Anion Ground State Research Articles

Temperature-Dependent Photoelectron Spectroscopy of Methyl Benzoate Anions: Observation of Steric Effect in o-Methyl Benzoate

Temperature-dependent photoelectron spectra of benzoate anion (C6H5CO2(-)) and its three methyl-substituted isomers (o-, m-, p-CH3C6H4CO2(-)) have been obtained using a newly developed low-temperature photoelectron spectroscopy apparatus that features an electrospray source and a cryogenically controlled ion trap. Detachment channels due to removing electrons from the carboxylate group and benzene ring pi electrons were distinctly observed. Well-resolved vibrational structures were obtained in the lower binding energy region due to the OCO bending modes, except for o-CH3C6H4CO2(-), which yielded broad spectra even at the lowest ion trap temperature (18 K). Theoretical calculations revealed a large geometry change in the OCO angles between the anion and neutral ground states, consistent with the broad ground-state bands observed for all species. A strong steric effect was observed between the carboxylate and the methyl group in o-CH3C6H4CO2(-), such that the -CO2(-) group is pushed out of the plane of the benzene ring by approximately 25 degrees and its internal rotational barrier is significantly reduced. The low rotational barrier in o-CH3C6H4CO2(-), which makes it very difficult to be cooled vibrationally, and the strong coupling between the OCO bending and CO2 torsional modes yielded the broad PES spectra for this isomer. It is shown that there is no C-H...O hydrogen bond in o-CH3C6H4CO2(-), and the interaction between the carboxylate and methyl groups in this anion is found to be repulsive in nature.

Electronic effects in SERS by liquid water

AbstractBy exploiting the attenuated total reflection (ATR)–Otto configuration, surface‐enhanced Raman scattering of water at monocrystalline copper and silver electrodes in 0.1 M Na2SO4 was observed at cathodic electrode potentials with hydrogen evolution at a laser wavelength of 647.1 nm. The proposed enhancement model is based on the condition that the upshift of the Fermi level of the electrode and the absorption of one laser photon allow an electron of the electrode at the Fermi level to reach the lower level of the energy band of delocalized electrons in liquid water. The electron interaction with the water molecules in the first hydration shell of cations in the Helmholtz plane is so‐called impulse scattering, which is the short‐time, non‐resonant excursion to the anionic ground state of molecules known for a long time in low‐energy electron scattering from free molecules. Copyright © 2005 John Wiley & Sons, Ltd.

Electron Affinities of BN, NO and NF: Coupled Cluster and Multireference Configuration Interaction Calculations

The accurate adiabatic electron affinities (EA) of the BN, NO and NF molecules have been determined using the coupled cluster approach and multireference configuration interaction methods. By combining large doubly augmented correlation-consistent basis sets (through the sextuple zeta) and complete basis set extrapolations with corrections for core-valence correlation and relativistic effects, we find that the RCCSD(T) method gives EA(BN) = 3.153 eV in very close agreement with experiment and predicts EA(NF) = 0.247 eV. The RCCSD(T) and UCCSD(T) EA(NO) results, 0.008 and 0.031 eV, bracket the experimental value. For both the neutral and anionic ground state species the usual spectroscopic constants were derived.

Uncovering the hidden ground state of green fluorescent protein

The fluorescence properties of GFP are strongly influenced by the protonation states of its chromophore and nearby amino acid side chains. In the ground state, the GFP chromophore is neutral and absorbs in the near UV. Upon excitation, the chromophore is deprotonated, and the resulting anionic chromophore emits its green fluorescence. So far, only excited-state intermediates have been observed in the GFP photocycle. We have used ultrafast multipulse control spectroscopy to prepare and directly observe GFP's hidden anionic ground-state intermediates as an integral part of the photocycle. Combined with dispersed multichannel detection and advanced global analysis techniques, the existence of two distinct anionic ground-state intermediates, I(1) and I(2), has been unveiled. I(1) and I(2) absorb at 500 and 497 nm, respectively, and interconvert on a picosecond timescale. The I(2) intermediate has a lifetime of 400 ps, corresponding to a proton back-transfer process that regenerates the neutral ground state. Hydrogen/deuterium exchange of the protein leads to a significant increase of the I(1) and I(2) lifetimes, indicating that proton motion underlies their dynamics. We thus have assessed the complete chain of reaction intermediates and associated timescales that constitute the photocycle of GFP. Many elementary processes in biology rely on proton transfers that are limited by slow diffusional events, which seriously precludes their characterization. We have resolved the true reaction rate of a proton transfer in the molecular ground state of GFP, and our results may thus aid in the development of a generic understanding of proton transfer in biology.

Addition of water to Al5O4− determined by anion photoelectron spectroscopy and quantum chemical calculations

The anion photoelectron spectra of Al5O4- and Al5O5H2- are presented and interpreted within the context of quantum chemical calculations on these species. Experimentally, the electron affinities of these two molecules are determined to be 3.50(5) eV and 3.10(10) eV for the bare and hydrated cluster, respectively. The spectra show at least three electronic transitions crowded into a 1 eV energy window. Calculations on Al5O4- predict a highly symmetric near-planar structure with a singlet ground state. The neutral structure calculated to be most structurally similar to the ground state structure of the anion is predicted to lie 0.15 eV above the ground state structure of the neutral. The lowest energy neutral isomer does not have significant Franck-Condon overlap with the ground state of the anion. Dissociative addition of water to Al5O4- is energetically favored over physisorption. The ground state structure for the Al5O4- +H(2)O product forms when water adds to the central Al atom in Al5O4- with -H migration to one of the neighboring O atoms. Again, the ground state structures for the anion and neutral are very different, and the PE spectrum represents transitions to a higher-lying neutral structure from the ground state anion structure.

Jahn−Teller Effect versus Spin−Orbit Coupling in X2E CH3S: An ab Initio Study by the Equation-of-Motion Coupled Cluster Method and Multiconfiguration Quasi-Degenerate Second-Order Perturbation Theory

The equation-of-motion coupled cluster method (EOMIP) using a reference Slater determinant based on the CH3S--anion ground state with basis sets of triple- and quadruple-ζ quality was applied for numerical optimizations of geometries of stationary points on adiabatic Jahn−Teller surfaces of the X2E ground state of CH3S crossing at the C3v symmetry nuclear configuration. Calculations by the multireference configuration interaction method and multiconfiguration quasi-degenerate second-order perturbation (PT2) theory were also performed with use of a complete active space CASSCF reference wave function. The linear and quadratic Jahn−Teller constants were computed for each of three Jahn−Teller active modes. The potential surfaces corresponding to spin−orbit states 2E3/2 and 2E1/2 were obtained with eigenenergies of the full Breit−Pauli spin−orbit operator with a PT2−CASSCF reference wave function. The one- and two-electron scalar relativistic effects were included in CASSCF and spin−orbit calculations. The calculated Jahn−Teller stabilization energy, the barrier to pseudorotation, and the spin−orbit splitting are 93, 15, and 358 cm-1, respectively. The Jahn−Teller distortions are totally quenched by the strong spin−orbit coupling. The recommended values of the geometry parameters of CH3S are Re(CS) = 1.794 Å, Re(CH) = 1.087 Å, and αe(HCS) = 109.8°.

Negative Ion Photoelectron Spectroscopy of SiN-

Negative ion photoelectron spectra of SiN- have been recorded using the 355 nm (3.493 eV) and 266 nm (4.661 eV) photodetachment wavelengths. The spectra exhibit resolved vibrational features corresponding to transitions to the X 2Σ+ and A 2Π states of SiN. Franck−Condon analyses yield the first experimental spectroscopic parameters, re and ωe, for the anion ground state, X 1Σ+. We also determined the first experimental adiabatic electron affinity as 2.949 ± 0.008 eV. The anion dissociation energy D0(SiN-) is then obtained from the electron affinities of Si and SiN and the dissociation energy of the neutral.

C6- electronic relaxation dynamics probed via time-resolved photoelectron imaging.

Anion time-resolved photoelectron imaging has been used to investigate the electronic relaxation dynamics of C(6) (-) following excitation of the C (2)Pi(g)<--X (2)Pi(u) and 2 (2)Pi(g)<--X (2)Pi(u) 0(0) (0) transitions at 607 and 498 nm, respectively. Analysis of evolving photodetachment energy distributions reveals differing relaxation pathways from these prepared states. Specifically, the C (2)Pi(g) 0(0) level relaxes on a time scale of 620+/-30 fs to vibrationally hot ( approximately 2.0 eV) anion ground state both directly and indirectly through vibrationally excited levels of the intermediate-lying A (2)Sigma(g) (+) state that decay with a time scale of 2300+/-200 fs. In contrast, the 2 (2)Pi(g) 0(0) level relaxes much more quickly (<100 fs) to vibrationally hot ( approximately 2.5 eV) anion ground state directly and with transient population accumulation in the A (2)Sigma(g) (+), B (2)Sigma(u) (+), and C (2)Pi(g) electronic levels, as determined by spectral and time-scale analyses. This work also presents the experimental observation of the optically inaccessible B (2)Sigma(u) (+) state, which is found to have an electronic term value of 1.41+/-0.05 eV.

Species with negative electron affinity and standard DFT methodsElectronic supplementary information (ESI) available: (i) Details of calculations (total energies, zero point corrections, etc.) and correlations obtained with different basis sets. (ii) Color graphics: structure, dipole, SOMO diagram and spin densities. (iii) xyz coordinates of all species. See http://www.rsc.org/suppdata/cp/b4/b403898c/

Concerned about the feasibility of traditional bound-electron models to properly describe ground state anion surfaces, we calculated the negative electron affinity of a representative set of compounds, finding a good correlation between the experimental data and theoretical values obtained with the popular B3LYP functional and Pople′s relatively inexpensive standard basis sets (6-311+G(2df,p)//6-31+G*) in the cases of valence anions.

Ground- and excited-state proton transfer reaction of 3-hydroxyflavone in dimyristoylphosphatidylcholine liposome membrane.

3-Hydroxyflavone (3HF), a molecule that exhibits excited-state intramolecular proton transfer, has been studied for its fluorescence characteristics in dimyristoylphosphatidylcholine (DMPC) liposome membrane. 3HF partitions to the lipid bilayer membrane with a reasonably large partition coefficient. On excitation at 417 nm, a weak emission from the ground-state anion species was observed at 483 nm, whereas excitation at absorption maxima (345 nm) gives the usual intense fluorescence of the phototautomeric emission at 530 nm. In this article, we report the observation of a ground-state proton transfer reaction of 3HF in DMPC liposome membrane.

Understanding Nonequilibrium Solute and Solvent Motions through Molecular Projections: Computer Simulations of Solvation Dynamics in Liquid Tetrahydrofuran (THF)

In this paper, we investigate the solvation dynamics of the weakly polar organic solvent tetrahydrofuran (THF) via classical molecular dynamics simulation. We find that the relaxation dynamics of all of the rotational and translational degrees of freedom of neat THF occur on similar time scales and have similar power spectra, making it impossible to use spectral density analysis to discern which specific molecular motions are involved in solvation. Instead, we probe the molecular origins of solvation dynamics using a nonequilibrium projection formalism that we originally outlined in M. J. Bedard-Hearn et al., J. Phys. Chem. A 2003, 107 (24), 4773. Here, we expand this formalism and use it to study the nonequilibrium solvation dynamics for a model reaction in THF in which a charge is removed from an anionic Lennard-Jones (LJ) solute, leaving behind a smaller neutral atom. The solute parameters are chosen to model the photodetachment of an electron from a sodium anion, Na- → Na0, to compare to the results of ultrafast spectroscopic experiments of this reaction being performed in our lab. We are able to explain the hidden breakdown of linear response for this system that we uncovered in our previous work in terms of the dynamical properties of the neat liquid and the structural properties of the solutions. In particular, our nonequilibrium projection analysis shows that four distinct solvation mechanisms are operative: (1) a rapid relaxation (t ≤ 700 fs) caused by longitudinal translational motions that dramatically change the local solvation structure; (2) a slower relaxation (t > 700 fs) caused by diffusive longitudinal translational motions that completes the transformation of the long-range solute−solvent packing; (3) an early-time relaxation (t < 500 fs) caused by solvent rotational motions that destabilizes the (unoccupied) anion ground state via Coulomb interactions; (4) a longer time process (t ≥ 500 fs) caused by solvent rotational motions that increases the solvation energy gap. This last process results from the LJ interaction component of the solvent rotational motions, as the nonequilibrium dynamics bring smaller THF−oxygen sites closer to the excited solute in place of larger THF−methylene sites. Overall, the simulations indicate that nonequilibrium solvation dynamics involves cooperative motions of both the solute and solvent and consists of multiple competing relaxation processes that can affect the solvation energy gap in opposite directions.

Structural and electronic properties of iron monoxide clusters FenO and FenO− (n=2–6): A combined photoelectron spectroscopy and density functional theory study

We report a combined anion photoelectron spectroscopy and density functional theory (DFT) study on a series of iron monoxide clusters, FenO (n=2–6). Well-resolved photoelectron spectra were obtained for FenO− at variable detachment energies, allowing the ground state and numerous low-lying excited states of FenO to be observed. Sharp threshold photoelectron features were obtained for each species, which suggest rather small geometry changes between the anion and neutral ground states for the monoxide clusters and allows the electron affinities of the neutral clusters to be measured accurately. Extensive DFT calculations using the generalized gradient approximation were carried out for both FenO and FenO−. Optimized geometries of the ground and lowest excited states of both the anion and neutral species are reported along with the ground-state vibrational frequencies and fragmentation energies. Theoretical electron affinities were compared with the experimental measurements to verify the ground states of the iron monoxide clusters obtained from the DFT calculations.

Structure of the hexafluorobenzene anion

The D 6 h , D 2 h (two), C 2 v , and D 2 structures of the hexafluorobenzene radical anion (C 6F 6 −) were studied by using density functional B3LYP method in conjunction with 6-31+G(d) and 6-311+G(d) basis sets. The B3LYP geometry optimization and frequency analysis calculations indicate that the C 2v( 2 A 1) structure has the lowest energy and represents an energy minimum. Adiabatic electron affinity value of the C 6F 6 molecule with respect to the anionic ground state (C 2v( 2 A 1)) is predicted to be 0.72–0.76 eV, in good agreement with the experimental data. Isotropic hyperfine coupling constants on the F- and 13C-centers ( a(F) and a(C)) in the C 2v( 2 A 1) structure were calculated by using ab initio MP2 as well as B3LYP methods, and averages of the calculated a(F) and a(C) values are in good agreement with the experimental coupling values. All these results indicate that the C 6F 6 − anion has a C 2 v structure and is in the 2A 1 ground state. The B3LYP calculations confirm that the D 2( 2 A) structure be the transition state for the pseudo-rotation process of the C 2v( 2 A 1) structure.

Electronic and Structural Evolution of Monoiron Sulfur Clusters, FeSn- and FeSn (n = 1−6), from Anion Photoelectron Spectroscopy

We report a photoelectron spectroscopic investigation of a series of monoiron−sulfur clusters FeSn- (n = 1−6) at various photon energies. Vibrationally resolved spectra were measured for FeS- and FeS3-. A wealth of electronic structure information was obtained for FeS and were tentatively assigned, yielding a 5Δ ground state for FeS and a 7Σ+ and a 5Δ excited state at 0.675 and 1.106 eV above the ground state, respectively. Franck−Condon factor simulations were performed for the vibrationally resolved 5Δ ground state and the 5Δ excited state, yielding an Fe−S bond length of 2.18 and 2.29 A for the anion ground state and the 5Δ excited state, respectively, as well as a vibrational temperature of 180 K for the anion. The electron affinities (EA's) of FeSn were measured to be 1.725 ± 0.10, 3.222 ± 0.009, 2.898 ± 0.008, 3.129 ± 0.008, 3.262 ± 0.010, and 3.52 ± 0.02 eV for n = 1−6, respectively. A significant EA increase was only observed from FeS to FeS2, whereas all larger species FeSn (n = 3−6) possess EA's...

Photoelectron spectroscopy of PbS −

The photoelectron spectrum of the gas phase diatomic anion, PbS − has been recorded with 2.540 eV photons. Transitions from the X 2 Π PbS − anion ground state to the X 1 Σ neutral PbS ground state are observed and assigned. The adiabatic electron affinity of PbS was determined to be 8461 cm −1 (1.049 eV). From this value and an energetic cycle, the dissociation energy of PbS − dissociating into the ground states of Pb and S − was found to be 55.4 kcal/mole (2.40 eV). From a Franck–Condon analysis of the spectrum, the anion’s bond length, vibrational frequency, and anharmonicity constant were determined to be r e=2.390 A ̊ , ω e=367 cm −1 , ω eχ e=2.2 cm −1 , respectively.

High-resolution study of dissociative electron attachment to dipolar molecules atlow energies: CH2Br2 and CCl3Br

Using the laser photoelectron attachment method with an energy width ≤ 1 meV at lowenergies (≤ 172meV) and an electron beam method (energy width about 70 meV) at higherenergies (up to 2 eV), we have determined absolute cross sections σe (E)for dissociative electron attachment to the dipolar molecules CH2Br2 andCCl3Br by normalization to known electron attachment rate coefficients.At thresholds for vibrational excitation of the CH2Br2 molecule,the DA cross section exhibits pronounced structure due to couplingof the attachment process with scattering channels; below the ν3 = 1onset for the symmetric CBr2 stretch vibration, a clear vibrationalFeshbach resonance is observed. At low energies the cross sections σe (E)show an energy dependence between E−1/2 andE−1,in essential agreement with predictions of an extended Vogt–Wannier (EVW)capture model which includes the long-range electron–dipole interaction inaddition to the polarization force. The determined absolute values are, however,substantially smaller than those predicted by the EVW model. SemiempiricalR-matrixcalculations have been carried out which show that at low electron energies(E < 0.15eV) Br− formation from CCl3Br proceeds by s-wave attachment to the anionground state while a broad peak, observed around 0.6 eV and evolvingpredominantly into the Cl− channel, is due to an excited anion state. Comparisonsare made with cross sections and rate coefficients obtained in previousphotoelectron attachment work (TPSA) and in electron beam as well asswarm experiments. Based on our joint experimental results for σe (E),we report the electron temperature dependence of the rate coefficientske (Te) forfree electron attachment involving a Maxwellian electron ensemble and a gas at room temperature(TG = 300K).

Excited States of Iodide Anions in Water: A Comparison of the Electronic Structure in Clusters and in Bulk Solution

A new computational approach for calculating charger-transfer-to-solvent (CTTS) states of anions in polar solvents is presented. This is applied to the prototypical aqueous iodide system when the anion is placed in the interior or at the gas−liquid interface of a bulk water solution or hydrated in small gas phase clusters. The experimental vertical detachment energies and CTTS transition energies are quantitatively reproduced without any adjustable parameters. The representative shapes of bulk CTTS wave functions are shown for the first time and compared with cluster excited states. The calculations start with an equilibrium classical molecular dynamics simulation of the solvated anion, allowing for an extended sampling of initial configurations. In the next step, ab initio calculations at the MP2 level employing an extended diffuse basis set are performed for the anionic ground and lowest triplet state, as well as for the corresponding neutral system. It is argued that due to the small singlet−triplet splitting, the triplet state is a good model for the experimental CTTS state. The present calculations on aqueous iodide ion are made computationally feasible by replacing all water molecules (or all waters except for the first solvation shell) by fractional point charges. It is concluded that the bulk wave function is mainly defined by the instantaneous location of voids in the first solvation shell, which arise due to thermal disorder in liquid water. The key ingredient to CTTS binding in the bulk is the long-range electrostatic field due to the preexisting polarization of water molecules by the ground state iodide ion. This is very different from the situation in small water clusters, where the CTTS state is an order of magnitude more fragile due to the lack of long-range polarization. Therefore, it is argued that the electronic structure of small halide clusters cannot be directly extrapolated to the bulk.

Theoretical Study of the Dipole-Bound Excited States of I-(H2O)4

Photoexcitation of I-(H2O)n clusters can lead to charge transfer states with the excess electron localized on the water cluster, making these ideal systems for studying electron/water cluster dynamics. In the present study, the MP2 method is used to study the structure and infrared spectrum of the ground state of I-(H2O)4 and the CASPT2 method is used to characterize the low-lying electronically excited states of the complex. The calculated IR spectrum is in good agreement with experimental results, providing support for a crown-like structure of I-(H2O)4. The 21A excited state is predicted to be 29 meV below the ground state of the neutral cluster (at the geometry of the ground-state anion), and the vertical excitation energy for the 11A → 21A charge-transfer transition is calculated to be 4.60 eV, in good agreement with the experimental value of 4.5 eV. Potential energy curves for the relaxation of the photoexcited I-(H2O)4 cluster from the crown-like to the planar structure are presented.

Theoretical investigation of the ultrafast NeNePo spectroscopy of Au 4 and Ag 4 Clusters

Ultrafast ground state nuclear dynamics of Au4 and Ag4 is theoretically explored in the framework of negative ion - to neutral - to positive ion (NeNePo) pump-probe spectroscopy based on the ab initio Wigner distribution approach. This involves the preparation of a nonequilibrium neutral ensemble by pump induced photodetachment of a thermal anionic ground state distribution, gradient corrected DFT classical trajectory simulations “on the fly” on the neutral ground state, and detection of the relaxation process of the ensemble in the cationic ground state by a time-delayed probe pulse. In Au4, the initially prepared linear structure is close to a local minimum of the neutral state giving rise to characteristic vibrations in the signals for probe wavelength near the initial Franck-Condon transition. A timescale of ∼1 ps for the structural relaxation towards the stable rhombic D2h neutral isomer was determined by the increase of the signal for probe wavelength in vicinity of the vertical ionization energy of the rhombic structure. In contrast, the relaxation dynamics in Ag4 is characterized by normal mode vibrations since both the initially prepared anionic ground state and the neutral ground state have rhombic minimum geometries. Thus, time-resolved oscillations of pump-probe signals are fingerprints of structural behaviour which can be used experimentally for the identification of particular isomers in the framework of NeNePo spectroscopy.

Ab Initio Adiabatic Dynamics Combined with Wigner Distribution Approach to Femtosecond Pump−Probe Negative Ion to Neutral to Positive Ion (NeNePo) Spectroscopy of Ag2Au, Ag4, and Au4 Clusters

Ultrafast ground state nuclear dynamics of small coinage metal clusters is theoretically explored in the framework of negative ion to neutral to positive ion (NeNePo) femtosecond pump−probe spectroscopy with the aim to determine the scope and perspective of this technique. This approach involves the preparation of an initial ensemble in the anionic ground state of the cluster, one-photon detachment by the pump pulse, the propagation of the system on the neutral electronic ground state, and detection via the cationic ground state by a time-delayed ionizing probe pulse. The calculations of the NeNePo-ZEKE signals under the condition of zero kinetic energy electrons are based on the combination of the Wigner distribution approach and ab initio molecular dynamics (MD) “on the fly” in the framework of gradient-corrected DFT for the propagation of an ensemble of classical trajectories and involves the average over the entire phase space. We have chosen examples of Ag2Au and Au4 to investigate time scales and th...