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

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, 10316-10317. (Adapted by Hazra, M.K; Sinha, A.) Kolb, C.E. et al. J. Am. Chem. Soc. 1994, 116, 10314-10315.

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, 17444-53.

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/6-311++G(3df,3pd), with ZPE Mackenzie, R.B.; Dewberry, C.T.; Leopold, K.R. Science. 2015, 349, 58-61.

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

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

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

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)

(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

Spectroscopic constants for parent ASA   Parent A [MHz] 3630.8996(11) B [MHz] 1359.30700(28) C [MHz] 1235.79407(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] 241.093(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/6-311++G(3df,3pd) structure 𝐻 = 𝐻 𝑟𝑜𝑡 + 𝐻 𝑐𝑑 + 𝐻 𝑖𝑟 10

Comparison of fit to computational results   M06-2X/6-311++g(3df,3pd) Experimental % Difference A [MHz] 3652 3630.8996(11) -0.58 B [MHz] 1367 1359.30700(28) -0.55 C [MHz] 1243 1235.79407(18) -0.54 V3 [cm-1] 263 241.093(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 3635.08 3614.0543(17) 3651.26 3630.186(52) 3431.07 3414.345(22) B 1338.10 1330.78967(36) 1359.60 1352.09806(23) 1259.74 1253.47425(22) C 1216.83 1210.29056(27) 1236.52 1229.80579(28) 1140.29 1134.73145(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 3.886 A Kraitchmann: 3.8944(34) or 3.9349(41) A (within 0.05 A) All the calculated isotopic shifts were the same as what we observed

Spectroscopic constants for observed ASA isotopologues   Parent 13CH3COOSO2OH CH3COO34SO2OH CD3COOSO2OD A [MHz] 3630.8996(11) 3614.0543(17) 3630.186(52) 3414.345(22) B [MHz] 1359.30700(28) 1330.78967(36) 1352.09806(23) 1253.47425(22) C [MHz] 1235.79407(18) 1210.29056(27) 1229.80579(28) 1134.73145(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] 241.093(30) 241.080(42) 241.150(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/6-311++G(3df,3pd) structure (b) Parameter fixed at the value determined from the parent fit

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

Pathways for ASA formation Methyl rotation TS Cycloaddition TS CCSD(T)/CBS//M06-2X/6-311++G(3df,3pd) H3CCOOH-SO3 ASA 2nd order saddle point Near-zero barrier in both sequential and simultaneous formation pathways

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  

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

Acknowledgements Dr. Ken Leopold Dr. Becca Mackenzie CJ Smith

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, 465-479. Ouyang, B; Howard, B.J. Phys. Chem. Chem. Phys. 2008, 11, 366-373.

ASA pathways with and without ZPE

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, 4233-4249.

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.

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 2 + 1 2 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 2 + D pi2 − [ 𝑝 𝛼 −𝜌∗ 𝑃 𝑟 2 ∗ P x 2 − P y 2 + 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

Spectroscopic constants for observed ASA isotopologues   Parent 13CH3COOSO2OH CH3COO34SO2OH CD3COOSO2OD A [MHz] 3630.8996(11) 3614.0543(17) 3630.186(52) 3414.345(22) B [MHz] 1359.30700(28) 1330.78967(36) 1352.09806(23) 1253.47425(22) C [MHz] 1235.79407(18) 1210.29056(27) 1229.80579(28) 1134.73145(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] 241.093(30) 241.080(42) 241.150(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/6-311++G(3df,3pd) structure (b) Parameter fixed at the value determined from the parent fit 𝐻= 𝐻 𝑅𝑅 + 𝐻 𝐶𝐷 + 𝐻 𝐼𝑅

FSA computational work

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