Negative Ion Photoelectron Spectra Research Articles

Cyanonaphthalene and cyanonaphthyl radicals: Vibrational structures via computed negative ion photoelectron spectra and thermochemistry of 1- and 2-cyanonaphthalene.

A double harmonic oscillator model is applied to compute the negative ion photoelectron spectra (NIPES) of the 1- and 2-cyanonaphthalene (CNN) radical anions. The computed Franck-Condon factors utilize optimized structures and harmonic vibrational frequencies obtained using density functional theory with the B3LYP 6-311++G (2d,2p) basis set while considering the mode-mixing Duschinsky effects. To test the accuracy of our model, the NIPES of α and β naphthyl radical anions were computed, and a strong agreement between the slow electron velocity-map ion imaging spectra and the predicted spectra was found. The adiabatic electron affinities (EAs) of the ground singlet states (S0) in 1-CNN and 2-CNN are 0.856 and 0.798eV, respectively. The origin of the lowest-lying triplet (T1) states in 1-CNN and 2-CNN is found to be 3.226 and 3.266eV, resulting in singlet-triplet energy splittings (ΔEST) of 2.370 and 2.468eV, respectively. Both the NIPES for electron detachment to the S0 and T1 states exhibit well-resolved vibrational features, allowing for the assignment of several vibrational fundamental frequencies. Following deprotonation, several isomers are formed, with the most stable deprotonated radical anions in 1-CNN and 2-CNN, corresponding to the removal of the most acidic proton, with EAs of 2.062 and 2.16eV. The rich spectroscopic and thermochemical data obtained in the current study make the CNN radical anions and their deprotonated species interesting systems for investigation in gas-phase, negative-ion experiments.

Cyanocyclopentadiene-Annulated Polycyclic Aromatic Radical Anions: Predicted Negative Ion Photoelectron Spectra and Singlet-Triplet Energies of Cyanoindene and Cyanofluorene Radical Anions.

Isomer-specific negative ion photoelectron spectra (NIPES) of cyanoindene (C9H7CN) and cyanofluorene (C14H9N), acquired through the computation of Franck-Condon (FC) factors that utilize harmonic vibrational frequencies and normal mode vectors derived from density functional theory (DFT) at the B3LYP/aug-cc-pVQZ and 6-311++G(2d,2p) basis sets, are reported. The adiabatic electron affinity (EA) values of the ground singlet (S0) and the lowest lying triplet (T1) states are used to predict site-specific S0-T1 energies (ΔEST). The vibrational spectra of the S0 and T1 states are typified by ring distortion and ring C-C stretching vibrational progressions. Among all the S0 isomers in C9H7CN, the 2-cyanoindene (2-C9H7CN) is found to be the most stable at an EA of 0.716 eV, with the least stable isomer being the 1-C9H7CN at an EA of 0.208 eV. In C14H9N, the most stable S0 isomer, 2-cyanofluorene (2-C14H9N), has an EA of 0.781 eV. The least stable S0 isomer in C14H9N is the 9-C14H9N, with an EA of 0.364 eV. The FC calculations are designed to mimic simulations that would be performed to aid in the analysis of experimental spectra obtained in NIPE spectroscopic techniques. The vibrational spectra, adiabatic EAs, and ΔEST values reported in this study are intended to act as a guide for future gas-phase ion spectroscopic experiments and astronomical searches, especially with regard to the hitherto largely unexplored C14H9N isomers.

Anion-Activated Bases and Nucleophiles Characterized by Photoelectron Spectroscopy.

Negative ion photoelectron spectra at 20 K along with ab initio [CCSD(T)] and M06-2X density functional theory calculations are reported for a series of six basic and nucleophilic pyridine derivatives with an anionic substituent [i.e., 3- and 4-PyrBX3-, where X = F, 4-t-BuC6H4, 4-MeOC6H4, and 3,5-(MeO)2C6H3]. Vertical detachment energies (VDEs) of these charge-activated reagents span from 4.50-5.85 eV and are well reproduced by M06-2X/aug-cc-pVTZ and CCSD(T)/maug-cc-pVTZ computations. Surprisingly, the VDEs are found to correlate with the SN2 reactivity of the PPh4+ salts of the substituted pyridine anions with 1-iodooctane in dichloromethane. This provides an experimental measure of the nucleophilicity of these charge-activated anions, which represent a new class of chemical reagent.

Predicted Negative Ion Photoelectron Spectra of 1-, 2-, and 9-Cyanoanthracene Radical Anions and Computed Thermochemical Values of the Three Cyanoanthracene Isomers.

In this work, the negative ion photoelectron spectra of 1-, 2-, and 9-cyanoanthracene (anthracenecarbonitrile, ACN) radical anions, obtained via the calculation of Franck-Condon (FC) factors based on a harmonic oscillator model, are reported. The FC calculations utilize harmonic vibrational frequencies and normal mode vectors derived from density functional theory using the B3LYP/6-311++G (2d,2p) basis set. The removal of an electron from the doublet anion allows for the computation of the negative ion photoelectron spectra that represents the neutral ground singlet state (So) and the lowest triplet state (T1) in each of the three ACN molecules. The respective adiabatic electron affinity (EA) values for the So state in 1-, 2-, and 9-ACN isomers are calculated to be 1.353, 1.360, and 1.423 eV. The calculated EA of the 9-cyanoanthracene singlet isomer is in close agreement with the previously reported experimental value of 1.27 ± 0.1 eV. Calculations show that the T1 states in 1-, 2-, and 9-ACN are located 1.656, 1.663, and 1.599 eV above the So state. The calculated T1 negative ion spectra exhibit intense vibrational origins and weak FC activity beyond the origins, indicating little change in geometry following electron detachment from the doublet anionic state. Upon deprotonation, the EA values of the radical isomers increase by ∼400-700 meV, resulting in neutral deprotonated radicals with EAs between 1.740 and 2.220 eV. The calculated site-specific gas-phase acidity values of ACN isomers indicate that ACN molecules are more acidic than benzonitrile.

From Benzonitrile to Dicyanobenzenes: The Effect of an Additional CN Group on the Thermochemistry and Negative Ion Photoelectron Spectra of Dicyanobenzene Radical Anions.

The negative ion photoelectron spectra of 1,2-dicyanobenzene (o-DCNB), 1,3-dicyanobenzene (m-DCNB), and 1,4-dicyanobenzene (p-DCNB) radical anions (DCNB·-), acquired through the computation of Frack-Condon (FC) factors, are presented. The FC calculations utilize harmonic frequencies and normal mode vectors derived from density functional theory at the B3LYP/aug-cc-pVQZ basis set. All the totally symmetric vibrational modes are treated with Duschinsky rotations to yield neutral DCNBs in their singlet (So) and lowest triplet (T1) states, following an electron removal from the doublet anionic ground state. For the So state, the adiabatic electron affinities (EAs) for o-, m-, and p-DCNB are 1.179, 1.103, and 1.348 eV. The EAs for the lowest T1 state in o-, m-, and p-DCNB are 4.151, 4.185, and 4.208 eV, resulting in an So-T1 energy difference (ΔEST) of 2.973, 3.082, and 2.860 eV. A vibrational analysis reveals evidence of FC activity involving ring distortion, C-N bending, and ring C═C stretching vibrational progressions in both the So and T1 states. With the detection of cyanonaphthalene (C10H7CN) and cyanoindene (C9H7CN) in the interstellar medium (ISM), our results highlight the extent to which replacing a single hydrogen on an aromatic molecule with a cyano group, C≡N, can alter the vibrational structure of the molecule/radical anion. As such, dicyano-polyaromatic hydrocarbons may be reasonably robust in the ISM, making it appealing to search for them in future interstellar detection missions.

Li2[SeC2Se]·2NH3: A Crystalline Ammoniate with a –Se–C≡C–Se– Dianion

Reaction of Li2C2 with elemental selenium in a molar ratio of 1:2 in liquid ammonia led to the formation of the ammoniate Li2[SeC2Se]·2NH3. Its crystal structure was solved and refined from high-resolution synchrotron powder diffraction data (P21/c, Z = 4). It contains the -Se-C≡C-Se- anion, unprecedented in a crystalline material, whose existence was corroborated by IR/Raman spectra and electronic-structure theory, showing an almost perfect agreement with calculated spectra. Elaborated magnetic-bottle and velocity-map imaging photoelectron spectroscopic investigations show that the -Se-C≡C-Se• radical anion can be transferred to the gas phase, where it was analyzed by NIPE (Negative Ion Photoelectron) and VMI (Velocity-Map Imaging) spectra, which correlate nicely with simulated spectra based on 2Πu → 3Σg- and 2Πu → 1Σg+ transitions including spin-orbit couplings.

Negative Ion Photoelectron Spectra of Deprotonated Benzonitrile Isomers via Computation of Franck-Condon Factors.

Negative ion photoelectron spectra of ortho (o-), meta (m-), and para (p-) deprotonated benzonitrile (o-, m-, p-C6H4(CN)-) isomers as well as the associated thermochemical values corresponding to deprotonation at o-, m-, and p-positions in C6H5(CN) are presented. Quantum mechanical results based on the density functional theory (DFT) utilizing the aug-cc-pVQZ basis set indicate that the o-, m-, p-C6H4(CN)● radicals have electron affinity values (EAs) of 1.901, 1.778, and 1.789 eV, respectively. The computed Franck-Condon (FC) factors give rise to o-, m-, and p-C6H4(CN)- negative ion spectra with FC active ring distortion vibrational modes with harmonic vibrational frequencies of ∼450, 760, and 1000 cm-1 as the dominant vibrational progressions. Deprotonation at the o-, m-, and p-positions in C6H5(CN) results in calculated gas-phase acidity values (ΔacidH298Ko) of 383.9, 385.7, and 385.3 kcal mol-1, respectively. The calculated ΔacidH298Ko is in close agreement with the previously reported high-pressure mass spectrometry experimental value of 383.4.0 ± 4.4 kcal mol-1. The computed ΔacidH298Ko and EAs are utilized to estimate the bond dissociation energy (DH298(H-C6H4CN)) associated with the formation o-, m-, and p-C6H4(CN)● using the negative ion thermochemical cycle: DH298(C6H5CN) = ΔacidH298Ko (H-C6H4(CN) + EA (C6H5CN)● - IP(H). The respective values of DH298(H-C6H4CN) corresponding to the formation of ortho, meta, and para C6H4(CN) radicals are 114.15, 113.11, and 113.51 kcal mol-1.

Electron Affinity and Electronic Structure of Hexafluoroacetone (HFA) Revealed by Photodetaching the [HFA]•- Radical Anion.

A great deal of effort has been focused on developing a metal-free catalytic system for epoxidation of unreactive alkenes. Fluoroketones are thought as remarkably promising catalysts for epoxidation reactions. The combination of fluorinated alcohols and catalytic amounts of hexafluoroacetone (HFA) gives a versatile and effective medium for epoxidation of various olefins with hydrogen peroxide. However, the fundamental physicochemical properties of HFA remained largely unclear, although they were very important to understand the related interactions. Here, we performed a joint study on the electron affinity and electronic structure of HFA employing negative ion photoelectron (NIPE) spectroscopy and quantum chemistry calculations. Two distinct bands with complicated vibrational progressions were observed in the 193 nm NIPE spectrum. The adiabatic/vertical detachment energies (ADE/VDE) were derived to be 1.42/2.06 and 4.43/4.86 eV for the ground singlet state and excited triplet state, respectively. Using the optimized geometries and vibrational frequencies of the anion and the neutral, the Franck-Condon factors were calculated for electron detachments to produce HFA in its lowest singlet and triplet states. Good agreements are obtained hereby for both bands between the experimental and calculated NIPE spectra, when taking into account combination vibrational excitations, unequivocally revealing that HFA possesses a singlet ground state with a giant singlet-triplet energy difference (ΔEST). The electron affinity (EA) and ΔEST of HFA were therefore determined to be EA = 1.42 ± 0.02 eV and ΔEST = -3.01 eV.

Probing Orientation-Specific Charge-Dipole Interactions between Hexafluoroisopropanol and Halides: A Joint Photoelectron Spectroscopy and Theoretical Study.

The interactions between hexafluoroisopropanol (HFIP) and halogen anions X- (F-, Cl-, Br-, and I-) have been investigated using negative ion photoelectron (NIPE) spectroscopy and ab initio calculations. The measured NIPE spectrum of each [HFIP·X]- (X = Cl, Br, and I) complex shows a pattern identical to the corresponding X- by shifting to the high electron binding energy side, indicative of the formation of the [HFIP···X-] structure in which X- interacts with HFIP via charge-dipole interactions. However, the spectrum of [HFIP·F]- appears completely different from that of F- and is more similar to the spectrum of the deprotonated HFIP anion (HFIP-H-). The geometry and electron density calculations indicate that a neutral HF molecule is formed upon HFIP interacting with F- via proton transfer, rendering a stable structure of [HFIP-H···HF]-. Two conformers of [HFIP-H·HF]- with HFIP being in synperiplanar and antiperiplanar configurations, respectively, are observed, providing direct experimental evidences to show the distinctly different and orientation-specific interactions between HFIP and halide anions.

Negative Ion Photoelectron Spectroscopy Confirms the Prediction of a Singlet Ground State for the 1,8-Naphthoquinone Diradical.

Negative ion photoelectron (NIPE) spectra, with 193, 266, 300, and 355 nm photons, of the radical anion of 1,8-naphthoquinone (1,8-NQ•-) have been obtained at 20 K. The electron affinity of 1,8-NQ is determined from the first resolved peak in the NIPE spectrum to be 2.965 ± 0.005 eV. Franck-Condon factors (FCFs), calculated from the CASPT2/aug-cc-pVDZ optimized geometries, normal modes, and vibrational frequencies, successfully simulate the intensity and frequencies of the spectral features that are associated with the lowest two electronic states. The NIPE spectra of 1,8-NQ•- and the peak assignments, based on the computed FCFs, confirm the theoretical predictions that 1A1 is the ground state of 1,8-NQ and 3B2 is the first excited state. The spectra provide an experimental value of Δ EST = -0.6 kcal/mol, which is 2 kcal/mol smaller in magnitude than the (12/12)CASPT2/aug-cc-pVTZ calculated value of Δ EST = -2.6 kcal/mol.

Negative Ion Photoelectron Spectroscopy Confirms the Prediction of the Relative Energies of the Low-Lying Electronic States of 2,7-Naphthoquinone.

Cryogenic negative ion photoelectron (NIPE) spectra of the radical anion of 2,7-naphthoquinone (NQ•-) have been taken at 20 K, using 193, 240, 266, 300, and 355 nm lasers for electron detachment. The electron affinity of the NQ diradical is determined from the first resolved peak in the NIPE spectrum to be 2.880 ± 0.010 eV. CASPT2/aug-cc-pVDZ calculations predict with reasonable accuracy the positions of the 0-0 bands in the three lowest electronic states of NQ. In addition, the Franck-Condon factors calculated from the CASPT2/aug-cc-pVDZ optimized geometries, vibrational frequencies, and normal modes successfully simulate the vibrational structures in these bands. The NIPE spectrum of NQ•- confirms that, as predicted, 3B2 is the ground state, and the 1B2 and 1A1 states are, respectively, 12.7 and 16.4 kcal/mol higher in energy than the triplet ground state. The experimental value of Δ EST = 12.7 kcal/mol in NQ and the finding that 1B2 is the lower energy of the two singlet states confirm the results of the previous calculations on NQ. These calculations predicted an increase in Δ EST on the substitution of both methylene groups in 2,7-naphthoquinodimethane (NQDM) by oxygens in NQ, thus providing a dramatic contrast to the decrease of 17.5 kcal/mol in Δ EST found for substitution of one methylene group by one oxygen on going from trimethylenemethane (TMM) to oxyallyl (OXA).

Experimental and Theoretical Studies of the F• + H-F Transition-State Region by Photodetachment of [F-H-F

The transition-state (TS) region of the simplest heavy-light-heavy type of reaction, F• + H-F → F-H + F•, is investigated in this work by a joint experimental and theoretical approach. Photodetaching the bifluoride anion, [F···H···F]-, generates a negative ion photoelectron (NIPE) spectrum with three partially resolved bands in the electron binding energy (eBE) range of 5.4-7.0 eV. These bands correspond to the transition from the ground state of the anion to the electronic ground state of [F-H-F]• neutral, with associated vibrational excitations. The significant increase of eBE of the bifluoride anion, relative to that of F-, reflects a hydrogen bond energy between F- and HF of ∼46 kcal/mol. Theoretical modeling reveals that the antisymmetric motion of H between the two F atoms, near the TS on the neutral [F-H-F]• surface, dominates the observed three bands, while the F-H-F bending, F-F symmetric stretching modes, and the couplings between them are calculated to account for the breadth of the observed spectrum. From the NIPE spectrum, a lower limit on the activation enthalpy for F• + H-F → F-H + F• can be estimated to be ΔH‡ = 12 ± 2 kcal/mol, a value below that of ΔH‡ = 14.9 kcal/mol, given by our G4 calculations.

Negative ion photoelectron spectra of ISO3-, IS2O3-, and IS2O4- intermediates formed in interfacial reactions of ozone and iodide/sulfite aqueous microdroplets.

Three short-lived, anionic intermediates, ISO3-, IS2O3-, and IS2O4-, are detected during reactions between ozone and aqueous iodine/sulfur oxide microdroplets. These species may play an important role in ozone-driven inorganic aerosol formation; however their chemical properties remain largely unknown. This is the issue addressed in this work using negative ion photoelectron spectroscopy (NIPES) and ab initio modeling. The NIPE spectra reveal that all of the three anionic species are characterized by high adiabatic detachment energies (ADEs) - 4.62 ± 0.10, 4.52 ± 0.10, and 4.60 ± 0.10 eV for ISO3-, IS2O3-, and IS2O4-, respectively. Vibrational progressions with frequencies assigned to the S-O symmetric stretching modes are discernable in the ground state transition features. Density functional theory calculations show the presence of several low-lying isomers involving different bonding scenarios. Further analysis based on high level CCSD(T) calculations reveal that the lowest energy structures are characterized by the formation of I-S and S-S bonds and can be structurally viewed as SO3 linked with I, IS, and ISO for ISO3-, IS2O3-, and IS2O4-, respectively. The calculated ADEs and vertical detachment energies are in excellent agreement with the experimental results, further supporting the identified minimum energy structures. The obtained intrinsic molecular properties of these anionic intermediates and neutral radicals should be useful to help understand their photochemical reactions in the atmosphere.

Calculations of the energies of the low‐lying electronic states of dioxatrimethylenemethane (H2CCO2) and prediction of the negative ion photoelectron (NIPE) spectrum of its radical anion

In order to predict the energies of the low‐lying electronic states of the dioxa derivative of trimethylenemethane (DOTMM), we have carried out (U)B3LYP, CASPT2, and (U)CCSD(T) calculations, using the aug‐cc‐pVTZ basis set. Our calculations predict that DOTMM has a triplet ground state, with one unpaired electron occupying a b2 σ MO and the other a b1 π MO. An open‐shell singlet state, with the same orbital occupancy as the triplet, is calculated to lie very close to the 3A2 ground state. However, this open‐shell singlet (1A2) is predicted to be the transition structure for methylene rotation and to lead to the barrierless formation of an equivalent pair of α‐lactones. We also report the results of some calculations on the fragmentation of DOTMM to CH2 + CO2. Our predictions about DOTMM could be tested experimentally by generating the DOTMM•− radical anion in the gas phase and obtaining its negative ion photoelectron spectrum. Copyright © 2016 John Wiley & Sons, Ltd.

Deprotonated Dicarboxylic Acid Homodimers: Hydrogen Bonds and Atmospheric Implications.

Dicarboxylic acids represent an important class of water-soluble organic compounds found in the atmosphere. In this work we are studying properties of dicarboxylic acid homodimer complexes (HO2C(CH2)nCO2(-)[HO2C(CH2)nCO2H], n = 0-12), as potentially important intermediates in aerosol formation processes. Our approach is based on experimental data from negative ion photoelectron spectra of the dimer complexes combined with updated measurements of the corresponding monomer species. These results are analyzed with quantum-mechanical calculations, which provide further information about equilibrium structures, thermochemical parameters associated with the complex formation, and evaporation rates. We find that upon formation of the dimer complexes the electron binding energies increase by 1.3-1.7 eV (30.0-39.2 kcal/mol), indicating increased stability of the dimerized complexes. Calculations indicate that these dimer complexes are characterized by the presence of strong intermolecular hydrogen bonds with high binding energies and are thermodynamically favorable to form with low evaporation rates. Comparison with the previously studied HSO4(-)[HO2C(CH2)2CO2H] complex (J. Phys. Chem. Lett. 2013, 4, 779-785) shows that HO2C(CH2)2CO2(-)[HO2C(CH2)2CO2H] has very similar thermochemical properties. These results imply that dicarboxylic acids not only can contribute to the heterogeneous complexes formation involving sulfuric acid and dicarboxylic acids but also can promote the formation of homogeneous complexes by involving dicarboxylic acids themselves.

Periodicity, Electronic Structures, and Bonding of Gold Tetrahalides [AuX4]− (X = F, Cl, Br, I, At, Uus)

Systematic theoretical and experimental investigations have been performed to understand the periodicity, electronic structures, and bonding of gold halides using tetrahalide [AuX4](-) anions (X = F, Cl, Br, I, At, Uus). The [AuX4](-) (X = Cl, Br, I) anions were experimentally produced in the gas phase, and their negative-ion photoelectron spectra were obtained, exhibiting rich and well-resolved spectral peaks. As expected, Au-X bonds in such series contain generally increasing covalency when halogen ligands become heavier. We calculated the adiabatic electron detachment energies as well as vertical electron detachment energies using density functional theory methods with scalar relativistic and spin-orbit coupling effects. The computationally simulated photoelectron spectra are in good agreement with the experimental ones. Our results show that the trivalent Au(III) oxidation state becomes progressively less stable while Au(I) tends to be preferred when the halides become heavier along the Periodic Table. This series of molecules provides an example for manipulating the oxidation state of metals in complexes through ligand design.

Negative Ion Photoelectron Spectroscopy Confirms the Prediction that 1,2,4,5-Tetraoxatetramethylenebenzene Has a Singlet Ground State.

The negative ion photoelectron (NIPE) spectrum of 1,2,4,5-tetraoxatetramethylenebenzene radical anion (TOTMB(•-)) shows that, like the hydrocarbon, 1,2,4,5-tetramethylenebenzene (TMB), the TOTMB diradical has a singlet ground state and thus violates Hund's rule. The NIPE spectrum of TOTMB(•-) gives a value of -ΔEST = 3.5 ± 0.2 kcal/mol for the energy difference between the singlet and triplet states of TOTMB and a value of EA = 4.025 ± 0.010 eV for the electron affinity of TOTMB. (10/10)CASPT2 calculations are successful in predicting the singlet-triplet energy difference in TOTMB almost exactly, giving a computed value of -ΔEST = 3.6 kcal/mol. The same type of calculations predict -ΔEST = 6.1-6.3 kcal/mol in TMB. Thus, the calculated effect of the substitution of the four oxygens in TOTMB for the four methylene groups in TMB is very unusual, since the singlet state is selectively destabilized relative to the triplet state. The reason why TMB → TOTMB is predicted to result in a decrease in the size of -ΔEST is discussed.

The Unpredictability of Research Directions and the Synergy between Theory and Experiment in Physical‐Organic Chemistry

AbstractResearch projects in physical‐organic chemistry can go in unpredictable directions. One such project, described herein, began with the discovery of a large, imaginary, calculated vibrational frequency and soon led to the prediction that cyclobutanetetraone (CO)4 had a triplet ground state. Molecular orbital (MO) theory provided the understanding of why (CO)4 was calculated to have a planar triplet ground state and why, in contrast, (SiO)4 was calculated to have a tetrahedral singlet ground state. Qualitative MO theory, as well as calculations, also predicted that (CO)3, (CO)5, and (CO)6 should all be found to have singlet ground states. Testing experimentally these predictions about (CO)n molecules, n=3–6, led to a very fruitful collaboration with Dr. Xuebin Wang, who obtained the negative ion photoelectron (NIPE) spectra of the radical anions of these four molecules and also of (CS)4. Assignments of the signals in these five NIPE spectra required computing the vibrational progressions in them. The necessary acquisition of the ability to simulate the signals in NIPE spectra resulted in the successful analysis of the very complex spectrum of meta‐benzoquinone (MBQ) radical anion, which Dr. Wang had already obtained and published. Analysis of this spectrum confirmed the prediction, made more than 20 years previously, that the ordering of the two lowest singlet states in MBQ was reversed from that in meta‐benzoquinodimethane (MBQDM). The research described herein illustrates not only the unpredictability of where some research projects will eventually lead, but also the synergy between calculations and experiments in physical‐organic chemistry.

The negative ion photoelectron spectrum of cyclopropane-1,2,3-trione radical anion, (CO)3(•-)--a joint experimental and computational study.

Negative ion photoelectron (NIPE) spectra of the radical anion of cyclopropane-1,2,3-trione, (CO)3(•-), have been obtained at 20 K, using both 355 and 266 nm lasers for electron photodetachment. The spectra show broadened bands, due to the short lifetimes of both the singlet and triplet states of neutral (CO)3 and, to a lesser extent, to the vibrational progressions that accompany the photodetachment process. The smaller intensity of the band with the lower electron binding energy suggests that the singlet is the ground state of (CO)3. From the NIPE spectra, the electron affinity (EA) and the singlet-triplet energy gap of (CO)3 are estimated to be, respectively, EA = 3.1 ± 0.1 eV and ΔEST = -14 ± 3 kcal/mol. High-level, (U)CCSD(T)/aug-cc-pVQZ//(U)CCSD(T)/aug-cc-pVTZ, calculations give EA = 3.04 eV for the (1)A1' ground state of (CO)3 and ΔEST = -13.8 kcal/mol for the energy gap between the (1)A1' and (3)A2 states, in excellent agreement with values from the NIPE spectra. In addition, simulations of the vibrational structures for formation of these states of (CO)3 from the (2)A2″ state of (CO)3(•-) provide a good fit to the shapes of broad bands in the 266 nm NIPE spectrum. The NIPE spectrum of (CO)3(•-) and the analysis of the spectrum by high-quality electronic structure calculations demonstrate that NIPES can not only access and provide information about transition structures but NIPES can also access and provide information about hilltops on potential energy surfaces.

The Negative Ion Photoelectron Spectrum of meta-Benzoquinone Radical Anion (MBQ•–): A Joint Experimental and Computational Study

Negative ion photoelectron (NIPE) spectra of the radical anion of meta-benzoquinone (MBQ, m-OC6H4O) have been obtained at 20 K, using both 355 and 266 nm lasers for electron photodetachment. The spectra show well-resolved peaks and complex spectral patterns. The electron affinity of MBQ is determined from the first resolved peak to be 2.875 ± 0.010 eV. Single-point, CASPT2/aug-cc-pVTZ//CASPT2/aug-cc-pVDZ calculations predict accurately the positions of the 0-0 bands in the NIPE spectrum for formation of the four lowest electronic states of neutral MBQ from the (2)A2 state of MBQ(•-). In addition, the Franck-Condon factors that are computed from the CASPT2/aug-cc-pVDZ optimized geometries, vibrational frequencies, and normal mode vectors, successfully simulate the intensities and frequencies of the vibrational peaks in the NIPE spectrum that are associated with each of these electronic states. The successful simulation of the NIPE spectrum of MBQ(•-) allows the assignment of (3)B2 as the ground state of MBQ, followed by the (1)B2 and (1)A1 electronic states, respectively 9.0 ± 0.2 and 16.6 ± 0.2 kcal/mol higher in energy than the triplet. These experimental energy differences are in good agreement with the calculated values of 9.7 and 15.7 kcal/mol. The relative energies of these two singlet states in MBQ confirm the previous prediction that their relative energies would be reversed from those in meta-benzoquinodimethane (MBQDM, m-CH2C6H4CH2).