Charge-transfer-to-solvent Band Research Articles

Natural Charge-Transfer Analysis: Eliminating Spurious Charge-Transfer States in Time-Dependent Density Functional Theory via Diabatization, with Application to Projection-Based Embedding.

For many types of vertical excitation energies, linear-response time-dependent density functional theory (LR-TDDFT) offers a useful degree of accuracy combined with unrivaled computational efficiency, although charge-transfer excitation energies are often systematically and dramatically underestimated, especially for large systems and those that contain explicit solvent. As a result, low-energy electronic spectra of solution-phase chromophores often contain tens to hundreds of spurious charge-transfer states, making LR-TDDFT needlessly expensive in bulk solution. Intensity borrowing by these spurious states can affect intensities of the valence excitations, altering electronic bandshapes. At higher excitation energies, it is difficult to distinguish spurious charge-transfer states from genuine charge-transfer-to-solvent (CTTS) excitations. In this work, we introduce an automated diabatization that enables fast and effective screening of the CTTS acceptor space in bulk solution. Our procedure introduces "natural charge-transfer orbitals" that provide a means to isolate orbitals that are most likely to participate in a CTTS excitation. Projection of these orbitals onto solvent-centered virtual orbitals provides a criterion for defining the most important solvent molecules in a given excitation and be used as an automated subspace selection algorithm for projection-based embedding of a high-level description of the CTTS state in a lower-level description of its environment. We apply this method to an ab initio molecular dynamics trajectory of I-(aq) and report the lowest-energy CTTS band in the absorption spectrum. Our results are in excellent agreement with the experiment, and only one-third of the water molecules in the I-(H2O)96 simulation cell need to be described with LR-TDDFT to obtain excitation energies that are converged to <0.1 eV. The tools introduced herein will improve the accuracy, efficiency, and usability of LR-TDDFT in solution-phase environments.

A Correction Method for Attenuated Total Reflection-Far Ultraviolet Spectra Via the Use of Charge Transfer to Solvent Band Intensities of Iodide in the Ultraviolet Region.

Attenuated total reflection (ATR) spectra, which are often used in IR analysis, can be transformed into extinction and refraction spectra by Kramers-Kronig transformation (KKT) with Fresnel equations. However, it is often difficult to obtain correct optical indices due to the inherent instrumental functions. This paper proposes a simple practical method for correction of KKT with two parameters, which include all the effects of the instrumental function. In order to obtain the parameters of the instrumental function, absorption ratios of charge transfer to solvent (CTTS) transitions of aqueous iodide ions observed at 195 nm and 230 nm were used as a standard. The absorption indices calculated from the ATR spectra with the parameters correspond reasonably well to those given by the transmittance spectra not only in the UV region but also in the far-ultraviolet (FUV, 120-200 nm) region. By applying the corrected KKT to the ATR-FUV spectra of aqueous potassium halide solutions in the range of 0-2 M, correct features of the absorption spectra of KCl and KBr, whose CTTS bands are thought to be observed in FUV region, were confirmed. It is possible to use the parameters representing the instrument function as long as the instrument is not changed.

Environmental Broadening of the CTTS Bands: The Hexaammineruthenium(II) Complex in Aqueous Solution

Cluster ab initio quantum chemistry approach is developed to simulate the charge-transfer-to-solvent (CTTS) absorption band and satellite ligand field bands of hexaammineruthenium(II) ion in aqueous solution. Several cluster models, including 16, 21, and 38 water molecules, are explored for this purpose. TDDFT method with long-range corrected BLYP (LC-BLYP) functional is used to obtain the vertical transition characteristics, and DFT B3LYP is used for calculation of the ground state geometry and vibrational frequencies of the solvated complex. A simple harmonic bath model is employed to estimate the absorption bandwidths and coherence decay times with the parameters taken from the quantum chemistry calculations. The present approach provides rather reasonable estimates for the CTTS band position and shape, also giving an additional insight for the mechanism of the CTTS band broadening.

First principles multielectron mixed quantum/classical simulations in the condensed phase. II. The charge-transfer-to-solvent states of sodium anions in liquid tetrahydrofuran

Gas-phase atomic anions lack bound electronic excited states, yet in solution many of these anions exhibit intense absorption bands due to the presence of excited states, referred to as charge-transfer-to-solvent (CTTS) states that are bound only by the presence of the solvent. CTTS spectra thus serve as delicate probes of solute-solvent interactions, but the fact that they are created by the interactions of a solute with many solvent molecules makes them a challenge to describe theoretically. In this paper, we use mixed quantum/classical molecular dynamics with the two-electron Fourier-grid (2EFG) electronic structure method presented in the previous paper [W. J. Glover, R. E. Larsen, and B. J. Schwartz, J. Chem. Phys. 132, 144101 (2010)] to simulate the CTTS states of a sodium anion in liquid tetrahydrofuran, Na(-)/THF. Since our 2EFG method is based on configuration interaction with single and double excitations in a grid basis, it allows for an exact treatment of the two valence electrons of the sodium anion. To simulate Na(-)/THF, we first develop a new electron-THF pseudopotential, and we verify the accuracy of this potential by reproducing the experimental absorption spectrum of an excess electron in liquid THF with near quantitative accuracy. We also are able to reproduce the CTTS spectrum of Na(-)/THF and find that the CTTS states of Na(-) exhibit a Rydberg-like progression due to the pre-existing long-range solvent polarization around the anion. We also find that the CTTS states are highly mixed with the disjoint electronic states supported by naturally occurring solvent cavities that exist in liquid THF. This mixing explains why the solvated electrons that are ejected following CTTS excitation appear with their equilibrium absorption spectrum. The mixing of the CTTS and solvent-cavity states also explains why the recombination of the electron and its geminate Na(0) partner occurs on slower time scales when photoexciting in the blue compared to in the red of the CTTS band: blue excitation accesses CTTS states whose charge densities lies further from the position of the anion, whereas red excitation accesses CTTS states that lie primarily within the anion's first solvation shell. Finally, we see that the radial character of the CTTS states near the Na(+) core matches that of Na(0), explaining why the spectrum of this species appears instantly after photoexciting Na(-).

The roles of the solute and solvent cavities in charge-transfer-to-solvent dynamics: Ultrafast studies of potasside and sodide in diethyl ether

Although electron transfer reactions are among the most fundamental in chemistry, it is still not clear how to isolate the roles of the solute and solvent in moving charge between reactants in solution. In this paper, we address this question by comparing the ultrafast charge-transfer-to-solvent (CTTS) dynamics of potasside (K(-)) in diethyl ether (DEE) to those of sodide (Na(-)) in both DEE and tetrahydrofuran (THF). We find that for sodide in both DEE and THF, CTTS excitation leads to delayed ejection of a solvated electron that appears with its equilibrium absorption spectrum. This indicates that the ejected electrons are localized in pre-existing solvent traps, suggesting that the structure of liquid DEE is characterized by cavities that are favorably polarized to localize an excess electron, as has been previously shown is the case for liquid THF. We also find that the geminate recombination dynamics following CTTS excitation of sodide in THF and DEE are similar, suggesting that the nature of the CTTS excited states and their coupling to the electronic states supported by the naturally occurring solvent cavities are similar in the two solvents. In contrast, the geminate recombination dynamics of potasside and sodide in DEE are different, with red-edge excitation of the K(-) CTTS band producing a greater number of long-lived electrons than is seen following the corresponding red-edge excitation of the Na(-) CTTS band. This indicates that the CTTS excited states of K(-) are better able to couple to the electronic states supported by the naturally occurring solvent cavities, allowing us to compare the energetic positions of the potasside and sodide ground and CTTS excited states on a common absolute scale. Finally, we also observe a strong transient absorption following the CTTS excitation of potasside in DEE that correlates well with the 766 nm position of the gas-phase potassium D-line. The data indicate that CTTS excitation of alkali metal anions essentially instantaneously produces a gas-phase-like neutral alkali metal atom, which then spontaneously undergoes partial ejection of the remaining valence electron to form a neutral alkali metal cation:solvated electron tight-contact pair.

Na − photolysis in THF: Charge transfer to solvent studied from the donors perspective in &amp;lt;10fs detail

Two and three pulse photolysis experiments on terahydrofuran (THF) solutions of Na(-), utilizing hyperspectral probing, are described. The objective is to probe the extent and duration of energetic correlations between the primary charge transfer to solvent (CTTS) fragments which are an e(-) and Na(0). The latter is characterized by an intense visible absorption spectrum with fine structure reflecting the atom's immediate solvent environment. Pump-probe experiments with approximately 6 fs pulses show that for the majority of irradiated ions, the electron ejection and production of unperturbed Na(0) is effectively over in approximately 15 fs, with no precursors. Three pulse experiments further demonstrate this to be true for nearly all ions irradiated at 3 eV. Thus, the 400 nm data provide a detailed spectral record of the formation and subsequent solvation and polarization of neutral sodium bubbles in THF. Measures are presented for parametrizing the ensuing spectral evolution. In contrast, exciting at 1.5 eV, the red edge of the CTTS band leads to charge transfer with less than unity quantum efficiency. The complementary fraction of absorbing ions is photostable at 800 nm, presumably due to preferential solvent stabilization. Prompt secondary irradiation at 2 microm can complete ionization of that population leading to additional generation of Na(0) but exhibiting much more pronounced spectral structure. Thus, at low photon energies, a short lived correlated and bound electronic excited state is produced with significant yield. These results are discussed in terms of classical models for CTTS spectra, as well as more recent simulations and experiments concerning CTTS in this and other related systems.

Emergence of Charge-Transfer-to-Solvent Band in the Absorption Spectra of Hydrogen Halides on Ice Nanoparticles: Spectroscopic Evidence for Acidic Dissociation

Extensive ab initio calculations complemented by a photodissociation experiment at 193 nm elucidate the nature of hydrogen halide molecules bound on free ice nanoparticles. Electronic absorption spectra of small water clusters (up to 5 water molecules) and water clusters doped with hydrogen fluoride, hydrogen chloride and hydrogen bromide were calculated. The spectra were modeled at the time-dependent density functional (TDDFT) level of theory with the BHandHLYP functional using the reflection principle. We observe the emergence of a charge-transfer-to-solvent (CTTS) band in the absorption spectra upon the acidic dissociation of the hydrogen halides. The CTTS band provides a spectroscopically observable feature for the acidic dissociation. The calculated spectra were compared with our new experimental photodissociation data for larger water clusters doped with HCl and HBr. We conclude that HCl and HBr dissociate to a large extent on the surface of ice nanoparticles at temperatures near 120 K and photoactive products are formed. The acidic dissociation of HX leads to an enhancement by about 4 orders of magnitude of the HCl photolysis rate in the 200-300 nm region, which is potentially relevant for the halogen budget in the atmosphere.

Absence of a Signature of Aqueous I(2P1/2) after 200-nm Photodetachment of I-(aq)

Ultrafast pump-broadband probe spectroscopy was used to study the transient photoproducts following 200-nm photodetachment of I(-)(aq). Resonant detachment at 200 nm in the second charge-transfer-to-solvent (CTTS) band of I(-)(aq) is expected to produce an electron and iodine in its spin-orbit excited state, I*((2)P(1/2)). The transients in solution following photodetachment were probed from 200 to 620 nm. Along with strong absorption in the visible region due to solvated electrons and a strong bleach of the I(-)(aq) ground-state absorption, a weaker transient absorption near 260 nm was observed that is consistent with a previously assigned ground-state I((2)P(3/2)) charge-transfer band. However, no evidence was found for an equivalent I*(aq) charge-transfer absorption, and I((2)P(3/2)) was produced within the instrument response. This suggests either that I* is electronically relaxed in less than 300 fs or that excitation in the second CTTS band does not in fact lead to I*. The consequences for previous experimental work where I*(aq) production has been postulated, as well as for halogen electron ejection mechanisms, are discussed. In addition, the broad spectral coverage of this study reveals in the bleach recovery the rapid cooling of the solvent surrounding the re-formed iodide after geminate recombination of the iodine with the solvated electron.

Solvent Induced Symmetry Breaking and Relaxation Following Charge Transfer to Solvent Excitation of Na- in Tetrahydrofuran

Pulses of 30 fs are used to follow the dynamics of charge-transfer to solvent (CTTS) in Na _ dissolved in tetrahydrofuran (THF). The photobleach of this transition is demonstrated to be orientationally anisotropic, indicating that the CTTS band consists of a number of orthogonally oriented transitions. The pronounced anisotropy of the bleach demonstrates that interaction with the solvent orients the transition dipoles, presumably by breaking the spherical symmetry which characterizes the isotropically absorbing Na _ ( g ) . The duration of solvent induced symmetry breaking was measured, and it was shown to persist for more than a picosecond. Rapid changes both in absorption and its anisotropy, which were not reported earlier, are apparent throughout the visible and near-IR. These are interpreted to reflect the presence of a primary photoproduct-tentatively assigned as the CTTS excited state, which decays completely within 200 fs. Assignment of later intermediates, analyzing the source of absorption anisotropies, and understanding the role played by the solvent in molding the photoinduced dynamics of sodide in THF are discussed.

Chemical Reactions Involving Multi-Solvated Electrons on Liquid Beam Surface of CaI2 Alcohol Solutions

A continuous liquid flow (a liquid beam) of a calcium iodide (CaI2) solution in ethanol (EtOH) in a vacuum was irradiated with a UV laser at different wavelengths and powers. Product ions ejected from the solution surface following multiphoton excitation via the charge-transfer-to-solvent (CTTS) band of I- were analyzed by a time-of-flight mass spectrometer. Under irradiation of the laser at a weak power, solvated cluster ions CaOEt+ (EtOH)m (m = 0−6), CaOH+ (EtOH)m (m = 0−1), and CaI+ (EtOH)m (m = 0−6) (Class I) were found to be produced, whereas at an intense power, cluster ions containing neutral salts, [Can(OEt)i(OH)j(O)k(I)l]+ (Class II), were produced additionally. The relative abundances of the cluster ions of Class II increased as the irradiation laser power increased. The abundance was particularly high when the excitation laser was resonant to the CTTS band. It was concluded that more than one solvated electrons participate in the production of the Class II cluster ions.

Charge-transfer spectra of ferrocene in halocarbon solvents under photoexcitation

The changes in the electronic absorption spectra of ferrocene in the halocarbon solvents chloroform and carbontetrachloride have been investigated under photoexcitation in nitrogen atmosphere. Photoexcitations have been made with monochromatic light (using an Xe-source and a monochromator), at intervals of a few nanometers in the spectral range 220–750 nm. Analysing the spectra by a modified method the position of the charge-transfer-to-solvent (CTTS) band has been located for both the solvents. The position of the CTTS band in the case of carbontetrachloride solution located (320 nm) by the present study is different from the previously reported value (307 nm), while from the previous studies the position of the CTTS band in the case of the spectra of ferrocene in chloroform was not clear. From the present investigation, the changes in spectra after photoexcitation studied as a function, the concentration of ferrocene in the solution and the time (duration) of photoexcitations, have been observed to be systematic. Using the position of the new band (320 nm) for the CTTS transition in the case of carbontetrachloride, ionization potential of ferrocene has been estimated and the estimated value has shown excellent agreement with the experimental value indicating the exactness of the newly located CTTS band position.

Direct observation of charge-transfer-to-solvent (CTTS) reactions: Ultrafast dynamics of the photoexcited alkali metal anion sodide (Na−)

Charge-transfer-to-solvent (CTTS) transitions have been the subject of a great deal of interest recently because they represent the simplest possible charge transfer reaction: The CTTS electron transfer from an atomic ion to a cavity in the surrounding solvent involves only electronic degrees of freedom. Most of the work in this area, both experimental and theoretical, has focused on aqueous halides. Experimentally, however, halides make a challenging choice for studying the CTTS phenomenon because the relevant spectroscopic transitions are deep in the UV and because the charge-transfer dynamics can be monitored only indirectly through the appearance of the solvated electron. In this paper, we show that these difficulties can be overcome by taking advantage of the CTTS transitions in solutions of alkali metal anions, in particular, the near-IR CTTS band of sodide (Na−) in tetrahydrofuran (THF). Using femtosecond pump–probe techniques, we have been able to spectroscopically separate and identify transient absorption contributions not only from the solvated electron, but also from the bleaching dynamics of the Na− ground state and from the absorption of the neutral sodium atom. Perhaps most importantly, we also have been able to directly observe the decay of the Na−* excited CTTS state, providing the first direct measure of the electron transfer rate for any CTTS system. Taken together, the data at a variety of pump and probe wavelengths provide a direct test for several kinetic models of the CTTS process. The model which best fits the data assumes a delayed ejection of the electron from the CTTS excited state in ∼700 fs. Once ejected, a fraction of the electrons, which remain localized in the vicinity of the neutral sodium parent atom, recombine on a ∼1.5-ps time scale. The fraction of electrons that recombine depends sensitively on the choice of excitation wavelength, suggesting multiple pathways for charge transfer. The spectrum of the neutral sodium atom, which appears on the ∼700-fs charge-transfer time scale, matches well with a species of stochiometry (Na+, e−) that has been identified in the radiation chemistry literature. All the results are compared to previous studies of both CTTS dynamics and alkali metal solutions, and the implications for charge transfer are discussed.

Formation of Ca+ (EtOH)m from Alcohol Solutions of CaCl2

A continuous liquid flow of a calcium chloride (CaCl2) solution in ethanol (EtOH) in a vacuum (a liquid beam) was irradiated with a 266 nm laser, and ions ejected from the surface following multiphoton ionization via the CTTS (charge transfer to solvent) band of Cl- were observed by a time-of-flight mass spectrometer. A variety of core ions (Ca+, CaOEt+, CaOH+, CaCl+, H+, etc.) are formed by reactions involving Ca2+, solvated electrons, and solvent molecules after the CTTS excitation by the laser irradiation and are ejected into vacuum with several accompanying alcohol molecules. The proposed mechanism is verified by the change of the ion intensity with introduction of an electron scavenger, CHCl3, in the solution. The cluster ion, Ca+(EtOH)m, remains intact for m < 3, while it dissociates into CaOEt+(EtOH)m-1 for m ≥ 3. This size-dependent dissociation is simply explained by the energetics.

Precursor of the I aq− charge-transfer-to-solvent (CTTS) band in I−⋅(H2O)n clusters

Ultraviolet charge-transfer-to-solvent (CTTS) bands are general spectral features of inorganic salts dissolved in dipolar (e.g., aqueous) solvents. The bands originate from excitations of the anions, and the excited states are thought to involve a delocalized electronic state supported by the arrangement of solvent molecules around the ion in the ground state. These diffuse states have fascinated physical chemists since the 1930s, who first used continuum models to explain the phenomenon and then later refined the treatment to include local molecular interactions. In this paper, we report that CTTS bands evolve from features present in very small gas phase clusters, I−⋅(H2O)n, n=1–4, and discuss how localization of the continuum wave function of the ejected electron acts as a precursor to the bound excited state in the liquid.

EDTA and CN-Complexing Effect on the Kinetics, Spectral Properties, and Redox Properties of Ag10and Ag2+in Aqueous Solution

The first steps of silver clusters nucleation have been studied by pulse radiolysis in the presence of EDTA and CN- ligands. The successive rate constants and the transient absorption spectra corresponding to Ag10 and Ag2+ have been determined. It has been found that Ag10 and Ag2+ were complexed by the ligands. In addition to the usual charge-transfer-to-solvent (CTTS) bands, red shifted in the presence of EDTA and CN-, the complexed Ag10 and Ag2+ spectra exhibit an additional absorption band assigned to a metal-ligand-charge-transfer (MLCT). The red shift of the CTTS band of the silver atom, which is more pronounced when the ligand interaction is stronger, is correlated with the ligand-induced redox properties of the monomeric silver couple.

Vertical ionization potentials and CTTS energies for anions in water and acetonitrile

The photoelectron emission threshold energy values, E t, are collected for eleven anions in water and five anions in acetonitrite. The E t values are found to be linearly correlated with the charge-transfer-to-solvent (CTTS) energies, E CTTS. This correlation can be used to find CTTS bands for some of the anions for which it is difficult to distinguish the bands from the intramolecular transitions such as π → π ∗ . The difference, E t - E CTTS, is found to depend on the kind of the anion not being consistent with the result reported by Burg and Delahay.

A CTTS band in the spectrum of Ru(CN) 4−6 ion in aqueous solution

Environmental and temperature effects on the absorption spectrum of Ru(CN) 4− 6 ion in solution were used to detect the existence of a charge transfer to solvent (CTTS) band. The salt and solvent effects were related to the theory dealing with the environmental effects on CTTS bands of polyvalent ions. The formation of hydrated electrons from the photoexitation of the Ru(CN) 4− 6 in aqueous solutions was attributed to the involvement of the CTTS excited state.

Photoelectron Emission by Solutions. II

Experimental conditions were determined for study of photoelectron emission (PEE) by aqueous solutions of sodium and lithium ferrocyanide. PEE currents for emission in pure water vapor were corrected for an appreciable Schottky effect and for electron back scattering. The Thomson equation yielded emission currents of the correct order of magnitude, but average velocities of electrons at emission, deduced from this equation, were incorrect. This result is explained by failure to consider in the Thomson equation the cross section for electron–water-molecule collisions and the energy dependence of this quantity. The error arising from electron attachment to water molecules in the gas phase is evaluated. The PEE spectral response is a sigmoid curve in the 186–214-nm explored range. This shape is accounted for by fluctuations in solvation structure and their effect on the energy required for PEE. The spectral response curve yields the vertical work function of the emitter in solution, i.e., approximately 6.2 eV for ferrocyanide in aqueous solution (0.03–1.3M). This is believed to be the first direct determination of its kind for species in solution. The difference between the vertical work function and the energy for charge transfer to solvent (CTTS) is calculated from the Platzman–Franck polaron model. The energy for PEE is also calculated directly from this model. Agreement with experiment is satisfactory, but the assignment of the CTTS band for ferrocyanide is uncertain. Theory and previous work on CTTS spectra suggest PEE by other systems, e.g., the varified PEE by lithium iodide in acetonitrile.