Gas-Phase Reactions of Deprotonated Nucleobases with H, N, and O Atoms

Ion-molecule studies of energetic oxygen allotropes in flow tubes: O 2 ( v ) , O 2 ( a Δ g 1 ) , O 3 , and O

Starting in the 1960s, flow tube apparatuses have played a central role in the study of ion-molecule kinetics, allowing for immense chemical diversity of cationic, anionic, and neutral reactants. Here, we review studies of oxygen allotropes, excluding ground state O2 (X 3 ∑ g - ), and focusing instead on reactions of cations, anions, and metal chemi-ionization reactions with ground state atomic oxygen (O 3P), vibrationally excited molecular oxygen (O2(v)), electronically excited molecular oxygen (O2 (a 1 Δ g )), and ozone (O3). Historical outlines of work over several decades are given along with a focus on more recent work by our group at the Air Force Research Laboratory.

Tandem Mass Spectrometry of Perfluorocarboxylate Anions: Fragmentation Induced by Reactive Species Formed From Microwave Excited Hydrogen and Water Plasmas

RATIONALE

Polyfluoroalkyl substances (PFAS) like perfluorooctanoic acid have persistent environmental and physiological effects. This study investigates the degradation of CnF2n+1CO2 - (n = 1-7) with neutral radical fragmentation under oxygen attachment dissociation (OAD). Unique fragments absent from collision-induced dissociation (CID) are observed. Further, potential mechanisms are uncovered by density functional theory (DFT) calculations.

METHODS

From a standard mixture of PFAS, straight-chain perfluorinated carboxylic acids with carbon chain lengths of one to eight were separated via liquid chromatography and transferred to the gas phase via negative-mode electrospray ionisation. Each CnF2n+1CO2 - of interest was mass selected and fragmented via both CID and OAD in a quadrupole time-of-flight mass spectrometer. DFT optimisations of structures were performed at M06/6-31+g(d), and single point energy calculations were performed at M06-2X/aug-cc-pVTZ for C3F7CO2 -.

RESULTS

Decarboxylation was observed from both CID and OAD, but fluorine abstraction and hydroxyl addition only occurred with OAD. The DFT calculations suggest that C3F6 -• (m/z 150) is most likely formed from by H• attack onto a β- C-F bond, then loss of HF, finally decarboxylation. Further, C3F5O- (m/z 147) likely arises from C3F6 -• recombining with OH• to produce energised C3F6OH- ions, followed by α- or β- elimination of HF to give enolate and/or epoxide-type products.

CONCLUSIONS

OAD of C3F7CO2 - yields unique product ions C3F6 -• (m/z 150) and C3F5O- (m/z 147) absent from collision-induced dissociation. DFT calculations suggest an intricate pathway of H• attack onto a β C-F bond, then loss of HF, decarboxylation, recombination with OH•, and finally α- or β- elimination of HF to give the products.

The associative ionization of N(2P) + O(3P)

The rate constant of the associative ionization reaction N(2P) + O(3P) → NO+ + e- was measured using a flow tube apparatus. A flowing afterglow source was used to produce an ion/electron plasma containing a mixture of ions, including N2+, N3+, and N4+. Dissociative recombination of these species produced a population of nitrogen atoms, including N(2P). Charged species were rejected from the flow tube using an electrostatic grid, subsequent to which oxygen atoms were introduced, produced either using a discharge of helium and oxygen or via the titration of nitrogen atoms with NO. Only the title reaction can produce the NO+ observed after the introduction of O atoms. The resulting rate constant (8 ± 5 ×10-11 cm3 s-1) is larger than previously reported N(2P) + O disappearance rate constants (∼2 × 10-11 cm3 s-1). The possible errors in this or previous experiments are discussed. It is concluded that the N(2P) + O(3P) reaction proceeds almost entirely by associative ionization, with quenching to the 2D or 4S states as only minor processes.

Ion-molecule reactions of mass-selected ions

Gas-phase reactions of mass-selected ions with neutrals covers a very broad area of fundamental and applied mass spectrometry (MS). Oftentimes, ion-molecule reactions (IMR) can serve as a viable alternative to collision-induced dissociation and other ion dissociation techniques when using tandem MS. This review focuses on the literature pertaining applications of IMR since 2013. During the past decade considerable efforts have been made in analytical applications of IMR, including advances in one of the major techniques for characterization of unsaturated fatty acids and lipids, ozone-induced dissociation, and the development of a new technique for sequencing of large ions, hydrogen atom attachment/abstraction dissociation. Many advances have also been made in identifying gas-phase chemistry specific to a functional group in organic and biological compounds, which are useful in structure elucidation of analytes and differentiation of isomers/isobars. With "soft" ionization techniques like electrospray ionization having become mainstream for quite some time now, the efforts in the area of metal ion catalysis have firmly moved into exploring chemistry of ligated metal complexes in their "natural" oxidation states allowing to model individual steps of mechanisms in homogeneous catalysis, especially in combination with high-level DFT calculations. Finally, IMR continue to contribute to the body of knowledge in the area of chemistry of interstellar processes.

Low-temperature reaction dynamics of paramagnetic species in the gas phase

Radicals are abundant in a range of important gas-phase environments. They are prevalent in the atmosphere, in interstellar space, and in combustion processes. As such, understanding how radicals react is essential for the development of accurate models of the complex chemistry occurring in these gas-phase environments. By controlling the properties of the colliding reactants, we can also gain insights into how radical reactions occur on a fundamental level. Recent years have seen remarkable advances in the breadth of experimental methods successfully applied to the study of reaction dynamics involving paramagnetic species-from improvements to the well-known crossed molecular beams approach to newer techniques involving magnetically guided and decelerated beams. Coupled with ever-improving theoretical methods, quantum features are being observed and interesting insights into reaction dynamics are being uncovered in an increasingly diverse range of systems. In this highlight article, we explore some of the exciting recent developments in the study of chemical dynamics involving paramagnetic species. We focus on low-energy reactive collisions involving neutral radical species, where the reaction parameters are controlled. We conclude by identifying some of the limitations of current methods and exploring possible new directions for the field.

Associative detachment in anion-atom reactions involving a dipole-bound electron

Associative electronic detachment (AED) between anions and neutral atoms leads to the detachment of the anion’s electron resulting in the formation of a neutral molecule. It plays a key role in chemical reaction networks, like the interstellar medium, the Earth’s ionosphere and biochemical processes. Here, a class of AED involving a closed-shell anion (OH⁻) and alkali atoms (rubidium) is investigated by precisely controlling the fraction of electronically excited rubidium. Reaction with the ground state atom gives rise to a stable intermediate complex with an electron solely bound via dipolar forces. The stability of the complex is governed by the subtle interplay of diabatic and adiabatic couplings into the autodetachment manifold. The measured rate coefficients are in good agreement with ab initio calculations, revealing pronounced steric effects. For excited state rubidium, however, a lower reaction rate is observed, indicating dynamical stabilization processes suppressing the coupling into the autodetachment region. Our work provides a stringent test of ab initio calculations on anion-neutral collisions and constitutes a generic, conceptual framework for understanding electronic state dependent dynamics in AEDs.

Associative electronic detachment (AED) reactions of anions play a key role in many natural processes. Here, Hassan and colleagues investigate AED reactions between hydroxyl anions and ultracold rubidium atoms in a hybrid atom-ion trap, revealing different dynamics for collisions with ground and electronically excited state rubidium.