A TDDFT Computational Study of Platinum Complexes Bound to Nucleobases

The electronic spectra of a number of platinum (II) complexes bound to single nucleobases have been investigated using Time-Dependent Density Functional Theory (TDDFT). The calculated spectra obtained in this work have been benchmarked against recent gas-phase photo-dissociation spectra of platinum complex-nucleobase clusters. UV spectra have been calculated for a range of density functionals and basis sets to determine the best functional-basis set combination for reproducing the experimental spectra. The first series of TDDFT calculations conducted in this work investigated the electronic transitions of iodide ion-nucleobase clusters and their constituent “monomer” parts (i.e. the isolated iodide anion and isolated nucleobases). Calculations on the I-∙Nu clusters (Nu = Uracil, Thymine or Adenine) and isolated uracil, cytosine, thymine and adenine produced computed UV spectra and the associated electronic transitions were characterised by inspection of respective molecular orbitals. For the nucleobases, these orbitals were revealed to be mainly of ππ* character. The electronic transitions of the I-∙Nu clusters were dominated by excitations involving orbitals localised on the nucleobases. A second series of studies focused on the electronic transitions of isolated platinum (II) and platinum (IV) cyanide complexes as well as their clusters involving a single water molecule. The excited states of the Pt(CN)4,62-∙H2O complexes were found to involve only platinum localised orbitals. The final set of TDDFT calculations were performed on Pt(CN)4,62-∙M complexes (M = Uracil or Cytosine). The electronic transitions occurring in the Pt(CN)42-∙Uracil and Pt(CN)42-∙Cytosine complexes were found to be of a short-range charge transfer nature. Conversely, the electronic transitions of Pt(CN)42-∙Uracil involved uracil localised orbitals that were of ππ* character

Photoexcitation of iodide ion-pyrimidine clusters above the electron detachment threshold : Intracluster electron transfer versus nucleobase-centred excitations

Laser photodissociation spectroscopy of the I-·thymine (I-·T) and I-·cytosine (I-·C) nucleobase clusters has been conducted for the first time across the regions above the electron detachment thresholds to explore the excited states and photodissociation channels. Although photodepletion is strong, only weak ionic photofragment signals are observed, indicating that the clusters decay predominantly by electron detachment. The photodepletion spectra of the I-·T and I-·C clusters display a prominent dipole-bound excited state (I) in the vicinity of the vertical detachment energy (∼4.0 eV). Like the previously studied I-·uracil (I-·U) cluster [W. L. Li et al., J. Chem. Phys. 145, 044319 (2016)], the I-·T cluster also displays a second excited state (II) centred at 4.8 eV, which we similarly assign to a π-π∗ nucleobase-localized transition. However, no distinct higher-energy absorption bands are evident in the spectra of the I-·C. Time-dependent density functional theory (TDDFT) calculations are presented, showing that while each of the I-·T and I-·U clusters displays a single dominant π-π∗ nucleobase-localized transition, the corresponding π-π∗ nucleobase transitions for I-·C are split across three separate weaker electronic excitations. I- and deprotonated nucleobase anion photofragments are observed upon photoexcitation of both I-·U and I-·T, with the action spectra showing bands (at 4.0 and 4.8 eV) for both the I- and deprotonated nucleobase anion production. The photofragmentation behaviour of the I-·C cluster is distinctive as its I- photofragment displays a relatively flat profile above the expected vertical detachment energy. We discuss the observed photofragmentation profiles of the I-·pyrimidine clusters, in the context of the previous time-resolved measurements, and conclude that the observed photoexcitations are primarily consistent with intracluster electron transfer dominating in the near-threshold region, while nucleobase-centred excitations dominate close to 4.8 eV. TDDFT calculations suggest that charge-transfer transitions [Iodide n (5p6) → Uracil σ∗] may contribute to the cluster absorption profile across the scanned spectral region, and the possible role of these states is also discussed

Resonances in nitrobenzene probed by the electron attachment to neutral and by the photodetachment from anion

We probe resonances (transient anions) in nitrobenzene with the focus on the electron emission from these. Experimentally, we populate resonances in two ways: either by the impact of free electrons on the neutral molecule or by the photoexcitation of the bound molecular anion. These two excitation means lead to transient anions in different initial geometries. In both cases, the anions decay by electron emission and we record the electron spectra. Several types of emission are recognized, differing by the way in which the resulting molecule is vibrationally excited. In the excitation of specific vibrational modes, distinctly different modes are visible in electron collision and photodetachment experiments. The unspecific vibrational excitation, which leads to the emission of thermal electrons following the internal vibrational redistribution, shows similar features in both experiments. A model for the thermal emission based on a detailed balance principle agrees with the experimental findings very well. Finally, a similar behavior in the two experiments is also observed for a third type of electron emission, the vibrational autodetachment, which yields electrons with constant final energies over a broad range of excitation energies. The entrance channels for the vibrational autodetachment are examined in detail, and they point to a new mechanism involving a reverse valence to non-valence internal conversion

Investigation of Anion Resonances Using Photoelectron Spectroscopy and Density Functional Theory

The structure and dynamics of temporary excited states of anions (resonances) has been probed using a combination of photoelectron (PE) spectroscopy and computational methods. Such resonances are important in understanding electron-driven chemistry. Here, photoexcitation from an anion to an excited state of the anion that lies in the electronic continuum is closely related to the analogous electron impact resonances. A particular emphasis is placed on how non-covalent interactions in clusters affect these dynamics. Two-dimensional (2D) PE spectroscopy is employed which provides fingerprints of resonance dynamics. Additionally, computational methods are used to aid the interpretation of experimental results and to lay a foundation for future studies. To demonstrate the applicability of these methods, we have probed a range of different anionic systems of relevance to astro-, bio-, and plasma-chemistry as well as fundamental chemical reaction dynamics. We studied the dynamics of anthracene resonances, showing that resonances overall decay by electron detachment. Time-dependent density-functional theory (TDDFT) calculations in conjunction with a stabilisation method could assign all observed resonances. For the para-benzoquinone radical anion, the addition of a single water molecule was found to lead to a dramatic enhancement in the ability for resonances to form the ground-state anion. Larger water clusters similarly showed that groundstate formation was facile. Clusters of the para-benzoquinone radical with other parabenzoquinone molecules showed dissociation dynamics following the excitation of a resonance. Finally, the cluster of the iodide anion and trifluoromethyl iodide was studied as a reactive intermediate in an SN2 reaction, in which the stereochemistry has been reversed from the traditional backside attack to a frontside attack pre-reaction complex. Overall, the interplay between TDDFT and 2D PE spectroscopy is shown to provide exquisite insight into the electronic structure of complex anionic clusters and their resonances, despite the complex structure of many of these clusters. This provides a stepping stone to studying larger and more complex anionic systems

The effect of solvation on electron capture revealed using anion two-dimensional photoelectron spectroscopy

The reaction of low-energy electrons with neutral molecules to form anions plays an important role in chemistry, being involved in, for example, various biological and astrochemical processes. However, key aspects of electron–molecule interactions, such as the effect of incremental solvation on the initially excited electronic resonances, remain poorly understood. Here two-dimensional photoelectron spectroscopy of anionic anthracene and nitrogen-substituted derivatives—solvated by up to five water molecules—reveals that for an incoming electron, resonances red-shift with increasing hydration; but for the anion, the excitation energies of the resonances remain essentially the same. These complementary points of view show that the observed onset of enhanced anion formation for a specific cluster size is mediated by a bound excited state of the anion. Our findings suggest that polycyclic aromatic hydrocarbons may be more efficient at electron capture than previously predicted with important consequences for the ionization fraction in dense molecular clouds

Photoelectron spectroscopic study of I−·ICF3: a frontside attack SN2 pre-reaction complex

Photodetachment and 2D photoelectron spectra of the mass-selected I−·CF3I complex are presented together with electronic structure calculations. Calculations show that the I− is located at the iodine side of CF3I. Vertical and adiabatic detachment energies were measured at 4.03 and approximately 3.8 eV, respectively. The photoelectron spectra and molecular orbitals show a significant covalent bonding character in the cluster. The presence of electronic excited states is observed. Below threshold, iodide is generated which can be assigned to the photoexcitation of degenerate charge-transfer bands from the off-axis p-orbitals localised on iodide. Near the onset of two spin–orbit thresholds, bright excited states are seen in the experiment and calculations. Excitation of these leads to the formation of slow electrons. The spectroscopy of I−·CF3I is compared to the well-studied I−·CH3I cluster, a pre-reaction complex in the text-book I− + CH3I SN2 reaction. Despite the reversed stereodynamics (i.e. inversion of the CX3 between X = H and F) of the SN2 reaction, striking similarities are seen. Both complexes possess charge transfer excited states near their respective vertical detachment energies and exhibit vibrational structure in their photoelectron spectra. The strong binding is consistent with observations in crossed molecular beam studies and molecular dynamics simulations that suggest that iodine as a leaving group in an SN2 reaction affects the reaction dynamics

Low energy electron impact resonances of anthracene probed by 2D photoelectron imaging of its radical anion

Electron-molecule resonances of anthracene were probed by 2D photoelectron imaging of the corresponding radical anion up to 3.7 eV in the continuum. A number of resonances were observed in both the photoelectron spectra and angular distributions, and most resonances showed clear autodetachment dynamics. The resonances were assigned using density functional theory calculations and are consistent with the available literature. Competition between direct and autodetachment, as well as signatures of internal conversion between resonances, was observed for some resonances. For the 12B2g resonance, a small fraction of population recovers the ground electronic state as evidenced by thermionic emission. Recovery of the ground electronic state offers a route of producing anions in an electron–molecule reaction; however, the energy at which this occurs suggests that anthracene anions cannot be formed in the interstellar medium by electron capture through this resonance

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