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1. Photoelectron spectroscopy and thermochemistry of o -, m -, and p -methylenephenoxide anions

   https://doi.org/10.1039/C8CP05403G  

   Nelson, Daniel J. ; Gichuhi, Wilson K. ; Nichols, Charles M. ; Bierbaum, Veronica M. ; Lineberger, W. Carl ; Lehman, Julia H. ( October 2018 , Physical Chemistry Chemical Physics)

   The anionic products following (H + H + ) abstraction from o -, m -, and p -methylphenol (cresol) are investigated using flowing afterglow-selected ion flow tube (FA-SIFT) mass spectrometry and anion photoelectron spectroscopy (PES). The PES of the multiple anion isomers formed in this reaction are reported, including those for the most abundant isomers, o -, m - and p -methylenephenoxide distonic radical anions. The electron affinity (EA) of the ground triplet electronic state of neutral m -methylenephenoxyl diradical was measured to be 2.227 ± 0.008 eV. However, the ground singlet electronic states of o - and p -methylenephenoxylmore »were found to be significantly stabilized by their resonance forms as a substituted cyclohexadienone, resulting in measured EAs of 1.217 ± 0.012 and 1.096 ± 0.007 eV, respectively. Upon electron photodetachment, the resulting neutral molecules were shown to have Franck–Condon active ring distortion vibrational modes with measured frequencies of 570 ± 180 and 450 ± 80 cm −1 for the ortho and para isomers, respectively. Photodetachment to excited electronic states was also investigated for all isomers, where similar vibrational modes were found to be Franck–Condon active, and singlet–triplet splittings are reported. The thermochemistry of these molecules was investigated using FA-SIFT combined with the acid bracketing technique to yield values of 341.4 ± 4.3, 349.1 ± 3.0, and 341.4 ± 4.3 kcal mol −1 for the o -, m -, and p -methylenephenol radicals, respectively. Construction of a thermodynamic cycle allowed for an experimental determination of the bond dissociation energy of the O–H bond of m -methylenephenol radical to be 86 ± 4 kcal mol −1 , while this bond is significantly weaker for the ortho and para isomers at 55 ± 5 and 52 ± 5 kcal mol −1 , respectively. Additional EAs and vibrational frequencies are reported for several methylphenyloxyl diradical isomers, the negative ions of which are also formed by the reaction of cresol with O − .« less
2. Ab Initio Simulations of Poorly- and Well-Equilibrated (CH3CN)n¯ Cluster Anions: Assigning Experimental Photoelectron Peaks to Surface-Bound Electrons and Solvated Monomer and Dimer Anions

   https://doi.org/10.1021/acs.jpca.1c05855  

   Narvaez, W. A. ; Schwartz, B. J. ( January 2021 , The journal of physical chemistry)

   Excess electrons in liquid acetonitrile are of particular interest because they exist in two different forms in equilibrium: they can be present as traditional solvated electrons in a cavity, and they can form some type of solvated molecular anion. Studies of small acetonitrile cluster anions in the gas phase show two isomers with distinct vertical detachment energies, and it is tempting to presume that the two gas-phase cluster anion isomers are precursors of the two excess electron species present in bulk solution. In this paper, we perform DFT-based ab initio molecular dynamics simulations of acetonitrile cluster anions to understand themore »electronic species that are present and why they have different binding energies. Using a long-range-corrected density functional that was optimally tuned to describe acetonitrile cluster anion structures, we have theoretically explored the chemistry of (CH3CN)n¯ cluster anions with sizes n=5,7 and 10. Since the temperature of the experimental cluster anions is not known, we performed two sets of simulations that investigated how the way in which the cluster anions are prepared affects the excess electron binding motif: one set of simulations simply attached excess electrons to neutral (CH3CN)n clusters, providing little opportunity for the clusters to relax in the presence of the excess electron, while the other set allowed the cluster anions to thermally equilibrate near room temperature. We find that both sets of simulations show three distinct electron binding motifs: electrons can attach to the surface of the cluster (dipole-bound) or be present as either solvated monomer anions, CH3CN¯, or as solvated molecular dimer anions, (CH3CN)2¯. All three species have higher binding energies at larger cluster sizes. Thermal equilibration strongly favors the formation of the valence-bound molecular anions relative to surface-bound excess electrons, and the dimer anion becomes more stable than the monomer anion and surface-bound species as the cluster size increases. The calculated photoelectron spectra from our simulations in which there was poor thermal equilibration are in good agreement with experiment, suggesting assignment of the two experimental cluster anion isomers as the surface-bound electron and the solvated molecular dimer anion. The simulations also suggest that the shoulder seen experimentally on the low-energy isomer's detachment peak is not part of a vibronic progression but instead results from molecular monomer anions. Nowhere in the size range that we explore do we see evidence for a non-valence, cavity-bound interior-solvated electron, indicating that this species is likely only accessible at larger sizes with good thermal equilibration.« less
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 correspondingmore »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.« less
4. Valence-, Dipole- and Quadropole-Bound Electronically Excited States of Closed-Shell Anions Formed by Deprotonation of Cyano- and Ethynyl-Disubstituted Polycyclic Aromatic Hydrocarbons

   https://doi.org/10.3390/chemistry4010004  

   Strauss, Marie E. ; Santaloci, Taylor J. ; Fortenberry, Ryan C. ( March 2022 , Chemistry)

   Dicyano-functionalized benzene and naphthalene anion derivatives exhibit a relatively rich population of electronically excited states in stark contrast to many assumptions regarding the photophysics of anions in general. The present work has quantum chemically analyzed the potential electronically excited states of closed-shell anions created by replacing hydrogen atoms with valence-bound lone pairs in benzene and naphthalene difunctionalized with combinations of -CN and -C2H. Dicyanobenzene anion derivatives can exhibit dipole-bound excited states as long as the cyano groups are not in para position to one another. This also extends to cyanoethynylbenzene anions as well as deprotonated dicyano- and cyanoethynylnaphthalene anion derivatives.more »Diethynyl functionalization is less consistent. While large dipole moments are created in some cases for deprotonation on the -C2H group itself, the presence of electronically excited states beyond those that are dipole-bound is less consistent. Beyond these general trends, 2-dicyanonaphthalene-34 gives strong indication for exhibiting a quadrupole-bound excited state, and the 1-cyanoethynylnaphthalene-29 and -36 anion derivatives are shown to possess as many as two valence-bound excited states and one dipole-bound excited state. These photophysical properties may have an influence on regions where polycyclic aromatic hydrocarbons are known to exist such as in various astrochemical environments or even in combustion flames.« less
5. Electronically Excited States of Closed-Shell, Cyano-Functionalized Polycyclic Aromatic Hydrocarbon Anions

   https://doi.org/10.3390/chemistry3010022  

   Santaloci, Taylor J. ; Fortenberry, Ryan C. ( March 2021 , Chemistry)

   Few anions exhibit electronically excited states, and, if they do, the one or two possible excitations typically transpire beyond the visible spectrum into the near-infrared. These few, red-shifted electronic absorption features make anions tantalizing candidates as carriers of the diffuse interstellar bands (DIBs), a series of mostly unknown, astronomically ubiquitous absorption features documented for over a century. The recent interstellar detection of benzonitrile implies that cyano-functionalized polycyclic aromatic hydrocarbon (PAH) anions may be present in space. The presently reported quantum chemical work explores the electronic properties of deprotonated benzene, naphthalene, and anthracene anions functionalized with a single cyano group. Bothmore »the absorption and emission properties of the electronically excited states are explored. The findings show that the larger anions absorption and emission energies possess both valence and dipole bound excitations in the 450–900 nm range with oscillator strengths for both types of >1×10−4. The valence and dipole bound excited state transitions will produce slightly altered substructure from one another making them appear to originate with different molecules. The known interstellar presence of related molecules, the two differing natures of the excited states for each, and the wavelength range of peaks for these cyano-functionalized PAH anions are coincident with DIB properties. Finally, the methods utilized appear to be able to predict the presence of dipole-bound excited states to within a 1.0 meV window relative to the electron binding energy.« less