Gas-Phase Reactions of Microsolvated Fluoride Ions: An Investigation of Different Solvents

The gas-phase reactions of F^–(DMSO), F^–(CH₃CN), and F^–(C₆H₆) with <i>t</i>-butyl halides were investigated. Reaction rate constants, kinetic isotope effects, and product ion branching ratios were measured using the flowing afterglow selected ion flow tube technique (FA-SIFT). Additionally, the structure of F^–(DMSO) was investigated both computationally and experimentally, and two stable isomers were identified. The reactions generally proceed by elimination mechanisms; however, the reaction of F^–(C₆H₆) with <i>t</i>-butyl chloride occurs by a switching mechanism. These reactions are compared to previous studies of microsolvated reactions of <i>t</i>-butyl halides where the solvent molecules were polar, protic molecules

Gas-Phase Reactions of CF⁺ with Molecules of Interstellar Relevance

We have studied the gas-phase reactions of CF⁺ with 24 neutral species. Reaction rate constants and product branching fractions are measured at 298 K using a flowing afterglow-selected ion flow tube. Experimental work is supported by computational chemistry calculations to provide insight into the reactivity of classes of neutral molecules. Reactions of CF⁺ with small triatomic species and oxygen-containing organic molecules produce the stable molecule CO. The product branching fractions are discussed, and the potential energy surfaces for a few representative reactions are examined. CF⁺ is highly reactive with complex molecules and will likely be destroyed in dense environments in the interstellar medium. However, the lack of reactivity with small diatomic molecules will likely enable its survival in diffuse regions

Deprotonated Purine Dissociation: Experiments, Computations, and Astrobiological Implications

A central focus of astrobiology is the determination of abiotic formation routes to important biomolecules. The dissociation mechanisms of these molecules lend valuable insights into their synthesis pathways. Because of the detection of organic anions in the interstellar medium (ISM), it is imperative to study their role in these syntheses. This work aims to experimentally and computationally examine deprotonated adenine and guanine dissociation in an effort to illuminate potential anionic precursors to purine formation. Collision-induced dissociation (CID) products and their branching fractions are experimentally measured using an ion trap mass spectrometer. Deprotonated guanine dissociates primarily by deammoniation (97%) with minor losses of carbodiimide (HNCNH) and/or cyanamide (NH₂CN), and isocyanic acid (HNCO). Deprotonated adenine fragments by loss of hydrogen cyanide and/or isocyanide (HCN/HNC; 90%) and carbodiimide (HNCNH) and/or cyanamide (NH₂CN; 10%). Tandem mass spectrometry (MS^<i>n</i>) experiments reveal that deprotonated guanine fragments lose additional HCN and CO, while deprotonated adenine fragments successively lose HNC and HCN. Every neutral fragment observed in this study has been detected in the ISM, highlighting the potential for nucleobases such as these to form in such environments. Lastly, the acidity of abundant fragment ions is experimentally bracketed. Theoretical calculations at the B3LYP/6-311++G­(d,p) level of theory are performed to delineate the mechanisms of dissociation and analyze the energies of reactants, intermediates, transition states, and products of these CID processes

Reactions of Sulfur- and Oxygen-Containing Anions with Hydrogen Atoms: A Comparative Study

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 oxygen-containing 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

Investigating the α‑Effect in Gas-Phase S_N2 Reactions of Microsolvated Anions

The α-effectenhanced reactivity of nucleophiles with a lone-pair adjacent to the attacking centerwas recently demonstrated for gas-phase S_N2 reactions of HOO^–, supporting an intrinsic component of the α-effect. In the present work we explore the gas-phase reactivity of microsolvated nucleophiles in order to investigate in detail how the α-effect is influenced by solvent. We compare the gas-phase reactivity of the microsolvated α-nucleophile HOO^–(H₂O) to that of microsolvated normal alkoxy nucleophiles, RO^–(H₂O), in reaction with CH₃Cl using a flowing afterglow-selected ion flow tube instrument. The results reveal enhanced reactivity of HOO^–(H₂O) and clearly demonstrate the presence of an α-effect for the microsolvated α-nucleophile. The association of the nucleophile with a single water molecule results in a larger Brønsted β_nuc value than is the case for the unsolvated nucleophiles. Accordingly, the reactions of the microsolvated nucleophiles proceed through later transition states in which bond formation has progressed further. Calculations show a significant difference in solvent interaction for HOO^– relative to the normal nucleophiles at the transition states, indicating that differential solvation may well contribute to the α-effect. The reactions of the microsolvated anions with CH₃Cl can lead to formation of either the bare Cl^– anion or the Cl^–(H₂O) cluster. The product distributions show preferential formation of the Cl^– anion even though the formation of Cl^–(H₂O) would be favored thermodynamically. Although the structure of the HOO^–(H₂O) cluster resembles HO^–(HOOH), we demonstrate that HOO^– is the active nucleophile when the cluster reacts

Experimental and Theoretical Studies of the Reactivity and Thermochemistry of Dicyanamide: N(CN)₂^–

Dicyanamide [N­(CN)₂^–] is a common anionic component of ionic liquids, several of which have shown hypergolic reactivity upon mixing with white-fuming nitric acid. In this study, we explore the thermochemistry of dicyanamide and its reactivity with nitric acid and other molecules to gain insight into the initial stages of the hypergolic phenomenon. We have developed and utilized an electrospray ion source for our selected ion flow tube (SIFT) to generate the dicyanamide anion. We have explored the general reactivity of this ion with several neutral molecules and atoms. Dicyanamide does not show reactivity with O₂, H₂SO₄, H₂O₂, DBr, HCl, NH₃, N₂O, SO₂, COS, CO₂, CH₃OH, H₂O, CH₄, N₂, CF₄, or SF₆ (<i>k</i> < 1 × 10^–12 cm³/s); moreover, dicyanamide does not react with N atom, O atom, or electronically excited molecular oxygen (<i>k</i> < 5 × 10^–12 cm³/s), and our previous studies showed no reactivity with H atom. However, at 0.45 Torr helium, we observe the adduct of dicyanamide with nitric acid with an effective bimolecular rate constant of 2.7 × 10^–10 cm³/s. Intrinsically, dicyanamide is a very stable anion in the gas phase, as illustrated by its lack of reactivity, high electron-binding energy, and low proton affinity. The lack of reactivity of dicyanamide with H₂SO₄ gives an upper limit for the gas-phase deprotonation enthalpy of the parent compound (HNCNCN; <310 ± 3 kcal/mol). This limit is in agreement with theoretical calculations at the MP2/6-311++G­(d,p) level of theory, finding that Δ<i>H</i>_298 K(HNCNCN) = 308.5 kcal/mol. Dicyanamide has two different proton acceptor sites. Experimental and computational results indicate that it is lower in energy to protonate the terminal nitrile nitrogen than the central nitrogen. Although proton transfer to dicyanamide was not observed for any of the acidic molecules investigated here, the calculations on dicyanamide with one to three nitric acid molecules reveal that higher-order solvation can favor exothermic proton transfer. Furthermore, the formation of 1,5-dinitrobiuret, proposed to be the key intermediate during the hypergolic ignition of dicyanamide ionic liquids with nitric acid, is investigated by calculation of the reaction coordinate. Our results suggest that solvation dynamics of dicyanamide with nitric acid play an important role in hypergolic ignition and the interactions at the droplet/condensed-phase surface between the two hypergolic liquids are very important. Moreover, dicyanamide exists in the atmosphere of Saturn’s moon, Titan; the intrinsic stability of dicyanamide strongly suggests that it may exist in molecular clouds of the interstellar medium, especially in regions where other stable carbon–nitrogen anions have been detected

Reactions of Azine Anions with Nitrogen and Oxygen Atoms: Implications for Titan’s Upper Atmosphere and Interstellar Chemistry

Azines are important in many extraterrestrial environments, from the atmosphere of Titan to the interstellar medium. They have been implicated as possible carriers of the diffuse interstellar bands in astronomy, indicating their persistence in interstellar space. Most importantly, they constitute the basic building blocks of DNA and RNA, so their chemical reactivity in these environments has significant astrobiological implications. In addition, N and O atoms are widely observed in the ISM and in the ionospheres of planets and moons. However, the chemical reactions of molecular anions with abundant interstellar and atmospheric atomic species are largely unexplored. In this paper, gas-phase reactions of deprotonated anions of benzene, pyridine, pyridazine, pyrimidine, pyrazine, and s-triazine with N and O atoms are studied both experimentally and computationally. In all cases, the major reaction channel is associative electron detachment; these reactions are particularly important since they control the balance between negative ions and free electron densities. The reactions of the azine anions with N atoms exhibit larger rate constants than reactions of corresponding chain anions. The reactions of azine anions with O atoms are even more rapid, with complex product patterns for different reactants. The mechanisms are studied theoretically by employing density functional theory; spin conversion is found to be important in determining some product distributions. The rich gas-phase chemistry observed in this work provides a better understanding of ion-atom reactions and their contributions to ionospheric chemistry as well as the chemical processing that occurs in the boundary layers between diffuse and dense interstellar clouds

C–H Bond Strengths and Acidities in Aromatic Systems: Effects of Nitrogen Incorporation in Mono-, Di-, and Triazines

The negative ion chemistry of five azine molecules has been investigated using the combined experimental techniques of negative ion photoelectron spectroscopy to obtain electron affinities (EA) and tandem flowing afterglow-selected ion tube (FA-SIFT) mass spectrometry to obtain deprotonation enthalpies (Δ_acid<i>H</i>₂₉₈). The measured Δ_acid<i>H</i>₂₉₈ for the most acidic site of each azine species is combined with the EA of the corresponding radical in a thermochemical cycle to determine the corresponding C–H bond dissociation energy (BDE). The site-specific C–H BDE values of pyridine, 1,2-diazine, 1,3-diazine, 1,4-diazine, and 1,3,5-triazine are 110.4 ± 2.0, 111.3 ± 0.7, 113.4 ± 0.7, 107.5 ± 0.4, and 107.8 ± 0.7 kcal mol^–1, respectively. The application of complementary experimental methods, along with quantum chemical calculations, to a series of nitrogen-substituted azines sheds light on the influence of nitrogen atom substitution on the strength of C–H bonds in six-membered rings