Showing papers on "Solvent effects published in 2014"

Solvent Effects in Acid‐Catalyzed Biomass Conversion Reactions

Abstract: Reaction kinetics were studied to quantify the effects of polar aprotic organic solvents on the acid-catalyzed conversion of xylose into furfural. A solvent of particular importance is g-valerolactone (GVL), which leads to signifi- cant increases in reaction rates compared to water in addition to increased product selectivity. GVL has similar effects on the kinetics for the dehydration of 1,2-propanediol to propanal and for the hydrolysis of cellobiose to glucose. Based on results obtained for homogeneous Bronsted acid catalysts that span a range of pKa values, we suggest that an aprotic organic solvent affects the reaction kinetics by changing the stabiliza- tion of the acidic proton relative to the protonated transition state. This same behavior is displayed by strong solid Bronsted acid catalysts, such as H-mordenite and H-beta. The use of organic solvents is pervasive in the chemical industry, and recently it has been shown that organic solvents are beneficial in the chemical conversion of lignocellulosic biomass. (1-3) One such solvent is g-valerolactone (GVL), which can be produced from biomass and displays significant improvements in reaction performance for biomass conver- sion reactions compared to conversion in aqueous media, such as increased catalytic activity and higher selectivity to desired reaction products. (2, 3) Furthermore, we have reported that the simultaneous conversion of hemicellulose and cellulose can be achieved using GVL as a solvent in a single reactor, eliminating the need for pretreatment and/or sepa- ration steps. (4) Recently, we have taken advantage of accel- erated rates of cellulose and hemicellulose deconstruction in GVL-H2O solvent mixtures to develop a processing strategy to produce streams of C5 and C6 sugars (e.g., 130 g l � 1 ) from biomass. (5) Other polar aprotic solvents, such as g-lactones and tetrahydrofurans, have also shown comparable benefits to GVL in biomass conversion processes. (3) Herein, we report the effects of GVL and other polar aprotic solvents on acid-catalyzed biomass conversion reac- tions using acid catalysts that span a range of pKa values. The liquid-phase dehydration of xylose to furfural is catalyzed by Bronsted acids and serves as a probe reaction in the present study. We compare the reactivity trends displayed by these homogeneous acid catalysts in the liquid phase with the performance of solid acid catalysts, the latter of which have been shown to span a range of catalytic activities for the gas-

Influence of the solvent in the synthesis of zeolitic imidazolate framework-8 (ZIF-8) nanocrystals at room temperature

Abstract: The effect of the solvent on the synthesis process and on the nanocrystal characteristics of the zeolitic imidazolate framework-8 (ZIF-8) was investigated. A synthesis protocol at room temperature employing a series of aliphatic alcohols, water, dimethylformamide and acetone was employed. The results show that the solvent modifies the evolution of the reaction, altering the crystallization rates and nanocrystal sizes. Its hydrogen bond donation ability is the main factor that governs this effect. More precisely, the solvent modulates the formation of ZIF-8 nanocrystals with sizes in the range between 15 and 42 nm. When synthesized in alcohol and acetone, these nanocrystals form globular aggregates with sizes between 130 and 420 nm. In contrast, under the same synthesis conditions, when using water or dimethylformamide the ZIF phase is not developed. In alcohols other than methanol, the crystals develop pill-shaped morphologies with poorly defined facets. Moreover, a markedly fast growing kinetics is verified in these alcohols, leading to an ultra-fast crystallization of ZIF-8 in about 60 s. These findings provide new information about the role of the solvent in the synthesis process of nanoZIF-8, which can be useful for controlling the crystallization rates and nanocrystal sizes of this material.

Origin of 5‐Hydroxymethylfurfural Stability in Water/Dimethyl Sulfoxide Mixtures

Abstract: In the present work, we combined vibrational spectroscopy with electronic structure calculations to understand the solvation of HMF in DMSO, water, and DMSO/water mixtures and to provide insights into the observed hindrance of HMF rehydration and aldol condensation reactions if it is dissolved in DMSO/water mixtures. To achieve this goal, the attenuated total reflection FTIR spectra of a wide composition range of binary and ternary mixtures were measured, analyzed, and compared to the findings of ab initio DFT calculations. The effect of solvent on the HMF C-O and O-H vibrational modes reveals significant differences that are ascribed to different intermolecular interactions between HMF and DMSO or water. We also found that DMSO binds to HMF more strongly than water, and interactions with the HMF hydroxyl group are stronger than those with the HMF carbonyl group. We also showed the preferential solvation of HMF C-O groups by DMSO if HMF is dissolved in DMSO/water mixed solvent. Frontier molecular orbital theory was used to examine the influence of the solvent on side reactions. The results show that HMF solvation by DMSO increases its LUMO energy, which reduces its susceptibility to nucleophilic attack and minimizes undesirable hydration and humin-formation reactions. This result, together with the preferential solvation of HMF by DMSO, provide an explanation for the enhanced HMF stability in DMSO/water mixtures observed experimentally.

1H-NMR as a structural and analytical tool of intra- and intermolecular hydrogen bonds of phenol-containing natural products and model compounds.

Abstract: Experimental parameters that influence the resolution of 1H-NMR phenol OH signals are critically evaluated with emphasis on the effects of pH, temperature and nature of the solvents. Extremely sharp peaks (Δν1/2 ≤ 2 Hz) can be obtained under optimized experimental conditions which allow the application of 1H-13C HMBC-NMR experiments to reveal long range coupling constants of hydroxyl protons and, thus, to provide unequivocal assignment of the OH signals even in cases of complex polyphenol natural products. Intramolecular and intermolecular hydrogen bonds have a very significant effect on 1H OH chemical shifts which cover a region from 4.5 up to 19 ppm. Solvent effects on –OH proton chemical shifts, temperature coefficients (Δδ/ΔT), OH diffusion coefficients, and nJ(13C, O1H) coupling constants are evaluated as indicators of hydrogen bonding and solvation state of phenol –OH groups. Accurate 1H chemical shifts of the OH groups can be calculated using a combination of DFT and discrete solute-solvent hydrogen bond interaction at relatively inexpensive levels of theory, namely, DFT/B3LYP/6-311++G (2d,p). Excellent correlations between experimental 1H chemical shifts and those calculated at the ab initio level can provide a method of primary interest in order to obtain structural and conformational description of solute-solvent interactions at a molecular level. The use of the high resolution phenol hydroxyl group 1H-NMR spectral region provides a general method for the analysis of complex plant extracts without the need for the isolation of the individual components.

Competing Intramolecular vs. Intermolecular Hydrogen Bonds in Solution

Abstract: A hydrogen bond for a local-minimum-energy structure can be identified according to the definition of the International Union of Pure and Applied Chemistry (IUPAC recommendation 2011) or by finding a special bond critical point on the density map of the structure in the framework of the atoms-in-molecules theory. Nonetheless, a given structural conformation may be simply favored by electrostatic interactions. The present review surveys the in-solution competition of the conformations with intramolecular vs. intermolecular hydrogen bonds for different types of small organic molecules. In their most stable gas-phase structure, an intramolecular hydrogen bond is possible. In a protic solution, the intramolecular hydrogen bond may disrupt in favor of two solute-solvent intermolecular hydrogen bonds. The balance of the increased internal energy and the stabilizing effect of the solute-solvent interactions regulates the new conformer composition in the liquid phase. The review additionally considers the solvent effects on the stability of simple dimeric systems as revealed from molecular dynamics simulations or on the basis of the calculated potential of mean force curves. Finally, studies of the solvent effects on the type of the intermolecular hydrogen bond (neutral or ionic) in acid-base complexes have been surveyed.

1H and 13C NMR scaling factors for the calculation of chemical shifts in commonly used solvents using density functional theory

Abstract: Calculation of NMR chemical shifts and coupling constants using quantum mechanical calculations [density functional theory (DFT)], has become a very popular tool for the determination of conformation and the assignment of stereochemistry within a molecule. We present the scaling factors (linear regression parameters) from 10 DFT methods for 10 commonly used NMR solvents using the same set of reference compounds. The results were compared with the corresponding gas-phase calculations to assess the inclusion of the polarizable continuum model for solvent effects.

Metal-free hydrogenation catalyzed by an air-stable borane: use of solvent as a frustrated Lewis base.

Abstract: In recent years ‘frustrated Lewis pairs’ (FLPs) have been shown to be effective metal-free catalysts for the hydrogenation of many unsaturated substrates. Even so, limited functional-group tolerance restricts the range of solvents in which FLP-mediated reactions can be performed, with all FLP-mediated hydrogenations reported to date carried out in non-donor hydrocarbon or chlorinated solvents. Herein we report that the bulky Lewis acids B(C6Cl5)x(C6F5)3−x (x=0–3) are capable of heterolytic H2 activation in the strong-donor solvent THF, in the absence of any additional Lewis base. This allows metal-free catalytic hydrogenations to be performed in donor solvent media under mild conditions; these systems are particularly effective for the hydrogenation of weakly basic substrates, including the first examples of metal-free catalytic hydrogenation of furan heterocycles. The air-stability of the most effective borane, B(C6Cl5)(C6F5)2, makes this a practically simple reaction method.

Solvent Effects on the Growth Morphology and Phase Purity of CL-20

Abstract: The performance and stability of the high energy secondary explosive 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (HNIW, also known as CL-20) can be affected by factors including the phase purity of the bulk material, as well as the particle size, morphology, and defect density of the individual crystallites. Slow evaporation crystallization of CL-20 from 16 different single solvent and co-solvent systems was performed. The phase purity of the bulk material obtained was analyzed by powder X-ray diffraction, optical microscopy, and differential scanning calorimetry. These complementary methods confirmed that under most of the slow evaporation conditions examined, a concomitant mixture of two or more crystalline phases was usually obtained. Numerous individual crystal morphologies were determined using single crystal X-ray goniometry and compared against calculated BFDH morphologies. Examination of the packing interactions in the different CL-20 phases via Hirshfeld surface analysis provides some insight int...

Poly(quinoxaline-2,3-diyl)s Bearing (S)-3-Octyloxymethyl Side Chains as an Efficient Amplifier of Alkane Solvent Effect Leading to Switch of Main-Chain Helical Chirality

Abstract: Poly(quinoxaline-2,3-diyl) containing (S)-3-octyloxymethyl side chains was synthesized to investigate the induction of a single-handed helical sense to the main chain in various alkane solvents. The polymer showed an efficient solvent dependent helix inversion between n-octane (M-helix) and cyclooctane (P-helix). After a screening of alkane solvents, it was found that linear alkanes having large molecular aspect ratios induced M-helical structure, and branched or cyclic alkanes having small molecular aspect ratios induced P-helical structure. A polymer ligand containing (S)-3-octyloxymethyl side chains and diphenylphosphino pendants also exhibited solvent-dependent helical inversion between n-octane and cyclooctane, leading to the highly enantioselective production of the both enantiomeric product in a palladium-catalyzed asymmetric hydrosilylation reaction of styrene (R-product 94% ee in n-octane and S-product 90% ee in cyclooctane).

Intramolecular charge transfer of push-pull pyridinium salts in the singlet manifold.

Abstract: The solvent effect on the photophysical and photochemical properties of the iodides of three trans (E) isomers of 2-D-vinyl,1-methylpyridinium, where D is a donor group (4-dimethylaminophenyl, 3,4,5-trimethoxyphenyl and 1-pyrenyl), was studied by stationary and transient absorption techniques. The results obtained allowed the negative solvatochromism and relaxation pathways of the excited states in the singlet manifold to be reasonably interpreted. Resorting to ultrafast absorption techniques and DFT calculations allowed information on the excited state dynamics and the role of the solvent-controlled intramolecular charge transfer (ICT) processes to be obtained. The structure-dependent excited state dynamics in nonpolar solvents, where the ICT is slower than solvent rearrangement, and in polar solvents, where an opposite situation is operative, was thus explained. The push–pull character of the three compounds, particularly the anilino-derivative, suggests their potential application in optoelectronics.

Effects of Solvent on Polymorph Formation and Nucleation of Prasugrel Hydrochloride

Abstract: Three intrinsic properties of solvent were used to evaluate the effects of solvent on polymorph formation of prasugrel hydrochloride. In situ Raman spectroscopy, FTIR, and powder X-ray diffraction were used to characterize two solvent-free polymorphs and five solvates of prasugrel hydrochloride, the two of which were reported for the first time. Reactive crystallization in 24 different pure solvents was studied at 313.15 K. It was found that polymorph formation of prasugrel hydrochloride directly depends on the solvents used in the experiments. Form I was obtained in solvents with low values of hydrogen bond donor ability (HBD), while form II was obtained in solvents with high values of HBD. The thermodynamic and kinetic reasons for the solvent effects were explained by using the solubility data and the nucleation experiments. The solubilities of forms I and II were experimentally determined by a gravimetric method, and an equation based on the linear free energy approach for predicting solubility was app...

Liquid-phase hydrogenation of acetophenone over silica-supported Ni, Co and Cu catalysts: Influence of metal and solvent

Abstract: In this work, we studied the influence of solvent and metal nature on the liquid-phase hydrogenation of acetophenone (AP) over Ni/SiO2, Co/SiO2 and Cu/SiO2. Catalysts were prepared by wetness impregnation method with metal loads of about 7–8 wt%. Catalytic tests were performed in a batch reactor, at 363 K and 10 bar (H2), using 2-propanol (IPA), cyclohexane (CHX), toluene (TOL) and benzene (BEN) as solvents. Considering the three catalysts, the general pattern for the initial hydrogenation rate was: Ni/SiO2 > Co/SiO2 > Cu/SiO2, whereas the trend for selectivity to 1-phenylethanol (PHE) was just the opposite. AP can interact with nickel metal surface through both –C=O group and aromatic ring and thus the aromatic alcohol and saturated compounds were obtained. Instead, cobalt and copper metal surfaces interact preferentially with the –C=O group leading to selective hydrogenation of AP into PHE. In addition, an important interaction between –C–OH group of PHE and Co/SiO2 surface takes place, leading to rapid alcohol hydrogenolysis into ethylbenzene. The general activity pattern with the four solvents was: IPA > CHX ≥ TOL ≥ BEN. The magnitude of solvent influence on the catalytic performance strongly depended on the metal nature. The most significant solvent effect took place with Ni/SiO2, whereas the less noticeable influence was observed in the case of Cu/SiO2. From pseudo-homogenous kinetic modeling and temperature-programmed desorption, the following noteworthy observations arose: (1) IPA has a positive contribution by hydrogen transfer and/or AP activation by polarization; (2) the magnitude of the positive IPA influence on AP hydrogenation rate follows the trend: Ni/SiO2 > Co/SiO2 > Cu/SiO2; (3) CHX has a neutral contribution because of its weak adsorption on the metal phase and low interaction with reactant and products; (4) the effect of TOL and BEN is clearly negative for Ni/SiO2 due to blockage of active sites by strong adsorption of solvent on the metallic surface; (5) the effect due to strong adsorption of TOL and BEN is much less noticeable on Co/SiO2 and Cu/SiO2, as a consequence, the pattern for AP hydrogenation rates in BEN and TOL is Cu/SiO2 > Co/SiO2 > Ni/SiO2. Selectivity to PHE was less influenced by solvent nature. However, in the case of Ni/SiO2 and Co/SiO2, maximum PHE yields and selectivities increased with the solvent–metal interactions, mainly due to inhibition of the PHE hydrogenolysis. Cu/SiO2 was always 100% selective to PHE in all of the solvents. These results are clearly indicating that the magnitude of the solvent effect on catalytic performance strongly depends on the metal nature.

Substrate, molecular structure, and solvent effects in 2D self-assembly via hydrogen and halogen nonding

Abstract: Recently, halogen···halogen interactions have been demonstrated to stabilize two-dimensional supramolecular assemblies at the liquid–solid interface. Here we study the effect of changing the halogen, and report on the 2D supramolecular structures obtained by the adsorption of 2,4,6-tris(4-bromophenyl)-1,3,5-triazine (TBPT) and 2,4,6-tris(4-iodophenyl)-1,3,5-triazine (TIPT) on both highly oriented pyrolytic graphite and the (111) facet of a gold single crystal. These molecular systems were investigated by combining room-temperature scanning tunneling microscopy in ambient conditions with density functional theory, and are compared to results reported in the literature for the similar molecules 1,3,5-tri(4-bromophenyl)benzene (TBPB) and 1,3,5-tri(4-iodophenyl)benzene (TIPB). We find that the substrate exerts a much stronger effect than the nature of the halogen atoms in the molecular building blocks. Our results indicate that the triazine core, which renders TBPT and TIPT stiff and planar, leads to stronger adsorption energies and hence structures that are different from those found for TBPB and TIPB. On the reconstructed Au(111) surface we find that the TBPT network is sensitive to the fcc- and hcp-stacked regions, indicating a significant substrate effect. This makes TBPT the first molecule reported to form a continuous monolayer at room temperature in which molecular packing is altered on the differently reconstructed regions of the Au(111) surface. Solvent-dependent polymorphs with solvent coadsorption were observed for TBPT on HOPG. This is the first example of a multicomponent self-assembled molecular networks involving the rare cyclic, hydrogen-bonded hexamer of carboxylic groups, R66(24) synthon.

Reaction Pathways of Alkoxyl Radicals. The Role of Solvent Effects on C-C Bond Fragmentation and Hydrogen Atom Transfer Reactions

Abstract: This account describes the results of our recent mechanistic studies on unimolecular C–C bond fragmentation (β-scission and O-neophyl rearrangement) and bimolecular hydrogen atom transfer (HAT) reactions of alkoxyl radicals. Particular attention is devoted to the study of solvent effects on these reactions by means of time-resolved techniques such as laser flash photolysis. Information is provided on the effect of ring substituents and of the solvent on the spectral properties of arylcarbinyloxyl radicals and on their reactivity in β-scission and O-neophyl rearrangement reactions, showing that a change in solvent can influence the fragmentation reactivity and selectivity. Detailed information has also been obtained on the role of the substrate structure and of the solvent on HAT reactions involving the cumyloxyl radical, showing the importance of solvent hydrogen bond interactions with the substrate and/or the radical on these processes, and providing a general mechanistic description of the kinetic solvent effects observed in HAT reactions from C–H bonds, as well as expanding on the previously available description for HAT from phenolic O–H bonds. The possible application of these findings to synthetically useful C–H functionalization procedures is discussed. 1 Introduction 2 C–C β-Scission Reactions 3 O-Neophyl Rearrangement 4 Hydrogen Atom Transfer (HAT) Reactions 5 Concluding Remarks

Halogen bonding in solution.

Abstract: Because of its expected applicability for modulation of molecular recognition phenomena in chemistry and biology, halogen bonding has lately attracted rapidly increasing interest. As most of these processes proceed in solution, the understanding of the influence of solvents on the interaction is of utmost importance. In addition, solution studies provide fundamental insights into the nature of halogen bonding, including, for example, the relative importance of charge transfer, dispersion, and electrostatics forces. Herein, a selection of halogen bonding literature is reviewed with the discussion focusing on the solvent effect and the electronic characteristics of halogen bonded complexes. Hence, charged and neutral systems together with two- and three-center bonds are presented in separate sub-sections. Solvent polarity is shown to have a slight stabilizing effect on neutral, two-center halogen bonds while strongly destabilizes charged, two-center complexes. It does not greatly influence the geometry of three-center halogen bonds, even though polar solvents facilitate dissociation of the counter-ion of charged three-center bonds. The charged three-center bonds are strengthened by increased environment polarity. Solvents possessing hydrogen bond donor functionalities efficiently destabilize all types of halogen bonds, primarily because of halogen vs hydrogen bond competition. A purely electrostatic model is insufficient for the description of halogen bonds in polar systems whereas it may give reasonable correlation to experimental data obtained in noninteracting, apolar solvents. Whereas dispersion plays a significant role for neutral, two-center halogen bonds, charged halogen bond complexes possess a significant charge transfer characteristic.

An experimental and theoretical study of the hetero Diels–Alder reactions between (E)-2-aryl-1-cyano-1-nitroethenes and ethyl vinyl ether: one-step or zwitterionic, two-step mechanism?

Abstract: The mechanistic aspects of the hetero Diels–Alder reactions between strongly electrophilic (E)-2-aryl-1-cyano-1-nitroethenes and ethyl vinyl ether have been analyzed on the basis of kinetic experimental results as well as quantum chemical simulations of reaction paths. These reactions may theoretically proceed via one step or stepwise, zwitterionic mechanism. It was found that the conversion of addends into adducts carried out via polar, but not zwitterionic mechanism. This is confirmed by kinetic substituent and solvent effects. Subsequently, a detailed analysis of internal reaction coordinate trajectories shows that the title reactions should be considered as examples of “two-stage one-step” cycloadditions according to Domingo terminology.

Substrate, molecular structure, and solvent effects in 2D self-assembly via hydrogen and halogen bonding

Abstract: Recently, halogen···halogen interactions have been demonstrated to stabilize two-dimensional supramolecular assemblies at the liquid–solid interface. Here we study the effect of changing the halogen, and report on the 2D supramolecular structures obtained by the adsorption of 2,4,6-tris(4-bromophenyl)-1,3,5-triazine (TBPT) and 2,4,6-tris(4-iodophenyl)-1,3,5-triazine (TIPT) on both highly oriented pyrolytic graphite and the (111) facet of a gold single crystal. These molecular systems were investigated by combining room-temperature scanning tunneling microscopy in ambient conditions with density functional theory, and are compared to results reported in the literature for the similar molecules 1,3,5-tri(4-bromophenyl)benzene (TBPB) and 1,3,5-tri(4-iodophenyl)benzene (TIPB). We find that the substrate exerts a much stronger effect than the nature of the halogen atoms in the molecular building blocks. Our results indicate that the triazine core, which renders TBPT and TIPT stiff and planar, leads to stronger adsorption energies and hence structures that are different from those found for TBPB and TIPB. On the reconstructed Au(111) surface we find that the TBPT network is sensitive to the fcc- and hcp-stacked regions, indicating a significant substrate effect. This makes TBPT the first molecule reported to form a continuous monolayer at room temperature in which molecular packing is altered on the differently reconstructed regions of the Au(111) surface. Solvent-dependent polymorphs with solvent coadsorption were observed for TBPT on HOPG. This is the first example of a multicomponent self-assembled molecular networks involving the rare cyclic, hydrogen-bonded hexamer of carboxylic groups, R 6 6 (24) synthon.

Deep-blue luminescent compound that emits efficiently both in solution and solid state with considerable blue-shift upon aggregation

Abstract: We have designed and synthesized a novel deep-blue luminescent compound (WuFDA), which can emit efficiently both in solution and in the aggregated state; the fluorescence emission maximum of WuFDA in the aggregated state shows a remarkable blue-shift from that shown in the dilute solution. Moreover, WuFDA shows interesting fluorescent behaviour in mixed solvent systems of THF–H2O, and a solvent effect in different polar solvents. These unusual phenomena have been interpreted by studies of the single crystalline structure of WuFDA and density functional theory (DFT) investigations. The special chemical structure (containing a planar conjugated fluorene core and strong polar groups (–NH and –F)) and their location in the molecule formed a typical non-planar D–π–A molecule with strong internal charge transfer (ICT). Such a polar molecule possesses the ability to form hydrogen bond interactions with itself, as well as with the polar solvents. Both the ICT effect and the intermolecular hydrogen bond interactions play very important roles in the geometrical manner of aggregation and the fluorescent behaviours of the WuFDA. In addition, WuFDA exhibited good thermal stability, with 5% and 10% weight-loss temperatures (Td) of WuFDA in nitrogen of 399 and 414 °C, respectively, and the glass transition temperature (Tg) was about 161 °C.

UV Transition Moments of Tyrosine

Abstract: To assist polarized-light spectroscopy for protein-structure analysis, the UV spectrum of p-cresol, the chromophore of tyrosine, was studied with respect to transition moment directions and perturbation by solvent environment. From linear dichroism (LD) spectra of p-cresol aligned in stretched matrices of poly(vinyl alcohol) and polyethylene, the lowest pi-pi* transition (L-b) is found to have pure polarization over its entire absorption (250-300 nm) with a transition moment perpendicular to the symmetry axis (C-1-C-4), both in polar and nonpolar environments. For the second transition (L-a), polarized parallel with the symmetry axis, a certain admixture of intensity with orthogonal polarization is noticed, depending on the environment. While the L-b spectrum in cydohexane shows a pronounced vibrational structure, it is blurred in methanol, which can be modeled as due to many microscopic polar environments. With the use of quantum mechanical (QM) calculations, the transition moments and solvent effects were analyzed with the B3LYP and omega B97X-D functionals in cyclohexane, water, and methanol using a combination of implicit and explicit solvent models. The blurred L-b band is explained by solvent hydrogen bonds, where both accepting and donating a hydrogen causes energy shifts. The inhomogeneous solvent-shift sensitivity in combination with robust polarization can be exploited for analyzing tyrosine orientation distributions in protein complexes using LD spectroscopy.

Dipolar correlations in structured solvents under nanoconfinement

Abstract: We study electrostatic correlations in structured solvents confined to nanoscale systems. We derive variational equations of Netz-Orland type for a model liquid composed of finite size dipoles. These equations are solved for both dilute solvents and solvents at physiological concentrations in a slit nanopore geometry. Correlation effects are of major importance for the dielectric reduction and anisotropy of the solvent resulting from dipole image interactions and also lead to a reduction of van der Waals attractions between low dielectric bodies. Finally, by comparison with other recently developed self-consistent theories and experiments, we scrutinize the effect of solvent-membrane interactions on the differential capacitance of the charged liquid in contact with low dielectric substrates. The interfacial solvent depletion driven by solvent-image interactions plays the major role in the observed low values of the experimental capacitance data, while non-locality associated with the extended charge structure of solvent molecules only brings a minor contribution.

Solvent effect on the crystal morphology of 2,6-diamino-3,5-dinitropyridine-1-oxide: a molecular dynamics simulation study.

Abstract: The attachment energy (AE) calculations were performed to predict the growth morphology of 2,6-diamino-3,5-dinitropyridine-1-oxide (ANPyO) in vacuum. The molecular dynamics (MD) method was applied to simulate the interaction of trifluoroacetic acid solvent with the habit faces and the corrected AE model was adopted to predict the growth habit of ANPyO in the solvent. The results indicate that the growth morphology of ANPyO in vacuum is dominated by (1 1 0), (1 0 0), (1 0 −1) and (1 1 −2) faces. The corrected AE energies change in the order of (1 1 0) > (1 0 −1) > (1 1 −2) > (1 0 0), which causes the crystal morphology to become very close to a flake in trifluoroacetic acid solvent and accords well with the results obtained from experiments. The radial distribution function analysis shows that the solvent molecules adsorb on the ANPyO faces mainly via the solvent–crystal face interactions of hydrogen bonds, Coulomb and Van der Waals forces. In addition to the above results, the analysis of diffusion coefficient of trifluoroacetic acid molecules on the crystal growth faces shows that the growth habit is also affected by the diffusion capacity of trifluoroacetic acid molecules. These suggestions may be useful for the formulation design of ANPyO.

Redox chemistry of selenenic acids and the insight it brings on transition state geometry in the reactions of peroxyl radicals.

Abstract: The redox chemistry of selenenic acids has been explored for the first time using a persistent selenenic acid, 9-triptyceneselenenic acid (RSeOH), and the results have been compared with those we recently obtained with its lighter chalcogen analogue, 9-triptycenesulfenic acid (RSOH). Specifically, the selenenyl radical was characterized by EPR spectroscopy and equilibrated with a phenoxyl radical of known stability in order to determine the O–H bond dissociation enthalpy of RSeOH (80.9 ± 0.8 kcal/mol): ca. 9 kcal/mol stronger than in RSOH. Kinetic measurements of the reactions of RSeOH with peroxyl radicals demonstrate that it readily undergoes H-atom transfer reactions (e.g., k = 1.7 × 105 M–1 s–1 in PhCl), which are subject to kinetic solvent effects and kinetic isotope effects similar to RSOH and other good H-atom donors. Interestingly, the rate constants for these reactions are only 18- and 5-fold smaller than those measured for RSOH in PhCl and CH3CN, respectively, despite being 9 kcal/mol less exoth...