Dipole bound, nucleic acid base anions studied via negative ion photoelectron spectroscopy

The valence electron affinity of uracil determined by anion cluster photoelectron spectroscopy

The unoccupied π* orbitals of the nucleobases are considered to play important roles in low-energy electron attachment to DNA, inducing damage. While the lowest anionic valence state is vertically unbound in all neutral nucleobases, it remains unclear even for the simplest nucleobase, uracil (U), whether its valence anion (U-) is adiabatically bound, which has important implications on the efficacy of damage processes. Using anion photoelectron spectroscopy, we demonstrate that the valence electron affinity (EAV) of U can be accurately measured within weakly solvating clusters, U-(Ar)n and U-(N2)n. Through extrapolation to the isolated U limit, we show that EAV = -2 ± 18 meV. We discuss these findings in the context of electron attachment to U and its reorganization energy, and more generally establish guidance for the determination of molecular electron affinities from the photoelectron spectroscopy of anion clusters.

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.

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.

Time-resolved photoelectron spectroscopy of iodide-4-thiouracil cluster: The ππ* state as a doorway for electron attachment

The photophysics of thiobases-nucleobases in which one or more oxygen atoms are replaced with sulfur atoms- vary greatly depending on the location of sulfonation. Not only are direct dynamics of a neutral thiobase impacted, but also the dynamics of excess electron accommodation. In this work, time-resolved photoelectron spectroscopy is used to measure binary anionic clusters of iodide and 4-thiouracil, I- · 4TU. We investigate charge transfer dynamics driven by excitation at 3.88 eV, corresponding to the lowest ππ* transition of the thiouracil, and at 4.16 eV, near the cluster vertical detachment energy. The photoexcited state dynamics are probed by photodetachment with 1.55 and 3.14 eV pulses. Excitation at 3.88 eV leads to a signal from a valence-bound ion only, indicating a charge accommodation mechanism that does not involve a dipole-bound anion as an intermediate. Excitation at 4.16 eV rapidly gives rise to dipole-bound and valence-bound ion signals, with a second rise in the valence-bound signal corresponding to the decay of the dipole-bound signal. The dynamics associated with the low energy ππ* excitation of 4-thiouracil provide a clear experimental proof for the importance of localized excitation and electron backfilling in halide-nucleobase clusters.

Excited-state chemistry of the nitromethane anion mediated by the dipole-bound states revealed by photofragment action spectroscopy

We report the first experimental observation of the excited dipole-bound state (DBS) of the cryogenically cooled nitromethane anion (CH3NO2-), where the excess electron is loosely attached to the singlet or triplet neutral-core. Photofragment and photodetachment action spectra have been employed for the dynamic exploration of Feshbach resonances located even far above the electron detachment threshold, giving excitation profiles from the ground anionic state (D0) to the DBSs which match quite well with the spectral structures of the photoelectron spectra. This indicates that the electron transfer from the nonvalence orbital (of DBS) to the valence orbital (of anion) is mainly responsible for the anionic fragmentation channels, giving strong evidence for that the DBS plays a dynamic doorway-role in the anionic fragmentation reactions. Photofragment action spectra have also been obtained for the anionic clusters of (CH3NO2)2-, (CH3NO2)3-, or (CH3NO2·H2O)-, giving the relative yields of various fragments as a function of the excitation energy for each cluster. The absorption profiles of the anionic clusters exhibit substantial blue-shifts compared to the bare nitromethane anion as their ground states are much stabilized by solvation. The anionic fragmentation pattern varies among different clusters, giving essential clues for the thorough understanding of the whole anionic dynamics such as the dynamic role of the short-lived nonvalence-bound states of the clusters.

How Accurately Can DFT Describe Non-valence Anions?

Weakly bound non-valence anions are molecular systems where the excess electron stabilizes in a very diffuse orbital whose size, shape, and binding energy (∼1-100 meV) are governed by the long-range electrostatic potential of the molecule. Its binding energy comes mainly from charge-dipole or charge-multipole interactions or dispersion forces. While highly correlated methods, like coupled cluster methods, are considered to be the state of the art for describing anionic systems, especially when the electron lies in a very diffuse orbital, we consider here the possibility to use DFT-based calculations. In such molecular anions, the outer electron experiences long-range exchange and correlation interactions. We show that DFT can describe long-range bound states provided that a correct asymptotic exchange and correlation potential is used, namely, that from a range-separated hybrid functional. This opens an alternative to the computationally demanding highly correlated method calculations. It is also suggested that the study of weakly bound anions could help in the construction of new DFT potentials to study systems where nonlocal effects are significant.

Low-Energy Shape Resonances of a Nucleobase in Water

When high-energy radiation passes through aqueous material, low-energy electrons are produced which cause DNA damage. Electronic states of anionic nucleobases have been suggested as an entrance channel to capture the electron. However, identifying these electronic resonances have been restricted to gas-phase electron-nucleobase studies and offer limited insight into the resonances available within the aqueous environment of DNA. Here, resonance and detachment energies of the micro-hydrated uracil pyrimidine nucleobase anion are determined by two-dimensional photoelectron spectroscopy and are shown to extrapolate linearly with cluster size. This extrapolation allows the corresponding resonance and detachment energies to be determined for uracil in aqueous solution as well as the reorganization energy associated with electron capture. Two shape resonances are clearly identified that can capture low-energy electrons and subsequently form the radical anion by solvent stabilization and internal conversion to the ground electronic state. The resonances and their dynamics probed here are the nucleobase-centered doorway states for low-energy electron capture and damage in DNA.

Electron attachment dynamics following UV excitation of iodide-2-thiouracil complexes

The dynamics of low energy electron attachment to the thio-substituted uracil analog 2-thiouracil are investigated using time-resolved photoelectron spectroscopy (TRPES) of iodide·2-thiouracil (I -·2TU­­) binary clusters. In these experiments, the anions are excited at pump energies of 4.16 and 4.73 eV, and the ensuing dynamics are probed by photodetachment at 1.59 and 3.18 eV. Upon excitation near the vertical detachment energy (4.16 eV), dipole bound (DB) and valence bound (VB) anion signals appear almost instantaneously, and the DB state of the 2TU anion undergoes ultrafast decay (~50 fs). At 4.73 eV, there is no evidence for a DB state, but features attributed to two VB states are seen. The transient negative ions formed by photoexcitation decay by autodetachment and I- fragmentation. The I- dissociation rates and their dependence on excitation energy agree reasonably well with the Rice-Ramsperger-Kassel-Marcus calculations. Notable differences with respect to TRPES of the related iodide-uracil anion are observed and discussed.

Electrophilic Properties of 2'-Deoxyadenosine···Thymine Dimer: Photoelectron Spectroscopy and DFT Studies

The anion radical of the 2'-deoxyadenosine···thymine (dAT^•-) pair has been investigated experimentally and theoretically in the gas phase. By employing negative-ion photoelectron spectroscopy (PES), we have registered a spectrum typical for the valence-bound anion, featuring a broad peak at the electron-binding energy (EBE) between ∼1.5 and 2.2 eV with the maximum at ∼1.9 eV. The measured value of the adiabatic electron affinity (AEA) for dAT was estimated to be ∼1.1 eV. Calculations performed at the M06-2X/6-31++G(d,p) level revealed that the structure, where thymine is coordinated to the sugar of dA by two hydrogen bonds, is responsible for the observed PES signal. The AEA_G and the vertical detachment energy of 0.91 and 1.68 eV, respectively, calculated for this structure reproduce the experimental values well. The role of the possible proton transfer in the stabilization of anionic radical complexes is discussed.

Observation of an Excited Dipole-Bound State in a Diatomic Anion

We report the observation of σ-type and π-type excited dipole-bound states (DBSs) in cryogenically cooled potassium iodide (KI) anions for the first time. Two DBSs were observed 39.7(10) meV and 5.0(12) meV below the photodetachment threshold via the resonant two-photon detachment. The different photoelectron angular distributions and binding energies suggest that the two DBSs are of different types. The existence of one σ-type and one π-type DBS in the KI anion was also supported by the high-level ab initio theoretical calculations. The excellent agreement between experimental and theoretical results confirms the prediction that a dipolar molecule with a large enough dipole moment can have an excited DBS.

Modeling the Photoelectron Angular Distributions of Molecular Anions: Roles of the Basis Set, Orbital Choice, and Geometry

A study investigating the effect of the basis set, orbital choice, and geometry on the modeling of photoelectron angular distributions (PADs) of molecular anions is presented. Experimental and modeled PADs for a number of molecular anions, including both closed- and open-shell systems, are considered. Guidelines are suggested for chemists who wish to design calculations to capture the correct chemical physics of the anisotropy of photodetachment, while balancing the computational cost associated with larger molecular anions.

Investigating the mapping of chromophore excitations onto the electron detachment spectrum: photodissociation spectroscopy of iodide ion-thiouracil clusters

Laser photodissociation spectroscopy (3.1-5.7 eV) has been applied to iodide complexes of the non-native nucleobases, 2-thiouracil (2-TU), 4-thiouracil (4-TU) and 2,4-thiouracil (2,4-TU), to probe the excited states and intracluster electron transfer as a function of sulphur atom substitution. Photodepletion is strong for all clusters (I-·2-TU, I-·4-TU and I-·2,4-TU) and is dominated by electron detachment processes. For I-·4-TU and I-·2,4-TU, photodecay is accompanied by formation of the respective molecular anions, 4-TU- and 2,4-TU-, behaviour that is not found for other nucleobases. Notably, the I-·2TU complex does not fragment with formation of its molecular anion. We attribute the novel formation of 4-TU- and 2,4-TU- to the fact that these valence anions are significantly more stable than 2-TU-. We observe further similar behaviour for I-·4-TU and I-·2,4-TU relating to the general profile of their photodepletion spectra, since both strongly resemble the intrinsic absorption spectra of the respective uncomplexed thiouracil molecule. This indicates that the nucleobase chromophore excitations are determining the clusters' spectral profile. In contrast, the I-·2-TU photodepletion spectrum is dominated by the electron detachment profile, with the near-threshold dipole-bound excited state being the only distinct spectral feature. We discuss these observations in the context of differences in the dipole moments of the thionucleobases, and their impact on the coupling of nucleobase-centred transitions onto the electron detachment spectrum.

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.

Electron and hole interactions with P, Z, and P:Z and the formation of mutagenic products by proton transfer reactions

Z would act as an electron acceptor and P would capture a hole in the unnatural DNA. The latter process would produce mutagenic products via a proton transfer reaction.

Time-resolved dynamics in iodide-uracil-water clusters upon excitation of the nucleobase

The dynamics of iodide-uracil-water (I^-·U·H₂O) clusters following π-π^* excitation of the nucleobase are probed using time-resolved photoelectron spectroscopy. Photoexcitation of this cluster at 4.77 eV results in electron transfer from the iodide moiety to the uracil, creating a valence-bound anion within the cross correlation of the pump and probe laser pulses. This species can decay by a number of channels, including autodetachment and dissociation to I^- or larger anion fragments. Comparison of the energetics of the photoexcited cluster and its decay dynamics with those of the bare iodide-uracil (I^-·U) complex provides a sensitive probe of the effects of microhydration on these species.

High-resolution photoelectron imaging and resonant photoelectron spectroscopy via noncovalently bound excited states of cryogenically cooled anions

Valence-bound anions with polar neutral cores (μ > ∼2.5 D) can support dipole-bound excited states below the detachment threshold. These dipole-bound states (DBSs) are highly diffuse and the weakly bound electron in the DBS can be readily autodetached via vibronic coupling. Excited DBSs can be observed in photodetachment spectroscopy using a tunable laser. Tuning the detachment laser to above-threshold vibrational resonances yields vibrationally enhanced resonant photoelectron spectra, which are highly non-Franck-Condon with much richer vibrational information. This perspective describes recent advances in the studies of excited DBSs of cryogenically cooled anions using high-resolution photoelectron imaging (PEI) and resonant photoelectron spectroscopy (rPES). The basic features of dipole-bound excited states and highly non-Franck-Condon resonant photoelectron spectra will be discussed. The power of rPES to yield rich vibrational information beyond conventional PES will be highlighted, especially for low-frequency and Franck-Condon-inactive vibrational modes, which are otherwise not accessible from non-resonant conventional PES. Mode-selectivity and intra-molecular rescattering have been observed during the vibrationally induced autodetachment. Conformer-specific rPES is possible due to the different dipole-bound excited states of molecular conformers with polar neutral cores. For molecules with μ ≪ 2.5 D or without dipole moments, but large quadrupole moments, excited quadrupole-bound states can exist, which can also be used to conduct rPES.

On the stability of a dipole-bound state in the presence of a molecule

Dipole-bound states (DBSs) are diffuse non-valence molecular orbitals of anions where the electron is bound by the permanent dipole moment of the neutral core. Here, an experimental study of the stability of such orbitals under the influence of a perturbing molecular alkyl chain is presented. Photodetachment action and photoelectron imaging spectroscopy of five para-substituted phenolate anions with progressively longer alkyl chains show that the DBS survives in all cases, suggesting that the perturbation of the orbital is not critical to the existence of the DBS.

Time-resolved radiation chemistry: femtosecond photoelectron spectroscopy of electron attachment and photodissociation dynamics in iodide-nucleobase clusters

Iodide-nucleobase (I-·N) clusters studied by time-resolved photoelectron spectroscopy (TRPES) are an opportune model system for examining radiative damage of DNA induced by low-energy electrons. By initiating charge transfer from iodide to the nucleobase and following the dynamics of the resulting transient negative ions (TNIs) with femtosecond time resolution, TRPES provides a novel window into the chemistry triggered by the attachment of low-energy electrons to nucleobases. In this Perspective, we examine and compare the dynamics of electron attachment, autodetachment, and photodissociation in a variety of I-·N clusters, including iodide-uracil (I-·U), iodide-thymine (I-·T), iodide-uracil-water (I-·U·H2O), and iodide-adenine (I-·A), to develop a more unified representation of our understanding of nucleobase TNIs. The experiments probe whether dipole-bound or valence-bound TNIs are formed initially and the subsequent time evolution of these species. We also provide an outlook for forthcoming applications of TRPES to larger iodide-containing complexes to enable the further investigation of microhydration dynamics in nucleobases, as well as electron attachment and photodissociation in more complex nucleic acid constituents.

Solvation Effects on Dissociative Electron Attachment to Thymine

Ionizing radiation can excite the cellular medium to produce secondary electrons that can subsequently cause damage to DNA. The damage is believed to occur via dissociative electron attachment (DEA). In DEA, the electron is captured by a molecule in a resonant antibonding state and a transient negative ion is formed. If this ion survives against electron autodetachment, then bonds within the molecule may dissociate as energy is transferred from the electronic degrees of freedom into vibrational modes of the molecule. We present a model for studying the effect that transferring kinetic energy into the vibrational modes of a molecule in this way has on a DNA nucleobase. We show that when the base is in an aqueous environment, dissociation is affected by interactions with the surrounding water molecules. In particular, hydrogen bonding between the nucleobase and the solvent can suppress the dissociative channel.

Observation of the dipole- and quadrupole-bound anions of 1,4-dicyanocyclohexane

Quadrupole-bound anions are negative ions in which their excess electrons are loosely bound by long-range electron-quadrupole attractions.

Probing the coupling of a dipole-bound electron with a molecular core

A dipolar molecule can weakly bind an electron in a diffuse orbital. However, the spin-orbit coupling between this weakly bound electron and the electrons in the molecular core is not known. Here we probe this coupling using the linear C₂P^- anion with the ³Σ⁺ ground state, which possesses dipole-bound excited states because neutral C₂P (²Π) has a sufficiently large dipole moment. Photodetachment spectroscopy and resonant photoelectron spectroscopy are used to probe the nature of the dipole-bound states. Two dipole-bound excited states are observed with a binding energy of 37 cm^-1, corresponding to the two spin-orbit states of neutral C₂P (²Π_1/2 and ²Π_3/2). The current study demonstrates that the weakly bound electron in the dipole-bound excited states of C₂P^- is not spin-coupled to the electrons in the C₂P core and can be considered as a quasi-free electron.

Accurate Predictions of Electron Binding Energies of Dipole-Bound Anions via Quantum Monte Carlo Methods

Neutral molecules with sufficiently large dipole moments can bind electrons in diffuse nonvalence orbitals with most of their charge density far from the nuclei, forming so-called dipole-bound anions. Because long-range correlation effects play an important role in the binding of an excess electron and overall binding energies are often only on the order of 10s-100s of wave numbers, predictively modeling dipole-bound anions remains a challenge. Here, we demonstrate that quantum Monte Carlo methods can accurately characterize molecular dipole-bound anions with near-threshold dipole moments. We also show that correlated sampling Auxiliary Field Quantum Monte Carlo is particularly well-suited for resolving the fine energy differences between the neutral and anionic species. These results shed light on the fundamental limitations of quantum Monte Carlo methods and pave the way toward using them for the study of weakly bound species that are too large to model using traditional electron structure methods.

Dynamics of electron attachment and photodissociation in iodide-uracil-water clusters via time-resolved photoelectron imaging

The dynamics of low energy electron attachment to monohydrated uracil are investigated using time-resolved photoelectron imaging to excite and probe iodide-uracil-water (I^-·U·H₂O) clusters. Upon photoexcitation of I^-·U·H₂O at 4.38 eV, near the measured cluster vertical detachment energy of 4.40 eV ± 0.05 eV, formation of both the dipole bound (DB) anion and valence bound (VB) anion of I^-·U·H₂O is observed and characterized using a probe photon energy of 1.58 eV. The measured binding energies for both anions are larger than those of the non-hydrated iodide-uracil (I^-·U) counterparts, indicating that the presence of water stabilizes the transient negative ions. The VB anion exhibits a somewhat delayed 400 fs rise when compared to I^-·U, suggesting that partial conversion of the DB anion to form the VB anion at early times is promoted by the water molecule. At a higher probe photon energy, 3.14 eV, I^- re-formation is measured to be the major photodissociation channel. This product exhibits a bi-exponential rise; it is likely that the fast component arises from DB anion decay by internal conversion to the anion ground state followed by dissociation to I^-, and the slow component arises from internal conversion of the VB anion.

Observation of Excited Quadrupole-Bound States in Cold Anions

We report the first observation of an excited quadrupole-bound state (QBS) in an anion. High-resolution photoelectron imaging of cryogenically cooled 4-cyanophenoxide (4CP^{-}) anions yields an electron detachment threshold of 24 927  cm^{-1}. The photodetachment spectrum reveals a resonant transition 20  cm^{-1} below the detachment threshold, which is attributed to an excited QBS of 4CP^{-} because neutral 4CP has a large quadrupole moment with a negligible dipole moment. The QBS is confirmed by observation of seventeen above-threshold resonances due to autodetachment from vibrational levels of the QBS.

Photodissociation dynamics of the iodide-uracil (I(-)U) complex

Photofragment action spectroscopy and femtosecond time-resolved photoelectron imaging are utilized to probe the dissociation channels in iodide-uracil (I(-) ⋅ U) binary clusters upon photoexcitation. The photofragment action spectra show strong I(-) and weak [U-H](-) ion signal upon photoexcitation. The action spectra show two bands for I(-) and [U-H](-) production peaking around 4.0 and 4.8 eV. Time-resolved experiments measured the rate of I(-) production resulting from excitation of the two bands. At 4.03 eV and 4.72 eV, the photoelectron signal from I(-) exhibits rise times of 86 ± 7 ps and 36 ± 3 ps, respectively. Electronic structure calculations indicate that the lower energy band, which encompasses the vertical detachment energy (4.11 eV) of I(-)U, corresponds to excitation of a dipole-bound state of the complex, while the higher energy band is primarily a π-π(∗) excitation on the uracil moiety. Although the nature of the two excited states is very different, the long lifetimes for I(-) production suggest that this channel results from internal conversion to the I(-) ⋅ U ground state followed by evaporation of I(-). This hypothesis was tested by comparing the dissociation rates to Rice-Ramsperger-Kassel-Marcus calculations.

Complete-active-space second-order perturbation theory (CASPT2//CASSCF) study of the dissociative electron attachment in canonical DNA nucleobases caused by low-energy electrons (0-3 eV)

Low-energy (0-3 eV) ballistic electrons originated during the irradiation of biological material can interact with DNA/RNA nucleobases yielding transient-anion species which undergo decompositions. Since the discovery that these reactions can eventually lead to strand breaking of the DNA chains, great efforts have been dedicated to their study. The main fragmentation at the 0-3 eV energy range is the ejection of a hydrogen atom from the specific nitrogen positions. In the present study, the methodological approach introduced in a previous work on uracil [I. González-Ramírez et al., J. Chem. Theory Comput. 8, 2769-2776 (2012)] is employed to study the DNA canonical nucleobases fragmentations of N-H bonds induced by low-energy electrons. The approach is based on minimum energy path and linear interpolation of internal coordinates computations along the N-H dissociation channels carried out at the complete-active-space self-consistent field//complete-active-space second-order perturbation theory level. On the basis of the calculated theoretical quantities, new assignations for the adenine and cytosine anion yield curves are provided. In addition, the π1 (-) and π2 (-) states of the pyrimidine nucleobases are expected to produce the temporary anions at electron energies close to 1 and 2 eV, respectively. Finally, the present theoretical results do not allow to discard neither the dipole-bound nor the valence-bound mechanisms in the range of energies explored, suggesting that both possibilities may coexist in the experiments carried out with the isolated nucleobases.

Microhydration Prevents Fragmentation of Uracil and Thymine by Low-Energy Electrons

When ionizing radiation passes biological matter, a large number of secondary electrons with very low energies (<3 eV) is produced. It is known that such electrons cause an efficient fragmentation of isolated nucleobases via dissociative electron attachment. We present an experimental study of the electron attachment to microhydrated nucleobases. Our novel approach allows significant control over the hydration of molecules studied in the molecular beam. We directly show for the first time that the presence of a few water molecules suppresses the dissociative channel and leads exclusively to formation of intact molecular and hydrated anions. The suppression of fragmentation is ascribed to caging-like effects and fast energy transfer to the solvent. This is in contrast with theoretical prediction that microhydration strongly enhances the fragmentation of nucleobases. The current observation impacts mechanisms of reductive DNA strand breaks proposed to date on the basis of gas-phase experiments.

Electron accommodation dynamics in the DNA base thymine

The dynamics of electron attachment to the DNA base thymine are investigated using femtosecond time-resolved photoelectron imaging of the gas phase iodide-thymine (I(-)T) complex. An ultraviolet pump pulse ejects an electron from the iodide and prepares an iodine-thymine temporary negative ion that is photodetached with a near-IR probe pulse. The resulting photoelectrons are analyzed with velocity-map imaging. At excitation energies ranging from -120 meV to +90 meV with respect to the vertical detachment energy (VDE) of 4.05 eV for I(-)T, both the dipole-bound and valence-bound negative ions of thymine are observed. A slightly longer rise time for the valence-bound state than the dipole-bound state suggests that some of the dipole-bound anions convert to valence-bound species. No evidence is seen for a dipole-bound anion of thymine at higher excitation energies, in the range of 0.6 eV above the I(-)T VDE, which suggests that if the dipole-bound anion acts as a "doorway" to the valence-bound anion, it only does so at excitation energies near the VDE of the complex.

Negative ion states of 5-bromouracil and 5-iodouracil

The valence anion states of the potential radiosensitisers 5-bromouracil and 5-iodouracil were investigated through elastic scattering calculations. These compounds have rich spectra of negative ion states that trigger off different mechanisms for dissociative electron attachment. For each molecule, we obtained a bound π* anion, two π* shape resonances and a low lying σ* anion state, in addition to a dipole-bound state (the latter was obtained using bound-state techniques). The σ* anion, formed by electron attachment to an anti-bonding carbon-halogen orbital, was found to have resonant character in 5-bromouracil, and bound-state character in 5-iodouracil. The present calculations place the σCBr* resonance around 0.7 eV, considerably below the energy inferred from the electron transmission data (1.3 eV). The signature of this anion state, not evident in the measurements, would be obscured by the large background arising from the dipolar interaction, not by the strong signature of the π2*, as presumed. Our results support the π2* resonance as a precursor state to dissociative electron attachment around 1.5 eV in both 5-bromouracil and 5-iodouracil, while the interplay among π1*, σ* and dipole-bound states would be expected close to 0 eV. We also discuss the suppression of the hydrogen elimination channels in these species.

Time-resolved radiation chemistry: dynamics of electron attachment to uracil following UV excitation of iodide-uracil complexes

Electron attachment to uracil was investigated by applying time-resolved photoelectron imaging to iodide-uracil (I(-)U) complexes. In these studies, an ultraviolet pump pulse initiated charge transfer from the iodide to the uracil, and the resulting dynamics of the uracil temporary negative ion were probed. Five different excitation energies were used, 4.00 eV, 4.07 eV, 4.14 eV, 4.21 eV, and 4.66 eV. At the four lowest excitation energies, which lie near the vertical detachment energy of the I(-)U complex (4.11 eV), signatures of both the dipole bound (DB) as well as the valence bound (VB) anion of uracil were observed. In contrast, only the VB anion was observed at 4.66 eV, in agreement with previous experiments in this higher energy range. The early-time dynamics of both states were highly excitation energy dependent. The rise time of the DB anion signal was ∼250 fs at 4.00 eV and 4.07 eV, ∼120 fs at 4.14 eV and cross-correlation limited at 4.21 eV. The VB anion rise time also changed with excitation energy, ranging from 200 to 300 fs for excitation energies 4.00-4.21 eV, to a cross-correlation limited time at 4.66 eV. The results suggest that the DB state acts as a "doorway" state to the VB anion at 4.00-4.21 eV, while direct attachment to the VB anion occurs at 4.66 eV.

Probing the vibrational spectroscopy of the deprotonated thymine radical by photodetachment and state-selective autodetachment photoelectron spectroscopy via dipole-bound states

Deprotonated thymine can exist in two different forms, depending on which of its two N sites is deprotonated: N1[T-H]^- or N3[T-H]^-. Here we report a photodetachment study of the N1[T-H]^- isomer cooled in a cryogenic ion trap and the observation of an excited dipole-bound state. Eighteen vibrational levels of the dipole-bound state are observed, and its vibrational ground state is found to be 238 ± 5 cm^-1 below the detachment threshold of N1[T-H]^-. The electron affinity of the deprotonated thymine radical (N1[T-H]˙) is measured accurately to be 26 322 ± 5 cm^-1 (3.2635 ± 0.0006 eV). By tuning the detachment laser to the sixteen vibrational levels of the dipole-bound state that are above the detachment threshold, highly non-Franck-Condon resonant-enhanced photoelectron spectra are obtained due to state- and mode-selective vibrational autodetachment. Much richer vibrational information is obtained for the deprotonated thymine radical from the photodetachment and resonant-enhanced photoelectron spectroscopy. Eleven fundamental vibrational frequencies in the low-frequency regime are obtained for the N1[T-H]˙ radical, including the two lowest-frequency internal rotational modes of the methyl group at 70 ± 8 cm^-1 and 92 ± 5 cm^-1.

Valence-bound and diffuse-bound anions of 5-azauracil

Structures, isomerization energies, and electron binding energies of 5-azauracil and its anions have been calculated ab initio with perturbative, coupled-cluster, and electron-propagator methods. Tautomeric structures, including those produced by proton transfer to a CH group, have been considered. Dyson orbitals and pole strengths from electron-propagator calculations validated a simple, molecular-orbital picture of anion formation. In one case, an electron may enter a delocalized π orbital, yielding a valence-bound (VB) anion with a puckered ring structure. The corresponding electron affinity is 0.27 eV; the vertical electron detachment energy (VEDE) of this anion 1.05 eV. An electron also may enter a molecular orbital that lies outside the nuclear framework, resulting in a diffuse-bound (DB) anion. In the latter case, the electron affinity is 0.06 eV and the VEDE of the DB anion is 0.09 eV. Another VB isomer that is only 0.02 eV more stable than the neutral molecule has a VEDE of 2.0 eV.

Photoelectron spectroscopic and computational study of hydrated pyrimidine anions

The stabilization of the pyrimidine anion by the addition of water molecules is studied experimentally using photoelectron spectroscopy of mass-selected hydrated pyrimidine clusters and computationally using quantum-mechanical electronic structure theory. Although the pyrimidine molecular anion is not observed experimentally, the addition of a single water molecule is sufficient to impart a positive electron affinity. The sequential hydration data have been used to extrapolate to -0.22 eV for the electron affinity of neutral pyrimidine, which agrees very well with previous observations. These results for pyrimidine are consistent with previous studies of the hydrated cluster anions of uridine, cytidine, thymine, adenine, uracil, and naphthalene. This commonality suggests a universal effect of sequential hydration on the electron affinity of similar molecules.