Acetic sulfuric anhydride, CHCOOSOOH, was produced by the reaction of SO and CHCOOH in a supersonic jet. Four isotopologues were observed by microwave spectroscopy. Spectra of both A and E internal rotor states were observed and analyzed, yielding a value of 241.093(30) cm 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 π + π + σ cycloaddition reaction within the CHCOOH-SO complex. The equilibrium orientation of the methyl group relative to the O═C-C plane is different in the anhydride and in the CHCOOH-SO complex, indicating that the -CH 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 CHCOOH-SO complex to CHCOOSOOH 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 SO to form a carboxylic sulfuric anhydride in the gas phase. Possible connections to atmospheric aerosol formation are discussed.
The rotational spectrum of acrylic sulfuric anhydride (CH═CHCOOSOOH, 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 π + π + σ cycloaddition reaction within the acrylic acid-SO 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 CH═CHCOOSOOH are 19.0 and 18.8 kcal/mol lower in energy than that of SO + their corresponding CH═CHCOOH precursor conformation. The zero-point-corrected transition state energies for formation of the s-trans and s-cis anhydrides are 0.22 and 0.33 kcal/mol lower than those of the complexes of SO 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 demonstrating that carboxylic acids readily add to SO in the gas phase to produce the corresponding carboxylic sulfuric anhydride.
Trifluoroacetic sulfuric anhydride (CF 3 COOSO 2 OH, 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 CF 3 COOH or CF 3 COOD with SO 3 . 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 CF 3 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 CF 3 relative to the CO bond changes upon formation of the anhydride, indicating that any path connecting the equilibrium structures of CF 3 COOH and CF 3 COOSO 2 OH 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 CF 3 COOH•••SO 3 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 SO 3 with nonfluorinated 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 + SO 3 → RCOOSO 2 OH readily occurs in the gas phase and is not restricted to acids with hydrocarbon R groups.
The purpose of this study was to assess the feasibility to determine fetal blood oxygen saturation (sO 2 ) with T 2 -weighted MR sequences using a fetal sheep model. T 2 measurements were performed on a 1.5-T scanner using a T 2 preparation pulse in combination with a three-dimensional balanced steady-state free precession sequence repeated at different echo times. Eight sheep fetuses were examined during a control, hypoxic, and recovery phase to perform T 2 -weighted scans of the fetal blood in the heart. Signal intensities in the left and right ventricle were measured to calculate the MR blood sO 2 . During each phase, fetal carotid artery sO 2 was directly measured and correlated with MR sO 2 . A Bland-Altman plot was performed. Fetal carotid artery sO 2 was 69% sO 2 during control, 16% sO 2 during hypoxemia, and 67% sO 2 during recovery. Mean values of the MR sO 2 were 49% sO 2 and 40% sO 2 for control, 6% sO 2 and 3% sO 2 for hypoxemia, and 51% sO 2 and 43% sO 2 for recovery in left ventricle and right ventricle, respectively. Mean values of fetal carotid artery sO 2 and MR sO 2 were highly correlated (left ventricle: r 5 0.87, right ventricle: r 5 0.89). According to the Bland-Altman plot, MR sO 2 was lower compared to fetal carotid artery sO 2 (left ventricle: 15%, right ventricle: 20%). Based on our preliminary results, it seems to be possible to assess fetal sO 2 with MR oximetry. Magn Reson Med 64:32-41,
Carboxylic acids react with sulfur trioxide to form carboxylic sulfuric anhydrides, RCOOSO 2 OH. In this article, new supersonic jet microwave spectra are presented for the anhydride derived from propiolic acid (HCCCOOH), and recent work on a series of carboxylic sulfuric anhydrides is reviewed. For the propiolic acid derivative, computed minimumenergy structures are reported for both the anhydride ( H C C C O O S O 2 O H ) a n d i t s p r e c u r s o r c o m p l e x (HCCCOOH−SO 3 ), and additional CCSD(T)/CBS(D-T)// M06-2X/6-311++G(3df,3pd) calculations indicate that, after zero-point energy corrections, the barrier to anhydride formation is effectively zero. These results are similar to those for other carboxylic sulfuric anhydrides studied and are consistent with their rapid production under supersonic jet conditions. Carboxylic sulfuric anhydrides, as a class, have not been widely characterized in the chemical literature and thus their study represents a new feature of the chemistry of sulfur oxides and oxyacids. As such, structural and energetic features of the carboxylic sulfuric anhydrides derived from formic, acetic, acrylic, trifluoroacetic, propiolic, pinic, and benzoic acids are compared. Computed vibrational frequencies are provided as Supporting Information and should be useful for possible future observation by infrared and/or Raman spectroscopy. Statistical thermodynamics is used to estimate the equilibrium constants for the formation reactions at a series of temperatures, and the results indicate values ranging from ∼10 4 atm −1 for formic acid at 288 K to over 10 11 atm −1 for benzoic acid at 217 K. We speculate that carboxylic sulfuric anhydrides could be active species in the Earth's atmosphere and atmospheric concentrations have, therefore, been estimated assuming an equilibrium state. These estimates are subject to significant uncertainties in the atmospheric SO 3 and carboxylic acid concentrations but may be as high as 10 7 molecules/cm 3 in some locations. Related calculations suggest that equilibrium anhydride concentrations may exceed those of the sulfuric acid precursors SO 3 −H 2 O and SO 3 −(H 2 O) 2 by several orders of magnitude. Kinetic modeling will ultimately be necessary to fully assess the role, if any, of carboxylic sulfuric anhydrides in atmospheric processes.
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