Photoelectron Angular Distributions of Pyridinide: A Benchmark Application of the Mixed s–p Model to a Truly Polyatomic Anion

Culberson, Lori M.; Blackstone, Christopher C.; Sanov, Andrei

doi:10.1021/jp402507v

Cited by 19 publications

(45 citation statements)

References 36 publications

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“…The X̃states, on the other hand, have localized s-p hybrid Dyson orbitals, which allows for partial l = 0 detachment and retained intensity at small eKE. 41 Calculated photodetachment cross sections, shown in Figure 3, are consistent with this threshold behavior for the X̃and Ãstates of both species.…”

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confidence: 69%

Vibrational and Electronic Structure of the α- and β-Naphthyl Radicals via Slow Photoelectron Velocity-Map Imaging

et al. 2015

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Slow photoelectron velocity-map imaging (SEVI) spectroscopy has been used to study the vibronic structure of gas-phase α-and β-naphthyl radicals (C 10 H 7 ). SEVI of cryogenically cooled anions yields spectra with <4 cm −1 resolution, allowing for the observation and interpretation of congested vibrational structure. Isomerspecific photoelectron spectra of detachment to the radical ground electronic states show detailed structure, allowing assignment of vibrational fundamental frequencies. Transitions to the first excited states of both radical isomers are also observed; vibronic coupling and photodetachment threshold effects are considered to explain the structure of the excited bands. P olycyclic aromatic hydrocarbons (PAHs) are of major importance in wide-ranging areas of chemistry. PAHs are involved in the combustion of organic matter 1 and subsequent soot formation, 2,3 while neutral, ionized, hydrogenated, and dehydrogenated PAHs are likely constituents of the interstellar medium (ISM). 4,5 PAHs are possible sources of mid-infrared emission features in the ISM 6−8 and have been considered tentative candidates for carriers of diffuse interstellar bands (DIBs) for many years. 9−11 No conclusive evidence of small PAHs as DIB carriers exists to date, despite much work searching for matches between laboratory spectra and astronomical data. 12,13 Decomposition of large interstellar PAHs may lead to the formation of carbon chains and hydrocarbon radicals in space. 14 In this Communication, we report high-resolution anion photoelectron spectra of α-and β-naphthyl, C 10 H 7 − , whose structures are shown in Figure 1. Naphthalene is the simplest PAH; its derivatives are therefore tractable models for the behavior of larger aromatic systems.There is a solid body of theoretical work on the electronic structure, geometries, and vibrations of the naphthyl radicals and anions 5,15−20 and some calculations of their reactivity in the context of combustion. 21,22 Experimental characterization is sparser. Reed and Kass 23 and Lardin et al. 24 measured the electron affinities (EAs) of the α-and β-naphthyl radicals through kinetic methods and calculated the α and β C−H bond dissociation energies of naphthalene. Both studies found the α anion to be lower in energy than the β anion by several kJ/mol, favoring α formation in deprotonation of naphthalene.Anion photoelectron spectroscopy (PES) is a powerful technique for probing the vibronic structure of neutral radicals through photodetachment of a closed-shell anion. 25,26 Ervin et al. 18 measured the photoelectron spectrum of C 10 H 7 − at 300 K and with a resolution of ∼100 cm −1 . The authors reported a congested, partially resolved spectrum of the radical ground state with an EA of 1.403(15) eV. By comparison to Franck− Condon (FC) simulations, the spectrum was assigned to an 11:1 α:β isomer ratio. Substantial enrichment in α-naphthyl is consistent with the authors' use of nonspecific deprotonation of naphthalene to generate anions.Slow photoelectron velocity-map ...

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confidence: 69%

Vibrational and Electronic Structure of the α- and β-Naphthyl Radicals via Slow Photoelectron Velocity-Map Imaging

et al. 2015

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“…2. The Dyson orbitals for the e X states, on the other hand, have localized s-p hybrid character, enabling partial l = 0 detachment, and significant intensity at small eKE (40). Photodetachment cross sections calculated as a function of eKE are shown in Fig.…”

Section: Resultsmentioning

confidence: 99%

Isomer-specific vibronic structure of the 9-, 1-, and 2-anthracenyl radicals via slow photoelectron velocity-map imaging

Weichman

DeVine

Levine

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

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Polycyclic aromatic hydrocarbons, in various charge and protonation states, are key compounds relevant to combustion chemistry and astrochemistry. Here, we probe the vibrational and electronic spectroscopy of gas-phase 9-, 1-, and 2-anthracenyl radicals (C 14 H 9 ) by photodetachment of the corresponding cryogenically cooled anions via slow photoelectron velocity-map imaging (cryo-SEVI). The use of a newly designed velocity-map imaging lens in combination with ion cooling yields photoelectron spectra with <2 cm −1 resolution. Isomer selection of the anions is achieved using gasphase synthesis techniques, resulting in observation and interpretation of detailed vibronic structure of the ground and lowest excited states for the three anthracenyl radical isomers. The groundstate bands yield electron affinities and vibrational frequencies for several Franck-Condon active modes of the 9-, 1-, and 2-anthracenyl radicals; term energies of the first excited states of these species are also measured. Spectra are interpreted through comparison with ab initio quantum chemistry calculations, Franck-Condon simulations, and calculations of threshold photodetachment cross sections and anisotropies. Experimental measures of the subtle differences in energetics and relative stabilities of these radical isomers are of interest from the perspective of fundamental physical organic chemistry and aid in understanding their behavior and reactivity in interstellar and combustion environments. Additionally, spectroscopic characterization of these species in the laboratory is essential for their potential identification in astrochemical data.polycyclic aromatic hydrocarbons | anion photoelectron spectroscopy | velocity-map imaging | vibronic structure P olycyclic aromatic hydrocarbons (PAHs) are an important class of species in many areas of chemistry. They are major components in coal (1) and in soot formed from combustion of organic matter (2, 3). PAHs are therefore common environmental pollutants and have well-documented mutagenic and carcinogenic biological activity (4, 5). PAHs are also believed to be abundant in the interstellar medium (6) and may be carriers of the anomalous IR emission bands (7-9). Recent molecular beam studies indicate that PAH growth can proceed through cold collisions of smaller hydrocarbons under interstellar conditions (10, 11). Individual PAH molecules can subsequently provide nucleation sites for amorphous graphitic grains (9). Interstellar PAHs and their clusters therefore bridge the gap between small carbonaceous molecules and larger particles, analogous to their role in soot condensation in combustion environments (12).In space, PAH species are likely to exist as an equilibrium of neutral and ionic charge states, with varying degrees of hydrogenation and dehydrogenation (13-15). Models of dense interstellar clouds find that anionic PAHs are the major carriers of negative charge, rather than free electrons (16). Closed-shell, singly deprotonated PAH carbanions have large electron affinities compared with ...

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“…Modeling the Photoelectron Angular Distributions. The PADs for cyclic deprotonated anions can be described with good accuracy and conceptual insight using the mixed s−p model, 9,29,30 which is a partial case of the general mixing theory presented recently. 31 The mixed s−p model approximates the MO of the parent anion, from which the observed photoelectrons are (nominally) emitted, with a model function ψ sp , defined as a superposition of one s and one p-type functions, with the (1 − γ p ) 1/2 and (γ p ) 1/2 amplitudes, respectively.…”

Section: Discussionmentioning

confidence: 99%

“…32,35 Our own past work has shown that the choice of cos δ 2,0 = 0.95 instead of 1 results in only minor changes in the mixed s−p model predictions. 9 In view of these findings and acknowledging that the phase shift exists but is small, we will set cos δ 2,0 = 0.95 throughout the analysis. This choice is somewhat arbitrary but does not affect the overall trends discussed.…”

Section: Discussionmentioning

confidence: 99%

Aromatic Stabilization and Hybridization Trends in Photoelectron Imaging of Heterocyclic Radicals and Anions

et al. 2015

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We examine the photoelectron spectra and laboratory-frame angular distributions in the photodetachment of furanide (C4H3O(-)), thiophenide (C4H3S(-)), and thiazolide (C3H2NS(-)) and compare the results to the previously reported studies of pyridinide (C5H4N(-)) and oxazolide (C3H2NO(-)). Using the mixed s-p model for the angular distributions, the results are interpreted in terms of the effective fractional p character of the highest-occupied molecular orbitals of these heterocyclic anions, revealing trends related to the aromaticity. We conclude that aromatic stabilization across a series of systems may be tracked using the photoelectron angular distributions. In addition, we report an improved (higher-precision) electron affinity (EA) for the thiophenyl radical, EA((•)C4H3S) = 2.089(8) eV. The EA of thiazolyl falls within the 2.5(1) eV range, but it is not clear if this determination corresponds to the 2- or 5-cyclic species or the 2-ring-open isomer. These results are analyzed in conjunction with the properties of other heterocyclic radicals (pyridinyl, furanyl, and oxazolyl) and interpreted in terms of the C-H bond dissociation energies (BDEs) of the corresponding closed-shell molecules. The BDEs of all five-membered-ring heterocyclics studied fall within the 116-120 kcal/mol range, contrasting the lower BDE = 110.4(2.0) kcal/mol of the more aromatic six-membered-ring pyridine. The observed aromaticity trends are consistent with the findings derived from the anion photoelectron angular distributions.

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Photoelectron Angular Distributions of Pyridinide: A Benchmark Application of the Mixed s–p Model to a Truly Polyatomic Anion

Cited by 19 publications

References 36 publications

Vibrational and Electronic Structure of the α- and β-Naphthyl Radicals via Slow Photoelectron Velocity-Map Imaging

Vibrational and Electronic Structure of the α- and β-Naphthyl Radicals via Slow Photoelectron Velocity-Map Imaging

Isomer-specific vibronic structure of the 9-, 1-, and 2-anthracenyl radicals via slow photoelectron velocity-map imaging

Aromatic Stabilization and Hybridization Trends in Photoelectron Imaging of Heterocyclic Radicals and Anions

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