4-Bromobiphenyl: Long-lived molecular anion formation and competition between electron detachment and dissociation

Interaction of low-energy electrons with radiosensitizers

We provide an experimentalist's perspective on the present state-of-the-art in the studies of low-energy electron interactions with common radiosensitizers, including compounds used in combined chemo-radiation therapy and their model systems. Low-energy electrons are important secondary species formed during the interaction of ionizing radiation with matter. Their role in the radiation chemistry of living organisms has become an important topic for more than 20 years. With the increasing number of works and reviews in the field, we would like to focus here on a very narrow area of compounds that have been shown to have radio-sensitizing properties on the one hand, and high reactivity towards low-energy electrons on the other hand. Gas phase experiments studying electron attachment to isolated molecules and environmental effects on reaction dynamics are reviewed for modified DNA components, nitroimidazoles, and organometallics. In the end, we provide a perspective on the future directions that may be important for transferring the fundamental knowledge about the processes induced by low-energy electrons into practice in the field of rational design of agents for concomitant chemo-radiation therapy.

Long-lived molecular anions of brominated diphenyl ethers

Resonance electron attachment in a series of brominated diphenyl ethers, namely 4-bromodiphenyl ether (BDPE), 4-bromophenyl ether (BPE), and decabromodiphenyl ether (DBDE), was investigated in the gas phase by means of dissociative electron attachment spectroscopy. In addition to channels of dissociation into stable fragments, long-lived molecular negative ions with an average lifetime relative to autodetachment of the order of 60 µs were found for the last two molecules. In the case of BDPE and BPE, the most intense dissociation channel is the bromine anion, and for DBDE-the [C6Br5O]- anion. The [C6Br5O]- anion sequentially decomposes with the elimination of the bromide anion on a microsecond time scale, which is confirmed by the registration of metastable ions with an apparent mass of 12.8 a.m.u. The electron affinity of the studied molecules and the appearance energy of fragment ions were estimated with CAM-B3LYP/6-311+G(d,p).

Resonant electron capture by 5-Br-2′-deoxyuridine

The results of the study of resonant electron capture by molecules of 5-Br-2′-deoxyuridine (BrdUrd) over the range of electron energies from near zero to 14 eV are described. In the thermal energy range, long-lived molecular negative ions, unstable with respect to autoneutralization and dehalogenation, have been registered. Examination of the kinetics of these decay processes led us to the conclusion that the most probable structure for molecular negative ions is that with an extended C–Br bond, which was predicted earlier using quantum-chemical calculations. Estimates have shown that the BrdUrd molecule owns a significant electronic affinity of 0.93–1.38 eV. The most intense fragmentation channel leads to the abundant formation of Br− ions. The dissociative electron attachment cross section for Br− ions formation was estimated to amount to no less than 1.65 × 10−15 cm2, indirectly implying a fairly intense formation of complementary highly reactive deoxyuridine-5-yl particles. These particles are known to be responsible for the radiosensitizing properties of BrdUrd.

Dissociative electron attachment to hexachlorobenzene

Gas phase molecules of hexachlorobenzene (C 6 Cl 6 ) were investigated by means of Dissociative Electron Attachment Spectroscopy (DEAS). Three channels of molecular negative ions decay have been recorded: Cl- and Cl 2 - as well as electron detachment (t a ~250 ms at 343 K). All of these decay channels exhibit temperature dependence. The electron affinity was estimated within the framework of the simple Arrhenius model (EA=1.6-1.9 eV), as well as quantum-chemically at various levels (EA a =0.9-1.0 eV). The reasons for the discrepancy between the obtained estimates of the EA value and the results of previous works (EA from 1 to 2 eV) are discussed.

Non-covalent anion structures in dissociative electron attachment to some brominated biphenyls

The present work combines experiment and theory to reveal the behavior of bromo-substituted-biphenyls after an electron attachment. We experimentally determine anion lifetimes using an electron attachment-magnetic sector mass spectrometer instrument. Branching ratios of dissociative electron attachment fragments on longer timescales are determined using the electron attachment-quadrupole mass spectrometer instrument. In all cases, fragmentation is low: Only the Br^- and [M-Br]^- ions are detected, and [M-H]^- is observed only in the case of 4-Br-biphenyl and parent anion lifetimes as long as 165 µs are observed. Such lifetimes are contradictory to the dissociation rates of 2- and 4-bromobiphenyl, as measured by the pulse radiolysis method to be 3.2 × 10¹⁰ and >5 × 10¹⁰ s^-1, respectively. The discrepancy is plausibly explained by our calculation of the potential energy surface of the dissociating anion. Isolated in vacuum, the bromide anion can orbit the polarized aromatic radical at a long distance. A series of local minima on the potential energy surface allows for a roaming mechanism prolonging the detection time of such weakly bound complex anions. The present results illuminate the behavior recently observed in a series of bromo-substituted compounds of biological as well as technological relevance.

Electron attachment to microhydrated 4-nitro- and 4-bromo-thiophenol

We investigate the effect of microhydration on electron attachment to thiophenols with halogen (Br) and nitro (NO₂) functional groups in the para position. We focus on the formation of anions upon the attachment of low-energy electrons with energies below 8 eV to heterogeneous clusters of the thiophenols with water. For nitro-thiophenol (NTP), the primary reaction channel observed is the associative electron attachment, irrespective of the microhydration. On the other hand, bromothiophenol (BTP) fragments significantly upon the electron attachment, producing Br^- and (BTP-H)^- anions. Microhydration suppresses fragmentation of both molecules, however in bromothiophenol, the Br^- channel remains intense and Br(H₂O)_n^- hydrated fragment clusters are observed. The results are supported by the reaction energetics obtained from ab initio calculations. Different dissociation dynamics of NTP and BTP can be related to different products of their plasmon induced reactions on Au nanoparticles. Computational modeling of the simplified BTP(H₂O) system indicates that the electron attachment products reflect the structure of neutral precursor clusters - the anion dissociation dynamics is controlled by the hydration site.

Electron attachment to isolated and microhydrated favipiravir

Electron attachment and its equivalent in complex environments, single-electron reduction, are important in many biological processes. Here, we experimentally study the electron attachment to favipiravir, a well-known antiviral agent. Electron attachment spectroscopy is used to explore the energetics of associative (AEA) and dissociative (DEA) electron attachment to isolated favipiravir. AEA dominates the interaction and the yields of the fragment anions after DEA are an order of magnitude lower than that of the parent anion. DEA primary proceeds via decomposition of the CONH2 functional group, which is supported by reaction threshold calculations using ab initio methods. Mass spectrometry of small favipiravir-water clusters demonstrates that a lot of energy is transferred to the solvent upon electron attachment. The energy gained upon electron attachment, and the high stability of the parent anion were previously suggested as important properties for the action of several electron-affinic radiosensitizers. If any of these mechanisms cause synergism in chemo-radiation therapy, favipiravir could be repurposed as a radiosensitizer.

5-Nitro-2,4-Dichloropyrimidine as an Universal Model for Low-Energy Electron Processes Relevant for Radiosensitization

We report experimental results of low-energy electron interactions with.

Dissociative Electron Attachment to 2,3,6,7,10,11-Hexabromotriphenylene

2,3,6,7,10,11-Hexabromotriphenylene (HBTP) and 2,3,6,7,10-pentabromotriphenylene (PBTP) were investigated by means of dissociative electron attachment spectroscopy (DEAS). The dominant decay channel of the transient molecular negative ions consists of elimination of Br^- with resonances in the low electron energy region. Formation of long-lived parent anions with autodetachment lifetime τ_a = 310 μs is observed at thermal electron energies. The adiabatic electron affinities, EA_a = 1.12 ± 0.1 eV in HBTP and 1.09 ± 0.1 eV in PBTP, evaluated using a simple Arrhenius approach are in good agreement with those predicted by DFT (XYG3/Def2-TZVPP//PBE0/Def2-TZVPP) calculations.

Structural rearrangements as relaxation pathway for molecular negative ions formed via vibrational Feshbach resonance

The low-energy (0-15 eV) resonance electron interaction with two organic acids, oxaloacetic and α-ketoglutaric, is studied under gas-phase conditions using dissociative electron attachment spectroscopy. The most unexpected observation is the long-lived (microseconds) molecular negative ions formed by thermal electron attachment via the vibrational Feshbach resonance mechanism in both compounds. Unlike oxaloacetic acid, for which only one slow (microseconds) dissociative decay is detected, as many as five metastable negative ions are observed for α-ketoglutaric acid. These results are analyzed using density functional theory calculations and estimations of electron affinity using the experimental electron detachment times. The results are of considerable interest for understanding the fundamental mechanisms responsible for the dynamics of highly excited negative ions and the transformation pathways of biologically relevant molecules stimulated by excess electron attachment.