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

Abstract

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.

A theoretical study on the formation mechanism of carboxylic sulfuric anhydride and its potential role in new particle formation

“…1(c), which is consistent with the mechanism proposed in recent studies. 30,31 3.2 Gas phase cycloaddition reaction of SO 3 to DTAA According to the predicted reaction complex of SO 3 and DTAA, reaction mechanism as well as the energy prole were further calculated. There are two different carboxyl groups (-COOH) in DTAA molecule shown in Fig.…”

“…28 Furthermore, recent studies have shown that except the dominant reaction of sulfur trioxide (SO 3 ) with water vapor in the troposphere, 29 cycloaddition reaction of SO 3 to carboxylic acids is a possible formation mechanism of carboxylic sulfuric anhydrides which would have lower vapor pressure than the corresponding carboxylic acids and have more inter-molecular interaction sites. [30][31][32] It is hence speculated that the contribution of DTAA to new particle formation may be improved by the reaction with SO 3 to generate diaterpenylic acetate sulfuric anhydride (DTASA, C 10 H 16 O 9 S).…”

Mechanistic Insight into the Reaction of Organic Acids with SO3 at the Air–Water Interface

“…This reaction also involves a low free‐energy barrier of activation (2.5 kcal mol −1 for FA, Figure S1). It provides a mechanism for incorporating organic matter into aerosol particles …”

“…Although both of the above mechanisms for the reaction between organic acids and SO 3 can explain the enhanced particle formation to some extent, they are still limited to the gas phase. In the atmosphere, liquid phases are abundant in the form of clouds, aerosols, or sea‐salt particles.…”

Conformational effect on the almost free internal rotation in 4-hexyn-3-ol studied by microwave spectroscopy and quantum chemistry

“…Due to higher resolution and more accurately determined molecular parameters in comparison to, for example, fluorescence or electronic spectroscopy, many barriers to internal rotation have been determined by this method. [1][2][3] The barrier height varies in a wide range, but could be classified in three main classes. The first class comprises molecules with high barriers (>600 cm −1 ) with ethyl chloride as a typical example (1200 cm −1 ).…”