Brian K. Decker

GAS-PHASE REACTIVITY OF HS2H+. AND S2+.: AN INVESTIGATION OF THE GAS BASICITY AND PROTON AFFINITY OF HS2.

Abstract The reactions of HS2H+· with the series of reference bases: H2S, CH2O, C3H3F3O (1,1,1-trifluoropropanone), C2H5I, C6H4F2 (o-difluorobenzene), HCO2H, C4H8 (trans-2-butene), c-C3H6, n-C3H6, CH3OH, CH3SH, and C2H5OH, with proton affinities ranging from 168.5–185.6 kcal/mol, have been investigated with a selected ion flow tube (SIFT) to bracket the gas basicity (GB) and proton affinity (PA) of the hydrothiosulfeno radical (HS2·). The recently developed thermokinetic method of Bouchoux et al. [Int. J. Mass Spectrom. Ion Processes 153 (1996) 37] applied to the data gives GB(HS2·) = 169.8 ± 2.2 kcal/mol and PA(HS2·) = 178.0 ± 2.4 kcal/mol, consistent with but more accurate than the simple bracketing procedure. The proton affinity is used to calculate the enthalpy of formation of HS2·, giving ΔHf○298(HS2·) = 25.0 ± 2.5 kcal/mol; this result is compared with the relatively few other reported determinations of this quantity. The HS2· radical is of potential importance to chemical processes in interstellar clouds (ISC), as well as to fuel refinery and atmospheric chemistry. The HS2H+· reactions and a parallel study of the reactions of S2+· are discussed.

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

Abstract We present a systematic study of the reactions of SO+·(X 2Πr), 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+·.

Selected ion flow tube studies of S2+ reactions with a series of organic molecules

Abstract A selected ion flow tube (SIFT) has been used to study the reactions of S2+ with a series of organic molecules (as well as H2, CO, NH3, NO and NO2). These include the hydrocarbons, C2H4, C2H6, CH2CCH2, CH3CHCH2 and C3H8; alcohols and thiols, CH3OH, C2H5OH, CH3SH and C2H5SH; ethers (CH3)2O and (C2H5)2O; aldehydes and ketones, CH3CHO, C2H5CHO and (CH3)2CO; and carboxylic acids and esters, HCO2H, HCO2CH3, HCO2C2H5, CH3CO2H, CH3CO2CH3, CH3CO2C2H5, C2H5CO2H, C2H5CO2CH3 and C2H5CO2C2H5. The rate coefficients are generally close to the collisional values, with exceptions among the reactions involving the smaller molecules. Most prevalent are abstraction reactions leading to formation of the thiosulfeno radical, HS2, or its protonated form; three-body associations; and channels leading to formation of the acetyl and propionyl cations, CH3CO+ and C2H5CO+, respectively. Only in reactions involving the alkenes is cleavage of the SS bond of S2+ observed. The isomeric molecules in the data set generally react very differently, as would be expected from reactivity controlled by the position and complexity of the functional groups. The data are discussed in terms of reaction mechanisms, thermodynamics, and implications for interstellar chemistry.

Product channeling in the reactions of CS+(X 2Σ+) with simple carboxylic acids and esters

Abstract In the present work, a selected ion flow tube was used to study the reactions of CS + ( X 2 Σ + ) with H 2 , CO, and a series of carboxylic acids and esters, RCO 2 R′ (R, R′ = H, CH 3 , C 2 H 5 ), 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 RCO 2 R′ 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 C S bond did not occur. The reactions of CS + with the RCO 2 R′ series are compared with the analogous reactions of S 2 + , SO + , S + , H 3 O + , NO + , and O 2 + 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 NO 2 and C 3 H 4 (allene) at 296 ± 4 K are presented.

Pulsed technique for observing infrared emissions from ionic gas phase reactions at low reactant ion concentrations

A technique has been developed to detect infrared emissions from the products of ionic reactions in plasmas. The technique employs dual-phase digital lock-in amplification and cold filtering to permit the detection of the weak infrared chemiluminescence (IRCL) with a solid-state detector. A novel method of cleanly modulating plasma chemiluminescence by the pulsed introduction of reagent gases has been developed and implemented. This new technique has been tested by studying the well-characterized H-atom reactions, H+Cl2→HCl(v=0–4)+Cl and H+NO2→OH(v=0–3)+NO. Rotational and vibrational distributions have been measured for these two reactions and are presented and compared with previous determinations. Additionally, the associative electron detachment reaction, H+Cl−→HCl(v=0–2)+e, has been studied, demonstrating that IRCL can be collected from reactions occurring at a low number density approaching that of the plasma ionization (∼4×1010 cm−3). The resolution, and hence, the information content of the collect...