Anion Photoelectron Spectra Research Articles

Structures of trimetallic molybdenum and tungsten suboxide cluster anions

Anion photoelectron spectra of Mo(3)O(y)(-) and W(3)O(y)(-) (y = 3-6) are reported and analyzed using density functional theory results in an attempt to determine whether electronic and structural trends in the less oxidized clusters (y = 3, 4) could elucidate the disparate chemical properties of the M(3)O(y)(-) (M = Mo, W, y = 5, 6) species. In general, cyclic structures are calculated to be more stable by at least 1 eV than extended structures, and the lowest energy structures calculated for the most reduced species favor M = O terminal bonds. While the numerous low-energy structures found for Mo(3)O(y)(-)/Mo(3)O(y) and W(3)O(y)(-)/W(3)O(y) were, in general, similar, various structures of W(3)O(y)(-)/W(3)O(y) were found to be energetically closer lying than analogous structures of Mo(3)O(y)(-)/Mo(3)O(y). Additionally, the Mo-O-Mo bridge bond was found to be a more stabilizing structural motif than the W-O-W bridge bond, with the oxygen center in the former having the highest negative charge. Based on this, the observation of trapped intermediates in reactions between Mo(3)O(y)(-) and water or CO(2) that are not observed in analogous W(3)O(y)(-) reactivity studies may be partially attributed to the role of bridge bond fluxionality.

Aluminum Avoids the Central Position in AlB9–and AlB10–: Photoelectron Spectroscopy and ab Initio Study

The structures and the electronic properties of two Al-doped boron clusters, AlB(9)(-) and AlB(10)(-), were investigated via joint photoelectron spectroscopy and high-level ab initio study. The photoelectron spectra of both anions are relatively broad and have no vibrational structure. The geometrical structures were established by unbiased global minimum searches using the Coalescence Kick method and comparison between the experimental and calculated vertical electron detachment energies. The results show that both clusters have quasi-planar structures and that the Al atom is located at the periphery. Chemical bonding analysis revealed that the global minimum structures of both anions can be described as doubly (σ- and π-) aromatic systems. The nona-coordinated wheel-type structure of AlB(9)(-) was found to be a relatively high-lying isomer, while a similar structure for the neutral AlB(9) cluster was previously shown to be either a global minimum or a low-lying isomer.

Photoelectron spectroscopic and computational studies of the Pt@Pb₁₀⁻¹ and Pt@Pb₁₂¹⁻/²⁻ anions.

A combination of anion photoelectron spectroscopy and density functional theory calculations has elucidated the geometric and electronic structure of gas-phase endohedral Pt/Pb cage cluster anions. The anions, Pt@Pb₁₀⁻¹ and Pt@Pb₁₂¹⁻ were prepared from "preassembled" clusters generated from crystalline samples of [K(2,2,2-crypt)]₂[Pt@Pb₁₂] that were brought into the gas phase using a unique infrared desorption/photoemission anion source. The use of crystalline [K(2,2,2-crypt)]₂[Pt@Pb₁₂] also provided access to K[Pt@Pb(n)](-) anions in the gas phase (i.e., the K⁺ salts of the Pt@Pb(n)²⁻ anions). Anion photoelectron spectra of Pt@Pb₁₀⁻¹, Pt@Pb₁₂¹⁻, and K[Pt@Pb₁₂]¹⁻ are presented. Extensive density functional theory calculations on Pt@Pb₁₀³⁻/²⁻/¹⁻/⁰ and Pt@Pb₁₂²⁻/¹⁻ provided candidate structures and anion photoelectron spectra for Pt@Pb₁₀⁻¹ and Pt@Pb₁₂¹⁻. Together, the calculated and measured photoelectron spectra show that Pt@Pb₁₀⁻¹ and Pt@Pb₁₂²⁻/¹⁻ endohedral complexes maintain their respective D(4d) and slightly distorted I(h) symmetries in the gas phase even for the charge states with open shell character. Aside from the fullerenes, the Pt@Pb₁₂²⁻ endohedral complex is the only bare cluster that has been structurally characterized in the solid state, solution, and the gas phase.

Photoelectron spectra of dihalomethyl anions: Testing the limits of normal mode analysis

We report the 364-nm negative ion photoelectron spectra of CHX(2)(-) and CDX(2)(-), where X = Cl, Br, and I. The pyramidal dihalomethyl anions undergo a large geometry change upon electron photodetachment to become nearly planar, resulting in multiple extended vibrational progressions in the photoelectron spectra. The normal mode analysis that successfully models photoelectron spectra when geometry changes are modest is unable to reproduce qualitatively the experimental data using physically reasonable parameters. Specifically, the harmonic normal mode analysis using Cartesian displacement coordinates results in much more C-H stretch excitation than is observed, leading to a simulated photoelectron spectrum that is much broader than that which is seen experimentally. A (2 + 1)-dimensional anharmonic coupled-mode analysis much better reproduces the observed vibrational structure. We obtain an estimate of the adiabatic electron affinity of each dihalomethyl radical studied. The electron affinity of CHCl(2) and CDCl(2) is 1.3(2) eV, of CHBr(2) and CDBr(2) is 1.9(2) eV, and of CHI(2) and CDI(2) is 1.9(2) eV. Analysis of the experimental spectra illustrates the limits of the conventional normal mode approach and shows the type of analysis required for substantial geometry changes when multiple modes are active upon photodetachment.

Anion photoelectron spectroscopy and ab initio calculations of the gas phase chloride–carbon monoxide complex: Cl−⋯CO

Abstract The first experimental anion photoelectron spectrum of the chloride–carbon monoxide complex is presented in combination with supporting ab initio calculations at the MP2 level of theory. The calculations reveal a previously unreported minimum for the neutral radical, namely the van der Waals Cl⋯OC linear complex. The experimental spectral feature is assigned to transitions from a Cl−⋯CO bent van der Waals complex to the linear Cl⋯CO neutral van der Waals radical. A stabilisation energy of 0.16 ± 0.01 eV is determined from the spectrum which is in line with the predicted cluster binding energies of 3.5 kcal mol−1 (0.15 eV) for the anion complex.

Theoretical study of geometric and electronic structures, and anion PES of the [formula omitted] ( n = 5–11; m = 1–3) clusters

The correlation between the features of the geometric and electronic structures and the anion photoelectron spectra (PES) of Al n Cs m - ( n = 5–11; m = 1–3) were investigated using DFT calculations. The geometric changes from the octahedral to the wheel-like (or semi-icosahedral) structure were found in the n = 8 and 9 Al n Cs − clusters, and the n = 7 and 8 Al n Cs 2 - and Al n Cs 3 - clusters. The calculated vertical detachment energy (VDE) was in good agreement with the corresponding anion PES. The Kohn–Sham orbital energy gap of the 3 p orbital region in the wheel-like structure was narrower than that in the octahedral structure.

Study of MoVOy (y = 2−5) Anion and Neutral Clusters using Anion Photoelectron Spectroscopy and Density Functional Theory Calculations

The vibrationally resolved anion photoelectron (PE) spectra of MoVO(y)(-) (y = 2 - 5) metal suboxide clusters are presented and analyzed in the context of density functional theory (DFT) calculations. The electronically congested spectra reflect an increase in cluster electron affinity with increasing oxidation state. Ion beam hole-burning results reveal the features in the PE spectra of MoVO(2)(-) and MoVO(4)(-) are a result of only one anion isomer, while at least two isomers contribute to electronic structure observed in the PE spectrum of MoVO(3)(-). Spectral features of the binary systems are compared to their pure analogs, Mo(2)O(y) and V(2)O(y). An attempt to characterize the anion and neutral electronic and molecular structures is made by comparison with results from DFT calculations. However, reconciliation between the cluster spectra and the calculated spectroscopic parameters is not as straightforward as in previous studies on similar systems (Yoder, B. L.; Maze, J. T.; Raghavachari, K.; Jarrold, C. C. J. Chem. Phys. 2005, 122, 094313 and Mayhall, N. J.; Rothgeb, D. W.; Hossain, E.; Raghavachari, K.; Jarrold, C. C. J. Chem. Phys. 2009, 130, 124313).

Theoretical Study of the Low Lying States of Ga2X (X = P, As)

Since the low lying electronic states of Ga(2)X (X = P, As) are nearly degenerate, an accurate determination of the electronic structure of these states was incomplete in the previous works. In this study, the geometry optimization and vibrational frequency calculation have been performed at the various levels of theory, using the effective core pseudopotentials of the Ga and As atoms and the associated cc-pVTZ basis sets. The ground state of Ga(2)P is found to be the (2)A' state correlating with the (2)B(2) state in C(2v) structure, which lies 0.031 eV above the global minimum at the RCCSD(T) level. The equilibrium of the (2)A(1) state is optimized above that of the (2)B(2) state, and the (2)B(1) state, predicted previously as the ground state, has a stable minimum above the optimized geometries of the former states. Concerning the low lying states of Ga(2)As, our results confirm in general the previous studies. Because all the states of both compounds can be considered monoconfigurational around their equilibrium, the RCCSD(T) method is well adapted to acurrately describe both systems. In turn, we have applied our obtained information to analyze the Ga(2)P(-) and Ga(2)As(-) anion photoelectron spectra, to which it remains difficult to appropriately assign the low lying states of Ga(2)P and Ga(2)As. As a consequence, our analysis supports the previous assignment for Ga(2)P, but contrasts partially to that for Ga(2)As.

Combined Experimental and Theoretical Investigation of Three-Dimensional, Nitrogen-Doped, Gallium Cluster Anions

Anion photoelectron spectra of Ga(x)N(y)(-) cluster anions, in which x = 4-12, y = 1 and x = 7-12, y = 2, were measured. Ab initio studies were conducted on Ga(x)N(y)(-) cluster anions in which x = 4-7, y = 1 and Ga(7)N(2)(-), providing their structures and electronic properties. The photoelectron spectra were interpreted in terms of the computational results. This allowed for identification of the isomers present in the beam experiments for specific Ga(x)N(-) cluster anions (x = 4-7). The unexpected presence of Ga(x)N(2)(-) species is also reported.

Do anionic titanium dioxide nano‐clusters reach bulk band gap? A density functional theory study

The electronic properties of both neutral and anionic (TiO(2))(n) (n = 1-10) clusters are investigated by extensive density functional theory calculations. The predicted electron detachment energies and excitation gaps of anionic clusters agree well with the original experimental anion photoelectron spectra (APES). It is shown that the old way to analyze APES tends to overestimate vertical excitation gaps (VGA) of large anionic clusters, due to the nature of multiple electronic origins for the higher APES bands. Moreover, the VGA of anionic TiO(2) clusters are evidently smaller than those of neutral clusters, which may also be the case for other metal oxide clusters with high electron affinity.

Angle-resolved photoelectron spectra of metal cluster anions within a many-body-theory approach

A consistent many-body theory based on the jellium model is applied for the description of angular resolved photoelectron spectra of metal clusters anions. The results of calculations demonstrate the dominant role of the many-body effects in the formation of angular distributions of photoelectrons emitted from sodium clusters and are in a good agreement with recent experimental data. The concrete comparison of theory and experiment has been performed for the photoionization of Na${}_{7}$${}^{\ensuremath{-}}$ and Na${}_{19}$${}^{\ensuremath{-}}$ anions being characterized by the entirely closed shells of delocalized electrons.

Molecular Structures and Energetics of the (ZrO2)n and (HfO2)n (n = 1−4) Clusters and Their Anions

The group IVB transition-metal dioxide clusters and their anions, (MO(2))(n) and (MO(2))(n)(-) (M = Zr, Hf; n = 1-4), are studied with coupled cluster (CCSD(T)) theory and density functional theory (DFT). Similar to the results for M = Ti, these oxide clusters have a number of low-lying isomeric structures, which can make it difficult to predict the ground electronic state especially for the anion. Electron affinities for the low-lying structures are calculated and compared with those for M = Ti. Electron affinities of these clusters depend strongly on the cluster structures. Anion photoelectron spectra are calculated for the monomer and dimer and demonstrate the possibility for structural identification at a spectral line width of <or=0.05 eV. Electron excitation energies from the low-lying states to the singlet and triplet excited states are calculated self-consistently, as well as by the time-dependent DFT and equation-of-motion coupled cluster (EOM-CCSD) methods. The calculated excitation energies are compared to the band energies of bulk oxides, indicating that the excitation energy is not yet converged for n = 4 for these clusters. The excitation energies of the low-lying isomeric clusters are less than the bulk metal oxide band gaps and suggest that these clusters could be useful photocatalysts with a visible light source.

Survey of Ar-Tagged Predissociation and Vibrationally Mediated Photodetachment Spectroscopies of the Vinylidene Anion, C2H2−

We report predissociation spectra of Ar-tagged C(2)H(2)(-) and C(2)D(2)(-) anions, and explore vibrationally mediated photodetachment from various vibrational levels of the bare C(2)H(2)(-) ion using velocity-map imaging. Intense photodetachment resonances are observed in the C-H stretching region that are strongly correlated with vibrational hot bands in the anion photoelectron spectra, indicating that one-color, resonant two-photon photodetachment (R2PD) is complicated by excitation of vibrationally excited states with autodetaching upper levels embedded in the continuum. Isolation of the R2PD spectrum was achieved using a two-color, IR-IR scheme in which vibrational excitation and photodetachment were carried out in two separate laser interaction regions.

Photoelectron spectroscopy of homogeneous nucleic acid base dimer anions

We report the photoelectron spectra of homogeneous dimer anions of the nucleobases: uracil, thymine, cytosine, adenine, and guanine, i.e., U(2)(-), T(2)(-), C(2)(-), A(2)(-), and G(2)(-) along with DFT calculations on U(2)(-) and T(2)(-). Based on these calculations the photoelectron spectrum of T(2)(-) was assigned as being due to both a proton transferred and a non-proton transferred isomer, while the photoelectron spectrum of U(2)(-) was assigned in terms of a single dominant barrier-free proton transferred isomer. Photoelectron spectra were also measured with a different source and on a different type of photoelectron spectrometer for U(2)(-), T(2)(-), A(2)(-), (1-MeT)(2)(-) and (1,3-Me(2)U)(2)(-).

Low temperature photoelectron spectra of water cluster anions

Photoelectron spectra of cold (10 K) size selected water cluster anions (H(2)O)(n) (-) and (D(2)O)(n) (-) have been measured in the size range n=20-120. A new isomer with a higher binding energy than the so-called isomer I has been identified, which appears in the size range n=25-30 and for (H(2)O)(n) (-) becomes dominant at n=46. Magic numbers observed in the mass spectra of the cluster anions provide evidence that this new isomer class consists of clusters with an internal electron.

Photoelectron Spectroscopy of Chloro-Substituted Phenylnitrene Anions

The 355 nm time-of-flight negative ion photoelectron spectra of (o-, m-, and p-chlorophenyl)nitrene radical anions are reported. Electron affinities are obtained from the photoelectron spectra, and are 1.79 +/- 0.05, 1.82 +/- 0.05, and 1.72 +/- 0.05 eV for the (o-, m-, and p-chlorophenyl)nitrenes, respectively. Singlet-triplet splittings are determined to be 14 +/- 2, 15 +/- 1, and 14 +/- 2 kcal/mol, respectively. The shapes of the photoelectron bands indicate resonance interactions in the singlet states for the ortho- and para-substituted isomers, which is attributed to quinoidal structures of the open-shell singlet states. Reanalysis of the photoelectron spectrum of phenylnitrene anion leads to a revised experimental singlet-triplet splitting of 14.8 kcal/mol in the unsubstituted phenylnitrene.

Search for Lowest-Energy Nonclassical Fullerenes III: C22

Density functional and second-order Møller-Plesset perturbation (MP2) methods were employed in the investigation of low-lying C22 isomers. All cage structures with four-, five-, six-, and seven-membered rings were examined with the monocyclic ring, bowl, and other noncage structures. Cage isomers were first identified via graph theoretical methods, and noncages were identified by basin-hopping methods. Initial isomer screenings were carried out at the PBE/DND level of theory. Low-lying isomers, within 0.6 eV of the predicted lowest-energy isomer, were further evaluated at the PBE1PBE/cc-pVTZ and MP2/cc-pVTZ levels. Our results confirm that the cage structures are more stable than the ring structure and the bowl structure. The lowest-energy structure for C22 is predicted to be the C22-1 cage containing one four-membered ring. Anion photoelectron and optical spectra of the six lowest-lying isomers are also computed.

Three approximations to the nonlocal and energy‐dependent correlation potential in electron propagator theory

AbstractThe historical dilemma that pits accuracy against interpretability in molecular electronic structure calculations may be resolved with electron propagator theory. In the interpretation of electron binding energies, electron propagator theory provides a systematically improvable description of electron correlation while retaining one‐electron concepts, such as Dyson orbitals, of wide‐ranging interpretive facility. The nonlocal, energy‐dependent correlation potential, known as the self‐energy, which occurs in the one‐electron equations of electron propagator theory may be approximated in practical calculations. The partial third‐order approximation has been employed in many calculations on large molecules. Applications to fragments of nucleic acids show how qualitative, orbital‐based insights emerge from calculations that suffice to quantitatively assign photoelectron spectra. A quasiparticle virtual orbital method for improving the efficiency of this method shows considerable promise. Simple perturbative approaches may be combined with renormalizations that incorporate final‐state orbital relaxation effects with the use of reference ensembles that correspond to Slater's transition state method. Such approximations produce useful predictions for core as well as valence electron binding energies. Another approximation that is based on highly correlated reference states and a renormalized treatment of the self‐energy shows considerable flexibility in the accurate prediction of electron binding energies. This method suffices to make definitive assignments of photoelectron spectra of double Rydberg anions, species whose ground state electronic structure requires assignment of electrons to nonvalence orbitals. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Electronic structures of MoWOy− and MoWOy determined by anion photoelectron spectroscopy and DFT calculations

The anion photoelectron spectra of MoWO(y)(-) (y=2-5) and density functional theory (DFT) calculations on MoWO(y)(-) and MoWO(y) are reported and compared to previous comparable studies on Mo(2)O(y)(-)/Mo(2)O(y) and W(2)O(y)(-)/W(2)O(y). The property governing the structure of the lowest energy MoWO(y) anion and neutral clusters is the stronger W-O bond relative to the Mo-O bond, which results in the stabilization of structures in which the Mo center is in a much lower oxidation state than the W center. Anion PE spectra show a much larger change in structure between anion and neutral states than what was observed in the pure Mo(2)O(y)(-) and W(2)O(y)(-) spectra. DFT calculations show increased single-metal localization of spin with respect to the pure metal oxide clusters.

Electronic structure of coordinatively unsaturated molybdenum and molybdenum oxide carbonyls

Results of density functional theory calculations on coordinatively unsaturated molybdenum carbonyl and molybdenum oxide carbonyl anion and neutral complexes observed in previous experimental studies [Wyrwas, Robertson, and Jarrold, J. Chem. Phys. 126, 214309 (2007)] and extended to related complexes are reported. The ground and low-lying electronic states were calculated for the most stable structures predicted for Mo(CO)(n)/Mo(CO)(n) (-) (n=1-3, 5 and 6), MoO(CO)(n)/MoO(CO)(n) (-) (n=0-3), and MoO(2)(CO)(n)/MoO(2)(CO)(n) (-) (n=0-2). Interesting trends are predicted with CO addition, electron addition, and oxidation of the Mo center. In all cases, anions have stronger Mo-CO bond energies, which is attributed to enhanced pi(CO) ( *) backdonation. This enhancement is more dramatic for the molybdenum oxo complexes because the highest occupied molecular orbitals shift from Mo to the pi(CO) ( *) backbonds with the addition of oxygen to the Mo center. Sequential addition of CO for all species results in a sequential stabilization of low spin states and a destabilization of higher spin states. Further, average Mo-CO bond lengths increase as carbonyls are sequentially added. This effect is attributed to fewer electrons per Mo-CO pi(CO) ( *) backbond. Finally, addition of O to Mo(CO)(n) appears to weaken the Mo-CO bonds, and addition of CO to MoO(n) appears to weaken Mo-O bonds. The calculations are validated by favorable agreement between the available measured anion photoelectron spectra and simulated spectra based only on calculated spectroscopic parameters (vibrational frequencies and normal coordinate displacements).