The ultrafast charge-transfer-to-solvent dynamics of iodide in tetrahydrofuran. 1. Exploring the roles of solvent and solute electronic structure in condensed-phase charge-transfer reactions

Although they represent the simplest possible charge-transfer reactions, the charge-transfer-to-solvent (CTTS) dynamics of atomic anions exhibit considerable complexity. For example, the CTTS dynamics of iodide in water are very different from those of sodide (Na-) in tetrahydrofuran (THF), leading to the question of the relative importance of the solvent and solute electronic structures in controlling charge-transfer dynamics. In this work, we address this issue by investigating the CTTS spectroscopy and dynamics of I- in THF, allowing us to make detailed comparisons to the previously studied I-/H2O and Na-/THF CTTS systems. Since THF is weakly polar, ion pairing with the counterion can have a substantial impact on the CTTS spectroscopy and dynamics of I- in this solvent. In this study, we have isolated "counterion-free" I- in THF by complexing the Na+ counterion with 18-crown-6 ether. Ultrafast pump-probe experiments reveal that THF-solvated electrons (e-THF) appear 380 +/- 60 fs following the CTTS excitation of "free" I- in THF. The absorption kinetics are identical at all probe wavelengths, indicating that the ejected electrons appear with no significant dynamic solvation but rather with their equilibrium absorption spectrum. After their initial appearance, ejected electrons do not exhibit any additional dynamics on time scales up to approximately 1 ns, indicating that geminate recombination of e-THF with its iodine atom partner does not occur. Competitive electron scavenging measurements demonstrate that the CTTS excited state of I- in THF is quite large and has contact with scavengers that are several nanometers away from the iodide ion. The ejection time and lack of electron solvation observed for I- in THF are similar to what is observed following CTTS excitation of Na- in THF. However, the relatively slow ejection time, the complete lack of dynamic solvation, and the large ejection distance/lack of recombination dynamics are in marked contrast to the CTTS dynamics observed for I- in water, in which fast electron ejection, substantial solvation, and appreciable recombination have been observed. These differences in dynamical behavior can be understood in terms of the presence of preexisting, electropositive cavities in liquid THF that are a natural part of its liquid structure; these cavities provide a mechanism for excited electrons to relocate to places in the liquid that can be nanometers away, explaining the large ejection distance and lack of recombination following the CTTS excitation of I- in THF. We argue that the lack of dynamic solvation observed following CTTS excitation of both I- and Na- in THF is a direct consequence of the fact that little additional relaxation is required once an excited electron nonadiabatically relaxes into one of the preexisting cavities. In contrast, liquid water contains no such cavities, and CTTS excitation of I- in water leads to local electron ejection that involves substantial solvent reorganization.     

Short-range and long-range solvent effects on charge-transfer-to-solvent transitions of I- and K+I- contact ion pair dissolved in supercritical ammonia

Vertical excitation and electron detachment energies associated with the optical absorption of iodide ions dissolved in supercritical ammonia at 420 K have been calculated in two limiting scenarios: as a solvated free I- ion and forming a K+I- contact ion pair (CIP). The evolution of the transition energies as a result of the gradual building up of the solvation structure was studied for each absorbing species as the solvent's density increased, i.e., changing the NH3 supercritical thermodynamic state. In both cases, if the solvent density is sufficiently high, photon absorption produces a spatially extended electron charge beyond the volume occupied by the solvated solute core; this excited state resembles a typical charge-transfer-to-solvent (CTTS) state. A combination of classical molecular dynamics simulations followed by quantum mechanical calculations for the ground, first-excited, and electron-detached electronic states have been carried out for the system consisting of one donor species (free I- ion or K+I- CIP) surrounded by ammonia molecules. Vertical excitation and electron detachment energies were obtained by averaging 100 randomly chosen microconfigurations along the molecular dynamics trajectory computed for each thermodynamic condition (fluid density). Short- and long-range contributions of the solvent-donor interaction upon the CTTS states of I- and K+I- were identified by performing additional electronic structure calculations where only the solvent interaction due to the first neighbor molecules was taken into account. These computations, together with previous experimental evidence that we collected for the system, have been used to analyze the solvent effects on the CTTS transition. In this paper we have established the following: (i) the CTTS electron of free I- ion or K+I- CIP presents similar features, and it gradually localizes in close proximity of the iodine parent atom when the ammonia density is increased; (ii) for the free I- ion, the short-range solvent interaction contributes to the stabilization of the ground state more than it does for the CTTS excited state, which is evidenced experimentally as a blueshift in the maximum absorption of the CTTS transition when the density is increased; (iii) this effect is less noticeable for the K+I- ion pair, because in this case a tight solvation structure, formed by four NH3 molecules wedged between the ions, appears at very low density and is very little affected by changes in the density; (iv) the long-range contribution to the solvent stabilization can be neglected for the K+I- CIP, since the main features of its electronic transition can be explained on the basis of the vicinity of the cation; (v) however, the long-range solvent field contribution is essential for the free I- ion to become an efficient CTTS donor upon photoexcitation, and this establishes a difference in the CTTS behavior of I- in bulk and in clusters.     

Watching Na atoms solvate into (Na+,e-) contact pairs: untangling the ultrafast charge-transfer-to-solvent dynamics of Na- in tetrahydrofuran (THF)

With the large dye molecules employed in typical studies of solvation dynamics, it is often difficult to separate the intramolecular relaxation of the dye from the relaxation associated with dynamic solvation. One way to avoid this difficulty is to study solvation dynamics using an atom as the solvation probe; because atoms have only electronic degrees of freedom, all of the observed spectroscopic dynamics must result from motions of the solvent. In this paper, we use ultrafast transient absorption spectroscopy to investigate the solvation dynamics of newly created sodium atoms that are formed following the charge transfer to solvent (CTTS) ejection of an electron from sodium anions (sodide) in liquid tetrahydrofuran (THF). Because the absorption spectra of the sodide reactant, the sodium atom, and the solvated electron products overlap, we first examined the dynamics of the ejected CTTS electron in the infrared to build a detailed model of the CTTS process that allowed us to subtract the spectroscopic contributions of the sodide bleach and the solvated electron and cleanly reveal the spectroscopy of the solvated atom. We find that the neutral sodium species created following CTTS excitation of sodide initially absorbs near 590 nm, the position of the gas-phase sodium D-line, suggesting that it only weakly interacts with the surrounding solvent. We then see a fast solvation process that causes a red-shift of the sodium atom's spectrum in approximately 230 fs, a time scale that matches well with the results of MD simulations of solvation dynamics in liquid THF. After the fast solvation is complete, the neutral sodium atoms undergo a chemical reaction that takes place in approximately 740 fs, as indicated by the observation of an isosbestic point and the creation of a species with a new spectrum. The spectrum of the species created after the reaction then red-shifts on a approximately 10-ps time scale to become the equilibrium spectrum of the THF-solvated sodium atom, which is known from radiation chemistry experiments to absorb near approximately 900 nm. There has been considerable debate as to whether this 900-nm absorbing species is better thought of as a solvated atom or a sodium cation:solvated electron contact pair, (Na+,e-). The fact that we observe the initially created neutral Na atom undergoing a chemical reaction to ultimately become the 900-nm absorbing species suggests that it is better assigned as (Na+,e-). The approximately 10-ps solvation time we observe for this species is an order of magnitude slower than any other solvation process previously observed in liquid THF, suggesting that this species interacts differently with the solvent than the large molecules that are typically used as solvation probes. Together, all of the results allow us to build the most detailed picture to date of the CTTS process of Na- in THF as well as to directly observe the solvation dynamics associated with single sodium atoms in solution.     

Na(-) photolysis in THF: Charge transfer to solvent studied from the donors perspective in <10 fs 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.     

Electron solvation in water clusters following charge transfer from iodide

The dynamics following charge transfer to solvent from iodide to a water cluster are studied using time-resolved photoelectron imaging of I-(H2O)n and I-(D2O)n clusters with n< or =28. The results show spontaneous conversion, on a time scale of approximately 1 ps, from water cluster anions with surface-bound electrons to structures in which the excess electron is more strongly bound and possibly more internalized within the solvent network. The resulting dynamics provide valuable insight into the electron solvation dynamics in water clusters and the relative stabilities between recently observed isomers of water cluster anions.     

Manipulating the production and recombination of electrons during electron transfer: Femtosecond control of the charge-transfer-to-solvent (CTTS) dynamics of the sodium anion

The scavenging of a solvated electron represents the simplest possible electron-transfer (ET) reaction. In this work, we show how a sequence of femtosecond laser pulses can be used to manipulate an ET reaction that has only electronic degrees of freedom: the scavenging of a solvated electron by a single atom in solution. Solvated electrons in tetrahydrofuran are created via photodetachment using the charge-transfer-to-solvent (CTTS) transition of sodide (Na(-)). The CTTS process ejects electrons to well-defined distances, leading to three possible initial geometries for the back ET reaction between the solvated electrons and their geminate sodium atom partners (Na(0)). Electrons that are ejected within the same solvent cavity as the sodium atom (immediate contact pairs) undergo back ET in approximately 1 ps. Electrons ejected one solvent shell away from the Na(0) (solvent-separated contact pairs) take hundreds of picoseconds to undergo back ET. Electrons ejected more than one solvent shell from the sodium atom (free solvated electrons) do not recombine on subnanosecond time scales. We manipulate the back ET reaction for each of these geometries by applying a "re-excitation" pulse to promote the localized solvated electron ground state into a highly delocalized excited-state wave function in the fluid's conduction band. We find that re-excitation of electrons in immediate contact pairs suppresses the back ET reaction. The kinetics at different probe wavelengths and in different solvents suggest that the recombination is suppressed because the excited electrons can relocalize into different solvent cavities upon relaxation to the ground state. Roughly one-third of the re-excited electrons do not collapse back into their original solvent cavities, and of these, the majority relocalize into a cavity one solvent shell away. In contrast to the behavior of the immediate pair electrons, re-excitation of electrons in solvent-separated contact pairs leads to an early time enhancement of the back ET reaction, followed by a longer-time recombination suppression. The recombination enhancement results from the improved overlap between the electron and the Na(0) one solvent shell away due to the delocalization of the wave function upon re-excitation. Once the excited state decays, however, the enhanced back ET is shut off, and some of the re-excited electrons relocalize even farther from their geminate partners, leading to a long-time suppression of the recombination; the rates for recombination enhancement and relocalization are comparable. Enhanced recombination is still observed even when the re-excitation pulse is applied hundreds of picoseconds after the initial CTTS photodetachment, verifying that solvent-separated contact pairs are long-lived, metastable entities. Taken together, all these results, combined with the simplicity and convenient spectroscopy of the sodide CTTS system, allow for an unprecedented degree of control that is a significant step toward building a full molecular-level picture of condensed-phase ET reactions.     

Mapping CTTS dynamics of Na- in tetrahydrofurane with ultrafast multichannel pump-probe spectroscopy

Using multichannel femtosecond spectroscopy we have followed Na- charge transfer to solvent (CTTS) dynamics in THF solution. Absorption of the primary photoproducts in the visible, resolved here for the first time, consists of an asymmetric triplet centered at 595 nm, which we assign to a metastable incompletely solvated neutral atomic sodium species. Decay of this feature within approximately 1 ps to a broad and structureless solvated neutral is accompanied by broadening and loss of spectral detail. Kinetic analysis shows that both the spectral structure and the decay of this band are independent of the excitation photon frequency in the range 400-800 nm. With different pump-probe polarizations the anisotropy in transient transmission has been charted and its variation with excitation wavelength surveyed. The anisotropies are assigned to the reactant bleach, indicating that due to solvent-induced symmetry breaking, the CTTS absorption band of Na- is made up of discreet orthogonally polarized sub bands. None of the anisotropy in transient absorption could be associated with the photoproduct triplet band even at the earliest measurable time delays. Along with the documented differences in the spatial distribution of ejected electrons across the tested excitation wavelength range, these results lead us to conclude that photoejection is extremely rapid, and that loss of correlations between the departing electron and its neutral core is faster than our time resolution of approximately 60 fs.     

Elucidating the initial dynamics of electron photodetachment from atoms in liquids using variably-time-delayed resonant multiphoton ionization

We study the photodetachment of electrons from sodium anions in room temperature liquid tetrahydrofuran (THF) using a new type of three-pulse pump-probe spectroscopy. Our experiments use two variably-time-delayed pulses for excitation in what is essentially a resonant 1+1 two-photon ionization: By varying the arrival time of the second excitation pulse, we can directly observe how solvent motions stabilize and trap the excited electron prior to electron detachment. Moreover, by varying the arrival times of the ionization (excitation) and probe pulses, we also can determine the fate of the photoionized electrons and the distance they are ejected from their parent Na atoms. We find that as solvent reorganization proceeds, the second excitation pulse becomes less effective at achieving photoionization, and that the solvent motions that stabilize the excited electron following the first excitation pulse occur over a time of approximately 450 fs. We also find that there is no spectroscopic evidence for significant solvent relaxation after detachment of the electron is complete. In combination with the results of previous experiments and molecular dynamics simulations, the data provide new insight into the role of the solvent in solution-phase electron detachment and charge-transfer-to-solvent reactions.     

One-photon photodetachment of I- in glycerol: spectra and yield of solvated electrons in the temperature range 329<or=T<or=536 K

Solvated electrons in glycerol were generated via a resonant one-photon photodetachment of the charge-transfer-to-solvent (CTTS) band of I- in glycerol (Gl) after irradiation with a 248 nm excimer laser. Optical absorption spectra of solvated electrons (esolv-) in Gl were recorded as a function of temperature (381相似文献   

Photofragment coincidence imaging of small I-(H2O)n clusters excited to the charge-transfer-to-solvent state

The photodissociation dynamics of small I-(H2O)n(n=2-5) clusters excited to their charge-transfer-to-solvent (CTTS) states have been studied using photofragment coincidence imaging. Upon excitation to the CTTS state, two photodissociation channels were observed. The major channel (approximately 90%) is a two-body process forming neutral I+(H2O)n photofragments, and the minor channel is a three-body process forming I+(H2O)n-1+H2O fragments. Both processes display translational energy [P(ET)] distributions peaking at ET=0 with little available energy partitioned into translation. Clusters excited to the detachment continuum rather than to the CTTS state display the same two channels with similar P(ET) distributions. The observation of similar P(ET) distributions from the two sets of experiments suggests that in the CTTS experiments, I atom loss occurs after autodetachment of the excited [I(H2O)n-]* cluster or, less probably, that the presence of the excess electron has little effect on the departing I atom.     

Electron solvation dynamics in I-(NH3)n clusters

Femtosecond photoelectron spectroscopy (FPES) is used to monitor the dynamics associated with the excitation of the charge-transfer-to-solvent (CTTS) precursor states in I-(NH3)n = 4-15 clusters. The FPE spectra imply that the weakly bound excess electron in the excited state undergoes partial solvation via solvent rearrangement on a time scale of 0.5-2 ps, and this partially solvated state decays by electron emission on a 10-50 ps time scale. Both the extent of solvation and the lifetimes increase gradually with cluster size, in contrast to the more abrupt size-dependent effects previously observed in I-(H2O)n clusters.     

Charge‐transfer‐to‐solvent absorption spectra of I−(H2O)3–5 at a finite temperature via simulation

Ground‐state equilibrium Born–Oppenheimer molecular dynamics on I^?(H₂O)_3–5 clusters at ～200 K are performed to sample configurations for calculating the charge‐transfer‐to‐solvent (CTTS) absorption spectra for these clusters. When there are more water molecules in clusters, the calculated CTTS spectra are found to become more intense with the absorption maxima shifting to higher energies, which is in agreement with experimental results. In addition, compared with the findings for optimized structures, the absorption energies of the iodide 5p orbitals are red‐shifted at ～200 K because, on average, the distances between the iodide and the dangling hydrogen atoms are increased at finite temperatures which weakens the interactions between the iodide and water molecules in the clusters. Moreover, the number of ionic hydrogen bonds in the clusters are also reduced. However, it is found that all dangling hydrogen atoms must be considered to obtain a good correlation between the CTTS excitation energy and the average distance between the iodide and the dangling hydrogen atoms, which indicates the existence of the strong interactions of the CTTS electron with all of the dangling hydrogen atoms.     

Moving solvated electrons with light: nonadiabatic mixed quantum/classical molecular dynamics simulations of the relocalization of photoexcited solvated electrons in tetrahydrofuran (THF)

Motivated by recent ultrafast spectroscopic experiments [Martini et al., Science 293, 462 (2001)], which suggest that photoexcited solvated electrons in tetrahydrofuran (THF) can relocalize (that is, return to equilibrium in solvent cavities far from where they started), we performed a series of nonequilibrium, nonadiabatic, mixed quantum/classical molecular dynamics simulations that mimic one-photon excitation of the THF-solvated electron. We find that as photoexcited THF-solvated electrons relax to their ground states either by continuous mixing from the excited state or via nonadiabatic transitions, approximately 30% of them relocalize into cavities that can be over 1 nm away from where they originated, in close agreement with the experiments. A detailed investigation shows that the ability of excited THF-solvated electrons to undergo photoinduced relocalization stems from the existence of preexisting cavity traps that are an intrinsic part of the structure of liquid THF. This explains why solvated electrons can undergo photoinduced relocalization in solvents like THF but not in solvents like water, which lack the preexisting traps necessary to stabilize the excited electron in other places in the fluid. We also find that even when they do not ultimately relocalize, photoexcited solvated electrons in THF temporarily visit other sites in the fluid, explaining why the photoexcitation of THF-solvated electrons is so efficient at promoting recombination with nearby scavengers. Overall, our study shows that the defining characteristic of a liquid that permits the photoassisted relocalization of solvated electrons is the existence of nascent cavities that are attractive to an excess electron; we propose that other such liquids can be found from classical computer simulations or neutron diffraction experiments.     

Charge-transfer-to-solvent-driven dissolution dynamics of I- (H2O)2-5 upon excitation: excited-state ab initio molecular dynamics simulations

In contrast to the extensive theoretical investigation of the solvation phenomena, the dissolution phenomena have hardly been investigated theoretically. Upon the excitation of hydrated halides, which are important substances in atmospheric chemistry, an excess electron transfers from the anionic precursor (halide anion) to the solvent and is stabilized by the water cluster. This results in the dissociation of hydrated halides into halide radicals and electron-water clusters. Here we demonstrate the charge-transfer-to-solvent (CTTS)-driven femtosecond-scale dissolution dynamics for I-(H2O)n=2-5 clusters using excited state (ES) ab initio molecular dynamics (AIMD) simulations employing the complete-active-space self-consistent-field (CASSCF) method. This study shows that after the iodine radical is released from I-(H2O)n=2-5, a simple population decay is observed for small clusters (2 相似文献   

Dynamical calculations of charge-transfer-to-solvent excited states of small I- (CH3CN)n clusters

Relaxation dynamics of photoexcited charge-transfer-to-solvent (CTTS) states for the I(-)(CH(3)CN)(n) (n = 2 and 3) clusters has been theoretically studied using electronic structure methods. First, we have calculated several lowest singlet and triplet potential energy surfaces using the multireference configuration interaction method. It was found that the character of the singlet CTTS excited-state potential surfaces is very similar to that of the triplet CTTS states. Due to a small singlet-triplet splitting, the lowest triplet potential energy surface was used as a good model to understand the dynamics of the photoexcited singlet CTTS states. We have carried out direct molecular dynamics simulations on the lowest triplet surface at the B3LYP level. When an I(-) anion is exteriorly solvated by CH(3)CN molecules, we found that the (CH(3)CN)(n)(-) anion cluster is effectively produced. In addition, when the I(-) anion is placed in the interior in I(-)(CH(3)CN)(n) clusters, photoexcitation gives an acetonitrile monomer anion plus neutral monomers. However, if the initial geometric configuration is distorted from the minimum structure, we also found that the (CH(3)CN)(2)(-) anion cluster, where an excess electron is internally trapped, is formed via I(-)(CH(3)CN)(2) + hnu --> I + (CH(3)CN)(2)(-) process.     

Preparation of a novel iodide-selective electrode based on iodide-miconazole ion-pair and its application to pharmaceutical analysis.

An iodide-miconazole ion-paired complex was used as a suitable ion-exchanger for the preparation of a plasticized-PVC membrane electrode. Among different solvent mediators tested, dioctylsebacate exhibited the proper response characteristics, including Nernstian slope of the calibration curve, fast response time and good reproducibility of the emf values. The electrode exhibits a Nernstian slope of -59.8 +/- 0.5 mV decade(-1) for I- ion over a concentration range of 1.0 x 10(-5) - 1.0 x 10(-2) M with a limit of detection of 7.0 x 10(-6) M. The electrode displays a good selectivity for I- with respect to a number of inorganic and organic species. It canbe used over a pH range of 2.5 - 8.5. The membrane sensor was successfully applied to the determination of iodide in water samples and blood serum, as well as in pharmaceutical products such as iodoquinol and thyroxin.     

Dynamics of charge-transfer-to-solvent precursor states in I- (water)n (n = 3-10) clusters studied with photoelectron imaging

The dynamics of charge-transfer-to-solvent states are studied in I- (H2O)(n=3-10) clusters and their deuterated counterparts using time-resolved photoelectron imaging. The photoelectron spectra for clusters with n > or = 5 reveal multiple time scales for dynamics after their electronic excitation. An increase in the vertical detachment energy (VDE) by several hundred millielectronvolts on a time scale of approximately 1 ps is attributed to stabilization of the excess electron, primarily through rearrangement of the solvent molecules, but a contribution to this stabilization from motion of the I atom cannot be ruled out. The VDE drops by approximately 50 meV on a time scale of tens of picoseconds; this is attributed to loss of the neutral iodine atom. Finally, the pump-probe signal decays with a time constant of 60 ps-3 ns, increasing with cluster size. This decay is commensurate with the growth of very slow electrons and is attributed to autodetachment. Smaller clusters (n = 3, 4) display simpler dynamics. Anisotropy parameters are reported for clusters n = 4-9.     

Electron photodetachment from iodide in ionic liquids through charge-transfer-to-solvent band excitation

Solvation of iodide and electrons in an ionic liquid (N,N,N-trimethyl-n-propylammonium bis(trifluoromethanesulfonyl)imide; TMPA-TFSI) was studied through the absorption spectra of the charge-transfer-to-solvent (CTTS) state of iodide and of solvated electrons. The interaction between the TMPA cation and iodide was strong, whereas electrons were weakly solvated in TMPA-TFSI. We followed electron photodetachment from iodide to the ionic liquid and formation of the solvated electrons by observing absorption in the visible and near-infrared regions using a nanosecond laser flash photolysis method. The quantum yield of the photodetachment in TMPA-TFSI was estimated to be 0.34, which is much higher than that in a high-concentration aqueous salt solution previously reported. We also examined a reaction of the solvated electrons with the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (Bmim-TFSI) as a solute in TMPA-TFSI. The reaction rate was determined to be 5.3 x 10(8) M(-1) s(-1). The electrons before full solvation (dry electrons) reacted with Bmim cations efficiently. These observations suggest that the electrons in TMPA-TFSI can move easily before solvation.     

Excited-state dynamics of (SO2)m clusters

A study of the excited-state dynamics of (SO2)m clusters following excitation by ultrafast laser pulses in the range of 4.5 eV (coupled 1A2, 1B1 states) and 9 eV (F band) is presented. The findings for the coupled 1A2 and 1B1 states are in good agreement with published computational work on the properties of these coupled states. A mechanism involving charge transfer to solvent is put forward as the source of the excited-state dynamics that follow the excitation of the SO2 F band within (SO2)m+1 clusters with m > 1. The proposed CTTS mechanism is supported by calculations of the energetics of the process and the observed trends in the excited-state lifetimes that correlate very well with the calculated energies.     

Microsolvation of the sodium and iodide ions and their ion pair in acetonitrile clusters: a theoretical study

The structural and thermodynamic properties of Na+(CH3CN)n, I-(CH3CN)n, and NaI(CH3CN)n clusters have been investigated by means of room-temperature Monte Carlo simulations with model potentials developed to reproduce the properties of small clusters predicted by quantum chemistry. Ions are found to adopt an interior solvation shell structure, with a first solvation shell containing approximately 6 and approximately 8 acetonitrile molecules for large Na+(CH3CN)n and I-(CH3CN)n clusters, respectively. Structural features of Na+(CH3CN)n are found to be similar to those of Na+(H2O)n clusters, but those of I-(CH3CN)n contrast with those of I-(H2O)n, for which "surface" solvation structures were observed. The potential of mean force calculations demonstrates that the NaI ion pair is thermodynamically stable with respect to ground-state ionic dissociation in acetonitrile clusters. The properties of NaI(CH3CN)n clusters exhibit some similarities with NaI(H2O)n clusters, with the existence of contact ion pair and solvent-separated ion pair structures, but, in contrast to water clusters, both types of ion pairs adopt a well-defined interior ionic solvation shell structure in acetonitrile clusters. Whereas contact ion pair species are thermodynamically favored in small clusters, solvent-separated ion pairs tend to become thermodynamically more stable above a cluster size of approximately 26. Hence, ground-state charge separation appears to occur at larger cluster sizes for acetonitrile clusters than for water clusters. We propose that the lack of a large Na+(CH3CN)n product signal in NaI(CH3CN)n multiphoton ionization experiments could arise from extensive stabilization of the ground ionic state by the solvent and possible inhibition of the photoexcitation mechanism, which may be less pronounced for NaI(H2O)n clusters because of surface solvation structures. Alternatively, increased solvent evaporation resulting from larger excess energies upon photoexcitation or major solvent reorganization on the ionized state could account for the observed solvent-selectivity in NaI cluster multiphoton ionization.