A Perfluorinated Carboxylic Sulfuric Anhydride: Microwave and

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

A Perfluorinated Carboxylic Sulfuric Anhydride: Microwave and Computational Studies of CFCOOSOOH 3

2

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.9b00300 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on January 30, 2019

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The Journal of Physical Chemistry

January 10, 2019 J. Phys. Chem. A Submitted

A Perfluorinated Carboxylic Sulfuric Anhydride: Microwave and Computational Studies of CF3COOSO2OH

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]

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Abstract Trifluoroacetic sulfuric anhydride (CF3COOSO2OH, TFASA) and its deuterated isotopologue have been observed by pulsed-nozzle Fourier transform microwave spectroscopy. TFASA was generated in situ in a supersonic expansion from the reaction of CF3COOH or CF3COOD with SO3. The spectrum, which was notably weaker than those of previously studied carboxylic sulfuric anhydrides, is that of a simple asymmetric rotor with no evidence of internal rotation of the CF3 group. Calculations at the M06-2X/6-311++G(3df,3pd) level indicate that the title compound is produced via a mechanism involving a concerted cycloaddition, analogous to that found for other carboxylic sulfuric anhydrides. The calculations further show that the equilibrium orientation of CF3 relative to the C=O bond changes upon formation of the anhydride, indicating that any path connecting the equilibrium structures of CF3COOH and CF3COOSO2OH

necessarily

includes

both

cycloaddition

and

internal

rotation.

CCSD(T)/complete basis set with double and triple extrapolation [CBS(D-T)] single-point energy calculations at key points on the potential surface indicate that the barrier to form TFASA from a putative CF3COOH···SO3 complex is about 1.2 kcal/mol after zero-point energy corrections. This value is significantly larger than the near-zero or slightly negative barriers previously reported for the reactions of SO3 with non-fluorinated carboxylic acids and likely accounts, at least in part, for the reduced spectral intensity. Thus, TFASA is a somewhat unique addition to the series of carboxylic sulfuric anhydrides studied to date. Theoretical values of certain structural parameters, atomic charges, and vibrational frequencies also support this point of view. Despite the differences, however, this work clearly demonstrates that the reaction RCOOH + SO3  RCOOSO2OH readily occurs in the gas phase and is not restricted to acids with hydrocarbon R groups.


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Introduction The potential impact of trifluoroacetic acid (TFA) in the environment has been a subject of numerous studies because it is a major degradation product of hydrochlorofluorocarbons (HCFC-123, HCFC-124), hydrofluorocarbons (HFC-134a), and other substitutes for ozonedepleting substances (HFO-1234yf).1-12 TFA is relatively inert in the atmosphere3,12,13 and has been observed in air and soil samples around the globe.4,5,14,15 Since it is otherwise stable and soluble in water, it is mainly removed from the atmosphere by precipitation and dry deposition but is resistant to subsequent degradation and therefore persists in the environment.3,8,9,11-14,16 While current TFA levels in aqueous environments are generally well below the toxic threshold, it is expected that TFA concentrations may continue to increase as refrigerants with high ozone depleting potential are phased out and replaced with those having shorter atmospheric lifetimes and higher TFA yields. 6-8,17 With the potential for a future rise in the production of atmospheric TFA, the exploration of its chemistry with other atmospherically active species is of considerable interest. Recent work in our laboratory has examined the formation of carboxylic sulfuric anhydrides from sulfur trioxide and non-fluorinated carboxylic acids.18-20 We have previously speculated that, if formed in the atmosphere, these compounds could play a role in the formation of atmospheric aerosol.18-21 For example, their hydrolysis in water-containing clusters or preexisting droplets would yield H2SO4 + RCOOH, thus providing a pathway to incorporate volatile organic compounds into aerosols or aerosol precursors. Another potential role involves the possibility that they may, themselves, act as seed species for aerosol nucleation.18 Recent computational work by Zhang et al.22 on the formation of 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA) atmospheric clusters has indicated that the formation of the corresponding carboxylic sulfuric anhydride would result in clusters of lower vapor pressure and higher stability than those containing MBTCA and H2SO4. Elm et al.23 have also indicated that MBTCA and H2SO4, alone, do not form clusters of sufficient stability to act as MBTCA-based nucleation precursors, and that the corresponding carboxylic sulfuric anhydrides might be viable alternates for providing the necessary enhancement in stability.



To date, we have reported the formation of formic sulfuric anhydride (HCOOSO2OH),18 acetic sulfuric anhydride (CH3COOSO2OH),19 and two conformers of acrylic sulfuric anhydride, (s-cis/s-trans-CH2CHCOOSO2OH),20 in supersonic jets containing SO3 and the corresponding carboxylic acid. Theoretical calculations have shown that these species are produced via an essentially barrierless π2 + π2 + σ2 cycloaddition reaction within the precursor RCOOH∙∙∙SO3 complex, and that the process is largely insensitive to the nature of the R group. We have also observed the CH3COOSO2OH∙∙∙H2O complex and showed that one water molecule is not sufficient to hydrolyze the anhydride in a cold molecular cluster.21 However, our work so far has involved only RCOOH species with hydrocarbon R groups, and in light of the potential rise in trifluoroacetic acid concentrations in the atmosphere, the chemistry involving halogenated analogues is a topic of some interest. In this paper, we report the microwave observation of trifluoroacetic sulfuric anhydride (CF3COOSO2OH, TFASA) formed in a supersonic jet containing CF3COOH and SO3. Density functional theory calculations are used to investigate the reaction mechanism and to show that internal rotation of the CF3 group must accompany a cycloaddition reaction within the putative CF3COOH∙∙∙SO3 complex. Possible formation pathways involving either simultaneous or sequential CF3 internal rotation and cycloaddition are presented, and differences in structure and energetics between TFASA and the previously studied carboxylic sulfuric anhydrides are discussed. TFASA is shown to be somewhat unique among the carboxylic sulfuric anhydrides studied to date, but despite the differences, this work further demonstrates that a variety of carboxylic acids can react with SO3 to produce their corresponding carboxylic sulfuric anhydride. Experimental Methods and Results Rotational spectra of TFASA were collected using a combined cavity24 and chirped-pulse25 Fourier transform microwave spectrometer, details of which have been given elsewhere.26,27 SO3 vapor was entrained in Ar by passing the carrier gas over a solid, polymerized sample of SO3 and then pulsed into the spectrometer through a stainless steel 0.8 mm diameter orifice cone nozzle at a stagnation pressure of 2.3 atm. Trifluoroacetic acid vapor was separately introduced along the axis of the expansion through a 0.012 in. ID hypodermic needle28 about 0.4 in. downstream from 4 ACS Paragon Plus Environment

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the 0.8 mm orifice as described in previous work.19-21 The vapor of trifluoroacetic acid was introduced by bubbling Ar through chilled (~0 °C) liquid trifluoroacetic acid (99%, Sigma Aldrich) at a backing pressure of 0.6 atm. Initial attempts to collect TFASA spectra were carried out using the chirped-pulse instrument, but they were unsuccessful at providing sufficient data to achieve preliminary spectral assignments despite apparently sufficient monomer signals. Therefore, searches were conducted using the cavity mode of the spectrometer, which offers higher sensitivity. Spectra were collected for the parent and deuterated isotopologues, with the latter produced by using an isotopically enriched sample of deuterated trifluoroacetic acid (99%), also obtained from Sigma Aldrich. Spectra of both isotopologues consisted of a-, b-, and c-type transitions, which was expected since dipole moment components are predicted along all three inertial axes (see next section). A cavity spectrum of the parent isotopologue showing two b-type transitions, which happen to lie within the same spectral window, is given in Figure 1. Consistent with the ground state of the trifluoroacetic acid monomer,29 no evidence of spectral doubling arising from CF3 internal rotation was observed. Spectra for each isotopologue were fit with an A-reduced Watson Hamiltonian in the Ir representation using the SPFIT program of Pickett.30 However, of the five quartic distortion constants, only J and JK were needed to fit the observed frequencies. Transitions were measured with a typical experimental accuracy of less than 5 kHz. Deuterium hyperfine structure was not resolved for the deuterated species, and therefore quadrupole coupling constants could not be obtained. The resulting spectroscopic constants are given in Table 1 and the assigned frequencies and their residuals from the least squares fits are provided as Supplementary Information. Computational Methods and Results All geometry optimizations and frequency calculations were done at the M06-2X/6311++G(3df,3pd) level of theory using the Gaussian09 suite of programs.31 This method was chosen to maintain consistency with our previous work on carboxylic sulfuric anhydrides.18-21 For comparison of relative energies with previous work, single-point electronic energies at the M06-2X optimized geometries were calculated at the CCSD(T) level using the complete basis set extrapolation scheme of Neese and Valeev between the ANO-pVDZ and ANO-pVTZ basis 5 ACS Paragon Plus Environment


sets.32 Corrections for zero-point energy were determined using the M06-2X/6-311++G(3df,3pd) frequencies. Minimum energy structures were calculated for both TFASA and the CF3COOH···SO3 complex. Both species are depicted in Figure 2. The minimum energy structure of TFASA (Figure 2b) lies 16.20 kcal/mol below the sum of the CF3COOH and SO3 monomer energies (14.44 kcal/mol with zero-point corrections), and 4.70 kcal/mol below that of the CF3COOH∙∙∙SO3 precursor complex (4.24 kcal/mol with zero-point corrections). Dipole moment components were predicted along all three inertial axes for the anhydride (μa, μb, μc = 1.02 D, 1.61 D, 0.94 D, respectively) and, as noted above, a-, b-, and c-type transitions were indeed observed. For the complex, the computed dipole moment components were μa, μb, μc = 1.57 D, 1.54 D, and 0.00 D, respectively. Table 2 compares the observed rotational constants and H/D isotope shifts with those predicted for both TFASA and the CF3COOH···SO3 precursor complex. The predicted rotational constants for TFASA are in excellent agreement with the experimental values, while those for CF3COOH···SO3 are not. Although the calculated isotope shifts for TFASA are in slightly better agreement with the observed values, the agreement is acceptable for either species, and thus the shifts do not provide an unambiguous diagnostic. A comparison of the optimized geometries for TFASA and the CF3COOH∙∙∙SO3 precursor complex shows that the C=O bond migrates from one oxygen to another during the cycloaddition. In both species, however, the in-plane C-F is cis to the C=O. Therefore, the CF3 must rotate 60° relative to its equilibrium orientation in the complex to reach its equilibrium orientation in TFASA. A similar result was found for acetic sulfuric anhydride for which no transition state involving a single coordinate could directly connect the precursor complex and carboxylic sulfuric anhydride. Thus, an analogous approach was carried out here in order to assess the possible TFASA formation pathways. Starting with the optimized CF3COOH∙∙∙SO3 geometry, the CF3 was rotated 60° and a transition state structure corresponding to CF3 internal rotation was optimized. This structure lies 0.38 kcal/mol (133 cm1) above CF3COOH∙∙∙SO3 and provides an estimate for the CF3 internal rotation barrier in the precursor complex. From the CF3 internal rotation transition state structure, the geometry was adjusted to resemble the cycloaddition transition state and was subsequently optimized. This results in a transition state 6 ACS Paragon Plus Environment

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structure corresponding only to the cycloaddition, as the CF3 group is already in the optimal position for the anhydride. The reaction barrier is taken as the energy of the cycloaddition transition state structure relative to CF3COOH∙∙∙SO3, which is then 3.29 kcal/mol (1.17 kcal/mol with zero-point corrections). The internal rotation barrier for the anhydride itself is 0.78 kcal/mol. In addition, a second-order saddle point structure was optimized so that the CF3 internal rotation and cycloaddition occur simultaneously. The formation of TFASA going through this path results in a 3.39 kcal/mol barrier without zero-point corrections (1.21 kcal/mol with corrections). Figure 3 summarizes the energetics at key points on the intermolecular potential energy surface. Although we are unable to conclude whether the formation of TFASA proceeds through a sequential or simultaneous pathway, we note that the energetics are similar along either route. Discussion Two pieces of evidence support the assignment of the observed spectrum to CF3COOSO2OH. First, as shown in Table 2 and noted above, the observed rotational constants are in much better agreement with those calculated for CF3COOSO2OH than for the CF3COOHSO3 complex. Errors in the predicted spectroscopic constants at the same level of theory for all other observed carboxylic sulfuric anhydrides studied have been within 1% of the experimentally determined constants,18-20 and the results here follow the same trend. Indeed, correcting the predicted TFASA constants for the apparent systematic error in the computations greatly aided initial cavity searches since observed transitions were typically found within only a few MHz of the adjusted predictions. The second piece of evidence is the observation of c-type rotational transitions. Since the calculated value of c is 0.94 D for CF3COOSO2OH but zero for CF3COOHSO3, the latter is readily ruled out. As noted above, the deuterium isotope shifts given at the bottom of the table are consistent with CF3COOSO2OH but do not distinguish it from CF3COOHSO3. The calculated structure of TFASA (Figure 2b) is similar to that of the carboxylic sulfuric anhydrides studied previously, featuring a six membered ring closed by an intramolecular hydrogen bond. However, while the geometries are similar, there are some subtle but distinct 7 ACS Paragon Plus Environment


differences. These are revealed by examining the intramolecular hydrogen bond defined by the O7∙∙∙H8 distance and O7∙∙∙H8-O3 angle. Table 3 contains these internal hydrogen bond lengths and angles from the M06-2X optimized geometries of the carboxylic sulfuric anhydrides of trifluoroacetic, formic, acetic, s-cis-acrylic, and s-trans-acrylic acids. The hydrogen bond length in TFASA is longest among the entries and has a value that is 0.072 Å longer than that of the next closest member of the series. This suggests that TFASA contains the weakest internal hydrogen bond of the set. Moreover, the O7∙∙∙H8-O3 angle is at the low end of the series, with a value that is 4° less than the next smallest value. This is also likely related to the strength of the internal hydrogen bond, though we note that for a six membered ring involving different bond lengths, it is unclear what an “optimum” hydrogen bond angle should be. The differences among the anhydrides were further investigated by examining the atomic charge on O7, the C5=O7 vibrational stretching frequency, and the C5=O7 bond length. Computational results are given in Table 4. It may be seen that across a variety of methods of population analysis, the C=O oxygen in TFASA is consistently the least negative among the anhydrides. Though the effect is small, the trend across methods is uniform, suggesting that the carbonyl oxygen in TFASA should act as a weaker hydrogen bond acceptor. Interestingly, the atomic charges for the acetic and acrylic acid derivatives are very similar and are the most negative in the series. Moreover, they have longer C=O bonds and, with the exception of the acetic acid derivative, have lower C=O stretching frequencies than the other anhydrides. These observations would be consistent with hyperconjugation (in the case of the acetic acid derivative) or resonance stabilization (in the case of acrylic acid derivative), both of which would act to emphasize resonance structures with a single CO bond and additional negative charge on the oxygen. Such structures are not possible with the formic acid or trifluoroacetic acid derivatives, thus rendering the internal hydrogen bonds weaker. For TFASA, inductive effects due to the fluorines likely also decrease the negative charge on the oxygen and contribute to a diminished hydrogen bond strength. It is interesting to note that the spectra of all of the previously studied carboxylic sulfuric anhydrides were prominent in chirped-pulse experiments, but that the TFASA spectrum was almost entirely absent from the broadband data for the same number of averaged free induction 8 ACS Paragon Plus Environment

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decay signals. One factor that may have contributed to this disparity is the difference in molecular dipole moments. Specifically, the largest dipole moment component (on the b-inertial axis) calculated for TFASA is 1.6 D compared to 3 to 5 D (on the a-inertial axis) for the other carboxylic sulfuric anhydrides. Another factor may involve energetics. Table 5 summarizes the energy differences labeled in Figure 3 and includes the equivalent results from previous calculations on the other carboxylic sulfuric anhydrides. The most striking difference between TFASA and the other anhydride species can be seen in a comparison of the reaction barriers. For the previously studied systems, the computational results predict an essentially barrierless reaction within the putative RCOOHSO3 complex. In the case of TFASA, however, the calculated barrier is 1.2 kcal/mol regardless of whether the CF3 internal rotation and cycloaddition occur sequentially or simultaneously. In either case, the relatively large moment of inertia of the CF3 unit (relative to, say, CH3) further hinders the rotation and hence the formation of the product. Additionally, the CF3COOH∙∙∙SO3 precursor complex has the lowest binding energy in the series and, therefore, may attain a lower population in the jet. Higher temperatures and more collisions, which are characteristic of the early part of the supersonic expansion, are clearly conducive to surmounting the activation barrier. However, lower temperatures are more likely to produce higher concentrations of precursor complex. Thus, it is not clear where in the expansion most of the anhydride is formed. Despite this uncertainty, however, it clearly is formed, though differences in the activation and binding energies may influence the amount of product that is ultimately produced. Nevertheless, the reaction is rapid: since the collisional phase of the supersonic expansion lasts only a few tens of microseconds, it is likely that under the conditions of our experiments, the production takes place within that time frame. Conclusion Trifluoroacetic sulfuric anhydride was produced in a supersonic jet from the reaction between CF3COOH and SO3 and observed by microwave spectroscopy. Calculations at the M06-2X/6311++G(3df,3pd) level have shown that the reaction proceeds via a concerted cycloaddition mechanism and that the equilibrium orientation of the CF3 must change in passing from CF3COOH∙∙∙SO3 to CF3COOSO2OH. Regardless of whether the CF3 internal rotation and cycloaddition happen sequentially or simultaneously, the calculated activation barrier is about 1.2 kcal/mol using CCSD(T)/CBS(D-T) electronic energies with zero-point corrections. This 9 ACS Paragon Plus Environment


value is significantly higher than those previously calculated for other carboxylic sulfuric anhydrides. Moreover, both the binding energy of the complex and the magnitude of the formation energy of TFASA with respect to the free monomers are the smallest among the anhydrides studied to date. Compared with the other RCOOSO2OH species, the theoretical results also indicate that TFASA has the weakest internal hydrogen bond. As a likely combined result of the less favorable energetics and smaller dipole moment components, the spectrum of TFASA was noticeably weaker than those of previously studied carboxylic sulfuric anhydrides. Nevertheless, its observation demonstrates that the reaction of RCOOH + SO3 to produce RCOOSO2OH is not restricted to hydrocarbon R groups. Therefore, in light of a potential future rise in atmospheric trifluoroacetic acid concentrations, it may be of interest to consider the possibility of the reaction between CF3COOH and SO3. If formed in the atmosphere, carboxylic sulfuric anhydrides could play a role in the production of aerosols and may be of interest to include in atmospheric models. Supplementary Information Available The Supporting Information (tables of transition frequencies, assignments, and residuals from the least squares fits; computed Cartesian coordinates 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 Supercomputing Institute. A.K.H. was supported by a Lester C. and Joan M. Krogh Fellowship, administered through the University of Minnesota.


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22. Zhang, H.; Wang, W.; Pi, S.; Liu, L.; Li, H.; Chen, Y.; Zhang, Y.; Zhang, X.; Li, Z. Gas Phase Transformation from Organic Acid to Organic Sulfuric Anhydride: Possibility and Atmospheric Fate in the Initial New Particle Formation. Chemosphere 2018, 212, 504–512. 23. Elm, J.; Myllys, N.; Olenius, T.; Halonen, R.; Kurtén, T.; Vehkamäki, H. Formation of Atmospheric Molecular Clusters Consisting of Sulfuric Acid and C8H12O6 Tricarboxylic Acid. Phys. Chem. Chem. Phys. 2017, 19, 4877–4886. 24. 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. 25. 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, 1–14. 26. 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, 12549– 12556. 27. Dewberry, C. T.; Mackenzie, R. B.; Green, S.; Leopold, K. R. 3D-Printed Slit Nozzles for Fourier Transform Microwave Spectroscopy. Rev. Sci. Instrum. 2015, 86, 06510717. 28. 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. 29. Stolwijk, V.M.; van Eijck, B.P. Microwave Spectra and Barriers to Internal rotation of Trifluoroacetic Acid and Trifluoroacetyl Fluoride, J. Mol. Spectrosc. 1985, 113, 196-207. 30. Pickett, H. M. The Fitting and Prediction of Vibration-Rotation Spectra with Spin Interactions, J. Mol. Spectrosc. 1991, 148, 371-377. 31. 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. Gaussian, Inc.: Wallingford CT 2013. 32. 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.



Table 1. Spectroscopic constants for the parent and deuterated isotopologues of TFASAa CF3COOSO2OH CF3COOSO2OD A [MHz] 2143.52445(77) 2112.2821(16) B [MHz] 588.825340(95) 586.69984(15) C [MHz] 557.74490(10) 554.28418(13) ΔJ [kHz] 0.01577(94) 0.0192(14) ΔJK [kHz] 0.122(10) 0.117(17) b N 25 19 RMS [kHz]c 3.0 2.3 (a) Numbers in parentheses are one standard error in the least squares fit. (b) Number of lines included in the fit. (c) Root mean square deviation of the residuals.


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Table 2. Comparison of theoretical results for TFASA and TFA∙∙∙SO3 with experimental constants Observed A [MHz] B [MHz] C [MHz] a [D] b [D] c [D]

2143.52445(77) 588.825340(95) 557.74490(10)

 0a

Shiftb

A [MHz] B [MHz] C [MHz]

Observed 31.2424(18) 2.12550(18) 3.46072(16)

CF3COOSO2OH Calculated (Obs.Calc.) (% Difference) 15 (0.7%) 3 (0.5%) 3 (0.6%)

2159 592 561 1.02 1.61 0.94

Deuterium Isotope Shifts CF3COOSO2OH(D) 33 2 4

CF3COOHSO3 Calculated (Obs.-Calc.) (% Difference) 2190 508 486 1.57 1.54 0.00

47 (2%) 81 (14%) 72 (13%)

CF3COOH(D)SO3 30 0 2

(a) From the observation of c-type rotational transitions. (b) Value for D minus value for H.



Table 3. Comparison of calculated intramolecular hydrogen bond distances and angles in RCOOSO2OHa,b Carboxylic Sulfuric Anhydride

R(O7∙∙∙H8) [Å]

∠(O7∙∙∙H8-O3) [deg]

CF3COOSO2OHc 2.153 118.0 HCOOSO2OHd 2.081 121.8 e CH3COOSO2OH 2.013 125.2 s-trans-H2C=CHCOOSO2OHf 2.000 126.0 f s-cis-H2C=CHCOOSO2OH 1.995 126.1 (a) Derived from optimized M06-2X/6-311++G(3df,3pd) geometries. (b) Using atomic numbering as shown in Figure 2. (c) This work. (d) Values of computed structures from reference [18]. (e) Values of computed structures from reference [19]. (f) Values of computed structures from reference [20].


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Table 4. Partial atomic charge on the carbonyl oxygen atom (O7) in carboxylic sulfuric anhydrides calculated with the Hirshfeld, Merz-Singh-Kollman (MK), and Natural Population Analysis (NPA) methods, and theoretical carbonyl stretching frequencies and bond lengthsa,b Carboxylic sulfuric anhydride

Hirshfeld

MK

NPA

ν(C5=O7) [cm-1]

CF3COOSO2OHc HCOOSO2OHd CH3COOSO2OHe s-trans-H2C=CHCOOSO2OHf s-cis- H2C=CHCOOSO2OHf

-0.22 -0.23 -0.24 -0.24 -0.24

-0.42 -0.47 -0.51 -0.50 -0.49

-0.55 -0.58 -0.60 -0.60 -0.61

1917 1860 1877 1849 1848

R(C5=O7) [Å] 1.186 1.189 1.194 1.198 1.198

(a) Population analysis and frequency calculations performed with M06-2X/6-311++G(3df,3pd) on geometries optimized at the same level of theory. Frequencies and distances for all but CF3COOSO2OH are taken from calculations originally performed in conjunction with the prior work referenced. (b) Using atomic numbering as shown in Figure 2b. (c) This work. (d) Reference 18. (e) Reference 19. (f) Reference 20.



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Table 5. Energetics relevant to the formation of various carboxylic sulfuric anhydridesa,b A B C D Carboxylic sulfuric anhydride species CF3COOSO2OHc HCOOSO2OHe CH3COOSO2OHf s-trans-H2C=CHCOOSO2OHg s-cis- H2C=CHCOOSO2OHg

Binding energy of precursor complex

Barrier to anhydride formation

Energy relative to precursor complex

Energy relative to monomers

10.2 12.6 15.7 16.1 16.1

1.17(1.21)d 0.26 0.057(-0.12)d -0.22 0.33

4.2 3.9 3.1 2.7 2.9

14.4 16.5 18.9 18.8 19.0

(a) All energies in kcal/mol calculated at CCSD(T)/CBS(D-T)//M06-2X/6-311++G(3df,3pd) with zero-point corrections from M06-2X/6-311++G(3df,3pd) frequencies. (b) Letters in the first row refer to the energy differences as indicated in Figure 3. (c) This work (d) Value outside of parentheses corresponds to the barrier of the sequential pathway of the CF3 or CH3 transition state followed by the cycloaddition transition state. Value inside the parentheses corresponds to the barrier of the simultaneous pathway in which both CF3 or CH3 rotation and cycloaddition occur simultaneously via a second order saddle point. (e) Reference 18 (f) Reference 19 (g) Reference 20


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Figure Captions Figure 1. Cavity spectrum of 909←818, and 717←606 for parent TFASA resulting from the average of 2,000 free induction decay signals. Figure 2. Optimized geometry of (a) the CF3COOH···SO3 complex and (b) TFASA from M062X/6-311++G(3df,3pd) calculations. Figure 3. Potential energy surface showing the formation of TFASA starting from its corresponding free monomers, TFA + SO3. The energy sum of the isolated monomers is defined as the zero of energy. Electronic energies are calculated at the CCSD(T)/CBS(D-T)//M06-2X/6311++G(3df,3pd) level of theory with zero-point corrections from M06-2X frequencies. Structures shown with an arrow over the CF3 have an imaginary frequency corresponding to CF3 internal rotation. Structures with three arrows shown within the complex have an imaginary frequency corresponding to the cycloaddition that precedes the formation of the anhydride. The sequential pathway where CF3 internal rotation is followed by cycloaddition is shown in black. The pathway highlighted in red proceeds via a second order saddle point in which CF3 internal rotation and cycloaddition occur simultaneously. The barriers to formation for the sequential and simultaneous pathways are 1.17 kcal/mol and 1.21 kcal/mol, respectively (3.29 kcal/mol and 3.39 kcal/mol without zero-point corrections). The energy differences labeled by letters A-D are summarized in Table 5.



Figure 1.


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Figure 2.

(a)

(b)



Figure 3.


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TOC Graphic



Figure 1. Cavity spectrum of 909←818, and 717←606 for parent TFASA resulting from the average of 2,000 free induction decay signals. 172x131mm (150 x 150 DPI)

ACS Paragon Plus Environment

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Figure 2. Optimized geometry of (a) the CF3COOH···SO3 complex and (b) TFASA from M06-2X/6311++G(3df,3pd) calculations. 81x101mm (150 x 150 DPI)



Figure 3. Potential energy surface showing the formation of TFASA starting from its corresponding free monomers, TFA + SO3. The energy sum of the isolated monomers is defined as the zero of energy. Electronic energies are calculated at the CCSD(T)/CBS(D-T)//M06-2X/6-311++G(3df,3pd) level of theory with zero-point corrections from M06-2X frequencies. Structures shown with an arrow over the CF3 have an imaginary frequency corresponding to CF3 internal rotation. Structures with three arrows shown within the complex have an imaginary frequency corresponding to the cycloaddition that precedes the formation of the anhydride. The sequential pathway where CF3 internal rotation is followed by cycloaddition is shown in black. The pathway highlighted in red proceeds via a second order saddle point in which CF3 internal rotation and cycloaddition occur simultaneously. The barriers to formation for the sequential and simultaneous pathways are 1.17 kcal/mol and 1.21 kcal/mol, respectively (3.29 kcal/mol and 3.39 kcal/mol without zero-point corrections). The energy differences labeled by letters A-D are summarized in Table 5. 276x226mm (150 x 150 DPI)


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70x45mm (150 x 150 DPI)


A Perfluorinated Carboxylic Sulfuric Anhydride: Microwave and

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