Showing papers on "Solvent effects published in 2005"

Unusually stable palladium(IV) complexes: detailed mechanistic investigation of C-O bond-forming reductive elimination.

Abstract: This communication describes the synthesis of a family of unusually stable palladium(IV) complexes containing two chelating 2-phenylpyridine ligands and two benzoates. These complexes undergo clean C−O bond-forming reductive elimination upon heating, and the mechanism of this catalytically relevant process has been studied in detail. Solvent effects, crossover experiments, Eyring plots (which show ΔS⧧ of −1.4 ± 1.9 and 4.2 ± 1.4 in CDCl3 and DMSO, respectively), and Hammett analysis (which shows ρ = −1.36 ± 0.04 upon substitution of the para-benzoate substituent) all suggest that reductive elimination does not proceed via initial dissociation of a benzoate ligand. Instead, an unusual mechanism involving pre-equilibrium dissociation of the N-arm of the phenylpyridine ligand is proposed.

Solvent Effects on the Two-Photon Absorption of Distyrylbenzene Chromophores

Abstract: A series of organic- and water-soluble distyrylbenzene-based two-photon absorption (TPA) fluorophores containing dialkylamino donor groups at the termini was designed, synthesized, and characterized. The central core was systematically substituted to modulate intramolecular charge transfer (ICT). These molecules allow an examination of solvent effects on the TPA cross section (delta) and on the TPA action cross section. In toluene, the delta values follow the order of ICT strength. The effect of solvent on delta is nonmonotonic: maximum delta was measured in an intermediate polarity solvent (THF) and was lowest in water. We failed to find a correlation between the observed solvent effect and previous theoretical predictions. Hydrogen bonding to the donor groups and aggregation of the optical units in water, which are not included in calculational analysis, may be responsible for the discrepancies between experimental results and theory.

Polar Attributes of Supercritical Carbon Dioxide

Abstract: Supercritical carbon dioxide (scCO2) is increasingly promoted as an environmentally benign alternative to conventional organic solvents. The supercritical state bridges the gap between liquid and gaseous states by offering gaslike diffusion rates and liquidlike solvent densities, thereby enabling potential opportunities as a reaction and separation medium in chemical industry. Understanding the solvent behavior of liquid and scCO2 is of critical importance to enable the design of CO2-philic molecular systems and to expand the use of these solvent systems to a wider range of chemical processes. Historically CO2 was treated as a nonpolar solvent, primarily because of its low dielectric constant and zero molecular dipole moment. CO2 has also been described as a quadrupolar solvent because of its significant quadrupole moment. Recent studies suggest that, as far as the microscopic solvent behavior of CO2 is concerned, CO2 has the potential to act as both a weak Lewis acid and Lewis base. Also, strong theoreti...

Solvent Effect on Crystal Polymorphism: Why Addition of Methanol or Ethanol to Aqueous Solutions Induces the Precipitation of the Least Stable β Form of Glycine

Abstract: Crystal polymorphism, which embodies the ability of molecules to form diverse packing arrangements displaying different physical and chemical characteristics, is of paramount importance in fields such as pharmacology, solid-state chemistry, and material science. However, the conditions to induce the precipitation of various (metastable) polymorphs is invariably achieved by “mix and try” methods, which are kinetically driven. Various factors should be considered in trying to understand these complex processes, for example, the formation of structured clusters in solution prior to crystallization, the structure of growing surfaces that delineate emerging nuclei, the interaction between these surfaces and the solvent, as well as solvent–solute and solute– solute interactions. Herein, we attempt to unravel some of these factors to rationalize the preferred crystallization of the b form of glycine (gly) in water–alcohol solutions as opposed to the more stable a or g polymorphs. The thermodynamic stability of the three polymorphs of glycine at room temperature is in the order g>a> b. The a form (space group P21/n), grown from supersaturated aqueous solutions (33.3 g/100 mL water) at 25 8C, has a bipyramidal habit and is composed of centrosymmetric bilayers formed by strong NH···O hydrogen-bonding interactions between cyclic hydrogen-bonded zwitterionic molecular pairs. These bilayers are related along the b axis by glide symmetry through weak CH···O interactions (Figure 1a). Previous studies indicated that a-gly crystallizes primarily in aqueous solutions through hydrogen-bonded cyclic dimer growth units: Diffusion-coefficient measurements of supersaturated aqueous solutions of glycine point to the formation of clusters with an average of 1.8 molecular growth units. Furthermore, atomic force microscopy (AFM) and phase interferometry microscopy measurements established that steps approximately 1 nm in size were formed, which correspond to the thickness of a glycine bilayer. Grazingincidence X-ray diffraction studies on growing a-gly {010}

The merits of the frozen-density embedding scheme to model solvatochromic shifts

Abstract: We investigate the usefulness of a frozen-density embedding scheme within density-functional theory [J. Phys. Chem. 97, 8050 (1993)] for the calculation of solvatochromic shifts. The frozen-density calculations, particularly of excitation energies have two clear advantages over the standard supermolecule calculations: (i) calculations for much larger systems are feasible, since the time-consuming time-dependent density functional theory (TDDFT) part is carried out in a limited molecular orbital space, while the effect of the surroundings is still included at a quantum mechanical level. This allows a large number of solvent molecules to be included and thus affords both specific and nonspecific solvent effects to be modeled. (ii) Only excitations of the system of interest, i.e., the selected embedded system, are calculated. This allows an easy analysis and interpretation of the results. In TDDFT calculations, it avoids unphysical results introduced by spurious mixings with the artificially too low charge-t...

Abnormal solvent effects on hydrogen atom abstraction. 3. Novel kinetics in sequential proton loss electron transfer chemistry.

Abstract: A prolonged search involving several dozen phenols, each in numerous solvents, for an ArOH/2,2-diphenyl-1-picrylhydrazyl (dpph•) reaction that is first-order in ArOH but zero-order in dpph• has reached a successful conclusion. These unusual kinetics are followed by 2,2‘-methylene-bis(4-methyl-6-tert-butylphenol), BIS, in five solvents (acetonitrile, benzonitrile, acetone, cyclohexanone, and DMSO). In 15 other solvents the reactions were first-order in both BIS and dpph• (i.e., the reactions followed “normal” kinetics). The zero-order kinetics indicate that in the five named solvents the BIS/dpph• reaction occurs by sequential proton loss electron transfer (SPLET). This mechanism is not uncommon for ArOH/dpph• reactions in solvents that support ionization, and normal kinetics have always been observed previously (see Litwinienko, G.; Ingold, K. U. J. Org. Chem. 2003, 68, 3433 and Litwinienko, G.; Ingold, K. U. J. Org. Chem. 2004, 69, 5888). The zero-order kinetics found for the BIS/dpph• reaction in five s...

Theoretical investigation of C--H hydroxylation by (N4Py)Fe(IV)=O(2+): an oxidant more powerful than P450?

Abstract: DFT calculations of C−H hydroxylation by a synthetic nonheme oxoiron(IV) oxidant supported by a neutral pentadentate N5 ligand show that this reagent is intrinsically more reactive than compound I of P450. This nonheme iron oxidant is predicted to exhibit stereoselective reactions, strong solvent effect, and involve multistate reactivity with spin-state crossing.

Collective solvent coordinates for the infrared spectrum of HOD in D2O based on an ab initio electrostatic map.

Abstract: An ab initio MP2 vibrational Hamiltonian of HOD in an external electrostatic potential parametrized by the electric field and its gradient-tensor is constructed. By combining it with the fluctuating electric field induced by the D2O solvent obtained from molecular dynamics simulations, we calculate the infrared absorption of the O-H stretch. The resulting solvent shift and infrared line shape for three force fields (TIP4P, SPC/E, and SW) are in good agreement with the experiment. A collective coordinate response for the solvent effect is constructed by identifying the main electrostatic field and gradient components contributing to the line shape. This allows a realistic stochastic Liouville equation simulation of the line shapes which is not restricted to Gaussian frequency fluctuations.

Solvent effects on the vibrational activity and photodynamics of the green fluorescent protein chromophore : A quantum-chemical study

Abstract: Vibrational activities in the Raman and resonance Raman spectra of the cationic, neutral, and anionic forms of 4‘-hydroxybenzylidene-2,3-dimethyl-imidazolinone, a model compound for the green fluorescent protein chromophore, have been obtained from quantum-chemical calculations in vacuo and with the inclusion of solvent effects through the polarizable continuum model. It is found that inclusion of solvent effects improves slightly the agreement with experimental data for the cationic and neutral forms, whose spectra are qualitatively well-described already by calculations in vacuo. In contrast, inclusion of solvent effects is crucial to reproduce correctly the activities of the anionic form. The structural effects of solvation are remarkable both in the ground and in the lowest excited state of the anionic chromophore and influence not only the vibrational activity but also the photodynamics of the lowest excited state. CASPT2//CASSCF photoreaction paths, computed by including solvent effects at the CASSC...

An explicit quantum chemical method for modeling large solvation shells applied to aminocoumarin C151.

Abstract: The absorption spectra of aminocoumarin C151 in water and n-hexane solution are investigated by an explicit quantum chemical solvent model. We improved the efficiency of the frozen-density embedding scheme, as used in a former study on solvatochromism (J. Chem. Phys. 2005, 122, 094115) to describe very large solvent shells. The computer time used in this new implementation scales approximately linearly (with a low prefactor) with the number of solvent molecules. We test the ability of the frozen-density embedding to describe specific solvent effects due to hydrogen bonding for a small example system, as well as the convergence of the excitation energy with the number of solvent molecules considered in the solvation shell. Calculations with up to 500 water molecules (1500 atoms) in the solvent system are carried out. The absorption spectra are studied for C151 in aqueous or n-hexane solution for direct comparison with experimental data. To obtain snapshots of the dye molecule in solution, for which subsequent excitation energies are calculated, we use a classical molecular dynamics (MD) simulation with a force field adapted to first-principles calculations. In the calculation of solvatochromic shifts between solvents of different polarity, the vertical excitation energy obtained at the equilibrium structure of the isolated chromophore is sometimes taken as a guess for the excitation energy in a nonpolar solvent. Our results show that this is, in general, not an appropriate assumption. This is mainly due to the fact that the solute dynamics is neglected. The experimental shift between n-hexane and water as solvents is qualitatively reproduced, even by the simplest embedding approximation, and the results can be improved by a partial polarization of the frozen density. It is shown that the shift is mainly due to the electronic effect of the water molecules, and the structural effects are similar in n-hexane and water. By including water molecules, which might be directly involved in the excitation, in the embedded region, an agreement with experimental values within 0.05 eV is achieved.

Amphiphilic Graft Copolymer in a Selective Solvent: Intramolecular Structures and Conformational Transitions

Abstract: We present a scaling theory for the equilibrium conformations of amphiphilic comblike (graft-) copolymer in dilute solution. We examine intramolecular self-assembly upon inferior solvent strength for the backbone. We demonstrate that at sufficiently dense grafting the backbone collapses via formation of a pearl necklace of intrachain micelles that are stabilized by steric repulsion between coronas. This behavior is reminiscent to the Rayleigh instability in hydrophobic polyelectrolyte, though occurs in a neutral macromolecule. The equilibrium local structure and the persistence length of comblike copolymer are analyzed as a function of solvent strength. Repulsion between micellar coronas leads to swelling of comblike macromolecule on larger scales and ensures the thermodynamic stability of the solution with respect to phase separation. In the case of sparse grafting we predict formation of not only spherical but also cylindrical intramolecular micelles and lamellar mesophases.

A molecular dynamics computer simulation study of room-temperature ionic liquids. II. Equilibrium and nonequilibrium solvation dynamics.

Abstract: The molecular dynamics (MD) simulation study of solvation structure and free energetics in 1-ethyl-3-methylimidazolium chloride and 1-ethyl-3-methylimidazolium hexafluorophosphate using a probe solute in the preceding article [Y. Shim, M. Y. Choi and H. J. Kim, J. Chem. Phys. 122, 044510 (2005)] is extended to investigate dynamic properties of these liquids. Solvent fluctuation dynamics near equilibrium are studied via MD and associated time-dependent friction is analyzed via the generalized Langevin equation. Nonequilibrium solvent relaxation following an instantaneous change in the solute charge distribution and accompanying solvent structure reorganization are also investigated. Both equilibrium and nonequilibrium solvation dynamics are characterized by at least two vastly different time scales--a subpicosecond inertial regime followed by a slow diffusive regime. Solvent regions contributing to the subpicosecond nonequilibrium relaxation are found to vary significantly with initial solvation configurations, especially near the solute. If the solvent density near the solute is sufficiently high at the outset of the relaxation, subpicosecond dynamics are mainly governed by the motions of a few ions close to the solute. By contrast, in the case of a low local density, solvent ions located not only close to but also relatively far from the solute participate in the subpicosecond relaxation. Despite this difference, linear response holds reasonably well in both ionic liquids.

Solvent Effects on the UV (n → π*) and NMR (13C and 17O) Spectra of Acetone in Aqueous Solution. An Integrated Car−Parrinello and DFT/PCM Approach

Abstract: The accurate reproduction of ultraviolet and nuclear magnetic resonance spectra of acetone in aqueous solution is used as a test of an integrated computational tool rooted in the density functional...

Criteria for Flame‐Spray Synthesis of Hollow, Shell‐Like, or Inhomogeneous Oxides

Abstract: The influence of metal precursor and solvent composition on the morphology of SiO2, Bi2O3, and other oxide particles made by flame spray pyrolysis (FSP) was investigated. Silica precursors with boiling points Tbp=299–548 K dissolved in xylene were used as well as different solvents (Tbp=308–557 K) with tetraethyl-orthosilicate (TEOS) as the silica precursor. For Bi2O3, nonvolatile bismuth nitrate pentahydrate was dissolved in solvents with Tbp=338–468 K. Product powders were characterized by nitrogen adsorption, X-ray diffraction, and scanning and transmission electron microscopy. From these data as well as from the literature of FSP synthesis of Bi2O3, CeO2, MgO, ZnO, Fe2O3, Y2O3, Al2O3, and Mg–Al spinel, it is inferred that hollow/inhomogeneous particles are formed at low combustion enthalpy densities and when the solvent boiling point is comparable or smaller than the precursor melting or decomposition point.

Two-photon absorption in solution by means of time-dependent density-functional theory and the polarizable continuum model

Abstract: We present the first study of two-photon absorption (TPA) of solvated molecules based on direct evaluation of TPA cross sections from the quadratic response of time-dependent perturbations. A set of prototypical two-photon (TP) chromophores has been selected and analyzed: a pure pi system (t-stilbene) and its substituted homologs obtained employing a donor (D) and an acceptor (A) group to probe the solvent effects along the series pi, D-pi-D, A-pi-D, and A-pi-A. For the selected systems we have calculated the TPA cross sections in different environments by means of the polarizable continuum model. The data have been analyzed to evaluate how the structural and environmental parameters contribute to the final two-photon absorption cross section. These include molecular structure, geometry relaxation in solution, polarity, and refractive index of the solvent. The performances of the three common functionals SVWN, BLYP, and B3LYP have been compared. The results show a significant solvent dependence of the TPA cross section and an unusual trend when passing from cyclohexane to water. The data have also been rationalized in terms of the main orbital excitations leading to the transitions. Finally, trends along the series have been described and comparison with experiments and previous calculations has been drawn.

The use of heat transfer fluids in the synthesis of high-quality CdSe quantum dots, core/shell quantum dots, and quantum rods

Abstract: Fluorescent semiconductor nanoparticles, or quantum dots, have potential uses as an optical material, in which the optoelectronic properties can be tuned precisely by particle size. Advances in chemical synthesis have led to improvements in size and shape control, cost, and safety. A limiting step in large-scale production is identified to be the raw materials cost, in which a common synthesis solvent, octadecene, accounts for most of the materials cost for a batch of CdSe quantum dots. Thus, less expensive solvents are needed. In this paper, we identify heat transfer fluids, a class of organic liquids commonly used in chemical process industries to transport heat between unit operations, as alternative solvents for quantum dot synthesis. We specifically show that two heat transfer fluids can be used successfully in the synthesis of CdSe quantum dots with uniform particle sizes. We show that the synthesis chemistry for CdSe/CdS core/shell quantum dots and CdSe quantum rods can also be performed in heat transfer fluids. With the aid of a population balance model, we interpret the effect of different HT fluids on QD growth kinetics in terms of solvent effects, i.e., solvent viscosity, CdSe bulk solubility in the solvent, and surface free energy.

Approximate Additivity of Anion−π Interactions: An Ab Initio Study on Anion−π, Anion−π2 and Anion−π3 Complexes

Abstract: We have studied the additivity of the anion−π interaction using high level ab initio calculations. We have optimized chloride and bromide complexes with one, two and three aromatic units (such as trifluoro-s-triazine and s-triazine). We have analyzed the interaction using the atoms in molecules theory and studied the charge transfer using several methods for deriving atomic charges. The results revealed additivities of both the geometries and the binding energies. We have also proposed a neutral receptor for chloride based on multiple anion−π interactions. Finally, we have simulated solvent effects within the self-consistent reaction field model.

Solvent effect on synthesis of indium tin oxide nano-powders by a solvothermal process

Abstract: Well-crystallized and nano-sized indium tin oxide (ITO) powders were prepared by a solvothermal process from a mixed solution consisting of indium nitrate and tin chloride. The characteristics of the solvothermally synthesized ITO powders were studied to investigate the contribution of the processing variables (solvent, working temperature, working pressure) on the physico-chemical properties. The results of this study show that the nano-sized blue ITO powders with a single phase could be directly obtained by changing the medium from water to organic solvent. Comparison of the yellow ITO powders synthesized by the coprecipitation using calcination of precipitates and blue ITO powders synthesized by the solvothermal process indicated that the solvothermal process led to an in situ reduction reaction. Solvent viscosity in the solvothermal process strongly influences both electrical and optical properties of ITO materials for transparent conducting oxides (TCOs). The size of synthesized ITO particles increased with increasing solvent viscosity due to steric hindrance. Moreover, the shape of the synthesized ITO particles became more square with increasing solvent viscosity, because the preferred orientation of ITO materials for crystallization is the fastest in the 〈1 0 0〉 direction.

Solvent effects on IR and VCD spectra of helical peptides: DFT-based static spectral simulations with explicit water.

Abstract: Simulations of IR and VCD spectra are carried out for model alpha-helical, 3(10)-helical, and 3(1)-helical (polyProII-like) oligopeptides, with up to 21 amide groups, and including explicit consideration of effects of directly hydrogen-bonded solvent (water). Parameters used were obtained from ab initio density functional theory (DFT) computations of force field, atomic polar and axial tensors for oligopeptides of 5 to 7 amides, whose structures were constrained in (phi,psi) to target the secondary structure type but otherwise fully optimized. By comparison with experimental data as well as with calculations for identical but isolated (gas phase) peptides, the computed effects of an inner shell of aqueous solvent on the vibrational spectra of helical oligopeptides are illustrated. The interaction with solvent causes significant frequency shifts of the amide bands, but only minor changes in the characteristic IR intensity distributions and splittings and the VCD band shapes. Better agreement with experimental band shapes is achieved for the alpha-helical amide I' (N-deuterated) VCD by inclusion of explicit solvent in the calculations. Some improvements are also observed in theoretical VCD predictions for 13C labeled alpha-helical peptides when solvated models are used. However, the qualitative isotopic splitting patterns are preserved and just shifted in frequency due to consistent, solvent independent interamide coupling constants. The critical match of experiment and theory for relative positions of transitions in peptides with specifically separated 13C=O labels, including and neglecting solvent, confirms the stability of the coupling interactions. Despite these solvation effects, the calculated VCD band shape of the amide I mode is shown to be a reliable conformational probe, since it remains basically insensitive to frequency shifts caused by environment. Thus theoretical VCD simulations, even vacuum calculations, are shown to provide useful spectral predictions for solution-phase peptides.

Mechanisms of formation of 8-oxoguanine due to reactions of one and two OH* radicals and the H2O2 molecule with guanine: A quantum computational study.

Abstract: Mechanisms of formation of the mutagenic product 8-oxoguanine (8OG) due to reactions of guanine with two separate OH* radicals and with H2O2 were investigated at the B3LYP/6-31G, B3LYP/6-311++G, and B3LYP/AUG-cc-pVDZ levels of theory. Single point energy calculations were carried out with the MP2/AUG-cc-pVDZ method employing the optimized geometries at the B3LYP/AUG-cc-pVDZ level. Solvent effect was treated using the PCM and IEF-PCM models. Reactions of two separate OH* radicals and H2O2 with the C2 position of 5-methylimidazole (5MI) were investigated taking 5MI as a model to study reactions at the C8 position of guanine. The addition reaction of an OH* radical at the C8 position of guanine is found to be nearly barrierless while the corresponding adduct is quite stable. The reaction of a second OH* radical at the C8 position of guanine leading to the formation of 8OG complexed with a water molecule can take place according to two different mechanisms, involving two steps each. According to one mechanism, at the first step, 8-hydroxyguanine (8OHG) complexed with a water molecule is formed ,while at the second step, 8OHG is tautomerized to 8OG. In the other mechanism, at the first step, an intermediate complexed (IC) with a water molecule is formed, the five-membered ring of which is open, while at the second step, the five-membered ring is closed and a hydrogen bonded complex of 8OG with a water molecule is formed. The reaction of H2O2 with guanine leading to the formation of 8OG complexed with a water molecule can also take place in accordance with two different mechanisms having two steps each. At the first step of one mechanism, H2O2 is dissociated into two OH* groups that react with guanine to form the same IC as that formed in the reaction with two separate OH* radicals, and the subsequent step of this mechanism is also the same as that of the reaction of guanine with two separate OH* radicals. At the first step of the other mechanism of the reaction of guanine with H2O2, the latter molecule is dissociated into a hydrogen atom and an OOH* group which become bonded to the N7 and C8 atoms of guanine, respectively. At the second step of this mechanism, the OOH* group is dissociated into an oxygen atom and an OH* group, the former becomes bonded to the C8 atom of guanine while the latter abstracts the H8 atom bonded to C8, thus producing 8OG complexed with a water molecule. Solvent effects of the aqueous medium on certain reaction barriers and released energies are appreciable. 5MI works as a satisfactory model for a qualitative study of the reactions of two separate OH* radicals or H2O2 occurring at the C8 position of guanine.