Facile Formation of Acetic Sulfuric Anhydride: Microwave Spectrum

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Facile Formation of Acetic Sulfuric Anhydride: Microwave Spectrum, Internal Rotation, and Theoretical Calculations Anna K. Huff, Rebecca B Mackenzie, Christopher J. Smith, and Kenneth R. Leopold J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b05105 • Publication Date (Web): 10 Jul 2017 Downloaded from http://pubs.acs.org on July 12, 2017

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The Journal of Physical Chemistry

June 28, 2017 J. Phys. Chem. A Revised

Facile Formation of Acetic Sulfuric Anhydride: Microwave Spectrum, Internal Rotation, and Theoretical Calculations

Anna K. Huff, Rebecca B. Mackenzie, C.J. Smith, and Kenneth R. Leopold*

Department of Chemistry, University of Minnesota, 207 Pleasant St., SE, Minneapolis, MN 55455

* Corresponding Author: E-mail: [email protected]; (612) 625-6072

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Abstract Acetic sulfuric anhydride, CH3COOSO2OH, has been produced by the reaction of SO3 and CH3COOH in a supersonic jet. Four isotopologues have been observed by microwave spectroscopy. Spectra of both A and E internal rotor states have been observed and analyzed, yielding a value of 241.093(30) cm−1 for the methyl group internal rotation barrier of the parent species.

Similar

values

were

obtained

for

the

other

isotopologues

studied.

M06-2X/6-311++G(3df,3pd) calculations indicate that the formation of the anhydride proceeds via a π2 + π2 + σ2 cycloaddition reaction within the CH3COOH−SO3 complex. The equilibrium orientation of the methyl group relative to the O=C-C plane is different in the anhydride and in the CH3COOH−SO3 complex, indicating that the −CH3 internal rotation accompanies the cycloaddition reaction. The energies of key points on the potential energy surface were calculated using CCSD(T)/complete basis set with double and triple extrapolation [CBS/(D-T)], and the transformation from the CH3COOH−SO3 complex to CH3COOSO2OH is shown to be nearly barrierless regardless of the orientation of the methyl group. This study provides the second experimental observation of the reaction between a carboxylic acid and SO3 to form a carboxylic sulfuric anhydride in the gas phase. Possible connections to atmospheric aerosol formation are discussed.


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Introduction An important advance in atmospheric science has been the recognition of molecular clusters as vital players in a variety of atmospheric processes. The homogeneous nucleation of atmospheric aerosol, for example, necessarily proceeds through stages involving small molecular aggregates whose physiochemical properties influence critical cluster size, nucleation rates, and the development of reliable climate models.1,2 Clusters can also act as catalysts and intermediates in atmospheric chemical reactions3,4 and their formation has been shown to alter the photochemistry and photophysics of a number of atmospherically active species.5,6

Sulfur chemistry is an arena in which the importance of clusters has been particularly evident. Sulfuric acid is a key species in the formation of critical nucleation clusters,1,7 and its production in the atmosphere is widely understood to proceed via the hydrolysis of SO3. While early work proposed a mechanism involving intramolecular proton transfer within the SO3−H2O complex,8,9 subsequent studies indicated that its production likely involves a SO3−(H2O)2 intermediate.10-12 The addition of a second water molecule to SO3−H2O lowers the activation barrier for H2SO4 production, resulting in a substantial enhancement in reaction rate and accounting for the observed second order dependence on water. Interestingly, the reverse process, i.e., the photodecomposition of H2SO4 and H2SO4−H2O, has been the subject of parallel investigations which have led to the identification of a novel mechanism involving vibrational overtone excitation.13,14

Several recent theoretical studies have indicated that a number of other species present in the atmosphere can serve the same function as the second water in SO3−(H2O)2 in facilitating the formation of H2SO4 from SO3 and H2O. For example, computational work by Hazra and Sinha suggested that the substitution of one water molecule in SO3−(H2O)2 by formic acid virtually eliminates the activation barrier for hydrolysis of the SO3 and may, therefore, constitute an alternate pathway for sulfuric acid formation in the atmosphere.15 A similar conclusion has been drawn by Long et al.16 Other calculations indicate that sulfuric acid itself can catalyze the hydration of SO317 and that clusters containing more than two water molecules may also participate.18



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Motivated by the possible role of formic acid, we recently reported a supersonic jet study that had initially been aimed at observing complexes involving SO3, H2O, and HCOOH.19 To our surprise, however, that work resulted in the observation of a facile π2 + π2 + σ2 cycloaddition reaction between formic acid and sulfur trioxide, viz.,

O

C

H

O

S O

H

O

H

O

O

S

C

O

O

O

(1) H

O

FSA

HCOOH−SO3

The product, formic sulfuric anhydride (FSA), was observed by microwave spectroscopy in a supersonic jet and further characterized using density functional theory and CCSD(T) calculations. These calculations indicated that the barrier to FSA formation, starting with the HCOOH−SO3 weakly bound complex, is effectively zero, consistent with its rapid formation during the first few tens of microseconds of the supersonic expansion. Further calculations on the analogous reactions involving benzoic and pinic acids gave similar results. This was interpreted as indicating that the above reaction is general for carboxylic acids. 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. Such a mechanism is shown in reactions 2-4.

RCOOH + SO3 → RCOOH−SO3

(2)

RCOOH−SO3 → RCOOSO2OH

(3)

RCOOSO2OH + H2O(g and/or l) → H2SO4( g and/or aq) + RCOOH(g and/or aq)

(4)

To date, no further experimental confirmation of a gas phase carboxylic sulfuric anhydride has been reported. Thus, in this paper, we describe the formation and microwave spectroscopic observation of acetic sulfuric anhydride (ASA) in a supersonic jet. Density functional theory and CCSD(T) calculations are used to investigate the formation mechanism and the results indicate that the reaction proceeds via a process similar to that of equation 1. Methyl group internal 4 ACS Paragon Plus Environment

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rotation plays a complicating role, but does not alter the barrierless nature of the process, and the results support our previous conjecture that the RCOOH + SO3 reaction is both facile and general.

Experimental Methods and Results Spectra were recorded using a tandem cavity20 and chirped-pulse21 Fourier transform microwave spectrometer, details of which have been given elsewhere.22,23 Argon was passed over solid, polymerized SO3 and pulsed into the spectrometer through a 0.8 mm diameter orifice at a stagnation pressure of 2.3 atm. The expansion was immediately guided through a 0.5″ stainless steel cone, which terminates in a 0.19″ diameter orifice. Along this channel, a few millimeters downstream of the 0.8 mm orifice, acetic acid vapor was introduced through a 0.012″ diameter hypodermic needle.24 The acetic acid vapor was carried by a stream of Ar flowing through liquid acetic acid (98%) at a backing pressure of 0.6 atm. A 6-18 GHz chirped pulse spectrum was obtained in 3 GHz intervals with a 90 µs collection time, each averaged over 10,000 free induction decay signals. This spectrum, which facilitated the initial identification of ASA lines, is shown in Figure 1. A strong a-type spectrum corresponding to ASA was readily identified on the basis of predictions, and a number of weaker c-type transitions were later predicted and observed using the cavity spectrometer. No b-type transitions were observed. Cavity spectra for the 34S isotopologue were recorded in natural abundance and the experimental isotopic shifts in the rotational constants matched their predicted values. Isotopically enriched samples of 13

CH3COOH (65%

13

C) and CD3COOD (98% D) were used to generate their respective ASA

isotopologues. For these isotopologues, spectra were readily observed using the chirped pulse spectrometer, and no further lines were measured in cavity mode.

Rotational transitions from J = 2←1 to J = 6←5 were recorded for the parent isotopologue while J = 3←2 to J = 7←6 transitions were observed for both the 13C-substituted, and deuterated ASA isotopologues. For these three isotopologues, transitions involving K−1= 0−3 were assigned for at least one J level. Only transitions from J = 3←2 to J = 6←5 were measured for the

34

S

isotopologue with K−1 = 0−2. For all species studied, both A and E states arising from hindered internal rotation of the methyl group were observed.



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All spectra were fit using the XIAM internal rotation fitting program from Hartwig25 which implements the extended internal axis method introduced by Woods.26 The Hamiltonian includes rigid rotor, centrifugal distortion, and internal rotation terms, viz., ) ) ) ) H = H rot + H cd + H ir

(5)

) where H ir describes the internal rotation of the methyl group and contains Fo (the rotational

constant of the methyl group about its symmetry axis) and V3, the three-fold barrier to internal rotation. This term also depends on δ and ε, the polar angles that define the position of the rotor axis in the principal axis system of the molecule. Specifically, δ is the angle between the CH3 top axis and the a-axis of the molecule and ε is the angle between the b-axis and the projection of the top axis onto the b-c plane.

Some details of the spectral analysis are as follows: Initial fits indicated that Fo was highly correlated with V3 and therefore was held fixed in subsequent fits at its computationally derived value of 158.757 GHz.27 For the CD3 species, the computed value of 79.4396 GHz was used. While the fitted values of ε and δ are in good agreement with computational results of 3.0 and 28.9°, respectively (next section), in the case of the deuterated isotopologue, ε could not be satisfactorily fit and was fixed to the value determined for the parent.28 Inclusion of the internal rotation-overall rotation distortion terms (∆iJ, ∆iK, and ∆i-) was necessary to bring all of the calculated transition frequencies into good agreement with the experimental results. However, despite various attempts, ∆iJ, ∆iK, and ∆i- were not well determined for the

34

S or deuterated

isotopologue fits and were fixed to their parent values. Attempts to include a V6 term in the analysis did not yield a statistically determined value and therefore this parameter was not incorporated in the final analysis. Results of the final fits for all isotopologues studied are given in Table 1 where it is seen that the standard deviations in the computed residuals are excellent. The slightly larger standard deviation in the residuals for the deuterated species in comparison with the other isotopologues is likely due to unresolved deuterium hyperfine structure.


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Computational Methods and Results All optimizations and frequency calculations were done at the M06-2X/6-311++G(3df,3pd) level of theory using the Gaussian suite of programs.29 This level was chosen because it proved successful in comparison with MP2/6-311++G(3df,3pd) calculations performed in our prior work on HCOOSO2OH.19 Single-point electronic energies were subsequently computed using CCSD(T) with the complete basis set extrapolation of Neese and Valeev between the ANOpVDZ and ANO-pVTZ basis sets.30 Unless otherwise specified, the energies quoted are from these calculations. The equilibrium structure of ASA is shown in Figure 2 and important numerical results are given in Table 2, where it may be seen that the calculated rotational constants are within 0.6% of the observed values. Calculations were also performed for the CH3COOH−SO3 complex. Cartesian coordinates for all relevant structures are provided as Supplementary Material. The minimum energy structure of ASA lies 20.7 kcal/mol lower than the sum of the CH3COOH and SO3 monomer energies and 3.5 kcal/mol lower than that of the precursor complex, CH3COOH−SO3. The largest dipole moment component is along the ainertial axis (µa = 4.39 D), thus predicting a prominent a-type rotational spectrum. The b and c dipole moment components are much smaller (0.002 D and 0.90 D, respectively), the former being consistent with the absence of b-type transitions among our recorded spectra.

The internal rotation barrier of the methyl group on ASA was also estimated. To do so, the methyl group was rotated 60° from its equilibrium orientation and a transition state was optimized in which the imaginary frequency corresponds to methyl group rotation. This structure lies 227cm−1 above the minimum energy structure (without zero point corrections) and the energy difference represents the peak-to-peak difference in the 3-fold periodic potential for internal rotation of the methyl group. It is therefore directly comparable to the fitted value of V3 and is seen to be in good agreement with the experimental value of 241 cm−1 for the parent species.31

A comparison of the optimized ASA and CH3COOH−SO3 structures reveals that the equilibrium orientation of the CH3 group differs between them. In both structures, the in-plane CH bond is cis to the C=O bond. However, upon formation of the anhydride, the oxygen involved in the C=O bond changes, thus requiring the methyl group to rotate when the complex converts to the



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monomer. This difference is most easily seen in Figure 3, in which the equilibrium structures of CH3COOH−SO3 and ASA appear as structures I and V, respectively.

A computational search for a transition state connecting the CH3COOH−SO3 complex and ASA was also conducted. However, because of the difference in orientation of the methyl group between the two structures, the cycloaddition reaction is necessarily accompanied by internal rotation and as a result, no transition state (i.e., involving a single coordinate) could be found. Thus, the following approach was taken: Starting with the optimized geometry of the CH3COOH−SO3 complex, the methyl group was first rotated by 60° and a transition state was optimized (structure II in Figure 3). This structure is only 39 cm−1 higher in energy than that of I and corresponds to the internal rotation barrier of the methyl group in the complex. Starting from structure II, the geometry was adjusted such that a transition state corresponding to the cycloaddition was optimized, structure III in Figure 3. The barrier to formation of the anhydride without zero point corrections is 1.89 kcal/mol and is determined by the energy of structure III. When zero point corrections are included, structure III is 0.23 kcal/mol lower in energy than the complex and the small barrier of 0.06 kcal/mol is due to the zero point corrected internal rotation barrier of the methyl group (structure II).

Finally, the second order saddle point involving simultaneous internal rotation and cycloaddition was located and optimized (structure IV in Figure 3). Without zero point energy corrections, the barrier to conversion through this path is 2.10 kcal/mol. With zero point energy corrections, however, it is barrierless, with the second order saddle point lying 0.12 kcal/mol lower in energy than the complex. While we draw no specific conclusion as to whether the formation of ASA from CH3COOH−SO3 involves sequential or simultaneous internal rotation and cycloaddition, we note that both paths are energetically favorable.

Discussion As shown in Table 2, the calculated values for the spectroscopic parameters for ASA are in excellent agreement with the experimental results. Moreover, although not indicated in the table, the observed isotopic shifts in the rotational constants were also in good agreement with those


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predicted from the structure shown in Figure 2.32 Thus, there is no doubt as to the identity of the observed species. The 241 cm−1 internal rotation barrier in ASA is 43% higher than the 168 cm−1 barrier previously reported for acetic acid.33 Although the origin of the difference is not clear, we note that methyl group internal rotation barriers can vary significantly and can even be dependent on remote parts of the molecule in ways that are not arguably steric. For example, the 168 cm−1 barrier in CH3COOH falls to 138 cm−1 and 118 cm−1 in the hydrogen bonded complexes CH3COOH−H2O and CH3COOH−(H2O)2, respectively,34 despite the added waters occupying positions that would appear to be well isolated from the rotor. Interestingly, the barrier in the CH3COOH−SO3 complex is predicted to be only 39 cm−1. Goodman et al.35 have highlighted the difficulties associated with applying simple steric repulsion arguments to understanding internal rotation barriers, noting that full account of skeletal relaxation must be taken in order to achieve a correct understanding. Thus, we hesitate to take the additional step of offering a simplistic rationalization of the difference in barrier height between ASA and free acetic acid.

The observation of ASA provides a second example of the gas phase reaction between SO3 and a carboxylic acid and supports our previous conjecture19 that the process is not limited to formic acid. To the best of our knowledge, no prior gas phase observation or spectroscopic characterization of ASA has been reported. We note, however, that its sodium salt has been isolated,36,37 though ASA itself appears to be unstable with respect to rearrangement in nonaqueous media.38,39 The kinetics of CH3COOSO3− hydrolysis has also been investigated. 40 Salts of the form [M+][SO3OCHO−] dissolved in non-aqueous solvents have also been discussed in a patent for the synthesis of isoflavones.41

Spectral transitions of ASA were prominent in the chirped pulse spectrum and were quite strong when recorded on the cavity spectrometer, indicating that its formation in the supersonic jet is facile. This is consistent with the near-zero activation barrier shown in Figure 3B. Conditions in the jet change rapidly as the expansion proceeds, decreasing from room temperature to a typical temperature of nominally 2 K in only a few tens of microseconds. While the reaction certainly proceeds during the collisional phase of the expansion, i.e., prior to reaching this terminal 9 ACS Paragon Plus Environment


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temperature, it is presently unclear over what temperature range the reaction is actually taking place. Carboxylic acids are abundant in the atmosphere42,43 and their reaction with SO3 appears to be generally unrecognized. While the formation of atmospheric sulfuric acid is well modeled in terms of the reaction of SO3 with water,11,12 the mechanism appears to proceed through a trimeric intermediate, (H2O)2−SO3, and has an activation barrier of several kcal/mol (depending on whether the process considered involves H2O + SO3−H2O or (H2O)2 + SO3). In contrast, carboxylic sulfuric anhydrides require only the initial formation of a dimer and the activation barrier to subsequent reaction is nearly zero. It remains to be seen to what extent carboxylic sulfuric anhydrides play a role in the atmosphere. Even if typical concentrations of water are sufficient to dominate the consumption of atmospheric SO3, other roles for FSA, ASA, and their analogs may involve their ability to incorporate organic matter into aerosol particles, as indicated in equations 2–4. Regardless of whether or not gas phase carboxylic sulfuric anhydrides turn out to be atmospherically significant, to the best of our knowledge they appear to represent new sulfur chemistry and, as such, are interesting species to investigate.

Conclusion Acetic sulfuric anhydride (CH3COOSO2OH) has been observed by microwave spectroscopy in a supersonic expansion containing SO3 and CH3COOH. Computational work indicates that its formation proceeds via a π2 + π2 + σ2 cycloaddition reaction and that the zero point corrected activation barrier is nearly zero. The internal rotation barrier of the methyl group has been determined from the observation of A and E internal rotor states and the value obtained, 241 cm−1, is 43% larger than the corresponding value in the CH3COOH monomer. This work represents the second example in which a simple carboxylic acid is observed to readily react with SO3 to form a carboxylic sulfuric anhydride and supports our previous assertion that the gas phase reaction RCOOH + SO3 → RCOOSO2OH is a general one. Given the facile reaction between SO3 and carboxylic acids, the resulting anhydrides may be of interest in chemical models for atmospheric aerosol formation.


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Supporting Information Available The Supporting Information (tables of transition frequencies, assignments, and residuals from the least squares fits; computed Cartesian coordinates and energies for the minimum energy and transition state structures) is available free of charge on the ACS Publications website at xxxx.

Acknowledgments This work was supported by the National Science Foundation, Grant Nos. CHE-1266320 and CHE-1563324, and the Minnesota Supercomputer Institute. We thank Dr. Will Isley for valuable discussions about the computational aspects of this work.



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24. Canagaratna, M; Phillips, J.A.; Goodfriend, H.; Leopold, K.R. Structure and Bonding of the Sulfamic Acid Zwitterion: Microwave Spectrum of +H3N−SO3−. J. Am. Chem. Soc. 1996, 118, 5290-5295. 25. Hartwig, H.; Dreizler, H. The Microwave Spectrum of Trans-2,3-Dimethyloxirane in Torsional Excited States. Z. Naturforsch. 1996, 51a, 923–932. 26. Woods, R.C. A General Program for the Calculation of Internal Rotation Splittings in Microwave Spectroscopy. J. Mol. Spectrosc. 1966, 21, 4-24. 27. Trial fits with Fo fixed from 157 – 159 GHz led to a variation in the fitted value of V3 by only 1%. 28. Trial fits in which ε was constrained at values between 0 and 8° caused the fitted value of δ to change by no more than 0.15°. 29. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A., et al. Gaussian 09, Revision E1; Gaussian, Inc.: Wallingford, CT, 2009. 30. Neese, F.; Valeev, E.F. Revisiting the Atomic Natural Orbital Approach for Basis Sets: Robust Systematic Basis Sets for Explicitly Correlated and Conventional Correlated ab initio Methods. J. Chem. Theory Comput. 2011, 7, 33-43. 31. At the M06-2X level, the calculated barrier is 263 cm−1. 32. This was not only apparent via qualitative inspection of the observed rotational constants, but is reflected in the good agreement between the theoretical C-S distance, 3.886 Å, and the experimental value derived from a Kraitchman analysis 3.8944(34) Å (Table 2). Note that the b-coordinate of the sulfur atom obtained from this analysis is small, but if a second C-S distance is calculated using the opposite sign of the sulfur b-coordinate, a value of 3.9349(41) Å (Table 2) is obtained. While in worse agreement with the theory, the computational value is still within 0.05 Å of this result. Note that with the isotopic data available, the C-S distance is the only distance that can be determined by a Kraitchman analysis. 33. van Eijck, B. P.; van Opheusden, J.; van Schaik, M. M. M.; van Zoeren, E. Acetic Acid: Microwave Spectra, Internal Rotation and Substitution Structure. J. Mol. Spectrosc. 1981, 86, 465–479. 14 ACS Paragon Plus Environment

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34. Ouyang, B.; Howard, B.J. The Monohydrate and Dihydrate of Acetic Acid. A HighResolution Microwave Spectroscopic Study. Phys.Chem.Chem.Phys. 2009, 11, 366-373. 35. Goodman, L.; Pophristic, V.; Weinhold, F. Origin of Methyl Internal Rotation Barriers. Acc. Chem. Res. 1999, 32, 983-993. 36. Van Peski, A.J. Sur Les Anhydrides Mixtes D’Acide Sulfurique et de Carboacides. Rev. Trav. Chim. 1921, 40, 103-118. 37. Tanghe, L.J.; Brewer, R.J. Equilibrium between Sulfuric and Acetylsulfuric Acids in Acetic Acid-Acetic Anhydride. Anal. Chem. 1968, 40, 350 – 353. 38. Russell, J.; Cameron, A.E. Acidity Measurements with the Hydrogen Electrode in Mixtures of Acetic Acid and Acetic Anhydride. J. Am. Chem. Soc. 1938, 60, 1345 – 1348. 39. Enzo, M.; Fornaroli, M.; Giuffré, L.; Tempesti, E.; Sioli, G. Equilibria and Reactions of Acetylsulphuric Acid in Liquid Sulphur Dioxide. J.C.S. Perkin II 1976 (15), 1784-1789. 40. Benkovic, S.J.; Hevey, R.C. Studies in Sulfate Esters. V. Mechanism of Hydrolysis of Phenyl Phosphosulfate, A Model System for 3'-Phosphoadenosine 5'-Phosphosulfate. J. Am. Chem. Soc. 1970, 92, 4971−4977. 41. Burdick, D.C. “Process for the Preparation of Isoflavones”, International Publication Number W0 02/085881 A1, World Intellectual Property Organization, International Bureau (2002). 42. Khwaja, H.A. Atmospheric Concentrations of Carboxylic Acids and Related Compounds. Atmos. Environ. 1995, 29, 127-139. 43. Souza, S.R.; Vasconcellos, P.C.; Carvalho, L.R.F. Low Molecular Weight Carboxylic Acids in an Urban Atmosphere: Winter Measurements in São Paulo City, Brazil. Atmos. Environm. 1999, 33, 2563-2574.



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Table 1. Spectroscopic Constants for the Observed Isotopologues of ASAa 13 CH3COOSO2OH CH3COOSO2OH CH3COO34SO2OH A [MHz] 3630.8996(11) 3614.0543(17) 3630.186(52) B [MHz] 1359.30700(28) 1330.78967(36) 1352.09806(23) C [MHz] 1235.79407(18) 1210.29056(27) 1229.80579(28) ∆J [kHz] 0.0980(19) 0.0869(26) 0.0944(37) ∆JK [kHz] 0.694(11) 0.700(17) 0.767(51) 52.2(10) 56.2(14) 52.2e ∆iJ [kHz] ∆iK [kHz] -440(16) -470(21) -440e ∆i- [kHz] 12.5(13) 14.2(15) 12.5e 241.093(30) 241.080(42) 241.150(17) V3 [cm−1] ε [deg] 3.04(47) 2.52(69) 2.61(53) δ [deg] 33.122(34) 32.708(52) 33.207(25) b Fo[GHz] 158.757 158.757 158.757 c N 56 57 23 d σ [kHz] 0.8 1.0 1.4 (a) Numbers in parentheses are one standard error in the least squares fit. (b) Fixed to the value derived from the M06-2X/6-311++G(3df,3pd) structure (c) Number of lines included in the fit (d) Standard deviation of the residuals (e) Parameter fixed at the value determined from the parent fit


CD3COOSO2OD 3414.345(22) 1253.47425(22) 1134.73145(23) 0.0783(19) 0.575(16) 52.2e -440e 12.5e 234.57(13) 3.04e 31.59(22) 79.4396 55 3.7

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Table 2. Computational Results for ASA and Comparison with Experimental Valuesa M06-2X/6-311++G(3df,3pd) Experimental A [MHz] 3652 3630.899 (11) B [MHz] 1367 1359.30700(28) C [MHz] 1243 1235.79407(18) 263c 241.093(30) V3 [cm−1] 28.9 33.12(3) δ [deg] 3.00 3.04(47) ε [deg] 4.39 µa [D] 0.00b µb [D] 0.90 µc [D] C9-S1 Distance [Å] 3.886 3.8944(3) Internal H-Bond Length (O7-H8) [Å] 2.01 Internal H-Bond Angle (O7-H8-O3) [deg] 125 C5-O6-S1 Angle [deg] 120. (a) Values are for the parent isotopologue. (b) This value is not zero by symmetry. The calculated value is 0.0015 D. (c) Using the single point electronic energies calculated using CCSD(T), V3 = 227 cm−1.



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Figure Captions Figure 1. Portion of the 6-18 GHz chirped-pulse microwave spectrum of SO3 and acetic acid in Ar. The spectrum is an average of 10,000 free induction decay signals. ASA, Ar-SO3, and acetic acid monomer transitions are highlighted in red, blue, and green, respectively. Known instrumental artifacts have been removed from the spectrum.

Figure 2. The structure of ASA as determined by M06-2X/6-311++G(3df,3pd) geometry optimization.

Figure 3. Potential energy surface of the formation of ASA starting at the CH3COOH−SO3 complex. The zero of energy is defined as the sum of the free CH3COOH + SO3 energies. Structures with an imaginary frequency corresponding to a methyl rotation are indicated by an arrow over the methyl group. Structures with an imaginary frequency corresponding to the formation of the anhydride are indicated by the three arrows showing the cycloaddition. A sequential pathway (internal rotation followed by cycloaddition) is shown in black and a simultaneous formation of the anhydride through a second order saddle point is highlighted in red. (A) CCSD(T)/CBS//M06-2X/6-311++G(3df,3pd) electronic energies uncorrected for zeropoint energy (ZPE). The barriers to formation on the sequential and simultaneous pathways are 1.89 and 2.10 kcal/mol, respectively. (B) CCSD(T)/CBS//M06-2X/6-311++G(3df,3pd) electronic energies corrected with ZPE from M06-2X frequency calculations. The barriers to formation on the sequential and simultaneous pathways are 0.06 and −0.12 kcal/mol, respectively.


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Figure 1



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Figure 2


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Figure 3



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Facile Formation of Acetic Sulfuric Anhydride: Microwave Spectrum

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