Vertical Binding Energies Research Articles

The Hydrated Electron: A Seemingly Familiar Chemical and Biological Transient

Since the discovery of the hydrated electron in bulk water in 1962, the species has been the subject of intense research and speculation. For many decades even the basic features of the simplest of all chemical and biological transients and reactants--such as its structure, binding motifs, lifetimes, and binding energies--remained elusive. Recently, another milestone in the research of the hydrated electron was the determination of its vertical binding energy (VBE). Also a long-lived hydrated electron near the surface of liquid water has been discovered. The present Minireview discusses the implications and consequences of this and other new findings in addition to the emerging complex picture of a solvated electron in water.

Sodium Microsolvation in Ethanol: Common Features of Na(HO-R)n (R = H, CH3, C2H5) Clusters

Ethanol clusters are generated in a continuous He seeded supersonic expansion and doped with sodium atoms in a pick-up cell. By this method clusters of the type Na(C(2)H(5)OH)(n) are formed and characterized by determining size selectively their ionization potentials (IPs) for n = 2-40 in photoionization experiments. A continuous decrease to 3.1 eV is found from n = 2 to 6 and a constant value of 3.07 ± 0.06 eV for n = 10-40. This IP evolution is similar to the sodium-water and the sodium-methanol system. Quantum chemical calculations (B3LYP and MP2) of the IPs indicate adiabatic contributions to the photoionization process for the cluster sizes n = 4 and 5, which is similar to the sodium-methanol case. The results of the extrapolated IPs and the vertical binding energies (VEBs) of cluster anions are compared with the recently reported VEBs of solvated electrons in liquid water, methanol, and ethanol solutions in the range of 3.1-3.4 eV. The new results imply that the extrapolated VBEs of solvated electrons in anionic clusters match the VBE in liquid water, while they are about 0.5 eV too low for methanol. The influence of the presence of counterions on these findings is discussed.

Photoelectron spectroscopy of hydrated electrons

We report a systematic study of the photoelectron spectroscopy of hydrated electrons in liquid water jets using multiple precursors and photodetachment wavelengths. Hydrated electrons were generated in and detached from liquid microjets using two photons from a single nanosecond laser pulse at 266 or 213 nm. Solutions of 50 to 250 mM potassium hexacyanoferrate(II) or potassium iodide were used to provide precursor anions. All of our experimental conditions yield similar results, giving a mean vertical binding energy of 3.6 ± 0.1 eV at a temperature of ∼280 K, a slightly higher value than in recent reports.

Direct Measurement of Vertical Electron Binding Energies of Solvated Electrons in Methanol and Ethanol

Abstract Time-resolved photoelectron spectroscopy at ultralow kinetic energy was applied to liquid beams of NaI solutions in methanol and ethanol. Solvated electrons were formed from I− ions in these solutions by charge transfer to solvent reactions. The vertical electron binding energies of the solvated electrons were determined for the first time. Both of the binding energies were found to be 3.10 ± 0.1 eV. This is in reasonable agreement with the dielectric continuum model of solvated electrons using the previously measured vertical binding energy of a hydrated electron. This indicates that the cavity radii of solvated electrons in water, methanol, and ethanol are approximately the same (0.33–0.35 nm).

Binding energies, lifetimes and implications of bulk and interface solvated electrons in water

Solvated electrons in liquid water are one of the seemingly simplest, but most important, transients in chemistry and biology, but they have resisted disclosing important information about their energetics, binding motifs and dynamics. Here we report the first ultrafast liquid-jet photoelectron spectroscopy measurements of solvated electrons in liquid water. The results prove unequivocally the existence of solvated electrons bound at the water surface and of solvated electrons in the bulk solution, with vertical binding energies of 1.6eV and 3.3eV, respectively, and with lifetimes longer than 100 ps. The unexpectedly long lifetime of solvated electrons bound at the water surface is attributed to a free-energy barrier that separates surface and interior states. Beyond constituting important energetic and kinetic benchmark and reference data, the results also help to understand the mechanisms of a number of very efficient electron-transfer processes in nature.

Direct measurement of vertical binding energy of a hydrated electron

We present the first measurement of the vertical binding energy (VBE) of a hydrated electron in bulk water by the time-resolved photoelectron spectroscopy (TRPES) of the charge-transfer-to-solvent (CTTS) reaction in aqueous NaI solution. Our best estimate of VBE is 3.27 +/- 0.10 eV for H(2)O and 3.20 +/- 0.10 eV for D(2)O.

Introducing various ligands into superhalogen anions reduces their electronic stabilities

Abstract The vertical electron detachment energies (VDE) of six NaX 2 - anions (where X = F, Cl, Br) were calculated at the OVGF level with the 6-311++G(3df) basis sets. In all the cases studied the VDE exceeds the electron affinity of chlorine atom and thus those species were classified as superhalogen anions. The largest vertical binding energy was found for the NaF 2 - system (6.644 eV). The strong VDE dependence on the ligand type, ligand–central atom distance, and the character of the highest occupied molecular orbital (HOMO) was observed and discussed.

Hydrogen bonds in liquid water studied by photoelectron spectroscopy

The authors report on photoelectron emission spectroscopy measurements of the oxygen 1s orbital of liquid water, using a liquid microjet in ultrahigh vacuum. By suitably changing the soft x-ray photon energy, within 600-1200 eV, the electron probing depth can be considerably altered as to either predominantly access the surface or predominantly bulk water molecules. The absolute probing depth in liquid water was inferred from the evolution of the O1s signal and from comparison with aqueous salt solution. The presence of two distinctive components in the core-level photoelectron spectrum, with significantly different binding energies, is revealed. The dominant contribution, at a vertical binding energy of 538.1 eV, was found in bulk and surface sensitive spectra. A weaker component at 536.6 eV binding energy appears to be present only in bulk water. Hartree-Fock calculations of O1s binding energies in different geometric arrangements of the water network are presented to rationalize the experimental distribution of O1s electron binding energies.

Exploration of the Ca2+ Interaction Modes of the Nifedipine Calcium Channel Antagonist

A comprehensive study is carried out using quantum chemical computation and molecular dynamics (MD) simulations to gain insight into the interaction between Ca(2+) ions and the most important class of calcium channel antagonists--nifedipine. First, the chelating structures and energetic characters of nifedipine-Ca(2+) in the gas phase are explored, and 25 isomers are found. The most favorable chelating mode is a tridentate one, that is, Ca(2+) binds to two carbonyl O atoms and one nitryl O atom, where Ca(2+) is above the plane of the three O atoms to form a pyramidal structure. Accurate geometric structures, relative stabilities, vertical and adiabatic binding energies, and charge distributions are discussed. The differences in the geometries and energies among these isomers are analyzed from the contributions of chelating sites, electrostatics and polarizations, steric repulsions, and charge distributions. The interconversions among isomers with similar geometries and energies are also investigated because of the importance of the geometric transformation in the biological system. Furthermore, certain numbers of water molecules are added to the nifedipine-Ca(2+) system to probe the effect of water. A detailed study is performed on the hydrated geometries on the basis of the most stable isomer 1. Stepwise hydration can weaken the nifedipine-Ca(2+) interaction, and the chelating sites of nifedipine are gradually replaced by the added water molecules. Hexacoordination is found to be the most favorable geometry no matter how many water molecules were added, which can be verified by the MD simulations. The transfer of water molecules from the inner shell to the outer shell is also supported by MD simulations of the hexahydrated complexes.

Photoelectron imaging of large anionic methanol clusters: (MeOH)n− (n∼70–460)

Electron solvation in methanol anion clusters, (MeOH)(n) (-) (n approximately 70-460), is studied by photoelectron imaging. Two isomers are observed: methanol I, with vertical binding energies (VBE) ranging from 2-2.5 eV, and methanol II, with much lower VBE's between 0.2 and 0.5 eV. The VBE's of the two isomers depend linearly on n(-1/3) with nearly identical slopes. We propose that the excess electron is internally solvated in methanol I clusters, whereas in methanol II it resides in a dipole-bound surface-state. Evidence of an excited state accessible at 1.55 eV is observed for methanol I.

Observation of Large Water-Cluster Anions with Surface-Bound Excess Electrons

Anionic water clusters have long been studied to infer properties of the bulk hydrated electron. We used photoelectron imaging to characterize a class of (H2O)n- and (D2O)n- cluster anions (n </= 200 molecules) with vertical binding energies that are significantly lower than those previously recorded. The data are consistent with a structure in which the excess electron is bound to the surface of the cluster. This result implies that the excess electron in previously observed water-cluster anions, with higher vertical binding energies, was internally solvated. Thus, the properties of those clusters could be extrapolated to those of the bulk hydrated electron.

Cavity size effects on charge-transfer-to-solvent precursor excited states of internal halide water clusters X −(H 2O) 6

The cavity size of an internal X −(H 2O) n cluster is shown, via ab initio calculations, to determine how its charge-transfer-to-solvent precursor excited electron is distributed around the cavity. Thus, we find an optimal cavity size (OCS), at which the excited electron has a maximal vertical binding energy. We argue that the OCS is a consequence of the balance of the electron probability influx into the cavity and the interactions of the excited electron with the parent core and solvent molecules. The implications of these findings to the initial solvent response upon photo-excitation of an anion in liquids are also discussed.

Charge-transfer-to-solvent (CTTS) precursor states of X −(H 2O) n clusters (X=Cl, Br, I)

The CTTS precursor states in X −(H 2O) n clusters are studied by the ab initio molecular orbital method. We systematically compare the effects of the solvent structure and anion sizes on the excitation energy from the ground state to the first CTTS precursor state ( E CTTS) and the vertical binding energy (VBE) from the first CTTS precursor state. VBE is found to be more sensitive to solvent environmental changes than E CTTS. At n=3–5, VBE for a surface ion configuration is ordered as Cl − < Br − < I −, whereas E CTTS follows the opposite trend. More complex VBE variation is observed for the internal configuration, when n and anions are varied.

Iodine effect on the relaxation pathway of photoexcited I −(H 2O) n clusters

The iodine effect on the photoexcited I −(H 2O) n relaxation pathway is studied using ab initio calculations. Critical behavior of the vertical binding energy (VBE) of the excited I −(H 2O) n is seen at short distances between the iodine and the remaining (H 2O) n , which is correlated to the rapid experimental VBE increase within 1 ps (L. Lehr, M.T. Zanni, C. Frischkorn, R. Weinkauf, D.M. Neumark, Science 284 (1999) 635). Further comparisons of computed results with experimental data reveal that iodine detachment is a crucial step in the cluster relaxation dynamics, intrinsically different from the electron-detachment pathway of photoexcited aqueous iodide.

The electronic structure and chemical bonding of hypermetallic Al5C by ab initio calculations and anion photoelectron spectroscopy

The chemical structure and bonding of the hypermetallic Al5C and Al5C− species have been studied by photoelectron spectroscopy and ab initio calculations. Both Al5C (C2v, 2A1) and Al5C− (C2v, 1A1) are found to have planar structures that can be related to that of the planar square Al4C− by adding one Al+ ion or one Al atom to an edge of the square. The planarity of Al5C and Al5C− can be explained in terms of the structure of their highest occupied molecular orbitals which are ligand five-center one- or two-electron bonding MO, respectively, similar to the orbital responsible for the planarity of Al4C−. Four peaks were observed in the photoelectron spectra of Al5C− with vertical binding energies of 2.67, 2.91, 3.19, and 4.14 eV which compare well with the 2.68, 2.96, 3.27, and 4.35 eV calculated by the Green function method [OVGF/6-311+G(2df )]. The excellent agreement between the calculated and experimental electron affinity and excitation energies allow us to completely elucidate the geometrical and electronic structures of Al5C− and suggest the most likely structure for the Al5C molecule.

Photodetachment studies of extended excited states in I−Xen clusters (n=1–54)

We present a comprehensive experimental study of bound excited states in I−Xen clusters (n=1–54), using photoelectron spectroscopy and energy-dependent action spectroscopy. Starting at n=4, the electron detachment action spectra developed peaks lying in energy lower than the vertical binding energy. This behavior has been shown for both final spin states of the neutral iodine. It indicates the existence of bound electronic states extended over the xenon cluster. The peaks lying in energy below the J=1/2 detachment continuum of the iodine (n&amp;gt;4) were detected over the whole cluster size range of I−Xen using electron detachment action spectroscopy. For the bound states relating to the J=3/2 continuum, in the size range n=4–12, thermionic emission has marked the existence of bound states. For n&amp;gt;12, these states were stabilized and detected via two-photon excitations. The picture, unveiled from the above photoelectron and action spectra, is the gradual evolution of bound excited states on the xenon solvent cluster. The critical size for the binding of an excited electron is n=4. At the largest cluster interrogated, I−Xe54, the binding energy amounts to 170 meV. We compare these states to the ground electronic state of Xen− clusters, and discuss the differences between the experiments and model calculations.

Photoelectron spectroscopy of Cl−, Br−, and I− solvated in water clusters

We present the photoelectron-spectra of Cl−, Br−, and I−, solvated in water clusters-(H2O)n, where n is 1–7, 1–16, and 1–60, respectively, taken with 7.1 eV photon energy. The vertical binding energies of the solvated anions are used to extract the solvent electrostatic stabilization energies of the anion. The photoelectron spectra of the solvated I− indicate the formation of the first solvation layer with a coordination number of six. Ab initio calculations support solvation shell closure at n=6. This conclusion is not born-out by current molecular dynamics calculations. These calculations favor structures with a surface solvated anion (coordination number of 3–4) and reproduce (within 0.2 eV) our vertical binding energies. The fitting of the experimental binding energies of large I−(H2O)n to the models of classical electrostatic solvation is consistent with surface solvation. In the size range n=34–40 we have detected special cluster structures, with very low electrostatic stabilization.

Molecular dynamics simulations of aqueous ionic clusters using polarizable water

The solvation properties of a chlorine ion in small water clusters are investigated using state-of-the-art statistical mechanics. The simulations employ the polarizable water model developed recently by Dang [J. Chem. Phys. 97, 2659 (1992)]. The ion–water interaction potentials are defined such that the successive binding energies for the ionic clusters, and the solvation enthalpy, bulk vertical binding energy, and structural properties of the aqueous solution agree with the best available results obtained from experiments. Simulated vertical electron binding energies of the ionic clusters Cl−(H2O)n, (n=1–6) are found to be in modest agreement with data from recent photoelectron spectroscopy experiments. Minimum energy configurations for the clusters as a function of ion polarizability are compared with the recent quantum chemical calculations of Combariza, Kestner, and Jortner [Chem. Phys. Lett. 203, 423 (1993)]. Equilibrium cluster configurations at 200 K are described in terms of surface and interior solvation states for the ion, and are found to be dependent on the magnitude of the Cl− polarizability assumed in the simulations.

Stability of MX2−3 ions in the gas phase and when do ionic molecules have large ionization potentials

The present work aims at theoretical consideration of the geometrical and electronic structures of a homologous series of free MXmq−-type alkali halides (M=Li, Na, K; X=F, Cl; m=1,2,3; q=0,1,2) in order to get insight into their kinetic and electronic stability. At the ab initio Hartree–Fock self-consistent field (HF-SCF) level of theory, the lowest energy fragmentation channel leading to the decomposition of the dianions MX32− into MX2− and X− has been investigated. The potential energy surface was found to exhibit a broad, but flat energy barrier to fragmentation. These findings have been confirmed using results from configuration interaction calculations and the molecular dianions are predicted to be long-lived species formally existing in a metastable state. The stability of the gas-phase MX32− dianions and of the MX2− fragmentation products with respect to autodetachment of an extra electron has been investigated using ab initio HF-SCF and Green’s function methods. The inclusion of many-body effects by the latter was found to decrease the vertical binding energy of the extra electrons with respect to the Koopmans’ theorem HF-SCF result, but the extra electrons remain bound. At the ab initio level, the compounds studied in this work are predicted to be extremely ionic species. The variation of the properties through a series of MXmq− species has been investigated. Motivated by the overall large ionic character—in particular of the MX32− molecules—the properties of the systems have been studied using a previously derived theoretical approach, referred to as ionic model, based on (classical) electrostatic terms. The versatility of the ionic model scheme has been extended here to obtain information on the tendency of an ionic molecule to accomodate an additional electron.

Photoelectron spectroscopy of iodine anion solvated in water clusters

We have measured the photoelectron spectra of clusters of I- solvated in water up to fifteen water molecules. The vertical binding energy of the electron increases with cluster size and levels off in clusters containing more than six water molecules. These results strongly indicate that the first solvation layer around the ion consists of six water molecules. This picture is quantitatively consistent with the photoelectron spectroscopy result of I- solvated in bulk water.