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1.
J Phys Chem A ; 2020 Mar 05.
Artigo em Inglês | MEDLINE | ID: mdl-32091900

RESUMO

The anion photoelectron imaging spectra of an ion with m/z 85, generated under ion source conditions that optimize •OH production in a coexpansion with isoprene, are presented and analyzed with supporting calculations. A spectroscopic feature observed at a vertical electron detachment energy of 2.45 eV, which dominates the photoelectron spectrum measured at 3.495 eV photon energy, is consistent with the OH-·isoprene ion-molecule complex, while additional signal observed at lower electron binding energy can be attributed to other constitutional isomers. However, spectra measured over a 2.2-2.6 eV photon energy range, i.e., from near threshold of the predominant OH-·isoprene detachment feature through the vertical detachment energy, exhibit sharp features with common electron kinetic energies, suggesting autodetachment from a temporary anion prepared by photoexcitation. The photon energy independence of the electron kinetic energy of these features along with the low dipole moment predicted for the neutral •OH·isoprene van der Waals complex, suggest a complex photon-driven process. We present calculations supporting a hypothesis that near-threshold production of the •OH···isoprene reactive complex results in hydrogen abstraction of the isoprene molecule. The newly formed activated complex anion supports a dipole bound state that temporarily traps the near zero-kinetic energy electron and then autodetaches, encoding the low-frequency modes of the dehydrogenated neutral isoprene radical in the electron kinetic energies.

2.
J Chem Phys ; 152(5): 054301, 2020 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-32035442

RESUMO

Mixed-metal oxides have proven to be effective catalysts for the hydrogen evolution reaction, often outperforming either of the binary metal oxides. The reactivity of MnxMoOy - (x = 1, 2; y = 3, 4) clusters toward H2O was investigated via time-of-flight mass spectrometry with clear evidence of cluster oxidation and corresponding H2 production, specifically for MnxMoO3 - (x = 1, 2) clusters. Unlike previously studied MoxOy - clusters, which assumed a broad distribution of stoichiometries (typically x ≤ y ≤ 3x), both MnMoOy - and Mn2MoOy - preferentially formed y = 3 and 4 compositions in significant quantities under our source conditions. The electronic and molecular structures of the MnxMoOy (x = 1, 2; y = 3, 4) anion and neutral clusters were probed with anion photoelectron spectroscopy and analyzed with supporting density functional theory calculations. Our studies suggest that both metal centers are involved in initial cluster-water complex formation, while Mo is the center that undergoes oxidation; hence, reactivity terminates when Mo is saturated in its highest oxidation state of +6. Across these four clusters, Mn remains relatively reduced and is stable in a high-spin electronic configuration. The preferential reactivity of water molecules toward the Mo center rather than Mn is rationalized by the much lower relative oxophilicity of Mn.

3.
Acc Chem Res ; 52(11): 3265-3273, 2019 Nov 19.
Artigo em Inglês | MEDLINE | ID: mdl-31702894

RESUMO

Lanthanide (Ln) oxide clusters and molecular systems provide a bottom-up look at the electronic structures of the bulk materials because of close parallels in the patterns of Ln 4fN subshell occupancy between the molecular and bulk Ln2O3 size limits. At the same time, these clusters and molecules offer a challenge to the theory community to find appropriate and robust treatments for the 4fN patterns across the Ln series. Anion photoelectron (PE) spectroscopy provides a powerful experimental tool for studying these systems, mapping the energies of the ground and low-lying excited states of the neutral relative to the initial anion state, providing spectroscopic patterns that reflect the Ln 4fN occupancy. In this Account, we review our anion PE spectroscopic and computational studies on a range of small lanthanide molecules and cluster species. The PE spectra of LnO- (Ln = Ce, Pr, Sm, Eu) diatomic molecules show spectroscopic signatures associated with detachment of an electron from what can be described as a diffuse Ln 6s-like orbital. While the spectra of all four diatomics share this common transition, the fine structure in the transition becomes more complex with increasing 4f occupancy. This effect reflects increased coupling between the electrons occupying the corelike 4f and diffuse 6s orbitals with increasing N. Understanding the PE spectra of these diatomics sets the stage for interpreting the spectra of polyatomic molecular and cluster species. In general, the results confirm that the partial 4fN subshell occupancy is largely preserved between molecular and bulk oxides and borides. However, they also suggest that surfaces and edges of bulk materials may support a low-energy, diffuse Ln 6s band, in contrast to bulk interiors, in which the 6s band is destabilized relative to the 5d band. We also identify cases in which the molecular Ln centers have 4fN+1 occupancy rather than bulklike 4fN, which results in weaker Ln-O bonding. Specifically, Sm centers in mixed Ce-Sm oxides or in SmxOy- (y ≤ x) clusters have this higher 4fN+1 occupancy. The PE spectra of these particular species exhibit a striking increase in the relative intensities of excited-state transitions with decreasing photon energy (resulting in lower photoelectron kinetic energy). This is opposite of what is expected on the basis of the threshold laws that govern photodetachment. We relate this phenomenon to strong electron-neutral interactions unique to these complex electronic structures. The time scale of the interaction, which shakes up the electronic configuration of the neutral, increases with decreasing electron momentum. From a computational standpoint, we point out that special care must be taken when considering Ln cluster and molecular systems toward the center of the Ln series (e.g., Sm, Eu), where treatment of electrons explicitly or using an effective core potential can yield conflicting results on competing subshell occupancies. However, despite the complex electronic structures associated with partially filled 4fN subshells, we demonstrate that inexpensive and tractable calculations yield useful qualitative insight into the general electronic structural features.

4.
J Phys Chem A ; 123(33): 7261-7269, 2019 Aug 22.
Artigo em Inglês | MEDLINE | ID: mdl-31403804

RESUMO

A combined experimental and computational study of H2 reactions with small 98MoxSy- clusters ranging from subsulfide (x ∼ y) to hypersulfide (y > 2x) is presented. Results suggest that the subsulfides react with H2 primarily by insertion of a more reduced Mo center into the H-H bond, forming a dihydride product. We find that this reaction occurs up to Mo oxidation states of +4. For the subsulfides containing a second metal in a sufficiently low oxidation state, a second insertion of H2 occurs, leading to a tetrahydride product. The reaction mechanisms of the sulfides are found to be very similar, albeit slightly higher energetically to those of the analogous oxosulfides that are also observed at low abundances in the experiments. In addition, the experimental results show an overall reduction of hypersulfides in the presence of H2, suggesting loss of H2S neutral molecules.

5.
J Phys Chem A ; 123(10): 2040-2048, 2019 Mar 14.
Artigo em Inglês | MEDLINE | ID: mdl-30788965

RESUMO

The electronic and molecular structure of the CeB6 molecular unit has been probed by anion PE spectroscopy and DFT calculations to gain insight into structural and electronic relaxation on edge and corner sites of this ionic material. While boron in bulk lanthanide hexaboride materials assumes octahedral B63- units, the monomer assumes a less compact structure to delocalize the charge. Two competitive molecular structures were identified for the anion and neutral species, which include a boat-like structure and a planar or near-planar teardrop structure. Ce adopts different orbital occupancies in the two isomers; the boat-like structure has a 4f superconfiguration while the teardrop favors a 4f 6s occupancy. The B6 ligand in these structures carries a charge of -4 and -3, respectively. The teardrop structure, which was calculated to be isoenergetic with the boat structure, was most consistent with the experimental spectrum. B6-local orbitals crowd the energy window between the Ce 4f and 6s (HOMO) orbitals. A low-lying transition from the B-based orbitals is observed slightly less than 1 eV above the ground state. The results suggest that edge and corner conductivity involves stabilized, highly diffuse 6s orbitals or bands rather than the bulk-favored 5d band. High-spin and open-shell low-spin states were calculated to be very close in energy for both the anion and neutral, a characteristic that reflects how decoupled the 4f electron is from the B6 2p- and Ce 6s-based molecular orbitals.

6.
J Chem Phys ; 150(3): 034302, 2019 Jan 21.
Artigo em Inglês | MEDLINE | ID: mdl-30660161

RESUMO

Photoelectron imaging spectra of three alkenoxide radical anions (3-buten-1-oxide, 3-buten-2-oxide, and 2-propenoxide) are presented and analyzed with supporting results of density functional theory calculations. In all spectra, intense detachment features are observed at approximately 2 eV electron binding energy, which is similar to the electron affinities of saturated neutral alkoxy radicals [Ramond et al., J. Chem. Phys. 112, 1158 (2000)]. Photoelectron angular distributions suggest the presence of several overlapping transitions which are assigned to the X̃ and à states of multiple energetically competitive conformers. The term energy of the à state of the 2-propenoxy radical, 0.17 eV, is higher than that of 3-buten-2-oxy (0.13 eV) and 3-buten-1-oxy (0.05 eV) radicals. Comparing the butenoxy radicals, we infer that stronger interactions between the non-bonding O 2p orbitals and the π bond increase the splitting between the ground and the first excited state in the 3-buten-2-oxy radical relative to the 3-buten-1-oxy radical.

7.
J Phys Chem Lett ; 10(2): 144-149, 2019 Jan 17.
Artigo em Inglês | MEDLINE | ID: mdl-30569715

RESUMO

The photoelectron spectra of Sm2O- obtained over a range of photon energies exhibit anomalous changes in relative excited-state band intensities. Specifically, the excited-state transition intensities increase relative to the transition to the neutral ground state with decreasing photon energy, the opposite of what is expected from threshold effects. This phenomenon was previously observed in studies on several Sm-rich homo- and heterolanthanide oxides collected with two different harmonic outputs of a Nd:YAG (2.330 and 3.495 eV) [ J. Chem. Phys. 2017, 146, 194310]. We relate these anomalous intensities to populations of ground and excited anionic and neutrals states through the inspection of time-dependent perturbation theory within the adiabatic and sudden limits and for the first time show that transition intensities in photoelectron spectroscopy have a deep significance in gauging participation from excited states. We believe our results will have significance in the study of other electron-rich systems that have especially high density of accessible spin states.

8.
J Phys Chem A ; 122(51): 9894-9900, 2018 Dec 27.
Artigo em Inglês | MEDLINE | ID: mdl-30500186

RESUMO

A mass spectrometric analysis of the anionic and cationic species generated by laser ablation of graphite fluoride (GF) and graphite targets performed under identical sets of conditions is presented. Under conditions that produce typical C n- cluster mass distributions from ablation of graphite, the mass spectra of anionic species generated by ablation of GF are congested with overlapping stoichiometric patterns such as C nF2 n and C nF(2 n-2). Some of the molecular formulas for these clusters, such as C6F6, C6F12, and C7F8, are evocative of stable neutral fluorocarbons. Additionally, the GF-ablation generated mass peaks broaden at higher masses more than the graphite-based counterparts, which may indicate cluster fragmentation. Furthermore, a pattern of fragmentation via loss of CF2 is observed and is reminiscent of previous studies which determined CF2 loss during thermal decomposition. No species were seen in the mass spectra of the cationic species generated from laser ablation of GF, while, under the same conditions, typical C n+ cluster distributions were observed.

9.
J Phys Chem A ; 122(51): 9879-9885, 2018 Dec 27.
Artigo em Inglês | MEDLINE | ID: mdl-30518216

RESUMO

Boronyl (B≡O) ligands have garnered much attention as isoelectronic and isolobal analogues of CO and CN-, yet successful efforts in synthesizing metal boronyl complexes remain scarce. Anion photoelectron (PE) spectroscopy and density functional theory calculations were employed to investigate two small CeO2B x- ( x = 2, 3) complexes generated from laser ablation of a mixed Ce/B pressed powder target. The spectra reveal markedly different bonding upon incorporation of an additional B atom. Most interestingly, CeO2B2- was found to have a Ce(I) center coordinated to two monoanionic boronyl ligands in a bent geometry. This result was unexpected as previous studies suggest electron-rich metals are most suitable for stabilizing such ligands; furthermore, it is one of the first examples of an experimental metal-polyboronyl complex. Introducing another boron atom, however, favors a much different geometry in which Ce(II) coordinates an O2B33- unit through both the O and B atoms, which was evident in the markedly different PE spectra.

10.
J Chem Phys ; 149(5): 054305, 2018 Aug 07.
Artigo em Inglês | MEDLINE | ID: mdl-30089379

RESUMO

Lanthanide (Ln) oxide clusters have complex electronic structures arising from the partially occupied Ln 4f subshell. New anion photoelectron (PE) spectra of SmxCe3-xOy- (x = 0-3; y = 2-4) along with supporting results of density functional theory (DFT) calculations suggest interesting x and y-dependent Sm 4f subshell occupancy with implications for Sm-doped ionic conductivity of ceria, as well as the overall electronic structure of the heterometallic oxides. Specifically, the Sm centers in the heterometallic species have higher 4f subshell occupancy than the homonuclear Sm3Oy-/Sm3Oy clusters. The higher 4f subshell occupancy both weakens Sm-O bonds and destabilizes the 4f subshell relative to the predominantly O 2p bonding orbitals in the clusters. Parallels between the electronic structures of these small cluster systems with bulk oxides are explored. In addition, unusual changes in the excited state transition intensities, similar to those observed previously in the PE spectra of Sm2O- and Sm2O2- [J. O. Kafader et al., J. Chem. Phys. 146, 194310 (2017)], are also observed in the relative intensities of electronic transitions to excited neutral state bands in the PE spectra of SmxCe3-xOy- (x = 1-3; y = 2, 4). The new spectra suggest that the effect is enhanced with lower oxidation states and with an increasing number of Sm atoms, implying that the prevalence of electrons in the diffuse Sm 6s-based molecular orbitals and a more populated 4f subshell both contribute to this phenomenon. Finally, this work identifies challenges associated with affordable DFT calculations in treating the complex electronic structures exhibited by these systems, including the need for a more explicit treatment of strong coupling between the neutral and PE.

11.
Inorg Chem ; 57(15): 9167-9174, 2018 Aug 06.
Artigo em Inglês | MEDLINE | ID: mdl-30004678

RESUMO

We report a first-principles quantum chemical study of the mechanistic pathways for the hydrogen evolution reaction (HER) by the molecular electrocatalyst [(PY5Me2)Mo(S2)]2+. By determining the relative thermodynamics of many possible species, we propose a mechanism fully consistent with all experimental observations. We also show the presence of two close-lying spin surfaces with the high spin state having a slightly less favorable reactivity profile than the low spin state. The energy of the high spin state is related to the ease of reduction of the S2 moiety and can be disrupted by interaction between S2 and a Lewis base. From this understanding, an explanation for the nearly 400 000-fold increase in turnover frequency on Hg drop electrode compared to glassy carbon is demonstrated. A next-generation catalyst based on the same motif has been designed to stabilize the more reactive low spin state and improve catalytic function without the need of Hg. Calculations indicate that this new species would have greatly improved HER reactivity and operate at a similar overpotential as the original system.

12.
J Chem Phys ; 148(5): 054308, 2018 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-29421881

RESUMO

Recent studies on reactions between MoxOy- cluster anions and H2O/C2H4 mixtures revealed a complex web of addition, hydrogen evolution, and chemifragmentation reactions, with chemifragments unambiguously connected to cluster reactions with C2H4. To gain insight into the molecular-scale interactions along the chemifragmentation pathways, the anion photoelectron (PE) spectra of MoC2H2-, MoC4H4-, MoOC2H2-, and MoO2C2H2- formed directly in MoxOy- + C2H4 (x > 1; y ≥ x) reactions, along with supporting CCSD(T) and density functional theory calculations, are presented and analyzed. The complexes have spectra that are all consistent with η2-acetylene complexes, though for all but MoC4H4-, the possibility that vinylidene complexes are also present cannot be definitively ruled out. Structures that are consistent with the PE spectrum of MoC2H2- differ from the lowest energy structure, suggesting that the fragment formation is under kinetic control. The PE spectrum of MoO2C2H2- additionally exhibits evidence that photodissociation to MoO2- + C2H2 may be occurring. The results suggest that oxidative dehydrogenation of ethylene is initiated by Lewis acid/base interactions between the Mo centers in larger clusters and the π orbitals in ethylene.

13.
J Phys Chem A ; 122(1): 41-52, 2018 Jan 11.
Artigo em Inglês | MEDLINE | ID: mdl-29202242

RESUMO

To probe the mechanism of sacrificial reagents in catalytic processes, product distributions from MoxOy- clusters reacting individually with C2H4 and H2O are compared with those from reactions with a C2H4 + H2O mixture, with the thermodynamics explored computationally. These molecules were chosen to model production of H2 from H2O via H2O + C2H4 → H2 + CH3CHO, mediated by MoxOy- clusters. H2O is known to sequentially oxidize MoxOy- suboxide clusters while producing H2, resulting in less reactive clusters. MoxOy- (y ∼ x) clusters undergo chemi-fragmentation reactions with C2H4, with MoxOyC2Hz- complexes forming as the cluster oxidation state increases. Unique species observed in reactions with the C2H4 + H2O mixture, Mo2O5C2H2- and MoO3C2H4-, suggest that the internal energy gained in new Mo-O bond formation from oxidation by H2O opens additional reaction channels. C2H3O- is observed uniquely in reactions with the C2H4 + H2O mixture, giving indirect evidence of CH3CHO formation via the cluster mediated H2O + C2H4 → H2 + CH3CHO reaction; C2H3O- can form via dissociative electron attachment to CH3CHO. Calculations support mechanisms that invoke participation of two ethylene molecules on thermodynamically favorable pathways leading to experimentally observed products.

14.
J Chem Phys ; 147(10): 104303, 2017 Sep 14.
Artigo em Inglês | MEDLINE | ID: mdl-28915734

RESUMO

The anion photoelectron (PE) spectra of a range of small mono-cerium molecular species, along with the Ce2O4- and Ce3O6- stoichiometric clusters, are presented and analyzed with the support of density functional theory calculations. A common attribute of all of the neutral species is that the Ce centers in both the molecules and clusters are in the +4 oxidation state. In bulk ceria (CeO2), an unoccupied, narrow 4f band lies between the conventional valence (predominantly O 2p) and conduction (Ce 5d) bands. Within the CeO2-, CeO3H2-, and Ce(OH)4- series, the PE spectra and computational results suggest that the Ce 6s-based molecular orbital is the singly occupied HOMO in CeO2- but becomes destabilized as the Ce 4f-local orbital becomes stabilized with increasing coordination. CeO3-, a hyperoxide, undergoes photodissociation with 3.49 eV photon energy to form the stoichiometric neutral CeO2 and O-. In the CeO2-, Ce2O4- ,and Ce3O6- stoichiometric cluster series, the 6s destabilization with 4f stabilization is associated with increasing cluster size, suggesting that a bulk-like band structure may be realized with fairly small cluster sizes. The destabilization of the 6s-based molecular orbitals can be rationalized by their diffuse size relative to Ce-O bond lengths in a crystal structure, suggesting that 6s bands in the bulk may be relegated to the surface.

15.
J Phys Chem A ; 121(29): 5459-5467, 2017 Jul 27.
Artigo em Inglês | MEDLINE | ID: mdl-28671848

RESUMO

Anion photoelectron imaging was used to measure the photodetachment spectra of molecular complexes formed between O2- and a range of atmospherically relevant polar molecules, including species with a carbonyl group (acetone, formaldehyde) and alcohols (ethanol, propenol, butenol). Experimental spectra are analyzed using a combination of Franck-Condon simulations and electronic structure calculations. Strong charge-dipole interactions and H-bonding stabilize the complex anions relative to the neutrals, resulting in a ca. 1 eV increase in electron binding energy relative to bare O2-, an effect more pronounced in complexes with H-bonding. In addition, broken degeneracy of the O2-local πg orbitals in the complexes results in the stabilization of the low-lying excited O2 (a 1Δg)·[polar VOC] state relative to the ground O2 (X 3Σg-)·[polar VOC] state when compared to bare O2. The spectra of the O2-·[polar VOC] complexes exhibit less pronounced laser photoelectron angular distribution (PADs). The spectrum of O2-·formaldehyde is unique in terms of both spectral profile and PAD. On the basis of these experimental results in addition to computational results, the complex anion cannot be described as a distinct O2- anion partnered with an innocent solvent molecule; the molecules are more strongly coupled through charge delocalization. Overall, the results underscore how the symmetry of the O2 πg orbitals is broken by different polar partners, which may have implications for atmospheric photochemistry and models of solar radiation absorption that include collision-induced absorption.

16.
J Chem Phys ; 146(19): 194310, 2017 May 21.
Artigo em Inglês | MEDLINE | ID: mdl-28527471

RESUMO

The anion photoelectron (PE) spectra along with supporting results of density functional theory (DFT) calculations on SmO-, SmCeOy-, and Sm2Oy- (y = 1, 2) are reported and compared to previous results on CeO- [M. Ray et al., J. Chem. Phys. 142, 064305 (2015)] and Ce2Oy- (y = 1, 2) [J. O. Kafader et al., J. Chem. Phys. 145, 154306 (2016)]. Similar to the results on CexOy- clusters, the PE spectra of SmO-, SmCeOy-, and Sm2Oy- (y = 1, 2) all exhibit electronic transitions to the neutral ground state at approximately 1 eV e-BE. The Sm centers in SmO and Sm2O2 neutrals can be described with the 4f56s superconfiguration, which is analogous to CeO and Ce2O2 neutrals in which the Ce centers can be described with the 4f 6s superconfiguration (ZCe = ZSm - 4). The Sm center in CeSmO2, in contrast, has a 4f6 occupancy, while the Ce center maintains the 4f 6s superconfiguration. The less oxidized Sm centers in both Sm2O and SmCeO have 4f6 6s occupancies. The 4f6 subshell occupancy results in relatively weak Sm-O bond strengths. If this extra 4f occupancy also occurs in bulk Sm-doped ceria, it may play a role in the enhanced O2- ionic conductivity in Sm-doped ceria. Based on the results of DFT calculations, the heteronuclear Ce-Sm oxides have molecular orbitals that are distinctly localized Sm 4f, Sm 6s, Ce 4f, and Ce 6s orbitals. The relative intensity of two electronic bands in the PE spectrum of Sm2O- exhibits an unusual photon energy-dependence, and the PE spectrum of Sm2O2- exhibits a photon energy-dependent continuum signal between two electronic transitions. Several explanations, including the high magnetic moment of these suboxide species and the presence of low-lying quasi-bound anion states, are considered.

17.
J Chem Phys ; 145(15): 154306, 2016 Oct 21.
Artigo em Inglês | MEDLINE | ID: mdl-27782463

RESUMO

The anion photoelectron (PE) spectra of Ce2Oy- (y = 1, 2), Ce3Oy- (y = 0-4), Ce4Oy- (y = 0-2), and Ce5Oy- (y = 1, 2) are reported and analyzed with supporting results from density functional theory calculations. The PE spectra all exhibit an intense electronic transition to the neutral ground state, all falling in the range of 0.7 to 1.1 eV electron binding energy, with polarization dependence consistent with detachment from diffuse Ce 6s-based molecular orbitals. There is no monotonic increase in electron affinity with increasing oxidation. A qualitative picture of how electronic structure evolves with an oxidation state emerges from comparison between the spectra and the computational results. The electronic structure of the smallest metallic cluster observed in this study, Ce3, is similar to the bulk structure in terms of atomic orbital occupancy (4f 5d2 6s). Initial cerium cluster oxidation involves largely ionic bond formation via Ce 5d and O 2p orbital overlap (i.e., larger O 2p contribution), with Ce-O-Ce bridge bonding favored over Ce=O terminal bond formation. With subsequent oxidation, the Ce 5d-based molecular orbitals are depleted of electrons, with the highest occupied orbitals described as diffuse Ce 6s based molecular orbitals. In the y ≤ (x + 1) range of oxidation states, each Ce center has a singly occupied non-bonding 4f orbital. The PE spectrum of Ce3O4- is unique in that it exhibits a single nearly vertical transition. The highly symmetric structure predicted computationally is the same structure determined from Ce3O4+ IR predissociation spectra [A. M. Burow et al., Phys. Chem. Chem. Phys. 13, 19393 (2011)], indicating that this structure is stable in -1, 0, and +1 charge states. Spectra of clusters with x ≥ 3 exhibit considerable continuum signal above the ground state transition; the intensity of the continuum signal decreases with increasing oxidation. This feature is likely the result of numerous quasi-bound anion states or two-electron transitions possible in molecules with abundant nearly degenerate partially occupied orbitals.

18.
J Phys Chem A ; 120(40): 7828-7838, 2016 Oct 13.
Artigo em Inglês | MEDLINE | ID: mdl-27648607

RESUMO

The anion photoelectron imaging spectra of O2-·VOC and O4-·VOC (VOC = hexane, isoprene, benzene, and benzene-d6) complexes measured using 3.49 eV photon energy, along with the results of ab initio and density functional theory results are reported and analyzed. Photodetachment of these anionic complexes accesses neutrals that model collision complexes, offering a probe of the effects of symmetry-breaking collision events on the electronic structure of normally transparent neutral molecules. The energies of O2-·VOC spectral features compared to the bare O2- indicate that photodetachment of the anion accesses a modestly repulsive region of the O2-VOC potential energy surface, with subtle VOC dependence on the relative energies of the O2 (X 3Σg-)·VOC ground state and O2 (a 1Δg)·VOC excited state. In contrast, a significantly higher intensity of the transition to the O2 (a 1Δg)·VOC excited state relative to the O2 (X 3Σg-)·VOC ground state is observed for VOC = benzene, with a less pronounced effect observed for VOC = isoprene. Similar spectral effects are observed in the O4-·benzene and O4-·isoprene PE spectra. Several explanations are considered, with involvement of a temporary anion state emerging as the most plausible.

19.
J Chem Phys ; 145(4): 044317, 2016 Jul 28.
Artigo em Inglês | MEDLINE | ID: mdl-27475371

RESUMO

The electronic structures of several small Ce-Pt oxide complexes were explored using a combination of anion photoelectron (PE) spectroscopy and density functional theory calculations. Pt and Pt2 both accept electron density from CeO diatomic molecules, in which the cerium atom is in a lower-than-bulk oxidation state (+2 versus bulk +4). Neutral [CeO]Pt and [CeO]Pt2 complexes are therefore ionic, with electronic structures described qualitatively as [CeO(+2)]Pt(-2) and [CeO(+)]Pt2 (-), respectively. The associated anions are described qualitatively as [CeO(+)]Pt(-2) and [CeO(+)]Pt2 (-2), respectively. In both neutrals and anions, the most stable molecular structures determined by calculations feature a distinct CeO moiety, with the positively charged Ce center pointing toward the electron rich Pt or Pt2 moiety. Spectral simulations based on calculated spectroscopic parameters are in fair agreement with the spectra, validating the computationally determined structures. In contrast, when Pt is coupled with CeO2, which has no Ce-localized electrons that can readily be donated to Pt, the anion is described as [CeO2]Pt(-). The molecular structure predicted computationally suggests that it is governed by charge-dipole interactions. The neutral [CeO2]Pt complex lacks charge-dipole stabilizing interactions, and is predicted to be structurally very different from the anion, featuring a single Pt-O-Ce bridge bond. The PE spectra of several of the complexes exhibit evidence of photodissociation with Pt(-) daughter ion formation. The electronic structures of these complexes are related to local interactions in Pt-ceria catalyst-support systems.

20.
J Phys Chem A ; 120(9): 1508-19, 2016 Mar 10.
Artigo em Inglês | MEDLINE | ID: mdl-26878402

RESUMO

A systematic comparison of MxOy(-) + ROH (M = Mo vs W; R = Me vs Et) reaction rate coefficients and product distributions combined with results of calculations on weakly bound MxOy(-)·ROH complexes suggest that the overall reaction mechanism has three distinct steps, consistent with recently reported results on analogous MxOy(-) + H2O reactivity studies. MxOy(-) + ROH → MxOy+1(-) + RH oxidation reactions are observed for the least oxidized clusters, and MxOy(-) + ROH → MxOyROH(-) addition reactions are observed for clusters in intermediate oxidation states, as observed previously in MxOy(-) + H2O reactions. The first step is weakly bound complex formation, the rate of which is governed by the relative stability of the MxOy(-)·ROH charge-dipole complexes and the Lewis acid-base complexes. Calculations predict that MoxOy(-) clusters form more stable Lewis acid-base complexes than WxOy(-), and the stability of EtOH complexes is enhanced relative to MeOH. Consistent with this result, MoxOy(-) + ROH rate coefficients are higher than analogous WxOy(-) clusters. Rate coefficients range from 2.7 × 10(-13) cm(3) s(-1) for W3O8(-) + MeOH to 3.4 × 10(-11) cm(3) s(-1) for Mo2O4(-) + EtOH. Second, a covalently bound complex is formed, and anion photoelectron spectra of the several MxOyROH(-) addition products observed are consistent with hydroxyl-alkoxy structures that are formed readily from the Lewis acid-base complexes. Calculations indicate that addition products are trapped intermediates in the MxOy(-) + ROH → MxOy+1(-) + RH reaction, and the third step is rearrangement of the hydroxyl group to a metal hydride group to facilitate RH release. Trapped intermediates are more prevalent in MoxOy(-) reaction product distributions, indicating that the rate of this step is higher for WxOy+1RH(-) than for MoxOy+1RH(-). This result is consistent with previous computational studies on analogous MxOy(-) + H2O reactions predicting that barriers along the pathway in the rearrangement step are higher for MoxOy(-) reactions than for WxOy(-).

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