Enhancement of electron accepting ability of para-benzoquinone by a single water molecule

Dynamics of Anions: From Bound to Unbound States and Everything In Between

Gas-phase anions present an ideal playground for the exploration of excited-state dynamics. They offer control in terms of the mass, extent of solvation, internal temperature, and conformation. The application of a range of ion sources has opened the field to a vast array of anionic systems whose dynamics are important in areas ranging from biology to star formation. Here, we review recent experimental developments in the field of anion photodynamics, demonstrating the detailed insight into photodynamical and electron-capture processes that can be uncovered. We consider the electronic and nuclear ultrafast dynamics of electronically bound excited states along entire reaction coordinates; electronically unbound states showing that photochemical concepts, such as chromophores and Kasha's rule, are transferable to electron-driven chemistry; and nonvalence states that straddle the interface between bound and unbound states. Finally, we consider likely developments that are sure to keep the field of anion dynamics buoyant and impactful.

Parent anion radical formation in coenzyme Q0: Breaking ubiquinone family rules

We report electron attachment (EA) measurements for the parent anion radical formation from coenzyme Q0 (CoQ0) at low electron energies (<2 eV) along with quantum chemical calculations. CoQ0 may be considered a prototype for the electron withdrawing properties of the larger CoQ n molecules, in particular ubiquinone (CoQ10), an electron carrier in aerobic cell respiration. Herein, we show that the mechanisms for the parent anion radical formation of CoQ0 and CoQ n (n = 1,2,4) are remarkably distinct. Reported EA data for CoQ1, CoQ2, CoQ4 and para-benzoquinone indicated stabilization of the parent anion radicals around 1.2-1.4 eV. In contrast, we observe for the yield of the parent anion radical of CoQ0 a sharp peak at ∼ 0 eV, a shoulder at 0.07 eV and a peak around 0.49 eV. Although the mechanisms for the latter feature remain unclear, our calculations suggest that a dipole bound state (DBS) would account for the lower energy signals. Additionally, the isoprenoid side chains in CoQ n (n = 1,2,4) molecules seem to influence the DBS formation for these compounds. In contrast, the side chains enhance the parent anion radical stabilization around 1.4 eV. The absence of parent anion radical formation around 1.4 eV for CoQ0 can be attributed to the short auto-ionization lifetimes. The present results shed light on the underappreciated role played by the side chains in the stabilization of the parent anion radical. The isoprenoid tails should be viewed as co-responsible for the electron-accepting properties of ubiquinone, not mere spectators of electron transfer reactions.

Statistical vibrational autodetachment and radiative cooling rates of para-benzoquinone

We report measurements of the statistical vibrational autodetachment (VAD, also called thermionic emission) and radiative cooling rates of isolated para-benzoquinone (pBQ, C₆H₄O₂) radical anions using the cryogenic electrostatic ion storage ring facility DESIREE. The results are interpreted using master equation simulations with rate coefficients calculated using statistical detailed balance theory. The VAD rate is determined by measuring the time-dependent yield of neutral pBQ due to spontaneous electron emission from a highly-excited ensemble of anions formed in an electron-attachment ion source. Competition with radiative cooling quenches the VAD rate after a critical time of τ_c = 11.00(5) ms. Master equation simulations which reproduce the VAD yield provide an estimate of the initial effective vibrational temperature of the ions of 1100(20) K, and provide insight into the anion formation scenario. A second measurement of the radiative cooling rate of pBQ^- stored for up to 0.5 s was achieved using time-dependent photodetachment action spectroscopy across the ²A_u ← ²B_2g and ²B_2u ← ²B_2g transitions. The rate at which hot-band contributions fade from the action spectrum is quantified by non-negative matrix factorisation. This is found to be commensurate with the average vibrational energy extracted from the simulations, with 1/e lifetimes of 0.16(3) s and 0.1602(7) s, respectively. Implications for astrochemistry are discussed.

Effect of Microhydration on the Temporary Anion States of Pyrene

The influence of incremental hydration (≤4) on the electronic resonances of the pyrene anion is studied using two-dimensional photoelectron spectroscopy. The photoexcitation energies of the resonances do not change; therefore, from the anion's perspective, the resonances remain the same, but from the neutral's perspective of the electron-molecule reaction, the resonances decrease in energy by the binding energy of the water molecules. The autodetachment of the resonances shows that hydration has very little effect, showing that even the dynamics of most of the resonances are not impacted by hydration. Two specific resonances do show changes that are explained by the closing of specific autodetachment channels. The lowest-energy resonance leads to efficient electron capture as observed through thermionic emission and evaporation of water molecules (dissociative electron attachment). The implications of low-energy electron capture in dense molecular interstellar clouds are discussed.

Electron Attachment to Isolated Molecules as a Probe to Understand Mitochondrial Reductive Processes

This chapter describes the complementary experimental techniques Electron Transmission Spectroscopy and Dissociative Electron Attachment Spectroscopy, two of the most suitable means for investigating interactions between electrons and gas-phase molecules, resonance formation of temporary molecular negative ions, and their possible decay through the dissociative electron attachment (DEA) mechanism. The latter can be seen as the gas-phase counterpart of the transfer of a solvated electron in solution, accompanied by dissociation of the molecular anion, referred to as dissociative electron transfer (DET). DET takes place in vivo under reductive conditions, for instance, in the intermembrane space of mitochondria under interaction of xenobiotic molecules possessing high electron affinity with electrons "leaked" from the mitochondrial respiratory chain. A likely mechanism of the toxic activity of dichlorodiphenyltrichloroethane based on its DEA properties is briefly outlined, and compared with the well-established harmful effects of the model toxicant carbon tetrachloride ascribed to reductive dechlorination in a cellular ambient. A possible mechanism of the antioxidant activity of polyphenolic compounds present near the main site of superoxide anion production in mitochondria is also briefly discussed.

Ionizing radiation and natural constituents of living cells: Low-energy electron interaction with coenzyme Q analogs

Resonance electron attachment to short-tail analogs of coenzyme Q₁₀ is investigated in the electron energy range 0 eV-14 eV under gas-phase conditions by means of dissociative electron attachment spectroscopy. Formation of long-lived (milliseconds) molecular negative ions is detected at 1.2 eV, but not at thermal energy. A huge increase in the electron detachment time as compared with the reference para-benzoquinone (40 µs) is ascribed to the presence of the isoprene side chains. Elimination of a neutral CH₃ radical is found to be the most intense decay detected on the microsecond time scale. The results give some insight into the timescale of electron-driven processes stimulated in living tissues by high-energy radiation and are of importance in prospective fields of radiobiology and medicine.

Role of Nonvalence States in the Ultrafast Dynamics of Isolated Anions

Nonvalence states of neutral molecules (Rydberg states) play important roles in nonadiabatic dynamics of excited states. In anions, such nonadiabatic transitions between nonvalence and valence states have been much less explored even though they are believed to play important roles in electron capture and excited state dynamics of anions. The aim of this Feature Article is to provide an overview of recent experimental observations, based on time-resolved photoelectron imaging, of valence to nonvalence and nonvalence to valence transitions in anions and to demonstrate that such dynamics may be commonplace in the excited state dynamics of molecular anions and cluster anions.

Photoelectron spectroscopy of para-benzoquinone cluster anions

The photoelectron spectra of para-benzoquinone radical cluster anions, (pBQ)_n ^- (n = 2-4), taken at hv = 4.00 eV are presented and compared with the photoelectron spectrum of the monomer (n = 1). For all clusters, a direct detachment peak can be identified, and the incremental increase in the vertical detachment energy of ∼0.4 eV n^-1 predominantly reflects the increase in cohesion energy as the cluster size increases. For all clusters, excitation also leads to low energy electrons that are produced by thermionic emission from ground electronic state anionic species, indicating that resonances are excited at this photon energy. For n = 3 and 4, photoelectron features at lower binding energy are observed which can be assigned to photodetachment from pBQ^- for n = 3 and both pBQ^- and (pBQ)₂ ^- for n = 4. These observations indicate that the cluster dissociates on the time scale of the laser pulse (∼5 ns). The present results are discussed in the context of related quinone cluster anions.