Gas-phase reactivity of SO+·: a selected ion flow tube study

doi:10.1016/S1387-3806(99)00146-3

International Journal of Mass Spectrometry

Volumes 195–196, 21 January 2000, Pages 185-201

https://doi.org/10.1016/S1387-3806(99)00146-3 Get rights and content

Abstract

We present a systematic study of the reactions of SO^+·(X ²Π_r), an important ion in space plasmas, with organic molecules of interstellar interest. A selected ion flow tube has been used to investigate the reactions of SO^+· with CH₄, C₂H₆, C₃H₈, C₂H₂, C₂H₄, C₃H₄ (allene), n-C₃H₆, CH₃OH, C₂H₅OH, CH₃OCH₃, OCS, CH₂O, CH₃CHO, CH₃C(O)CH₃, HCO₂H, and HCO₂CH₃, and additionally the reactions of S₂^+· with C₂H₂ and O₂^+· with CH₄, C₂H₂, C₃H₄ (allene), n-C₃H₆, CH₃OCH₃, and HCO₂H at 294.5 ± 2.5 K. With just a few exceptions the reactions proceed at or near their theoretical collisional capture rates. Apart from the smaller and more saturated hydrocarbons and OCS, the reactions of SO^+· are dominated by heterogenic abstractions of R⁻ (R = H, OH, CH₃, OCH₃). Charge transfer, where it is exothermic, occurs in competition with the abstraction channels. Hydride abstraction is particularly prevalent, forming the thioperoxy radical, HSO^·, or its structural isomer, SOH^·. Hydroxide abstraction to form the hydroxysulfinyl radical, HOSO^·, occurs in some of the reactions with oxygen-bearing molecules. Where neutral, the abstraction products are inferred from the calculated reaction energetics; however, they are frequently detected directly in their protonated forms. This suggests a two-step reaction mechanism whereby competition for a proton occurs between leaving partners in the exit channel of the activated complex. In the reaction of SO^+· with HCO₂CH₃, the protonated methoxysulfinyl radical, CH₃OSOH^+·, is observed for the first time. The reactions of SO^+· with the smaller unsaturated hydrocarbons are more complex, and largely involve rupture of the S–O bond and a C–C bond to form products containing C–S and C–O bonds. The SO^+· reactions are discussed in terms of their mechanisms, product formation, thermodynamics, and interstellar significance, and are compared with the related reactions of S₂^+· and O₂^+·.

Introduction

Oxygen and sulfur are members of the select group of elements which are both cosmically abundant and disposed to forming chemical bonds (chiefly H, O, C, N, Mg, Si, Fe, and S, in decreasing order of solar abundance) [1]. Although sulfur is ranked below Mg, Si, and Fe in abundance, its participation in the ion/molecule chemistry of interstellar clouds is more extensive due to its higher first ionization energy (10.36 eV relative to <8.2 eV for Si, Fe, and Mg); thus, the atomic ions of H, O, C, N, S, and ions derived from these drive the bulk of synthetic gas phase ion/molecule chemistry which directs the chemical evolution of interstellar molecular clouds (ISC) [2], [3], [4]. Whereas atomic O⁺ does not persist in ISC due to its rapid reaction with H₂ [5], [6], ground-state atomic S⁺ is unreactive with both H₂ and CO [5], [6], the two major molecular constituents of ISC [2], [3], [4]. S⁺ in its ground ⁴S state is also unreactive with H₂O [5], [6]. Sulfur and oxygen in the gas phase in ISC are thought to combine primarily by the ion/molecule reaction $S^{+} + OH^{·} → SO^{+·} + H,$ which has not yet been studied in the laboratory. Reaction (1) is expected to be particularly important for the synthesis of SO^+· in cold, dark interstellar clouds, such as the Taurus Molecular Cloud (TMC-1) [7]. SO^+· is one of the 14 sulfur-containing compounds and one of the 13 ions which have been detected in ISC [8].

The sulfur monoxide molecular ion, SO^+·, itself reacts neither with H₂ nor with CO [6], and thus a consideration of its reactivity with minority species in ISC is necessary. Turner has made the most extensive astrophysical searches for SO^+· to date [7], [9], and has found this species to be abundant in a wide variety of interstellar sources [7]. His studies were motivated by the potential for SO^+· to be a tracer for shocked regions of ISC [7]. In his simple model of SO^+· chemistry in ISC, Turner considered electron/ion dissociative recombination to be the only important process that destroys SO^+· [9]. His model assumed a recombination rate coefficient of α_e(SO^+·) = 2 × 10⁻⁷ [T(K)/300]^−0.5 cm³ molecule⁻¹ s⁻¹ [9]; a plausible value, albeit one that has not yet been confirmed experimentally. No other destruction pathways for SO^+· were considered, which necessitated the assumption of a high electron density (the so-called “high metal” value) to make the chemical model consistent with the observed SO^+· column densities in many sources [9]. It is therefore important to investigate whether SO^+· can be destroyed by numerous fast ion/molecule reactions with observed and expected interstellar molecules.

From a more fundamental standpoint, the reactivity of SO^+· as compared with its isovalent “sister” species, S₂^+· and O₂^+·, is of considerable interest. The most important differences in reactivity among these three species are expected to stem from the essential electronic difference between the component atoms, S and O. The valence electrons of S are better shielded from the nuclear core potential than those of O, and hence, the promotion and ionization energies of electrons in molecular orbitals arising from S atoms are lower than in those arising from O atoms; for the same reason, the electronegativity of S (2.58, Pauling) is substantially lower than that of O (3.44, Pauling) [10]. In consequence, the recombination energies of S₂^+·, SO^+·, and O₂^+· are 9.36, 10.29, and 12.07 eV, respectively [11], and hence, the capacity for exothermic reaction progresses steadily in the order S₂^+· < SO^+· < O₂^+·. For reactions of such ions with polyatomic molecules, the prominence of charge (electron) transfer is also strongly correlated with the charge transfer exothermicity. This is because the high density of rovibronic states available in a polyatomic molecule permits a long-range charge transfer to occur with a high probability of favorable Franck-Condon factors. Here the probability of reaction essentially depends on the volume of phase space available to the reactants, and hence, on the reaction exothermicity. Such charge-transfer mechanisms in ion/molecule reactions have recently been reviewed [12].

As a consequence of the difference in electronegativity between S and O, SO^+· is a highly polar species, with an electric dipole moment in the ground X ²Π_r state of about 2.2 D [13]; the homonuclear S₂^+· and O₂^+· clearly lack permanent electric dipole moments. Although the overall collisional capture rates for the reactions of all three ions with neutral molecules will be dominated by the charge/permanent electric dipole and the charge/induced electric dipole interactions [14], the strong dipole of SO^+· is likely to distinguish that ion from S₂^+· and O₂^+· with regard to the intimate reaction mechanisms and the kinds of products formed from the activated complex. Therefore, a comparative study of the reactivities of S₂^+·, SO^+·, and O₂^+· will investigate the effects of recombination energy and charge separation on the rates and mechanisms of ion/molecule reactions.

Very few reactions of SO^+· have been studied previously in the laboratory. The standard ion/molecule reaction databases [5], [6] list only eight such reactions, i.e., with H₂, N, NH₃, O₂, H₂S, CO, SO₂, and SF₆. Four of these, i.e., with H₂, O₂, CO, and SO₂, do not proceed under standard conditions [6]. In contrast, many ion/molecule reactions of S₂^+· [5], [6], [15], [16], [17], [18] and O₂^+· [5], [6], [19], [20], [21], [22], [23], [24], [25] have been investigated, and hence are available for direct comparison with those of SO^+·. In the present work, a selected ion flow tube (SIFT) has been used in the first systematic study of the ion/molecule reactions of SO^+· with 16 organic molecules identified or likely to be present in ISC [8]: CH₄, C₂H₆, C₃H₈, C₂H₂, C₂H₄, C₃H₄ (allene), n-C₃H₆, CH₃OH, C₂H₅OH, CH₃OCH₃, OCS, CH₂O, CH₃CHO, CH₃C(O)CH₃, HCO₂H, and HCO₂CH₃. For completeness of comparison, the reactions of S₂^+· with C₂H₂ and O₂^+· with CH₄, C₂H₂, C₃H₄ (allene), n-C₃H₆, CH₃OCH₃, and HCO₂H have also been investigated (of these, only the reactions of O₂⁺ with CH₄ and HCO₂H have been studied previously under thermal conditions [5], [6], [20]). Rate coefficients and product distributions are presented for the reactions of all three ions with the 16 molecules, and the relative reactivities of S₂^+·, SO^+·, and O₂^+· are discussed in light of these results. For SO^+·, the reaction mechanisms, nature of the products formed, and implications for chemistry in ISC are also discussed.

Section snippets

Experimental

The SIFT technique has been described at length elsewhere [26]; the essentials are presented here, with emphasis on details particular to the present experiments. The SO^+· ions were efficiently generated by two methods: (a) directly by impact of 70 eV electrons on SO₂ in a low-pressure ion source (LPIS); and (b) by generation of S⁺ from 70 eV electron impact on CS₂ in the same source, followed in the flow tube by $S^{+} (^{4} S)+ O_{2} (X^{3} ∑_{g}^{−}) k_{2}^{(2)} → SO^{+·} (X^{2} Π_{r})+ O (^{3} P)+6.9 kcal mol^{−1}$ where k₂⁽²⁾ = 1.8 × 10⁻¹¹

Results and discussion

Rate coefficients, k_exp⁽²⁾, and fractional product distributions, f, for the reactions of SO^+· and the supplementary reactions of S₂^+· and O₂^+· measured in the present study are presented in Table 1. Capture rate coefficients, k_th⁽²⁾, calculated from the parameterized variational “transition state” theory of Su and Chesnavich [37] are presented for comparison with the experimentally-determined rate coefficients. Electric dipole polarizabilities and electric dipole moments for these

Summary and conclusions

The reactions of the diatomic π-radical cation SO^+· with 16 molecules representing examples of several classes of organic compound have been studied with a SIFT and compared with the reactions of S₂^+· and O₂^+· with the same species.

Reactions of SO^+· are generally characterized by heterogenic bonding to form π-radical abstraction products such as HSO^·/SOH^·, HOSO^·, CH₃SO^·, and CH₃OSO^·. Such products frequently appear in the ion product spectra in their protonated forms when the leaving partner

Acknowledgements

The National Science Foundation, Division of Astronomical Sciences, is gratefully acknowledged for funding this work under grant no. AST-9415485. The authors are extremely grateful to T. Daniel Crawford of the Center for Computational Quantum Chemistry at The University of Georgia for computing the dipole moment and Mulliken population of SO^+·, as well as the enthalpies of formation of the various isomeric forms of H₂SO^+· using coupled cluster ab initio methods.

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Citation Excerpt :
The remaining reactivity is represented by H-atom transfer, which is chiefly homolytic, forming HCS+. In addition to the reactions of CS+ presented here, those of S2+ [21], S+ [23], H3O+, NO+, and O2+ [24] with the entire RCO2R′ series (including C2H5CO2H), and of SO+ with HCO2H and HCO2CH3, also have been studied [22]. The behaviors of these ions are highly varied, reflecting fundamental differences in their electronic structures.
In the present work, a selected ion flow tube was used to study the reactions of CS⁺(X ²Σ⁺) with H₂, CO, and a series of carboxylic acids and esters, RCO₂R′ (R, R′ = H, CH₃, C₂H₅), at 296 ± 4 K. The CS⁺ ion is expected to be a reasonable ionic analogue of the neutral pseudohalogen, CN, because of its strong chemical bond and isovalency. The reactions of CS⁺ with the RCO₂R′ series were all fast at greater than 80% of the theoretical upper-limit ion/dipole capture rate coefficient. Numerous binary ion products were observed, the most prevalent being the acylium ion (RCO⁺), the radical ion of the neutral reactant, and HCS⁺; no ternary association products were observed. Despite the abundance of product channels, these reactions were simple mechanistically, proceeding either by the induction of lone pair electrons on the carboxyl O atoms, or by H-atom transfer, which was chiefly homolytic rather than heterolytic. Induction of an O-atom lone pair by the C-terminus of CS⁺ into a molecular orbital of the activated complex is believed to lead to product formation through an oxonium ylide intermediate. The CS⁺ was converted, with few exceptions, into the small product molecules OCS, CS, and HCS⁺. Except in one instance, rupture of the strong CS bond did not occur. The reactions of CS⁺ with the RCO₂R′ series are compared with the analogous reactions of S₂⁺, SO⁺, S⁺, H₃O⁺, NO⁺, and O₂⁺ which have been studied previously. The reactive behavior of CS⁺ is discussed with regard to the pseudohalogen character of this ion and its potential role in studies of chemical reaction dynamics. Additionally, the rate coefficients and product distributions for the reactions of S⁺ with NO₂ and C₃H₄ (allene) at 296 ± 4 K are presented.
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¹: Dedicated to the memory of Robert R. Squires.

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Original ArticlesGas-phase reactivity of SO+·: a selected ion flow tube study1

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Summary and conclusions

Acknowledgements

Int. J. Mass Spectrom.

Int. J. Mass Spectrom. Ion Processes

Chem. Phys. Lett.

Int. J. Mass Spectrom. Ion Processes

Int. J. Mass Spectrom. Ion Processes

Int. J. Mass Spectrom. Ion Processes

Int. J. Mass Spectrom. Ion Processes

Int. J. Mass Spectrom. Ion Phys.

Int. J. Mass Spectrom. Ion Phys.

Chem. Phys. Lett.

Chem. Phys. Lett.

Chem. Phys. Lett.

Chem. Phys. Lett.

Astrophys. J.

Annu. Rev. Phys. Chem.

J. Chem. Soc., Faraday Trans.

Chem. Rev.

Gas Phase Ion Molecule Reaction Rate Constants Through 1986

J. Phys. Chem. Ref. Data

Astrophys. J.

Astrophys. J.

The Elements

Recent Res. Devel. Physical Chem.

J. Chem. Phys.

Ber. Bunsenges. Phys. Chem.

J. Phys. Chem. A

Original Articles
Gas-phase reactivity of SO^+·: a selected ion flow tube study1