High-Resolution Photoelectron Spectroscopy of Vibrationally Excited Vinoxide Anions.

High-resolution photoelectron spectra of vibrationally pre-excited vinoxide anions (CH2CHO-) are reported using the recently developed IR-cryo-SEVI technique. This method is combined with a newly developed implementation of vibrational perturbation theory that can readily identify relevant anharmonic couplings among nearly degenerate vibrational states. IR-cryo-SEVI spectra are obtained by resonant infrared excitation of vinoxide anions via the fundamental C-O (ν4, 1566 cm-1) or isolated C-H (ν3, 2540 cm-1) stretching vibrations prior to photodetachment. Excitation of the ν4 mode leads to a well-resolved photoelectron spectrum that is in excellent agreement with a harmonic Franck-Condon simulation. Excitation of the higher-energy ν3 mode results in a more complicated spectrum that requires consideration of the calculated anharmonic resonances in both the anion and the neutral. From this analysis, information about the zeroth-order states that contribute to the nominal ν3 wave function in the anion is obtained. In the neutral, we observe anharmonic splitting of the ν3 fundamental into a polyad feature with peaks at 2737(22), 2 835(18), and 2910(12) cm-1, for which only the center frequency has been previously reported. Overall, 9 of the 12 fundamental frequencies of the vinoxy radical are extracted from the IR-cryo-SEVI and ground-state cryo-SEVI spectra, most of which are consistent with previous measurements. However, we provide a new estimate of the ν5 (CH2 scissoring) fundamental frequency at 1395(11) cm-1 and attribute the discrepancy with previously reported values to a Fermi resonance with the 2ν11 overtone (CH2 wagging).

Observation of resonances in the transition state region of the F + NH3 reaction using anion photoelectron spectroscopy.

The transition state of a chemical reaction is a dividing surface on the reaction potential energy surface (PES) between reactants and products and is thus of fundamental interest in understanding chemical reactivity. The transient nature of the transition state presents challenges to its experimental characterization. Transition-state spectroscopy experiments based on negative-ion photodetachment can provide a direct probe of this region of the PES, revealing the detailed vibrational structure associated with the transition state. Here we study the F + NH3 → HF + NH2 reaction using slow photoelectron velocity-map imaging spectroscopy of cryogenically cooled FNH3- anions. Reduced-dimensionality quantum dynamical simulations performed on a global PES show excellent agreement with the experimental results, enabling the assignment of spectral structure. Our combined experimental-theoretical study reveals a manifold of vibrational Feshbach resonances in the product well of the F + NH3 PES. At higher energies, the spectra identify features attributed to resonances localized across the transition state and into the reactant complex that may impact the bimolecular reaction dynamics.

High Resolution Photoelectron Spectroscopy of the Acetyl Anion.

High-resolution photoelectron spectra of cryogenically cooled acetyl anions (CH3CO-) obtained using slow photoelectron velocity-map imaging are reported. The high resolution of the photoelectron spectrum yields a refined electron affinity of 0.4352 ± 0.0012 eV for the acetyl radical as well as the observation of a new vibronic structure that is assigned based on ab initio calculations. Three vibrational frequencies of the neutral radical are measured to be 1047 ± 3 cm-1 (ν6), 834 ± 2 cm-1 (ν7), and 471 ± 1 cm-1 (ν8). This work represents the first experimental measurement of the ν6 frequency of the neutral. The measured electron affinity is used to calculate a refined value of 1641.35 ± 0.42 kJ mol-1 for the gas-phase acidity of acetaldehyde. Analysis of the photoelectron angular distributions provides insight into the character of the highest occupied molecular orbital of the anion, revealing a molecular orbital with strong d-character. Additionally, details of a new centroiding algorithm based on finite differences, which has the potential to decrease data acquisition times by an order of magnitude at no cost to accuracy, are provided.

Photoelectron spectroscopy of cryogenically cooled NiO2-via slow photoelectron velocity-map imaging.

High-resolution anion photoelectron spectra of cryogenically cooled NiO2- anions, obtained using slow photoelectron velocity-map imaging (cryo-SEVI), are presented in tandem with coupled cluster electronic structure calculations including relativistic effects. The experimental spectra encompass the X̃1Σg+ ← X̃2Πg, ã3Πg ← X̃2Πg, and Ã1Πg ← X̃2Πg photodetachment transitions of linear ONiO0/-, revealing previously unobserved vibrational structure in all three electronic bands. The high-resolution afforded by cryo-SEVI allows for the extraction of vibrational frequencies for each state, consistent with those previously measured in the ground state and in good agreement with scalar-relativistic coupled-cluster calculations. Previously unobserved vibrational structure is observed in the ã3Πg and Ã1Πg states and is tentatively assigned. Further, a refined electron affinity of 3.0464(7) eV for NiO2 is obtained as well as precise term energies for the ã and à states of NiO2 of 0.3982(7) and 0.7422(10) eV, respectively. Numerous Franck-Condon forbidden transitions involving the doubly degenerate ν2 bending mode are observed and ascribed to Herzberg-Teller coupling to an excited electronic state.

Structural Characterization of Nickel-Doped Aluminum Oxide Cations by Cryogenic Ion Trap Vibrational Spectroscopy.

Isolated nickel-doped aluminum oxide cations (NiOm)(Al2O3)n(AlO)+ with m = 1-2 and n = 1-3 are investigated by infrared photodissociation (IRPD) spectroscopy in combination with density functional theory and the single-component artificial force-induced reaction method. IRPD spectra of the corresponding He-tagged cations are reported in the 400-1200 cm-1 spectral range and assigned based on a comparison to calculated harmonic IR spectra of low-energy isomers. Simulated spectra of the lowest energy structures generally match the experimental spectra, but multiple isomers may contribute to the spectra of the m = 2 series. The identified structures of the oxides (m = 1) correspond to inserting a Ni-O moiety into an Al-O bond of the corresponding (Al2O3)1-3(AlO)+ cluster, yielding either a doubly or triply coordinated Ni2+ center. The m = 2 clusters prefer similar structures in which the additional O atom either is incorporated into a peroxide unit, leaving the oxidation state of the Ni2+ atom unchanged, or forms a biradical comprising a terminal oxygen radical anion Al-O•- and a Ni3+ species. These clusters represent model systems for under-coordinated Ni sites in alumina-supported Ni catalysts and should prove helpful in disentangling the mechanism of selective oxidative dehydrogenation of alkanes by Ni-doped catalysts.

Electronic structure of NdO via slow photoelectron velocity-map imaging spectroscopy of NdO--.

Electronically excited NdO is a possible product of the chemistry associated with the release of Nd into the ionosphere, and emission from these states may contribute to the observations following such experiments. To better characterize the energetics and spectroscopy of NdO, we report a combined experimental and theoretical study using slow photoelectron velocity-map imaging spectroscopy of cryogenically cooled NdO- anions (cryo-SEVI) supplemented by wave function-based quantum-chemical calculations. Using cryo-SEVI, we measure the electron affinity of NdO to be 1.0091(7) eV and resolve numerous transitions to low-lying electronic and vibrational states of NdO that are assigned with the aid of the electronic structure calculations. Additionally, temperature-dependent data suggest contributions from the (2)4.5 state of NdO- residing 2350 cm-1 above the ground anion state. Photodetachment to higher-lying excited states of NdO is also reported, which may help to clarify observations from prior release experiments.

High-Resolution Photoelectron Spectroscopy of Vibrationally Excited OH.

The effect of vibrational pre-excitation of anions on their photoelectron spectra is explored, combining slow photoelectron velocity-map imaging of cryogenically cooled anions (cryo-SEVI) with tunable IR radiation to pre-excite the anions. This new IR cryo-SEVI method is applied to OH- as a test system, where the R(0) transition of the hydroxyl anion (3591.53 cm-1) is pumped. Vibrational excitation induces a 30% depletion in photodetachment signal from the v = 0, J = 0 ground state of the anion and the appearance of all five allowed, rotationally resolved photodetachment transitions from the OH- (v = 1, J = 1) level, each with peak widths between 1 and 2 cm-1. By scanning the IR laser, IR cryo-SEVI can also serve as a novel action technique to obtain the vibrational spectrum of OH-, giving an experimental value for the R(0) transition of 3591(1.2) cm-1.

Unveiling the coexistence of cis- and trans-isomers in the hydrolysis of ZrO2: A coupled DFT and high-resolution photoelectron spectroscopy study.

High-resolution anion photoelectron spectroscopy of the ZrO3H2 - and ZrO3D2 - anions and complementary electronic structure calculations are used to investigate the reaction between zirconium dioxide and a single water molecule, ZrO2 0/- + H2O. Experimental spectra of ZrO3H2 - and ZrO3D2 - were obtained using slow photoelectron velocity-map imaging of cryogenically cooled anions, revealing the presence of two dissociative adduct conformers and yielding insight into the vibronic structure of the corresponding neutral species. Franck-Condon simulations for both the cis- and trans-dihydroxide structures are required to fully reproduce the experimental spectrum. Additionally, it was found that water-splitting is stabilized more by ZrO2 than TiO2, suggesting Zr-based catalysts are more reactive toward hydrolysis.

High-Resolution Photoelectron Spectroscopy of Cryogenically Cooled NO3Ì .

High-resolution anion photoelectron spectra of cryogenically cooled NO3̅ anions obtained using slow photoelectron velocity-map imaging are presented and provide new insight into the vibronic structure of the corresponding neutral radical. A combination of improved spectral resolution, measurement of energy-dependent intensity effects, temperature control, and comparison to theory allows for full assignment of the vibronic features observed in this spectrum. We obtain a refined electron affinity of 3.9289(14) eV for NO3. Further, the appearance of Franck-Condon forbidden transitions from vibrationally cold anions to neutral states with excitation along the NO3 ν4 mode confirms that these features arise from vibronic coupling with the B̃2E' excited state of NO3 and are not hot bands, as has been suggested. Together, the suite of experimental and simulated results provides clear evidence that the ν3 fundamental of NO3 resides near 1050 cm-1, addressing a long-standing controversy surrounding this vibrational assignment.

Photoelectron spectra of Al2O2- and Al3O3-via slow electron velocity-map imaging.

High-resolution photoelectron spectra of cryogenically-cooled Al2O2- and Al3O3- cluster anions are obtained using slow electron velocity-map imaging. These spectra show vibrationally-resolved detachment from the (X[combining tilde]2B3u) ground state of Al2O2- to the X[combining tilde]1Ag and ã3B3u neutral electronic states, giving an electron affinity of 1.87904(4) eV for neutral Al2O2 and a term energy of 0.4938(4) eV for the triplet excited state. Additionally, there is evidence for autodetachment from photoexcited anions as well as influences from vibronic coupling between excited states of the neutral Al2O2 cluster. Detachment from both the "kite" and "book" isomers of Al3O3- is observed, yielding electron affinities of 2.0626(4) and 2.792(3) eV for the corresponding neutral isomers. Experiments carried out at different anion temperatures suggest that the two anionic isomers are nearly isoenergetic but clearly identify the kite isomer as the global minimum structure, in contrast to prior studies. This finding is supported by density functional theory calculations, which show that the relative ordering of the anion isomers is sensitive to basis set size; calculations for the anion isomers at the B3LYP/aug-cc-pVQZ level find the kite isomer to lie 0.011 eV below the book isomer.

Slow photoelectron velocity-map imaging of cold C 7 - and C 9 - .

High-resolution anion photoelectron spectra of cryogenically cooled C7 - and C9 - clusters obtained using slow photoelectron velocity-map imaging are presented, providing insight into the vibronic structure of neutral C7 and C9. These spectra yield accurate measurements of vibrational frequencies for the neutral clusters as well as electron affinities of 3.3517(4) and 3.6766(14) eV for C7 and C9, respectively. In the C7 - spectrum, transitions involving the previously unreported v 1 and v 2 symmetric stretching modes, as well as the v 9, v 10, and v 11 asymmetric bending modes, are assigned. Spin-orbit splitting is observed for several transitions in this spectrum, giving an energy difference of 28(6) cm-1 between the Π 1 / 2 g 2 and Π 3 / 2 g 2 spin-orbit levels of the C7 - anion. In the spectrum of C9 -, transitions involving the previously unreported symmetric stretch v 1 and the asymmetric bend v 11 are observed. In both spectra, several features are assigned to Franck-Condon forbidden transitions involving the doubly degenerate v 10 and v 11 modes of C7 and the v 13 and v 14 modes of C9. The appearance of these transitions is attributed to Herzberg-Teller coupling between the electronic states of the neutral clusters. Additional FC-forbidden transitions to states previously observed in gas-phase infrared experiments are observed and attributed to vibronic coupling between the electronic states of the anion, resulting in non-totally symmetric character in the anion's full vibrational ground state. Finally, consideration of the energy dependence of detachment cross sections and Dyson orbital analyses reveal that addition of more carbon atoms to the linear chain results in photodetachment from delocalized molecular orbitals with increasing nodal structure, leading to threshold photodetachment cross sections that differ considerably from simple symmetry considerations.

High-resolution photoelectron spectroscopy of TiO3H2-: Probing the TiO2- + H2O dissociative adduct.

Slow electron velocity-map imaging spectroscopy of cryogenically cooled TiO3H2- anions is used to probe the simplest titania/water reaction, TiO20/- + H2O. The resultant spectra show vibrationally resolved structure assigned to detachment from the cis-dihydroxide TiO(OH)2- geometry based on density functional theory calculations, demonstrating that for the reaction of the anionic TiO2- monomer with a single water molecule, the dissociative adduct (where the water is split) is energetically preferred over a molecularly adsorbed geometry. This work represents a significant improvement in resolution over previous measurements, yielding an electron affinity of 1.2529(4) eV as well as several vibrational frequencies for neutral TiO(OH)2. The energy resolution of the current results combined with photoelectron angular distributions reveals Herzberg-Teller coupling-induced transitions to Franck-Condon forbidden vibrational levels of the neutral ground state. A comparison to the previously measured spectrum of bare TiO2- indicates that reaction with water stabilizes neutral TiO2 more than the anion, providing insight into the fundamental chemical interactions between titania and water.

Autodetachment from Vibrationally Excited Vinylidene Anions.

Slow electron velocity-map imaging of the cryogenically cooled H2CC¯ anion reveals a strong dependence of its high-resolution photoelectron spectrum on detachment photon energy in two specific ranges, from 4000 to 4125 cm-1 and near 5020 cm-1. This effect is attributed to vibrational excitation of the anion followed by autodetachment to H2CC + e¯. In the lower energy range, the electron kinetic energy (eKE) distributions are dominated by two features that occur at constant eKEs of 114(3) and 151.9(14) cm-1 rather than constant electron binding energies, as is typically seen for direct photodetachment. These features are attributed to ΔJ = ΔK = 0 autodetachment transitions from two vibrationally excited anion states. The higher energy resonance autodetaches to neutral eigenstates with amplitude in the theoretically predicted shallow well lying along the vinylidene-acetylene isomerization coordinate. Calculations provide assignments of all autodetaching anion states and show that the observed autodetachment is facilitated by an intersection of the anion and neutral surfaces.

Encoding of vinylidene isomerization in its anion photoelectron spectrum.

Vinylidene-acetylene isomerization is the prototypical example of a 1,2-hydrogen shift, one of the most important classes of isomerization reactions in organic chemistry. This reaction was investigated with quantum state specificity by high-resolution photoelectron spectroscopy of the vinylidene anions H2CC- and D2CC- and quantum dynamics calculations. Peaks in the photoelectron spectra are considerably narrower than in previous work and reveal subtleties in the isomerization dynamics of neutral vinylidene, as well as vibronic coupling with an excited state of vinylidene. Comparison with theory permits assignment of most spectral features to eigenstates dominated by vinylidene character. However, excitation of the ν6 in-plane rocking mode in H2CC results in appreciable tunneling-facilitated mixing with highly vibrationally excited states of acetylene, leading to broadening and/or spectral fine structure that is largely suppressed for analogous vibrational levels of D2CC.

Feshbach resonances in the exit channel of the F + CH3OH → HF + CH3O reaction observed using transition-state spectroscopy.

The transition state governs how chemical bonds form and cleave during a chemical reaction and its direct characterization is a long-standing challenge in physical chemistry. Transition state spectroscopy experiments based on negative-ion photodetachment provide a direct probe of the vibrational structure and metastable resonances that are characteristic of the reactive surface. Dynamical resonances are extremely sensitive to the topography of the reactive surface and provide an exceptional point of comparison with theory. Here we study the seven-atom F + CH3OH → HF + CH3O reaction using slow photoelectron velocity-map imaging spectroscopy of cryocooled CH3OHF- anions. These measurements reveal spectral features associated with a manifold of vibrational Feshbach resonances and bound states supported by the post-transition state potential well. Quantum dynamical calculations yield excellent agreement with the experimental results, allow the assignment of spectral structure and demonstrate that the key dynamics of complex bimolecular reactions can be captured with a relatively simple theoretical framework.

Slow photoelectron velocity-map imaging of cold tert-butyl peroxide.

Photoelectron spectra of cryogenically cooled X∼1A' tert-butyl peroxide anions are obtained using slow electron velocity-map imaging. The spectra show highly structured bands corresponding to detachment to the X∼2A″ and A∼2A' electronic states of the neutral radical and represent a notable improvement in resolution over previous photoelectron spectra. We report an electron affinity of 1.1962(20) eV and a term energy T0(A∼2A') of 0.9602(24) eV for the tert-butyl peroxy radical. New vibrational structure is resolved, providing several frequencies for both neutral states. Additionally, the threshold behavior of the photodetachment cross section is investigated within the context of Dyson orbital calculations.

A complementary palette of NanoCluster Beacons.

NanoCluster Beacons (NCBs), which use few-atom DNA-templated silver clusters as reporters, are a type of activatable molecular probes that are low-cost and easy to prepare. While NCBs provide a high fluorescence enhancement ratio upon activation, their activation colors are currently limited. Here we report a simple method to design NCBs with complementary emission colors, creating a set of multicolor probes for homogeneous, separation-free detection. By systematically altering the position and the number of cytosines in the cluster-nucleation sequence, we have tuned the activation colors of NCBs to green (C8-8, 460 nm/555 nm); yellow (C5-5, 525 nm/585 nm); red (C3-4, 580 nm/635 nm); and near-infrared (C3-3, 645 nm/695 nm). At the same NCB concentration, the activated yellow NCB (C5-5) was found to be 1.3 times brighter than the traditional red NCB (C3-4). Three of the four colors (green, yellow, and red) were relatively spectrally pure. We also found that subtle changes in the linker sequence (down to the single-nucleotide level) could significantly alter the emission spectrum pattern of an NCB. When the length of linker sequences was increased, the emission peaks were found to migrate in a periodic fashion, suggesting short-range interactions between silver clusters and nucleobases. Size exclusion chromatography results indicated that the activated NCBs are more compact than their native duplex forms. Our findings demonstrate the unique photophysical properties and environmental sensitivities of few-atom DNA-templated silver clusters, which are not seen before in common organic dyes or luminescent crystals.

DNA/RNA Detection Using DNA-Templated Few-Atom Silver Nanoclusters.

DNA-templated few-atom silver nanoclusters (DNA/Ag NCs) are a new class of organic/inorganic composite nanomaterials whose fluorescence emission can be tuned throughout the visible and near-IR range by simply programming the template sequences. Compared to organic dyes, DNA/Ag NCs can be brighter and more photostable. Compared to quantum dots, DNA/Ag NCs are smaller, less prone to blinking on long timescales, and do not have a toxic core. The preparation of DNA/Ag NCs is simple and there is no need to remove excess precursors as these precursors are non-fluorescent. Our recent discovery of the fluorogenic and color switching properties of DNA/Ag NCs have led to the invention of new molecular probes, termed NanoCluster Beacons (NCBs), for DNA detection, with the capability to differentiate single-nucleotide polymorphisms by emission colors. NCBs are inexpensive, easy to prepare, and compatible with commercial DNA synthesizers. Many other groups have also explored and taken advantage of the environment sensitivities of DNA/Ag NCs in creating new tools for DNA/RNA detection and single-nucleotide polymorphism identification. In this review, we summarize the recent trends in the use of DNA/Ag NCs for developing DNA/RNA sensors.