Nearby Solvent Research Articles

The role of solvent structure in the absorption spectrum of solvated electrons: Mixed quantum/classical simulations in tetrahydrofuran

In polar fluids such as water and methanol, the peak of the solvated electron's absorption spectrum in the red has been assigned as a sum of transitions between an s-like ground state and three nearly degenerate p-like excited states bound in a quasispherical cavity. In contrast, in weakly polar solvents such as tetrahydrofuran (THF), the solvated electron has an absorption spectrum that peaks in the mid-infrared, but no definitive assignment has been offered about the origins of the spectrum or the underlying structure. In this paper, we present the results of adiabatic mixed quantum/classical molecular dynamic simulations of the solvated electron in THF, and provide a detailed explanation of the THF-solvated electron's absorption spectrum and electronic structure. Using a classical solvent model and a fully quantum mechanical excess electron, our simulations show that although the ground and first excited states are bound in a quasispherical cavity, a multitude of other, nearby solvent cavities support numerous, nearly degenerate, bound excited states that have little Franck-Condon overlap with the ground state. We show that these solvent cavities, which are partially polarized so that they act as electron trapping sites, are an inherent property of the way THF molecules pack in the liquid. The absorption spectrum is thus assigned to a sum of bound-to-bound transitions between a localized ground state and multiple disjoint excited states scattered throughout the fluid. Furthermore, we find that the usual spherical harmonic labels (e.g., s-like, p-like) are not good descriptors of the excited-state wave functions of the solvated electron in THF. Our observation of multiple disjoint excited states is consistent with femtosecond pump-probe experiments in the literature that suggest that photoexcitation of solvated electrons in THF causes them to relocalize into solvent cavities far from where they originated.

Three Types of Physical Measurements Needed to Characterize Iron Oxide Nanoparticles for MRI and MRA: Magnetization, Relaxometry, and Light Scattering

Iron oxide nanoparticles, with monocrystalline cores (generally magnetite or maghemite) and coated with organic polymer to increase chemical stability and solubility, have generated widespread interest as negative contrast (susceptibility) agents for magnetic resonance imaging (MRI) (1,2). For negative agents, the requisite T2 shortening depends on the geometry of the physiologic structures that take up the agent (2) but is relatively insensitive to the size and structure of the nanoparticles themselves. Nanoparticles also have a potential as positive agents for magnetic resonance angiography (MRA) (3–6), an application that depends on T1 shortening in the vasculature by individual nanoparticles, provided that T2 is not too short (7). To optimize their suitability for MRA, the agents must be synthesized and characterized reproducibly, the size of the magnetic core must be carefully controlled, and the interaction of the organic coating with nearby solvent molecules must be understood (8). Here we describe a set of physical techniques for measuring the parameters that determine MRA efficacy: the magnetic moment and diameter d of the iron oxide cores, and the interaction of organic coating with solvent. We study magnetization (8) as a function of magnetic field B0; l/Tl nuclear magnetic relaxation dispersion (NMRD) profiles (9) for B0 from 0.24 mT to 1.2 T (0.01–50 MHz) and 1/T2 at 0.47 T (20 MHz); and solute translational diffusivity by photon correlation spectroscopy (PCS) (10). As demonstrated earlier (8), magnetization data, being thermodynamic, yield and the saturation magnetization of the particles, Msat. Combined with total iron content and the known density of the core, one obtains d. When these results are combined with NMRD data—which depend on and d and yield certain correlation times—one can detect any water-opaque organic “rind” and measure the diffusivity D of outer sphere solvent, a measure of viscous interactions between solvent and polymer. These interactions can be quantitated by PCS, which yields the effective radius of gyration of the polymers (11), including contributions from both polymer and viscously entrained water molecules. This trio of measurements lets one characterize the physical properties of solute nanoparticles self-consistently, deduce their behavior in MRA applications, and guide the development of improved agents. We report on two preparations of coated iron oxide nanoparticles: (a) CLARISCAN, a Nycomed Amersham product in clinical trials (8); and (b) MION-46L, a laboratory preparation (11) of MION-46 obtained from Massachusetts General Hospital.

Electron Transfer Involving Nonbonded Superexchange Interactions in Rigid Donor−Acceptor Arrays

Rigid intramolecular trichromophoric arrays based on a 4,5-diaminoxanthene bridge B were prepared in which the xanthene bridge orients A2 relative to D, so that nonbonded interactions between 1*D and A2 can be studied. The electron donor, D, is 4-(N-piperidinyl)naphthalene-1,8-dicarboximide (ANI), and the acceptors A1 and A2 are pyromellitimide (PI) and naphthalene-1,8:4,5-bis(dicarboximide) (NI), respectively, and X is either n-C8H17 or H. The torsional angles of the xanthene−imide single bonds orient the NI and PI acceptors approximately cofacial to one another. Femtosecond transient absorption measurements show that electron transfer from 1*ANI to NI occurs by nonbonded superexchange interactions that include contributions from both the n-C8H17 substituent on the NI acceptor and nearby solvent molecules. The structural rigidity of these compounds allows evaluation of the distance dependence of both charge separation and recombination. For charge separation, β = 1.1−1.3 A-1, and evidence exists for solv...

Subpicosecond Fluorescence Spectra of Tryptophan in Water

A femtosecond ultraviolet fluorescence spectrophotofluorometer has been exploited for the study of tryptophan and analogues. Using a time resolution better than 300 fs, time-resolved emission spectra have been constructed for TRP in water. They reveal an initial spectrum shifted 4900 cm-1 from the putative 0−0 band at 295 nm, along with a picosecond relaxation to a spectrum 412 cm-1 lower and 322 cm-1 wider. The time constant of this “solvent relaxation” of tryptophan in water was ∼1.2 ps, as expected for forced diffusive reorientation of nearby solvent molecules upon excitation. Emission anisotropy measurements on the same time scale, obtained at a wide range of excitation and emission wavelengths, yielded “r0” (apparent initial anisotropies) compatible with classical measurements in glassy solvents and single-exponential anisotropy decay. This implies that any state conversion must occur well within the impulse response of our instrument; eg., τIC < 300 fs. Even upon “red edge” (301 nm) excitation, the ...

Rotational energy relaxation of individual rotational states in liquids

The manner in which most molecules reorient in liquids bears little resemblance to the process in the gas phase. For small-moment-of-inertia species such as the hydrides, however, the observation of discrete spectroscopic lines corresponding to individual isolated-molecule quantum transitions suggests that one is actually seeing single-molecule dynamics perturbed only weakly by the environment—just as one sees with solution-phase vibrational behavior. We examine here the degree to which such individual rotational quantum states remain well defined in liquids by considering the rates of discrete energy-level-to-energy-level transitions in solution. For rotational quantum states that do preserve their free-rotor character in a liquid, we find that the transition rate between angular momentum states obeys a rotational Landau–Teller relation strikingly similar to the analogous expression for vibration: the rate is proportional to the liquid’s rotational friction evaluated at the transition frequency. Subsequent evaluation of this friction by classical linearized instantaneous-normal-mode theory suggests that we can understand this relationship by regarding the relaxation as a kind of resonant energy transfer between the solute and the solution modes. On specializing to the particular cases of H2 and D2 in Ar(l), we find that the most critical modes are those that move the light solute’s center of mass with respect to a single nearby solvent. This observation, in turn, suggests a generalization of instantaneous-normal-mode ideas that transcends both linear coupling and harmonic dynamics: an instantaneous-pair theory for the relaxation of higher-lying levels. By employing a linearized instantaneous-normal-mode theory of relaxation within the liquid band and an instantaneous-pair theory for higher-frequency relaxation, we find that the resonant-transfer paradigm is reasonably successful in reproducing molecular dynamics results spanning a wide range of different rotational states.

Solvent structure and perturbations in solutions of chemical and biological importance

Much of chemistry and biology happens in an aqueous environment. Although the water is thought to play a significant role in the various processes, our understanding of these solvent effects at the molecular level is limited. Recent developments In neutron scattering are changing this: we can now begin to see directly how different solutes are hydrated, and how the solute perturbs its nearby solvent. This article discusses recent results on the hydration of both polar and non-polar groups, and tentatively concludes that the classical

Reaction-field–supermolecule approach to calculation of solvent effects

The self-consistent reaction-field formalism has been implemented into the AM1 and MNDO molecular structure codes and has been used to calculate the solvation energies of water and ions in aqueous solution. The interaction of nearby solvent molecules with the solute has been included explicitly by combining the reaction-field formalism with the supermolecule model. The self-consistent reaction-field method was also used to study dielectric effects on tautomeric equilibria. The agreement with the experimental results is good and indicates the usefulness of the combined reaction-field–supermolecule approach for studying solvent effects on chemical processes.

Kinetic and spectroscopic study of triplet state and ionic pathways in the laser-induced photoexcitation of N-ethylcarbazole in fluid solutions

When fluid solutions of N-ethylcarbazole (NEC) are irradiated by 308-nm pulses from an excimer laser, transient triplets and ions may be observed in the microsecond time regime. The formation of ions is favored in more polar solvents such as acetonitrile (AcN) and that of triplets in nonpolar solvents such as paraffin oil. The quantum yield of triplet formation from initial excitation is 0.23. Triplet states decay through concurrent first- and second-order channels: the second-order process, triplet-triplet annihilation, leads to formation of excited singlet states with an efficiency of 0.12. Transient carbazolyl cations and anions display double-exponential decays. To account for this behavior, a mechanism involving initial electron donation by the multiply excited chromophore to a nearby NEC molecule or solvent molecule is proposed; subsequent decay occurs primarily by germinate recombination of the resulting ion pair. The quantum yield for cation production is 0.037. Delayed fluorescence is AcN also decays by a double-exponential decay curve with rate constants, suggesting that the emission arises mainly from ion recombination.

Excited states, electron-transfer reactions, and intermediates in bacterial photosynthetic reaction centers

The three-dimensional structure of a photosynthetic reaction center has recently been obtained at atomic resolution using x-ray crystallography by Deisenhofer, Epp, Miki, Huber, and Michel (J. Mol. Biol. 1984, 180, 385-398; Nature 1985, 318, 618-624). This breakthrough provides the fundamental structural information needed to understand the mechanisms of the initial energy- and electron-transfer steps in photosynthesis. The structure reveals the distances among the reactive bacteriochlorophylls and quinones as well as the location of all nearby solvent molecules, the amino acids of the reaction center protein. Thus, the reaction center provides a complex but well-defined solid-state reactive system for the study of fundamental physical and chemical processes with implications and applications well beyond this specific system. We review recent studies of the reaction intermediates and mechanism of electron transfer in which the energetics and reaction dynamics have been perturbed with external electric and magnetic fields. Electron-transfer mechanisms which have been proposed are reviewed critically in light of the available data, and electron transfer in the reaction center is compared with electron transfer in other biological and nonbiological systems.

2D heteronuclear NOE study of intermolecular interactions between a solute carbon-13 nucleus and nearby solvent protons

2D heteronuclear NOE study of intermolecular interactions between a solute carbon-13 nucleus and nearby solvent protons

Theory of the anomalous resonant absorption of DNA at microwave frequencies.

Aqueous solutions of oligopolymer DNA have been observed by Edwards, Davis, Swicord & Saffer to show structured absorption of microwave energy in the region of several gigahertz characteristic of an ordered series of compressional normal mode vibrations propagating on the polymer chain. Hydrodynamic coupling of such vibrations to the surrounding solvent would preclude the existence of sharp resonances. The inclusion of electromagnetic interactions with surrounding counter ions yields a richer space of possibilities for complex behavior of the combined system. A well defined resonant absorption peak appears when the molecular motion and the nearby solvent motion are even slightly decoupled. The microwave electric fields in the vicinity of the molecule provide a mechanism for such a decoupling not present for the case of electrically neutral solvent.

Applications of synchronously pumped dye-lasers to time-resolved emission and absorption spectroscopy

Methods of generation and measurement of tunable ultrashort light pulses from c.w. synchronously pumped dye-lasers are described. Conditions for optimal pulse shape are discussed and experiments to determine absolute dye-laser cavity length, pulse-to-pulse reproducibility and pulse substructure are described. Methods of applying synchronously pumped dye-lasers to time-resolved emission and absorption spectroscopy are described. Results on the orientational motion of oxazine in a variety of solvents are described and discussed, in terms of the hydrodynamic boundary condition. Fluorescence data on the solvent-dependent excited-state dynamics of anilinonaphthalene sulphonate are presented and discussed. It is suggested that the very rapid radiationless transition observed in aqueous solution corresponds to electron transfer from the excited molecule to a nearby solvent trap.

Intermediates in the photolysis of alkyl sulfides and disulfides in dilute glass matrices

Simple alkyl sulfides and disulfides have been photolysed at 253.7 nm in dilute rigid glasses of 3-methylpentane, and the reactions which occur both at 77 K and in the glass at higher temperatures have been studied using epr and optical spectroscopy. The end products of photolysis have been determined using glc at room temperature. In sulfides, C—S bond cleavage leads to 'hot' radical pairs that react with each other giving rise to the main diamagnetic products observed, or react with nearby solvent molecules in the matrix by hydrogen abstraction yielding RSH and the solvent derived radicals (M•) which dominate the initial epr spectrum at 77 K. Thiyl radicals (RS•) are produced in significant amounts only after the sulfide glasses have been warmed slightly to allow the cage reaction,[Formula: see text]to occur. Pethiyl radicals (RS2•) are not ordinarily observed in sulfide photolyses. Photolysis of disulfide-containing glasses gives rise to S—S bond rupture and mainly diamagnetic products, or to C—S bond cleavage resulting in the formation of solvent radicals and RS2. Very little RS• is generated on warming the matrices. As with ethyl mercaptan photolysis, radicals corresponding to α-hydrogen abstraction of the substrate are obtained for both ethyl sulfide and disulfide. The spectral characteristics and photolytic behaviour of the sulfur containing radical species have been investigated where possible.

Raman spectroscopic study of the valinomycin-KSCN complex

This paper reports the first Raman spectroscopic study of the potassium complex of the cation-specific antibiotic valinomycin. Complete Raman spectra (140 to 3600 cm −1) of crystalline valinomycin-KSCN and its CCl 4, CHCl 3 and C 2H 5OH solutions are presented and used to probe the structure of the complex in these environments. In all cases a single, narrow peak is observed in the ester CO stretch region (1750 to 1775 cm −1) which contrasts strongly with the broad bands observed in solutions of uncomplexed valinomycin. This is consistent with the presence of a single conformation in which all six ester CO groups co-ordinate an enclosed potassium ion. We find that although the ester CO stretch frequencies of the complex are similar in the solid state and in non-polar solution (~1770 cm −1) they are considerably different in the presence of polar solvents (~1756 cm −1); this may indicate that the complexed potassium ion is still free to interact with nearby solvent ions (and possibly its counterion) through gaps in the hydrophobic “shield” provided by the hydrocarbon residues of valinomycin. In contrast the amide CO frequencies of the complex (~1650 cm −1) are solvent-independent. These groups are apparently strongly hydrogen-bonded to provide a rather rigid, compact framework for the complex conformation.

Transport Processes in Liquid Alloys. II. The Electrical Force on an Ion

The electrolytic migration velocities of impurity ions in liquid metals suggest the existence of strong net electrical forces on these ions, although other mechanisms are possible. On the basis of a detailed model of a liquid metal the early theory of Skaupy is revived and revised. In dilute solution a total electrical force, proportional to the conductivity increment produced by a solute, is exerted on the solute and on nearby solvent when the electric field is applied. The part of the force on the solute ion may be opposed to the part on the solvent. Implications for the resistivity of liquid alloys are discussed, including a possible effect in very dilute solutions when the solute ions are further apart than one mean free path. The effective number of free electrons in pure liquid mercury is provisionally estimated to be 0.3 per atom.