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1. Spectroscopic signatures of states in the continuum characterized by a joint experimental and theoretical study of pyrrole

   https://doi.org/10.1063/5.0123603  

   Mukherjee, Madhubani; Ragesh Kumar, T. P.; Ranković, Miloš; Nag, Pamir; Fedor, Juraj; Krylov, Anna I. (November 2022, The Journal of Chemical Physics)

   We report a combined experimental and theoretical investigation of electron–molecule interactions using pyrrole as a model system. Experimental two-dimensional electron energy loss spectra (EELS) encode information about the vibrational states of the molecule as well as the position and structure of electronic resonances. The calculations using complex-valued extensions of equation-of-motion coupled-cluster theory (based on non-Hermitian quantum mechanics) facilitate the assignment of all major EELS features. We confirm the two previously described π resonances at about 2.5 and 3.5 eV (the calculations place these two states at 2.92 and 3.53 eV vertically and 2.63 and 3.27 eV adiabatically). The calculations also predict a low-lying resonance at 0.46 eV, which has a mixed character—of a dipole-bound state and σ* type. This resonance becomes stabilized at one quanta of the NH excitation, giving rise to the sharp feature at 0.9 eV in the corresponding EELS. Calculations of Franck–Condon factors explain the observed variations in the vibrational excitation patterns. The ability of theory to describe EELS provides a concrete illustration of the utility of non-Hermitian quantum chemistry, which extends such important concepts as potential energy surfaces and molecular orbitals to states embedded in the continuum. 

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2. The quest to uncover the nature of benzonitrile anion

   https://doi.org/10.1039/C9CP06484B  

   Gulania, Sahil; Jagau, Thomas-C.; Sanov, Andrei; Krylov, Anna I. (March 2020, Physical Chemistry Chemical Physics)

   Anionic states of benzonitrile are investigated by high-level electronic structure methods. The calculations using equation-of-motion coupled-cluster theory for electron-attached states confirm earlier conclusions drawn from the photodetachment experiments wherein the ground state of the anion is the valence 2 B 1 state, while the dipole bound state lies adiabatically ∼0.1 eV above. Inclusion of triple excitations and zero-point vibrational energies is important for recovering relative state correct ordering. The computed Franck–Condon factors and photodetachment cross-sections further confirm that the observed photodetachment spectrum originates from the valence anion. The valence anion is electronically bound at its equilibrium geometry, but it is metastable at the equilibrium geometry of the neutral. The dipole-bound state, which is the only bound anionic state at the neutral equilibrium geometry, may serve as a gateway state for capturing the electron. Thus, the emerging mechanistic picture entails electron capture via a dipole bound state, followed by non-adiabatic relaxation forming valence anions. 

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3. Electronically Excited States of Potential Interstellar, Anionic Building Blocks for Astrobiological Nucleic Acids

   https://doi.org/10.3389/fspas.2021.777107  

   Santaloci, Taylor J.; Strauss, Marie E.; Fortenberry, Ryan C. (November 2021, Frontiers in Astronomy and Space Sciences)

   Functionalizing deprotonated polycyclic aromatic hydrocarbon (PAH) anion derivatives gives rise to electronically excited states in the resulting anions. While functionalization with −OH and −C 2 H, done presently, does not result in the richness of electronically excited states as it does with −CN done previously, the presence of dipole-bound excited states and even some valence excited states are predicted in this quantum chemical analysis. Most notably, the more electron withdrawing −C 2 H group leads to valence excited states once the number of rings in the molecule reaches three. Dipole-bound excited states arise when the dipole moment of the corresponding neutral radical is large enough (likely around 2.0 D), and this is most pronounced when the hydrogen atom is removed from the functional group itself regardless of whether functionalized by a hydroxyl or enthynyl group. Deprotonatation of the hydroxyl group in the PAH creates a ketone with a delocalized highest occupied molecular orbital (HOMO) unlike deprotonation of a hydrogen on the ring where a localized lone pair on one of the carbon atoms serves as the HOMO. As a result, hydroxyl functionlization and subsequent deprotonation of PAHs creates molecules that begin to exhibit structures akin to nucleic acids. However, the electron withdrawing −C 2 H has more excited states than the electron donating −OH functionalized PAH. This implies that the −C 2 H electron withdrawing group can absorb a larger energy range of photons, which signifies an increasing likelihood of being stabilized in the harsh conditions of the interstellar medium. 

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

   https://doi.org/10.1039/c8cp07831a  

   Kunin, Alice; Neumark, Daniel M. (April 2019, Physical Chemistry Chemical Physics)

   null (Ed.)

   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·H 2 O), 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. 

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5. What does it take to stabilize a naphthalene anion?

   https://doi.org/10.1063/5.0230131  

   Ďurana, Jozef; Kocábková, Barbora; Rakovský, Jozef; Pysanenko, Andrij; Kresin, Vitaly; Fedor, Juraj; Fárník, Michal (September 2024, The Journal of Chemical Physics)

   We investigate attachment of slow electrons (0–10 eV) to naphthalene (Np) clusters in a crossed beam experiment. Supersonic expansions under different conditions using different buffer gases generate the clusters: in He, Ne, and low pressure Ar, neat (Np)N clusters are formed, while we also observe mixed clusters of naphthalene with rare-gas atoms in co-expansion with Ar above 0.5 bar and with Kr. Negatively charged (Np)n− and Rgm(Np)n− (Rg = Ar, Kr) clusters are analyzed by mass spectrometry, and electron energy dependent ion yields are measured. We show that the smallest stable naphthalene complex with an excess electron, the dimer (Np)2− anion, cannot be formed in a binary collision of a free electron with (Np)2 dimer, nor with (Np)3 trimer. Evaporation of a weakly bound Ar atom(s) from a mixed ArM(Np)2 cluster following electron attachment leads to the dimer (Np)2− anion. Larger (Np)n−, n > 3, transient cluster anions decay via evaporation of an Np unit on a timescale of tens of microseconds. The self-scavenging process opens around 6 eV, where a naphthalene unit is electronically excited by the incoming electron, which is slowed down and trapped. However, the transient negative ion is efficiently stabilized only in the mixed clusters, from which Ar atom(s) can be evaporated. 

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