Observation of Two Conformers of Acrylic Sulfuric Anhydride by

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Observation of Two Conformers of Acrylic Sulfuric Anhydride by Microwave Spectroscopy C.J. Smith, Anna K Huff, Rebecca B Mackenzie, and Kenneth R. Leopold J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b09833 • Publication Date (Web): 03 Nov 2017 Downloaded from http://pubs.acs.org on November 4, 2017

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The Journal of Physical Chemistry

October 30, 2017 J. Phys Chem. A Revised

Observation of Two Conformers of Acrylic Sulfuric Anhydride by Microwave Spectroscopy

C.J. Smith, Anna K. Huff, Rebecca B. Mackenzie, Kenneth R. Leopold* Department of Chemistry, University of Minnesota, 207 Pleasant St., SE, Minneapolis, Minnesota 55455, United States

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

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Abstract The rotational spectrum of acrylic sulfuric anhydride (CH2=CHCOOSO2OH, AcrSA) has been observed using pulsed-nozzle Fourier transform microwave spectroscopy. The species was produced from the reaction between acrylic acid and sulfur trioxide in a supersonic jet. Spectroscopic constants are reported for both the s-cis- and s-trans-AcrSA conformers of the parent and monodeuterated (OD) isotopologues. Geometries were optimized for both conformers using M06-2X/6-311++G(3df,3pd) methods. Single point energy calculations at the M06-2X geometries were calculated using the CCSD(T)/complete basis set method with double and triple extrapolation [CBS(D-T)]. Further calculations indicate that the anhydride results from a π2 + π2 + σ2 cycloaddition reaction within the acrylic acid – SO3 complex. Because the C=O double bond of the acrylic acid migrates from one of the COOH oxygens to the other during the reaction, the s-cis form of acrylic acid leads to the s-trans form of the anhydride and vice versa. With zero point energy corrections applied to the CCSD(T) energies, the s-cis and s-trans forms of CH2=CHCOOSO2OH are 19.0 kcal/mol and 18.8 kcal/mol lower in energy than that of SO3 + their corresponding CH2=CHCOOH precursor conformation. The zero point corrected transition state energies for formation of the s-trans and s-cis anhydrides are 0.22 kcal/mol and 0.33 kcal/mol lower than those of the complexes of SO3 with s-cis and s-trans acrylic acid, respectively, indicating that the reaction is essentially barrierless. This system adds to a growing body of examples which demonstrate that carboxylic acids readily add to SO3 in the gas phase to produce the corresponding carboxylic sulfuric anhydride.


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Introduction The formation and activity of atmospheric aerosol is an active area of research.1-4 Interest in this topic stems from the ability of aerosol particles to modulate the earth’s energy budget via reflection and absorption of solar radiation, their function as cloud condensation nuclei, and their role as sites for heterogeneous chemistry. There is now considerable evidence that aerosol formation in the atmosphere is a multi-component process, and that a wide range of substances can serve to stabilize precursor clusters during the early stages of nucleation.5-8 Indeed, while great strides have been made in understanding the chemical composition of aerosol precursors,9 the chemical and dynamic complexity of the atmosphere is such that the modeling of new particle formation remains a challenging problem.

Despite the difficulties, sulfuric acid is widely recognized as a key component in atmospheric aerosol nucleation.9-11 Early work suggested the H2O−SO3 complex as the primary precursor to the formation of H2SO4 from H2O and SO3,12,13 but subsequent studies concluded that the activation energy for intramolecular proton transfer within the 1:1 complex was too large for the direct reaction to be of importance.14,15 Further theoretical and experimental work later indicated, that the hydration of SO3 can be catalyzed by the addition of a second H2O molecule to form an SO3−(H2O)2 intermediate,16-19 and more recent computational studies suggest that atmospheric species other than water can also catalyze the process.20-24 Most relevant to the present report is work by Hazra and Sinha20 and Long et al.23 indicating that replacement of the second water by formic acid to produce an SO3−H2O−HCOOH intermediate yields H2SO4 with essentially no barrier at all. Hazra and Sinha further argued that since the immediate product is a hydrogen bonded complex between HCOOH and H2SO4, the process may be a preliminary step en route to the formation of a critical nucleation cluster.

With this idea in mind, we recently set out to investigate complexes containing SO3, HCOOH, and H2SO4 by microwave spectroscopy in a supersonic jet. To our surprise, however, upon expanding a mixture of HCOOH and SO3 in argon, the most prominent spectral features observed were not due to weakly bound complexes, but rather to a new molecule, HCOOSO2OH (formic sulfuric anhydride, FSA), which we concluded was formed from a cycloaddition reaction between SO3 and HCOOH.25 Theoretical calculations suggested that the activation energy for the 3 ACS Paragon Plus Environment


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process is only 0.2 kcal/mol after zero point corrections, and furthermore that benzoic and pinic acids would undergo similar reactions with slightly negative activation energies. We speculated that the formation and subsequent hydrolysis of FSA (or other carboxylic sulfuric anhydrides) either in a water cluster or a pre-existing droplet, could provide a pathway for the incorporation of organic matter into atmospheric aerosol.25

Further experimental and theoretical work in our laboratory has identified the acetic acid analog of FSA (CH3COOSO2OH, ASA),26 providing additional experimental evidence that the reaction of SO3 + RCOOH can occur with a carboxylic acid other than HCOOH. Internal rotation of the methyl group raised some interesting issues insofar as its equilibrium orientation changes when CH3COOH is converted to ASA. Specifically, because the equilibrium orientation of the methyl group differs between acetic acid and acetic sulfuric anhydride, the cycloaddition reaction that forms the anhydride must be accompanied by internal rotation. Whether the two processes are sequential or simultaneous could not be addressed, but both paths were shown to have a negligible activation barrier.

In this paper, we examine the reaction of SO3 with acrylic acid (CH2=CHCOOH). Acrylic acid has two conformers, s-cis and s-trans (Figure 1), where the nomenclature follows the IUPAC designations in which s-cis and s-trans refer to the relative orientation of two double bonds separated by a single bond.27 The s-cis form is slightly more stable than the s-trans form, with estimates of the relative energies lying in the 0.2 to 0.6 kcal/mol range.28-31 Note that for both of these forms, the syn orientation of the OH and carbonyl group is the energetically preferred arrangement. We show that the reaction of SO3 with acrylic acid produces both s-cis and s-trans conformers of acrylic sulfuric anhydride (AcrSA), though our calculations identify transition states that connect the s-cis form of the monomer with the s-trans form of the anhydride and vice versa.

Acrylic acid is an important industrial chemical, and while we are unaware of specific determinations of its concentration in the atmosphere, numerous reports of its release appear on the EPA’s Toxic Release Inventory.32,33 Studies aimed at understanding its atmospheric degradation have also been reported.34 Moreover, carboxylic acids in general are abundant in the 4 ACS Paragon Plus Environment

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atmosphere and their chemistry is of broad interest.35-38 This system provides an additional example of the formation of a carboxylic sulfuric anhydride and, as such, contributes to a general characterization of what appears to be a relatively unrecognized reaction of carboxylic acids with SO3.

Experimental Methods and Results Rotational spectra of AcrSA were obtained using a Fourier transform microwave (FTMW) spectrometer with combined cavity39 and chirped-pulse40 capabilities. Details of the instrument have been described elsewhere,41,42 and the molecular source design was the same as that used in our previous work on acetic sulfuric anhydride.26 SO3 was introduced into the system by passing argon over a reservoir of solid, polymerized SO3, which was then expanded at a stagnation pressure of 1.3 atm through a 0.8 mm diameter nozzle into the spectrometer. Acrylic acid vapor entrained in argon, at a pressure of 0.7 atm, was added through a continuous flow line, which terminates in an “injection needle” of inner diameter 0.012 inches.43 The needle was placed a few millimeters downstream of the nozzle orifice so as to introduce the sample along the axis of the expansion. A 5:1 acrylic acid/D2O mixture was used to collect H2C=CHCOOSO2OD rotational spectra with the same apparatus. Acrylic acid and SO3 were obtained from Sigma-Aldrich and D2O was obtained from Cambridge Isotope Laboratories, Inc. Initially, spectra were collected from 6 – 18 GHz in 3 GHz segments using our broadband chirped-pulse system. For each segment, 10,000 free induction decay signals were averaged, each with a 90 µs data collection time. The observed spectrum is shown in Figure 2a. Once transitions of known species were removed from consideration, spectra of the s-cis and s-trans forms of the anhydride were identified using predicted structures (see next section) and were each fit to the A-reduced Watson Hamiltonian44 using the SPFIT program of Pickett.45 Fewer than four unassigned features remained in the chirped pulse spectrum, none of which could be assigned to the SO3−acrylic acid van der Waals complex. (See next section for calculated structures.) Preliminary fits utilized only a-type transitions, but with non-zero dipole moment components predicted along all three inertial axes for both conformers, b- and c-type transitions were also anticipated. After switching to our narrowband cavity spectrometer, the weaker b- and c-type transitions were readily observed, and higher resolution measurements of several a-type 5 ACS Paragon Plus Environment


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transitions were obtained. The monodeuterated isotopologue was also readily observed with the cavity system, with which transitions were recorded between 5.6 and 10.6 GHz. Deuterium hyperfine structure was not resolved in these spectra and therefore no nuclear quadrupole coupling constants were obtained. For all measurements using the cavity mode, the sample was pulsed at a rate of 4 Hz and four free induction decay signals were collected per pulse, each with a data collection time of 140.8 µs. Transition frequencies measured on the chirped-pulse system were typically accurate to ~30 kHz. Measurements made on the cavity system were accurate to ~4 kHz. A recording of the 312 ← 202 transition of the s-cis conformer taken on the cavity spectrometer is shown in Figure 2b. Spectroscopic constants are given in Table 1. The slightly larger rms residual for the s-trans parent species appears to be related to a number of the higher J, K-1 transitions included. However, since it is still significantly less than the estimated uncertainty in the chirped-pulse measurements, no attempts were made to pursue the issue. The centrifugal distortion constants appear to show a curious isotope dependence, though this, too, may be an artifact of the number and identity of measured transitions. Observed frequencies, assignments, and residuals from the least squares fits are provided as Supporting Information.

Computational Methods and Results All structures were optimized at the M06-2X/6-311++G(3df,3pd) level of theory46 using the Gaussian suite of programs.47 This level proved successful in our prior work on the formic25 and acetic26 sulfuric anhydrides and was applied here to be consistent with the previous studies. The equilibrium structures of s-cis- and s-trans-AcrSA, as well as those of both conformers of the acrylic acid − SO3 weakly bound complex, are shown in Figure 3. Chemically significant results for all four species are summarized in Table 2 and Cartesian coordinates for all relevant structures are given as Supporting Information. To improve the quality of the computed energies, single point calculations were performed using M06-2X geometries with the CCSD(T)/complete basis set with double and triple extrapolation [CBS(D-T)] method.48 Zero-point energy corrections were applied using the frequencies obtained from the M06-2X calculations. A summary of the energy calculations is presented in Table 3.

Computational searches for transition states connecting the putative CH2=CH-COOH−SO3 complexes with their corresponding anhydrides were also conducted. Cyclic transition state 6 ACS Paragon Plus Environment

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structures analogous to those previously found for the RCOOH−SO3 → RCOOSO2OH (R = H, CH3) reaction were identified for both the s-cis- and s-trans-acrylic acid reactions. Transition state energies relative to the weakly bound SO3 complexes are included in Table 3, and key points on the potential energy surface are shown in Fig 4. The CCSD(T) barriers without zero point corrections are 1.8 and 1.9 kcal/mol for the formation of s-cis- and s-trans-AcrSA, respectively. Both barriers are eliminated with the inclusion of zero point energy, with the transition state energies 0.22 and 0.33 kcal/mol below that of the acrylic-SO3 complexes for s-cis and s-trans-acrylic acid, respectively. The imaginary frequencies at the transition states for the scis and s-trans conformers both correspond to a concerted reaction, fully analogous to that of the π2+ π2+σ2 cycloaddition mechanism previously proposed for the HCOOH25 and CH3COOH26 reactions. The overall formation energies for s-cis-AcrSA and s-trans-AcrSA from s-trans- and s-cis-acrylic acid are −19.0 and −18.8 kcal/mol, respectively. At the level of theory used, the scis/s-trans energy difference for the acrylic acid monomer is 0.25 kcal/mol, in agreement with the range reported in the literature.28-31 In contrast, the energy difference between the s-trans and s-cis conformers of AcrSA is virtually zero (0.02 kcal/mol).

The barrier to interconversion from the s-cis to s-trans form via rotation about the C−C single bond was also estimated by locating a transition state along the coordinate corresponding to O=C−C=C dihedral angle. For the acrylic acid monomer, the value obtained from CCSD(T) calculations with zero point energy corrections was 5.6 kcal/mol (6.3 kcal/mol uncorrected), in good agreement with previous estimates.28-31 For the anhydride at the same level of theory, a value of 6.2 kcal/mol (6.7 kcal/mol uncorrected) was obtained. For both acrylic acid and the anhydride, the dihedral angle at the transition state is very close to 90° (within 1°).

Discussion Table 4 compares the experimental rotational constants and their H/D shifts with those calculated for the four structures in Figure 3. Inspection of the table reveals that the values obtained for the species assigned to the s-cis anhydride are in excellent agreement with the computational results while, overall, in rather poor agreement with calculated values for the van der Waals complexes or the s-trans anhydride. The values for the A and B rotational constants, as well as their isotope shifts, are particularly compelling. We note that the C rotational constant for the deuterated 7 ACS Paragon Plus Environment


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species, and hence the isotope shift, do not provide as clean a comparison. (The calculated values for either the s-cis anhydride or the s-cis van der Waals complex could be construed as being reasonably commensurate with the experiment.) However, the other constants clearly rule out the van der Waals complex. Similar comparisons confirm the assignment for the s-trans anhydride. Thus, these results leave little doubt that the observed spectra correspond to the s-cis and s-trans conformers of AcrSA. That their formation in the jet is favorable is consistent with the low calculated activation barrier for conversion of the van der Waals complex to the anhydride, as well as with our experience with the formic and acetic acid analogues.

The reaction of SO3 + acrylic acid is energetically favorable. Specifically, relative to SO3 + acrylic acid, the anhydrides are 19 kcal/mol more stable than the isolated monomers. Note that in free acrylic acid, the s-cis conformer is lower in energy than the s-trans conformer by 0.25 kcal/mol while in the anhydride, the energies are essentially the same (at the level of theory used).

The definitive observation of acrylic sulfuric anhydride provides the third experimental demonstration that carboxylic acids readily react with SO3 in the gas phase to form a corresponding carboxylic sulfuric anhydride. In all cases, supporting theoretical calculations indicate a cycloaddition mechanism with a small or negative activation barrier. With HCOOH, the reaction is straightforward. With CH3COOH, the mechanism is essentially the same, though internal rotation of the methyl group introduces some complications which have been discussed.26 In the case of acrylic acid, the new feature is the existence of a pair of conformers derived from the two conformations of the parent acid. As noted above, the s-cis form of the acid gives rise to the s-trans form of the anhydride and vice versa. However, this does not arise from an internal rotation about the C5-C9 bond. Rather, it is because the carbon-oxygen double bond migrates from O6 to O7 and because the s-cis and s-trans conformers are named according to the dihedral angle between the two double bonds separated by the C5-C9 single bond (See Figure 4.).

A variety of carboxylic acids with concentrations ranging from a few hundredths to a few tens of parts per billion35,37,38 have been detected in the atmosphere, and their potential to enhance 8 ACS Paragon Plus Environment

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atmospheric nucleation has been discussed.49−52 Although we are unaware of any specific observations of acrylic acid, the generality of the cycloaddition mechanism for the RCOOH + SO3 reaction and the absence of any significant activation barrier suggest that carboxylic sulfuric anhydrides could also play a role in atmospheric chemistry. As we have noted previously, one such role may be serving as a vehicle to incorporate small organic molecules into atmospheric aerosol via hydrolysis in clusters or pre-existing aqueous droplets.

Conclusion The s-cis and s-trans conformers of acrylic sulfuric anhydride have been formed in a supersonic jet and observed by microwave spectroscopy. M06-2X/6-311++G(3df,3pd) calculations indicate that their formation proceeds via a π2+ π2+σ2 cycloaddition reaction within the acrylic acid − SO3 complex and that the zero-point corrected activation barrier is less than zero. CCSD(T)/CBS calculations at the optimized M06-2X geometries indicate that s-cis-AcrSA and s-trans-AcrSA are lower in energy than their corresponding free monomer precursors by 19.0 kcal/mol and 18.8 kcal/mol, respectively. Because the carbon-oxygen double bond migrates from the C=O oxygen of the COOH group to the OH oxygen during the cycloaddition, the s-cis conformer of acrylic acid produces the s-trans conformer of the anhydride and vice versa. This system provides a third experimental example of the facile reaction between SO3 and a carboxylic acid, suggesting that the cycloaddition reaction occurs for a variety of acids. Given the aggregate concentration of these organic acids in the atmosphere, carboxylic sulfuric anhydrides may play a role in atmospheric chemistry.

Acknowledgements This work was supported by the National Science Foundation (Grant Nos. CHE–1266320 and CHE 1563324) and the Minnesota Supercomputing Institute.

Supporting Information Available Tables of transition frequencies, assignments, and residuals from the least squares fits; tables of atomic Cartesian coordinates for calculated structures of s-cis and s-trans AcrSA and their precursor SO3 complexes and corresponding transition states.



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32.https://iaspub.epa.gov/triexplorer/release_fac?p_view=USFA&trilib=TRIQ1&sort=_VIEW_ &sort_fmt=1&state=All+states&county=All+counties&zipcode=&epa_region=&chemical=0 00079107&industry=ALL&YEAR=2015&tab_rpt=1&FLD=RELLBY&FLD=TSFDSP 33. See also http://www.toxipedia.org/display/toxipedia/Acrylic+Acid. 34. Teruel, M.A.; Blanco, M.B.; Luque, G.R. Atmospheric Fate of Acrylic Acid and Acrylonitrile: Rate Constants with Cl Atoms and OH Radicals in the Gas Phase. Atmos. Environ. 2007, 41, 5769-5777. 35. Veres, P.R.; Roberts, J.M.; Cochran, A.K.; Gilman, J.B.; Kuster, W.C.; Holloway, J.S.; Graus, M.; Flynn, J.; Lefer, B.; Warneke, C.; de Gouw, J. Evidence of Rapid Production of Organic Acids in an Urban Air Mass. Geophys. Res. Lett. 2011, 38, L17807. 36. Vaïtilingom, M.; Charbouillot, T.; Deguillaume, L.; Maisonobe, R.; Parazols, M.; Amato, P.; Sancelme, M.; Delort, A.-M. Atmospheric Chemistry of Carboxylic Acids: Microbial Implication versus Photochemistry. Atmos. Chem. Phys. 2011, 11, 8721-8733. 37. Khwaja, H.A. Atmospheric Concentrations of Carboxylic Acids and Related Compounds at a Semiurban Site. Atmos. Environ. 1995, 29, 127-139. 38. Finlayson-Pitts, B.J.; Pitts, Jr., J.N. Chemistry of the Upper and Lower Atmosphere, Theory Experiments, and Applications. Academic Press: San Diego 2000, and references therein. 39. Balle, T.J.; Flygare, W.H. Fabry–Perot Cavity Pulsed Fourier Transform Microwave Spectrometer with a Pulsed Nozzle Particle Source. Rev. Sci. Instrum. 1981, 52, 33-45. 40. Brown, G.G.; Dian, B.C.; Douglass, K.O.; Geyer, S.M.; Shipman, S.T.; Pate, B.H. A Broadband Fourier Transform Microwave Spectrometer Based on Chirped Pulse Excitation. Rev. Sci. Instrum. 2008, 79, 053103-1-13. 41. Phillips, J.A.; Canagaratna, M.; Goodfriend, H.; Grushow, A.; Almlöf, J.; Leopold, K.R. Microwave and ab Initio Investigation of HF−BF3. J. Am. Chem. Soc. 1995, 117, 1254912556. 42. Dewberry, C.T.; Mackenzie, R.B.; Green, S.; Leopold, K.R. 3D-Printed Slit Nozzles for Fourier Transform Microwave Spectroscopy. Rev. Sci. Instum. 2015, 86, 065107-1-7. 43. 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. 13 ACS Paragon Plus Environment


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44. Gordy, W.; Cook, R.L. Microwave Molecular Spectra, Third Ed., John Wiley and Sons, New York, 1984. 45. Pickett, H.M. The Fitting and Prediction of Vibration-Rotation Spectra with Spin Interactions J. Mol. Spectrosc. 1991, 148, 371-377. 46. Zhao, Y.; Truhlar, D.G. The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of Four M06-Class Functionals and 12 Other Functionals Theor. Chem. Accts. 2008 120, 215-241. 47. 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. 48. 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. 49. Zhang, R.; Suh, I.; Zhao, J.; Zhang, D.; Fortner, E.C.; Tie, X.; Molina, L.T.; Molina, M.J. Atmospheric New Particle Formation Enhanced by Organic Acids. Science 2004, 304, 14871490. 50. Zhang, R.; Wang, L.; Khalizov, A.F.;Zhao, J.; Zheng, J.; McGraw, R.L.; Molina, L.T. Formation of Nanoparticles of Blue Haze Enhanced by Anthropogenic Pollution. Proc. Nat. Acad. Sci. USA 2009, 106, 17650-17654. 51. Elm, J.; Kurtén, T.; Bilde, M.; Mikkelsen, K.V. J. Phys. Chem. A 2014, 118, 7892-7900. 52. Xu , Y.; Nadykto, A.B.; Yu, F.; Herb, J.; Wang, Wei. Interaction between Common Organic Acids and Trace Nucleation Species in the Earth’s Atmosphere. J. Phys. Chem. A 2010, 114, 387-396.


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Table 1: Spectroscopic Constants of Conformers and Isotopologues of AcrSAa s-trans-AcrSA s-cis-AcrSA s-trans-OD-AcrSA 2852.59731(50) 3639.31630(57) 2801.452(38) A (MHz) 1052.22420(32) 896.25135(15) 1049.76868(45) B (MHz) 905.04333(30) 837.56260(21) 899.14677(13) C (MHz) 0.0722(37) 0.0349(10) 0.0547(46) ∆J (kHz) 0.360(37) 0.595(20) 0.23(10) ∆JK (kHz) b 42 41 15 N 10 3 4 RMS (kHz) a Numbers in the parentheses are one standard error in the least squares fit. b Number of transition frequencies in the least squares fit.


s-cis-OD-AcrSA 3557.207(70) 895.21100(11) 833.24420(11) 0.0308(12) 0.671(24) 15 1


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Table 2: Computational Results for AcrSA and SO3⋯Acrylic Acid van der Waals Complexes s-cis anhydride A [MHz] 3667 B [MHz] 901 C [MHz] 842 5.08 µa [D] 0.26 µb [D] µc [D] 0.9 R(O7-H8) [Å] 1.997 125.9 ∠(O7-H8-O3) [deg] 119.7 ∠(C5-O6-S1) [deg] R(S1-O6) [Å] 1.627 R(C5-O6) Å] 1.371 (a) M06-2X/6-311++G(3df,3pd).

s-trans anhydride 2874 1057 910 4.6 0.35 0.9 2.00 125.9 119.7 1.624 1.371

s-cis vdw complex 2910 960 837 6.29 0.51 0.11 1.01 153.3 124.1 1.91 1.25


s-trans vdw complex 3711 839 787 6.82 0.28 0.13 1.01 153.7 123.7 1.91 1.25

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Table 3. Summary of Energetics for Complexes and Anhydrides of SO3 with Acrylic Acida Species M06-2X

Energy Difference (kcal/mol) M06-2X + Zero CCSD CCSD + Zero Point Corrections Point Corrections 17.4 17.6 16.1

s-trans-Acrylic + SO3 s-trans-Acrylic⋅⋅⋅SO3

18.9

s-cis-Acrylic + SO3 s-cis-Acrylic⋅⋅⋅SO3

18.9

17.4

17.6

16.1

s-trans-Acrylic + SO3 s-cis-AcrSA s-cis-Acrylic + SO3 s-trans-AcrSA s-trans-Acrylic⋅⋅⋅SO3(TS) s-trans-Acrylic⋅⋅⋅SO3 b s-cis-Acrylic⋅⋅⋅SO3(TS) s-cis-Acrylic⋅⋅⋅SO3c s-trans-Acrylic Acid s-cis-Acrylic Acid s-trans-AcrSA s-cis-AcrSA

22.3

20.5

20.8

19.0

22.2

20.3

20.7

18.8

1.6

−0.55

1.8

−0.33

1.7

−0.44

1.9

−0.22

0.20

0.22

0.24

0.25

−0.12

−0.002

−0.10

+0.02

Description

Complexation Energy, SO3 + strans-Acrylic Acid Complexation Energy, SO3 + s-cisAcrylic Acid s-cis-Anhydride Formation Energy s-trans-Anhydride Formation Energy Barrier Relative to strans Complex Barrier Relative to scis Complex Acrylic Acid s-cis/strans Difference Acr-SA s-cis-/s-trans Difference

C B H G E D A F

(a) Entry in the Table gives the energy difference between the species listed in the left-most column (top species minus bottom species, i.e., a positive number indicates that the top species listed in the row is higher energy). “+” refers to the isolated monomers. “⋅⋅⋅” refers to the weakly bound complex. Letters in the right-most column refers to the energy difference indicated in Figure 4. (b) Leads to s-cis-AcrSA. (c) Leads to s-trans-AcrSA.



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Table 4. Comparison between Observed and Theoreticalb H/D Isotope Shifts. Obs. s-cis Obs. s-trans Calc. s-cis Calc. s-trans Calc. s-cis Anhydride Anhydride Anhydride Anhydride vdw A(H) 3639.3163 2852.597 3667.120 2874.375 2910.117 A(D) 3557.207 2801.452 3580.361 2820.857 2857.950 Shifta −82.109 −51.145 −86.759 −53.518 −52.167 B(H) B(D) Shifta

896.251 895.211 −1.040

1052.224 1049.769 −2.456

901.388 900.067 −1.321

1057.159 1054.285 −2.874

959.975 959.950 −0.025

Calc. s-trans vdw 3710.810 3646.441 −64.369 838.758 838.556 −0.202

C(H) 837.563 905.043 842.265 909.674 836.764 786.569 C(D) 833.244 899.147 837.605 903.412 832.329 783.483 a Shift −4.318 −5.897 −4.660 −6.262 −4.435 −3.086 (a) Value of the rotational constant for the OD species minus that for the OH species. (b) Determined from M06-2X/6-311++G(3df,3pd) optimized geometries.


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Figure Captions Figure 1. M06-2X/6-311++G(3df,3pd) minimum energy structures of the s-cis and s-trans forms of acrylic acid.

Figure 2. (a) A 6-18 GHz chirped-pulse microwave spectrum of SO3 and acrylic acid in Ar. The spectrum is an average of 10,000 free induction decay signals. s-cis- and s-trans-AcrSA, Ar-SO3, as well as s-cis- and s-trans-acrylic acid monomer transitions are highlighted in red, blue, purple, green, and orange, respectively. Known instrumental artifacts have been removed from the spectrum. (b) A trace of the 312 ← 202 transition of the s-cis conformer recorded on the cavity spectrometer.

Figure 3. Equilibrium structures of the s-cis and s-trans conformers of AcrSA and the acrylic acid−SO3 complex obtained from M06-2X/6-311++G(3df, 3pd) calculations.

Figure 4. Zero point corrected CCSD(T)/CBS energies for key points on the SO3 + acrylic acid potential energy surface. The transformation of s-cis-acrylic acid + SO3 to s-trans-AcrSA is depicted in red. The transformation of s-trans-acrylic acid + SO3 to s-cis-ASA is depicted in blue. Zero point corrections were obtained from M06-2X/6-311++G(3df,3pd) calculations. See Table 3 for the energies A-H.



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Figure 1

s-cis-Acrylic Acid

s-trans-Acrylic Acid

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Figure 2

(a)

(b)


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Figure 3


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Figure 4



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Observation of Two Conformers of Acrylic Sulfuric Anhydride by

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