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Reactions of Sulfur and Oxygen Containing Anions with Hydrogen Atoms: A Comparative Study Ya-Ke Li, Zhe-Chen Wang, Sheng-Gui He, and Veronica M. Bierbaum J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02641 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017
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The Journal of Physical Chemistry Letters
Reactions of Sulfur and Oxygen Containing Anions with Hydrogen Atoms: A Comparative Study Ya-Ke Li, a,b,cZhe-Chen Wang, aSheng-Gui He, b,c and Veronica M. Bierbaum*a
a
Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, 80309,
United States b
State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of
Chemistry, Chinese Academy of Sciences, Beijing, 100190, China c
University of Chinese Academy of Sciences, Beijing, 100049, China
AUTHOR INFORMATION Corresponding Author *
Email:
[email protected] 1 ACS Paragon Plus Environment
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ABSTRACT
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Reactions of hydrogen atoms with small sulfur-containing anions, SCN−,
CH3COS−, C6H5COS−, −SCH2COOH, C6H5S−, 2-HOOCC6H4S−, and related oxygen-containing anions, OCN−, CH3COO−, C6H5COO−, HOCH2COO−, C6H5O−, 2-HOOCC6H4O−, have been studied both experimentally and computationally. The experimental results show that associative electron detachment (AED) is the only channel for the reactions. The rate constants for reactions between sulfur-containing anions and H atoms are generally higher than for the related oxygencontaining anions with the exception of the reaction of SCN−. The generally higher reactivity of the sulfur anions contrasts with previous results where AED reactivity was found to correlate with reaction exothermicity. Density functional theory calculations indicate that the reaction enthalpies, the characteristics of the reaction potential energy surfaces, and other structural and electronic factors can influence the reaction rate constants. This study indicates that organic sulfur anions can be more reactive than related oxygen anions in the interstellar medium where hydrogen atoms are abundant.
TOC GRAPHIC
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The field of interstellar chemistry began in the 1930s with the detection1-3 by optical means of CH, CH+, and CN. The observable interstellar clouds were believed to be low-density and essentially atomic in character with a small diatomic molecular component. The discovery of microwave emission from the inversion transition of ammonia4 located in the direction of the galactic center produced a fundamental change in our understanding of the interstellar medium from a low-density essentially atomic environment to a highly heterogeneous one with great density variations.5 Today the interstellar medium is known to be composed of atoms, molecules, ions, and dust grains. More than 200 molecular species, most of which are organic and include a wide variety of functional groups, have been detected in dense clouds, despite the harsh conditions of the interstellar medium (ISM). Sulfur-containing species have been observed in interstellar environments since the early 1970s.6-10 In addition to the wide variety of neutrals and positive ions, some negative ions,11-16 CN−, C3N−, C5N−, C4H−, C6H−, and C8H−, were also detected in the denser regions of the ISM in recent years. This suggests that negative ions are involved in the chemical reactions within interstellar clouds and play a role in the formation and distribution of species in the interstellar medium. Astronomers and spectroscopists are now also turning their attention to chemical species that incorporate third row elements including sulfur.17 Although sulfur-containing anions have not yet been detected, they are expected to exist in molecular clouds due to the relative abundance of sulfur and the predicted stability of the anions. It is, therefore, of interest to study the reactivity of sulfur-containing anions, especially with hydrogen atoms, the most abundant atomic species in the ISM. The reaction rate constants are important for astrochemical modeling and for characterizing the chemistry of the interstellar medium.
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Previous ion-atom studies have focused on the reactions between cations and H atoms,18-30 and more recent studies have characterized the reactivity of small negative ions with H atoms.31-37 In the current study, we report the reactivity of six sulfur-containing anions with hydrogen atoms using both experimental and computational methods; comparison with the rate constants for six related oxygen-containing anions demonstrates that several factors in addition to reaction exothermicity are critical in determining reactivity. The tandem electrospray ionization- or flowing afterglow-selected ion flow tube (ESI-SIFT or FA-SIFT) at the University of Colorado, Boulder was used to study the reactions of hydrogen atoms with small sulfur-containing anions, SCN−, CH3COS−, C6H5COS−, −SCH2COOH, C6H5S−, 2-HOOCC6H4S−, and related oxygen-containing counterparts, OCN−, CH3COO−, C6H5COO−, HOCH2COO−,
C6H5O−,
2-HOOCC6H4O−
(see
more
method
details
in
supporting
information).38,39,40 All of the anions are measured to be reactive with H atoms except C6H5COO−, HOCH2COO−, and 2-HOOCC6H4O−. Only an associative electron detachment channel, in which the H atom bonds to the anion and an electron is ejected (A− + H → AH + e−), was observed in the reactions. Although there are multiple possibilities for the neutral products in each case, the neutral species are not detected in our experiments. Here we consider the most probable products. The reaction products, rate constants, and reaction efficiencies41,42 for the studied reactions are summarized in Table 1. The rate constants for reactions between sulfurcontaining anions and H atoms are found to be generally higher than for the related oxygencontaining anions with the exception of the reaction of SCN−. Density functional theory (DFT) calculations B3LYP/6-311++g(d,p)43-45 using the Gaussian 09 program46 were tested (Table S1) and found to perform very well for the bond energies of CH, 4 ACS Paragon Plus Environment
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OH, SH, CC, CO, and CS in diatomic molecules. Therefore, this method was used to study the reaction exothermicities (Table 1) and the reaction mechanisms. In addition, we computed the electron affinities of SCN and OCN, which have been previously determined experimentally.47 Our calculated values, EA(SCN) = 3.53 eV and EA(OCN) = 3.60 eV, are in excellent agreement with the experimental values (3.54 eV and 3.61 eV, respectively) indicating that the computational approach and basis set are appropriate for these systems. The details about how an electron is lost from a reaction system are beyond the scope of the DFT calculations.48 Therefore, in this work, we only focus on the processes for approach of H atoms to the anions and the thermodynamic energy changes for the electron detachment steps. It is worth noting that in several reactions of this study the electron detachment processes are exothermic. This indicates that the related anion radicals are metastable states with short lifetimes and they were not detected in the experiments. The natural charge analysis of these anion radicals and the dipole moments of the neutral species are shown in the Supporting Information section 4. As shown in Table 1, all of the AED reactions are exothermic. The exothermicity of the reaction between SCN− and H is smaller than the exothermicity of reaction between OCN− and H. This may contribute to the lower rate constant for the SCN− and H reaction. However, the reactivities of the other sulfur-containing anions are higher than those of the corresponding oxygen-containing anions, even though their reaction exothermicities are lower. This suggests that reaction exothermicity is not the only factor influencing the reaction rate constant. We have used our computational studies to provide insight into these anion–H atom reactions. According to the reaction mechanisms, these reactions can be classified into two categories: the reactions of XCN‒ (X=S and O) with H atoms and the reactions of RX‒ (R≠H, X=S and O) with H atoms, including the reactions of CH3COS− (CH3COO−) + H, C6H5COS− (C6H5COO−) + H, 5 ACS Paragon Plus Environment
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−
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SCH2COOH (HOCH2COO−) + H, C6H5S− (C6H5O−) + H, 2-HOOCC6H4S− (2-HOOCC6H4O−) +
H. Table 1. Reactions of sulfur-containing anions and related oxygen-containing anions with H atoms studied with ESI-SIFT or FA-SIFT. Ionic reactant (A−)
Reaction products (AH + e−)
Rate constant kexpa (10−10 cm3 s−1)
Reaction efficiency (kexp / kLangevin) b
∆H0Kc (eV)
SCN−
SCNH + e−
0.29±0.11
0.02
-0.31
OCN−
SCNH + e−
3.3±0.4
0.17
-1.04
CH3COS−
CH3COSH + e−
8.9±0.9
0.46
-0.85
CH3COO−
CH3COOH + e−
3.6±0.8
0.19
-1.26
C6H5COS−
C6H5COSH + e−
7.9±0.5
0.41
-0.64
C6H5COO−
C6H5COOH + e−
