1. 1
MICROWAVE CHARACTERIZATION OF PROPIOLIC SULFURIC ANHYDRIDE AND TWO CONFORMERS OF ACRYLIC SULFURIC ANHYDRIDE
C.J. Smith, Anna Huff, Becca Mackenzie, and Ken Leopold
Ken Leopold Group
Department of Chemistry – UMN

2. Overview Background and possible relevance to atmospheric aerosols
Propiolic Sulfuric Anhydride (PSA)
Cis- and trans-Acrylic Sulfuric Anhydride (cis- and trans-AcrSA)
The outline of my talk will be as follows. First, I’ll briefly overview the motivation behind this work and possible relevance to atmospheric aerosols. I’ll then describe our experimental work on propiolic sulfuric anhydride as well as cis- and trans-Acrylic sulfuric anhydride.

3. Atmospheric Aerosol Particles
Affect atmosphere in several ways:
Sites of heterogeneous chemistry
Act as cloud condensation nuclei
Climate
Affect human health
Visibility
Atmospheric aerosol particle research is very active right now and these particles have been shown to affect the atmosphere in several ways: by visibility, they act as cloud condensation nuclei, and they affect climate. They have been also shown to have adverse affects on human health as well as act as sites for heterogeneous chemistry to occur in the atmosphere. While aerosol particles are formed through heterogenous chemistry and most of you may know how an aerosol particle forms, I want to just briefly overview the general mechanism.
M. Kulmala, Science, vol. 302, no. 5647, , (2003)

4. Atmospheric Aerosol Formation
Gases
Complexes
Solids & Liquids
𝑆 𝑂 3 ⋯ 𝐻 2 𝑂→ 𝐻 2 𝑆 𝑂 4
𝑆 𝑂 3 ⋯ 𝐻 2 𝑂⋯ 𝐻 2 𝑂→ 𝐻 2 𝑆 𝑂 4 ⋯ 𝐻 2 𝑂
We start with a seed molecule, which interacts with several of water molecules. As it interacts with more water molecules, it forms a cluster. Once this cluster becomes stable, it will continue to grow spontaneously into an aerosol particle. Now, we know that sulfuric acid is a major component in seed molecules. However, sulfuric acid and water alone DO NOT explain observed nucleation rates. Therefore, something else needs to be involved. We also know that during nucleation, organic acids and amines become incorporated into the aerosol particle. So, the big question is, do they participate in the early stages of aerosol formation? To further understand the early stages of the nucleation process, we need to look at how sulfuric acid forms in the atmosphere. Now, sulfuric acid isn’t directly emitted into that atmosphere. It’s formed from SO2, which comes from natural and anthropogenic sources, through a series of steps and converted to sulfuric acid. The step I want to highlight is the last step, the conversion of SO3 and H2O to form sulfuric acid.
Morokuma et al., J. Am. Chem. Soc. 116, 10316–10317 (1994).
Kolb et al., J. Am. Chem. Soc. 116, (1994).

5. The activation barrier for the conversion of SO3 and H2O to sulfuric acid is fairly large, approximately 32 kcal/mol. However, it’s been shown that the addition of a second water molecule, significantly drops that activation barrier! Since water isn’t the only molecule in the atmosphere, can other molecules have a similar effect??
Morokuma et al., J. Am. Chem. Soc. 116, 10316–10317 (1994).
Kolb et al.,J. Am. Chem. Soc. 116, (1994).

6. Alternate Pathway for H2SO4 Generation
Hazra and Sinha investigated this problem computationally, where they replaced the second water molecule with Formic Acid. Ultimately what they discovered was that the conversion of SO3 and H2O to sulfuric acid was essentially barrierless (as can be seen from the graph), when formic acid replaced the second water molecule. So based on this work, a former graduate student in our lab, Becca, wanted to investigate some of these complexes further! Long story short, she didn’t actually observe these complexes, but discovered something different and completely unexpected…..
M. K. Hazra and A. Sinha; J. Am. Chem. Soc., vol. 133, no. 43, pp , (2011)

7. Discovering Formic Sulfuric Anhydride (FSA)7
Discovering Formic Sulfuric Anhydride (FSA)
Different Organic Acids...Same Result?
FA + SO3 -2 -4 -6 -8
-18
-12
-14
-10
-16
Energy (kcal/mol)
H2O∙∙∙SO3
FA∙∙∙H2O TS
FSA
FA∙∙∙SO3 S O H C R +
She found that FSA is generated by the reaction of SO3 and formic acid through a 𝜋2+𝜋2+ 𝜎2 cycloaddition reaction. Her computational calculations showed that after Zero Point Energy corrections, the reaction between FA+SO3 to FSA is essentially barrierless! Furthermore, similar anhydride structures were obtained when benzoic acid and pinic acid were investigated using computational methods. So, due to her calculations and showing only one experimental observation of this reaction, I asked the question, “Is this reaction generalizable? Can other organic acids, both atmospherically relevant and in general, produce similar results?” Therefore, I decided to look at a simple and more general case first, PROPIOLIC ACID! Before I talk about this project, I want to briefly describe the instrument we utilize.
Can other organic acids produce similar results??
R. B. Mackenzie, C. T. Dewberry, and K. R. Leopold; Science, vol. 349, no. 6243, pp , (2015)

8. Fourier Transform Microwave Spectrometer
Sample
High Pressure
Diffusion Pump
10-5 torr
Gas pulse
Microwave radiation
The instrument that we use is our Fourier Transform Microwave Spectrometer (FTMW). It contains two different modes of operation, the cavity FTMW (located along the plane of the image) and the chirped-pulse FTMW (located into the plane of the picture).
Contains:
Cavity FTMW
Chirped-pulse FTMW

9. Tandem Cavity and Chirp Pulse FTMW9
Tandem Cavity and Chirp Pulse FTMW
Broadband, 3 GHz sweep
>30 kHz resolution and lower sensitivity
Narrowband, 1-2 MHz window
2-8 kHz resolution and high sensitivity
Cavity Mode
Chirp Mode
Our cavity mode uses 2 large spherical mirrors, which are set to form a resonant cavity. In switching to our chirp mode, we flip this foam arm up, eliminating and ringing between the mirrors that might otherwise occur during our chirped-pulse experiments. This foam arm allows us to switch between the two modes of operation easily AND without breaking vacuum.

10. Instrumental Setup SO3 pulsed in with Ar
SO3 in Ar
HC2COOH in Ar
SO3 pulsed in with Ar
2.3 atm
Propiolic acid was flowed in with Ar separately
0.6 atm
Using a similar set-up as Becca had used when she discovered FSA, we supersonically expanding SO3 in Ar and then injected propiolic acid in Ar in the early stages of the expansion via a hypodermic needle. However, we first had to predict a structure and some rotational constants so as to know where to search.
~4 K

11. Computational Results11
Computational Results
Predictions made using M06-2X / G(3df,3pd)
Name
Structure
Dipole Moments (D)
Rotational Constants (MHz)
Propiolic Sulfuric Anhydride
µa = 4.17
µb = 0.42
µc = 0.93
A = 3389
B = 957
C = 873
Based on our predictions, we should see a-type transitions as well as some c-type transitions. The predicted rotational constants are as followed and the energy of formation for PSA is ~-18.3 kcal/mol.

12. Propiolic Sulfuric Anhydride
Here is a 9-15 GHz chirp spectra we collected for Propiolic sulfuric anhydride and these were some of the transitions that matched the predicted a-type spectra. As you can see, these transitions match well with the predicted a-type spectra. Now, using cavity mode, we were able to find more a-type transitions as well as some c-type transitions and after fitting those transitions, were able to determine some spectroscopic constants.
GHz 9 15 11 13

13. Spectroscopic Constants
Propiolic Sulfuric Anhydride
Propiolic-OD Sulfuric Anhydride
34S-Propiolic Sulfuric Anhydride
Structure
A (MHz)
(45)
(31)
(94)
B (MHz)
(12)
(15)
(17)
C (MHz)
(11)
(17)
(16)
ΔJ (kHz)
0.0603(29)
0.0430(27)
0.0439(38)
ΔJK (kHz)
0.213(38)
0.286(28)
0.389(72) δj
0.0145(22) - N 30 17 13
RMS (kHz) 5 2
As you can see here, we fit the rotational constants and several distortional constants for the parent and 2 isotopologues, a deuterated sample and the S34 sample. The isotopically substituted samples also matched their calculated isotopic shifts extremely well, giving undeniable evidence that we have observed Propiolic Sulfuric Anhydride!

14. Comparison of Constants
Parent Propiolic Sulfuric Anhydride
Constant
Observed (MHz)
Theoretical (MHz)
Percent Difference A
(45)
3389
-0.5% B
(12)
957 C
(11)
873
After determining the spectroscopic constants, we compared the observed and theoretical rotational constants. As can be seen here, the percent difference between the two is less than 1%, showing that the predictions matched well with our observations.

15. Zero-Point Corrected Potential Energy Surface
+ SO3 -3
-18
-12
-15
Energy (kcal/mol)
Now, knowing that the simple case follows the general reaction, I decided to look at a more complicated example by further investigating this problem with acrylic acid.

16. Two conformers of Acrylic Acid
cis-Acrylic Acid
trans-Acrylic Acid
What makes this case more complicated is that this molecule has two conformers, cis- and trans-acrylic acid. The energy difference between the two conformers is ~0.2 kcal/mol.
0 kcal/mol
+0.2 kcal/mol
K. Bolton, D.G. Lister, J. Sheridan; J. Chem. Soc. Faraday Trans., vol. 2, no. 70, p. 113, (1974)

17. Computational Results17
Computational Results
Predictions made using M06-2X / G(3df,3pd)
Name
Structure
Dipole Moments (D)
Rotational Constants (MHz)
Cis-Acrylic Sulfuric Anhydride
µa = 4.60
µb = 0.35
µc = 0.90
A = 2874
B = 1057
C = 909
Trans-Acrylic Sulfuric Anhydride
µa = 5.08
µb = 0.26
A = 3667
B = 901
C = 842
Trans

18. Trans-Acrylic Sulfuric Anhydride Cis-Acrylic Sulfuric Anhydride18
Trans-Acrylic Sulfuric Anhydride
Cis-Acrylic Sulfuric Anhydride 6 18 10 14
GHz
Using the same experimental set-up as before, we then investigated cis- and trans- acrylic acid. As you can see, this spectra is slightly more complicated due to the two conformers, but we were able to find transitions that matched very well with our predictions. Here is a 6-18 GHz chirp spectra of cis-acrylic sulfuric anhydride. 6 18 10 14
GHz

19. Spectroscopic Constants19
Spectroscopic Constants
Constant
Cis-Acrylic Sulfuric Anhydride
Cis-Acrylic-OD Sulfuric Anhydride
Trans-Acrylic Sulfuric Anhydride
Trans-Acrylic-OD Sulfuric Anhydride
Structure
A (MHz)
(50)
(14)
(57)
(84)
B (MHz)
(32)
(16)
(15)
(14)
C (MHz)
(30)
(48)
(21)
(14)
ΔJ (kHz)
0.0722(37)
0.0547(17)
0.0349(10)
0.0307(15)
ΔJK (kHz)
0.360(37)
0.230(38)
0.595(20)
0.668(29) N 42 15 41
RMS (kHz) 10 4 3 1
Here are the spectroscopic constants after collecting and fitting transitions for the parent and two isotopologues, with, again, A, B, and C being the rotational constants. Again, the important takeaway here is that the isotopically substituted samples match their predicted isotopic shifts extremely well.
***Reason the RMS on cis-AcrSA is high is because many lines are from chirp experiments and we never looked at them on the cavity to resolve the lines further.

20. Comparison of Constants20
Comparison of Constants
Cis-Acrylic Sulfuric Anhydride
Constant
Observed (MHz)
Theoretical (MHz)
Percent Difference A
(50)
2874
-0.7% B
(32)
1057
-0.5% C
(50)
910
Trans-Acrylic Sulfuric Anhydride
As was seen in the chirp spectra for the 2 conformers the pattern was well matched. Here is a comparison of the observed and theoretical rotational constants. As you can see, the percent difference for all three molecules is less than 1% providing evidence for not only generating another sulfuric anhydride molecule, but that this reaction works for even more complicated examples!
Constant
Observed (MHz)
Theoretical (MHz)
Percent Difference A
(57)
3668
-0.8% B
(15)
901
-0.6% C
(21)
842
-0.5%

21. Zero-Point Corrected Potential Energy Surface21
Zero-Point Corrected Potential Energy Surface
+ SO3
+ SO3
Energy (kcal/mol)
Finally, after looking at the zero-point corrected potential energy surface, we see that both conformers for acrylic acid produce a barrierless transition to produce the sulfuric anhydride!

22. Conclusions and Next Step
𝑅𝐶𝑂𝑂𝐻+𝑆 𝑂 3 →𝑅𝐶𝑂𝑂𝐻−𝑆 𝑂 (1)
𝑅𝐶𝑂𝑂𝐻−𝑆 𝑂 3 →𝑅𝐶𝑂𝑂𝑆 𝑂 2 𝑂𝐻 (2)
𝑅𝐶𝑂𝑂𝑆 𝑂 2 𝑂𝐻+ 𝐻 2 𝑂 (𝑔 𝑜𝑟 𝑎𝑞) → 𝐻 2 𝑆 𝑂 4 (𝑔 𝑜𝑟 𝑎𝑞) +𝑅𝐶𝑂𝑂 𝐻 (𝑔 𝑜𝑟 𝑎𝑞) (3)
So, in thinking back to the bigger picture and asking, “Is the reaction between SO3 and any carboxylic acid a general reaction?”, I believe we have shown that it is! Both Propiolic and the two conformers of Acrylic acid react with SO3 to form the general sulfuric anhydride. Now, the next step we want to take is to hydrate these molecules and see if we generate a sulfuric acid-carboxylic acid complex. We also want to answer the question, “How relevant are these molecules atmospherically? To answer that question, we turn to statistical mechanics and thermodynamics.
****Mention we have done the water complexes!
We further speculated that the subsequent hydrolysis of carboxylic sulfuric anhydrides in a cluster or aqueous droplet could provide a route to the incorporation of low molecular weight organics into atmospheric aerosol.
***Even if this isn’t atmospherically relevant, we have discovered something new about sulfur chemistry!
**Lead into organic involvement (somewhere).

23. Acknowledgments Dr. Ken Leopold Dr. Becca Mackenzie Anna Huff Comet
With that, I would like to thank my adviser, Ken, and my group mates, Becca and Anna who helped collect and analyze the data for both projects. I would also like to thank our mascot Comet, for keeping us entertained and on task as well as NSF and the University of Minnesota for funding. I would be happy to take any questions.
Dr. Ken Leopold
Dr. Becca Mackenzie
Anna Huff
Comet