO3
H2SO4···HCOOH
The idea for this project started from work that Becca Mackenzie, a former graduate student in our lab, started a couple years ago when she was looking for complexes pertaining to formic acid, sulfur trioxide, and water. Specifically, we were anticipating complexes like formic-SO3, formic sulfuric, or formic-so3-h2o. But instead of observing these complexes she discovered this new molecule that formed readily in our setup, formic sulfuric anhydride or FSA. Computational work rationalized the facile formation of this molecule, that starting with the formic acid SO3 complex FSA is produced going through a pi2+pi2+sigma2 cycloaddition where the hydrogen is transferred and a sulfur-oxygen bond is formed
*To the best of our knowledge, this was a new molecule and a reaction that was not already reported in the literature
This was an interesting and exciting discovery, and we had also speculated possible effects it could have in the atmosphere if it did form there…
CCSD(T)/CBS//M06-2X/ G(3df,3pd), with ZPE
Mackenzie, R.B.; Dewberry, C.T.; Leopold, K.R. Science. 2015, 349,

5. Potential atmospheric impacts
Cluster or droplet R
HCOOH
RCOOH + R R
H2SO4
Incorporation of volatile organics into atmospheric aerosol
This type of molecule is relatively unknown in the literature, and since it forms in the gas phase from molecules that can be readily found in the atmosphere it’s interesting to consider the possible impacts it could have in atmospheric chemistry if FSA does form in the atmosphere
Since this is an acid anhydride, the next realistic step is for it to be hydrolyzed
There’s been a lot of work going into understanding how volatile organics are incorporated into atmospheric aerosol, and this simple scheme shows one possible route in which that could occur
*Assemble a body of evidence that this reaction is general
What happens if we use something besides formic acid as our carboxylic acid. Becca had done computational work with benzoic and pinic acid which showed that their reaction to form an analogous carboxylic sulfuric anhydride was barrierless. So from here, we wanted to experimentally establish a series of these carboxylic sulfuric anhydrides to show the generality of this reaction
First, we can imagine that if FSA forms in the atmosphere that it could be hydrolyzed to form sulfuric acid and return formic acid.
Production of H2SO4 in the atmosphere
Establish a series of carboxylic sulfuric anhydrides

6. Tandem cavity and chirped-pulse FTMW spectrometer
Narrowband, ~1 MHz increments
2-8 kHz resolution and high sensitivity
Basic setup for the arrangement of sample – pulsed nozzle into large vacuum chamber
Supersonic expansion promotes ‘on the fly’ reactions at cold temperatures (estimated 3-5 K), population of low rotational levels – example SO3-H2O
Cavity 4-8 kHz resolution vs ~50-90 kHz chirp resolution
Dual capabilities of narrow and broadband spectroscopy
Cavity is the more traditional setup, high resolution but can only scan in 3 MHz spectral windows
Chirped-pulse uses microwave horns to transmit and receive frequencies rather than standing wave between mirrors
Lower resolution, but can now collect 3 GHz windows in 15 minutes
Broadband, 3 GHz spectral windows
>30 kHz resolution and lower sensitivity

7. Pulsed nozzle source with on-the-fly mixing
Pulse Line
2.3 atm Ar
SO3
Continuous Flowline
0.67 atm Ar
Formation of complexes, or in this case hopefully ‘new’ monomers
RCOOH

8. Chirping for another carboxylic sulfuric anhydride
CF3COOH + SO3 → trifluoroacetic sulfuric anhydride? (TFASA)
CF3COOH
TFA
TFA-H2O
Background
6 GHz
18 GHz
We were going to start with acetic acid, but figured the fluorination of the make this easier for us to avoid internal rotation
No evidence for the formation of TFASA
FSA had formed readily resulting in strong signals in even the chirp spectra
No regular rotational patterns to fit to TFA-SO3 species
At this point, make sure that our operating conditions were still suitable to see what was seen before
And we could still see FSA blazing in all of its glory!
No chirp observation of TFASA
(But TFASA later observed in cavity)

9. (Still) Chirping for another carboxylic sulfuric anhydride
CH3COOH + SO3 → acetic sulfuric anhydride? (ASA)
3←2
7807 [MHz]
7762
4←3
303←202
5←4
6←5
322←221
321←220
6 GHz
18 GHz
That day accomplished a rough fit of spectra to the acetic sulfuric anhydride (ASA) prediction
Computational comparison of TFA+SO3 to AA+SO3 – why this makes sense and the implications for the cycloaddition mechanism (and atmospheric relevance)
Obtained rough fit of A state for ASA by end of the day

10. Spectroscopic constants for parent ASA
Parent
A [MHz]
(11)
B [MHz]
(28)
C [MHz]
(18)
ΔJ [kHz]
0.0980(19)
ΔJK [kHz]
0.694(11)
ΔiJ [kHz]
52.2(10)
ΔiK [kHz]
-440(16)
Δi- [kHz]
12.5(13)
V3 [cm-1]
(30)
ε [deg]
3.04(47)
δ [deg]
33.122(34)
Iα [uÅ2]a
3.183 N 56
σ [kHz]
0.8
303 ← 202 E
322 ← 221
321 ← 220 A E A E A
7762
7807
[MHz]
We did go back to collect cavity spectra of weaker c-types
Xiam fit, prediction – Hamiltonian includes terms for internal rotation and coupling of internal/overall rotation (includes barrier)
(a) Fixed to the value derived from the M06-2X/ G(3df,3pd) structure
𝐻 = 𝐻 𝑟𝑜𝑡 + 𝐻 𝑐𝑑 + 𝐻 𝑖𝑟 10

11. Comparison of fit to computational results
M06-2X/ g(3df,3pd)
Experimental
% Difference
A [MHz]
3652
(11)
-0.58
B [MHz]
1367
(28)
-0.55
C [MHz]
1243
(18)
-0.54
V3 [cm-1]
263
(30)
-9.1
δ [deg]
28.9
33.12(3) 13
ε[deg]
3.00
3.04(47)
1.3
13CH3COOSO2OH
CH3COO34SO2OH
CD3COOSO2OD
Prediction
Experimental A
(17)
(52)
(22) B
(36)
(23)
(22) C
(27)
(28)
(23)
% Difference
-0.58
-0.55
-0.54
% Difference
-0.58
-0.55
-0.54
% Difference
-0.49
-0.50
Polar angles defining the position of the internal rotor with respect to the principal axis system
M06-2X C-S distance A
Kraitchmann: (34) or (41) A (within 0.05 A)
All the calculated isotopic shifts were the same as what we observed

12. Spectroscopic constants for observed ASA isotopologues
Parent
13CH3COOSO2OH
CH3COO34SO2OH
CD3COOSO2OD
A [MHz]
(11)
(17)
(52)
(22)
B [MHz]
(28)
(36)
(23)
(22)
C [MHz]
(18)
(27)
(28)
(23)
ΔJ [kHz]
0.0980(19)
0.0869(26)
0.0944(37)
0.0783(19)
ΔJK [kHz]
0.694(11)
0.700(17)
0.767(51)
0.575(16)
ΔiJ [kHz]
52.2(10)
56.2(14)
52.2b
ΔiK [kHz]
-440(16)
-470(21)
-440b
Δi- [kHz]
12.5(13)
14.2(15)
12.5b
V3 [cm-1]
(30)
(42)
(17)
234.57(13)
ε [deg]
3.04(47)
2.52(69)
2.61(53)
3.04b
δ [deg]
33.122(34)
32.708(52)
33.207(25)
31.59(22)
Iα [uÅ2]a
3.183
6.362 N 56 57 23 55
σ [kHz]
0.8
1.0
1.4
3.7
For the 13C isotopologue we did go back to collect c-type lines, 34S is all cavity since this was in natural abundance
Xiam fit, prediction – Hamiltonian includes terms for internal rotation and coupling of internal/overall rotation (includes barrier)
(a) Fixed to the value derived from the M06-2X/ G(3df,3pd) structure
(b) Parameter fixed at the value determined from the parent fit

13. Computational work on ASA formation: Role of the methyl group
Cycloaddition
60° reorientation
Equilibrium orientation of the methyl group changes going from the complex to ASA
We turned to computational chemistry to help us understand our observations and a possible mechanism for this reaction. When we tried to find a transition state going from the AA-SO3 complex to the anhydride, we found that it seemed like two motions needed to happen – both the rotation of the methyl group and the cycloaddition in which the oxygen-sulfur bond is formed and the proton is transferred from the acid to SO3. While we’re not trying to imply that this process happens on way or the other sequentially or in one step, it’s clear that both paths are energetically favorable with a near zero barrier to formation.
H3CCOOH-SO3
ASA

14. Pathways for ASA formation
Methyl rotation TS
Cycloaddition TS
CCSD(T)/CBS//M06-2X/ G(3df,3pd)
H3CCOOH-SO3
ASA
2nd order saddle point
Near-zero barrier in both sequential and simultaneous formation pathways

15. Significance of ASA
Characterization of ASA supports the assertion that a wide variety of carboxylic acids react with SO3 in the gas phase to form carboxylic sulfuric anhydrides.
Experimentally characterized
Computational only R -H
-CH3
-CF3
-cis-C=CH2, -trans-C=CH2
-C≡CH 
Pinic acid
Benzoic acid  R 
And again, we did later observe TFASA (SO3+trifluoroacetic acid) in cavity spectra and that will be something we talk about next year  

16. Possible atmospheric relevance
Subsequent hydrolysis could provide a route for atmospheric H2SO4 production and incorporation of organic matter into atmospheric aerosol
RCOOH R
H2SO4
It is, after all, an anhydride so we can a realistic fate to be hydration or hydrolysis
RCOOH···H2SO4
RCOOSO2OH + H2O
RCOOSO2OH···H2O
Formation of carboxylic sulfuric anhydrides represents
new sulfur chemistry

17. Acknowledgements
Dr. Ken Leopold
Dr. Becca Mackenzie
CJ Smith

19. Acetic acid internal rotation barriers
241 cm-1
168 cm-1
*39 cm-1
(AA-SO3 complex 39 cm-1)
ASA 241 cm-1
Acetic acid 168 cm-1
AA-H2O 138 cm-1, AA-(H2O)2 118 cm-1
138 cm-1
118 cm-1
B. van Eijck. J. Mol. Spec. 1981, 86,
Ouyang, B; Howard, B.J. Phys. Chem. Chem. Phys. 2008, 11,

20. ASA pathways with and without ZPE

21. Atmospheric concentrations of carboxylic acids
SO3 on the order of ppm, H2O much larger (0.05%, 100’s of ppm)
There’s a lot of information in this table, but what I want to focus on is the concentrations listed for formic acid and acetic acid which are on the order of ppb. So this is an example of the abundance of certain carboxylic acids in the atmosphere. Recently, Hazra and Sinha computationally investigated the involvement of formic acid in the hydrolysis of SO3. And what they found was that compared to the SO3-H2O-H2O pathway here….
I’m going to begin with talking about where the motivation for this work came from, which starts with considering the production of atmospheric sulfuric acid. As we know, sulfuric acid is an important molecule in aerosol nucleation. a computational study by Hazra and Sinha who were investigating SO3 hydrolysis
Hydrolysis of SO3 through this trimeric intermediate was established as the primary route to form H2SO4
SO3-H2O-H2O ~6 kcal/mol, more stable than SO3-H2O than by at least 20 kcal/mol
Chebbi, A.; Carlier, P. Atmos. Env. 1996, 30,

22. Methyl internal rotation
𝐻 = 𝐻 𝑅𝑅 + 𝐻 𝐶𝐷 + 𝐻 𝐼𝑅
Rigid rotor
Centrifugal distortion
Internal rotation/
Overall rotation coupling α
The barrier can be crossed due to tunneling motion and results in two torsional states, A (non-degenerate) and E (doubly degenerate)
Picture of the triply degenerate configurations and barrier, A and E states
What does this mean?
The A and E states each have their own set of rotational levels
Cannot induce transitions crossing between A and E because forbidden to transition between the states because of different symmetry.
Results in doubling of the spectra
Rotational energy levels spaced differently in A vs E state
W. Gordy and R. Cook. Microwave molecular spectra, 3rd Ed.; Wiley: New York, 1984.

23. XIAM (Internal Axis Method)
𝐻 𝑅𝑅 = 𝐵 𝐽 ∗ 𝑃 2 + 𝐵 𝐾 ∗ 𝑃 𝑧 2 + 𝐵 − ( 𝑃 𝑥 2 − 𝑃 𝑦 2 )
principal axis system
𝐻 𝐶𝐷 =− 𝐷 𝐽 ∗ 𝑃 4 − 𝐷 𝐽𝐾 ∗ 𝑃 𝑧 2 ∗ 𝑃 2 − 𝐷 𝐾 ∗ 𝑃 𝑧 4
𝐻 𝐼𝑅 =𝐹 𝑝 𝛼 −𝜌∗ 𝑃 𝑟 𝑉 3 ∗(1− cos 𝑁𝛼 )
rho axis system
𝐻 𝐼𝑅𝐷 =2 𝐷 𝑝𝑖2𝐽 ∗ 𝑝 𝛼 −𝜌∗ 𝑃 𝑟 2 ∗ P 2 + D pi2K ∗ 𝑝 𝛼 −𝜌∗ 𝑃 𝑟 2 ∗ 𝑃 𝑧 2 + 𝑃 𝑧 2 ∗ 𝑝 𝛼 −𝜌∗ 𝑃 𝑟 D pi2 − [ 𝑝 𝛼 −𝜌∗ 𝑃 𝑟 2 ∗ P x 2 − P y P x 2 − P y 2 ∗ 𝑝 𝛼 −𝜌∗ 𝑃 𝑟 2
HID diagonalized in the RAS then rotated/transformed into the PAS
Hartwig, H. and Dreizler, H. Z. Naturforsch. 1996, 51a, 923

24. Spectroscopic constants for observed ASA isotopologues
Parent
13CH3COOSO2OH
CH3COO34SO2OH
CD3COOSO2OD
A [MHz]
(11)
(17)
(52)
(22)
B [MHz]
(28)
(36)
(23)
(22)
C [MHz]
(18)
(27)
(28)
(23)
ΔJ [kHz]
0.0980(19)
0.0869(26)
0.0944(37)
0.0783(19)
ΔJK [kHz]
0.694(11)
0.700(17)
0.767(51)
0.575(16)
ΔiJ [kHz]
52.2(10)
56.2(14)
52.2b
ΔiK [kHz]
-440(16)
-470(21)
-440b
Δi- [kHz]
12.5(13)
14.2(15)
12.5b
V3 [cm-1]
(30)
(42)
(17)
234.57(13)
ε [deg]
3.04(47)
2.52(69)
2.61(53)
3.04b
δ [deg]
33.122(34)
32.708(52)
33.207(25)
31.59(22)
Iα [uÅ2]a
3.183
6.362 N 56 57 23 55
σ [kHz]
0.8
1.0
1.4
3.7 c b ε a b δ
Xiam fit, prediction – Hamiltonian includes terms for internal rotation and coupling of internal/overall rotation (includes barrier)
(a) Fixed to the value derived from the M06-2X/ G(3df,3pd) structure
(b) Parameter fixed at the value determined from the parent fit
𝐻= 𝐻 𝑅𝑅 + 𝐻 𝐶𝐷 + 𝐻 𝐼𝑅

25. FSA computational work

26. Formation of ASA TS
TFA-SO3
0.63 kcal
-0.46 kcal
AA-SO3 TS
TFASA
ASA