Solvent network at the transition state in the solvolysis of hindered sulfonyl compounds

Alcoholysis rates of unhindered benzenesulfonyl chlorides (X‐ArSO₂Cl, X = H‐; 4‐Br‐; 4‐Me‐) are similar in methanol; the same behavior is also observed in ethanol, whereas the reactivity order in iso‐propanol is 4 Me‐ < H‐ < 4‐Br‐. On the other hand, alcoholysis of sterically hindered arenesulfonyl chlorides (X‐ArSO₂Cl) (X = 2,4,6‐Me₃‐3‐NO₂‐; 2,6‐Me₂‐4‐tBu‐; 2,4,6‐Me₃‐; 2,3,5,6‐Me₄‐; 2,4,6‐iPr₃‐; 2,4‐Me₂‐; 2,4,6‐(OMe)₃‐) in all studied alcohols show a significant increase in reactivity, the so‐called positive steric effect. Most of the substrates showed a reaction order b ~ 2 with respect to the nucleophile in methanol and ethanol, and b ~ 3 in iso‐propanol. The correlation between reactivity and the Kirkwood function (1/ξ) gives negative sensitivity (U) for all systems. All substrates showed high sensitivity to media nucleophilicity that depends on Σσ_X. Obtained results suggest the alcoholysis of benzenesulfonyl chlorides proceeds through S_N2 mechanism where the transition state (TS) involves the participation of 2–3 alcohol molecules; such a TS can be cyclic, in the case of unbranched alcohols, or linear, for alcohols with bulkier hydrocarbon groups like iso‐propanol. To include the number of alcohol molecules playing such a role in the TS, the following terminology is proposed: cS_N2s_n for S_N2 reactions involving n solvent molecules in a cyclic (c) TS, where “s” stands for the solvent and “n” is either the closest integer or half‐integer to the reaction order relative to the solvent or, in computational studies, the proposed number of solvent molecules taking part in the TS, whereas S_N2s_n is proposed when the TS is not cyclic. Copyright © 2016 John Wiley & Sons, Ltd.     

Dielectric relaxation studies of alkanols solubilized by sodium dodecyl sulphate aqueous solutions

Considerable attention has been paid in recent years to the influence of alcohols on ionic micellar structures, alcohol co-surfactants most commonly employed in the preparation of microemulsions. Dielectric relaxation spectroscopy (DRS) is a versatile tool to monitor the dynamic process of such micellar systems. We report here the changes in the dielectric constant of the medium and the relaxation time of micellar solutions of sodium dodecyl sulphate (SDS) in water in the presence and absence of alkanols—hexan-1-ol, octan-l-ol, decan-l-ol and 1,2-ethanediol. The time domain dielectric data were obtained in the reflection mode in the frequency range of 10 MHz to 20 GHz using a HP54750A sampling oscilloscope and HP 54754A TDR plug-in-module. The sample is held at 303 K in a SMA cell with an effective pin length of 1.35 mm. We have determined the relaxation time (τ) using the Cole–Cole method. The relative viscosity ηr on the micellar solutions were also determined. The result shows that the dielectric constant increases linearly with SDS concentration in pure aqueous solutions up to 400 mM and decreases thereafter. The relaxation times are of the order of 15 ps to 25 ps and are far greater than that of Debye rotational relaxation of the monomer species. It is assigned to the dispersion step to water molecules surrounding to hydrophobic particles or the “bound water”. The addition of alcohols results in a linear increase in relaxation time in all the systems. As the concentration of SDS increases τ/ηr attains a near constant value showing that solubilization becomes more difficult at these concentrations. Our results show that the alcohol molecules are solubilized at the surface of the SDS micelle (i.e.) the micelle–water interface.     

Raman spectroscopy of alkali halide hydration: hydrogen bond relaxation and polarization

The solute–solvent interaction of salts has a striking impact on various biological and industrial processes but its mechanism remains yet mysterious despite intensive studies since 1888 when Franz Hofmeister established the salt series. A combination of confocal Raman spectroscopy and contact angle measurements has enabled us to resolve the hydrogen bond relaxation (O:H―O, HB) and the associated charge polarization dynamics at different molecular site because of alkali halides hydration. Results show consistently that salt hydration softens the O:H phonon but stiffens H―O phonon cooperatively. The extent of HB relaxation and polarization is proportional to the electronegativity difference and ionic radius, following the order of Hofmeister series: X (R/η) = I (2.2/2.5) > Br (1.96/2.8) > Cl (1.81/3.0) > F (1.33/4.0) ≈ 0 for anions, and Y(R/η) = Na (0.98/0.9) > K (1.33/0.8) > Rb (1.49/0.8) > Cs (1.65/0.8) for cations. Observations suggest that ions create each an electric field that aligns, stretches, and polarizes water molecules, which relaxes the O:H―O bond cooperatively, depresses the molecular dynamics, and enhances the hydration shell viscosity and the skin stress. Exercises also demonstrate that Raman spectroscopy performs as a powerful tool for probing the molecular‐site‐resolved HB network relaxation dynamics in terms of phonon stiffness, molecular fluctuation dynamics, and phonon abundance transition under external stimulus. Copyright © 2016 John Wiley & Sons, Ltd.     

Incoherent quasielastic neutron scattering from H2O and aqueous ZnCl2 solutions

A study of the diffusive motions of the protons in pure water and ZnCl₂ aqueous solutions has been performed, using incoherent quasielastic neutron scattering. It is shown that it is essential to take into account the rotational motion of the water molecules. The translational linewidth is conveniently fitted over the whole Q-range, using the Random Jump Diffusion model for which the jump length turns out to be roughly the same for pure H₂O and the saturated solution, fairly close to the distance between protons in the water molecule.     

Kinetics of the thermal decomposition reaction of diethylketone cyclic triperoxide in acetone–toluene and acetone–1‐propanol binary solvent mixtures

The thermal decomposition reaction of diethylketone triperoxide (DEKT) ca. 0.02 M was studied in binary mixtures of acetone–toluene and acetone–1‐propanol at 150 °C. Products of DEKT thermolysis in solution, detected by GC analysis, were diethylketone, bibenzyl and butane. The reactions were explored by GC at different solvent compositions and in each case the reactions followed a pseudo first order kinetic law, up to at least 90% peroxide conversion. The rate coefficient value of the reaction is affected by the solvent properties, showing an increase in the k_obs values with increases in the polarity of the solvent mixture in acetone–toluene systems. Changes in the rate coefficient values are probably caused by the presence of the apolar toluene solvent, which dominates the preferential solvation around the DEKT molecule through non‐specific interactions. In acetone–1‐propanol mixtures the solvation effect is slightly dominated by the specific interactions between the 1‐propanol and a polar intermediate specie represented by the biradical, initially formed. The rate coefficient value increases ca. 6% in the mixture with 0.1 mole fraction of 1‐propanol in comparison with the value in pure acetone; but no more changes in rate coefficient values are observed when the amount of the alcohol increases. The critical state of the reaction (intermediate biradical) is preferentially solvated by the 1‐propanol instead of acetone, but in mixtures of different composition, it is not possible to detect any effect on the reactivity for homolytic rupture of the O? O bond. Copyright © 2008 John Wiley & Sons, Ltd.     

Self-association behaviour of alcohols in diluted aqueous solutions

Summary Our recent studies related to the properties of alcohol/water mixtures show the occurrence of some kind of molecular aggregation in the water-rich region of composition beyond a threshold alcohol concentrationx ₂ ^*. The observed behaviour suggests that forx ₂<x ₂ ^* the alcohol molecules are essentially dispersed and surrounded by ?water cages? where the short-range order and microdynamic of water molecules are changed with respect to those of pure water. Alcohol molecules are in mutual contact at higher concentration only when almost all water is involved in hydration shells of alcohol molecules. The structural transition atx ₂ ^* resembles, for some aspect, the micellization process. The main results of these investigations are reviewed and discussed in this paper. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8 1994     

Coherent molecular vibrational dynamics of CH2 stretching modes in polyethylene studied by femtosecond‐CARS

We have studied the coherent molecular vibrational dynamics of CH₂ stretching modes in polyethylene by time‐resolved femtosecond coherent anti‐Stokes Raman spectroscopy. We observed that the coherent vibrational relaxation of symmetric CH₂ stretching modes in polyethylene at room temperature is much faster than that previously measured in polyvinyl alcohol. In addition, it was detected that, at low temperature, the coherent vibrational relaxation of the symmetric stretching modes evidently becomes slower compared with that at room temperature. These temperature‐dependent measurements enable us to discriminate the contribution of pure dephasing mechanism, due to phonons and two‐level systems in polymer, from the contribution of lifetime of the vibrational excited state to the coherent vibrational relaxation of CH₂ stretching modes. We conclude that the coherent vibrational relaxation of symmetric CH₂ stretching modes at room temperature consists of the contribution of lifetime and approximately 1.5 times larger contribution of pure dephasing. Copyright © 2015 John Wiley & Sons, Ltd.     

Combined measurement of X‐ray photon correlation spectroscopy and diffracted X‐ray tracking using pink beam X‐rays

Combined X‐ray photon correlation spectroscopy (XPCS) and diffracted X‐ray tracking (DXT) measurements of carbon‐black nanocrystals embedded in styrene–butadiene rubber were performed. From the intensity fluctuation of speckle patterns in a small‐angle scattering region (XPCS), dynamical information relating to the translational motion can be obtained, and the rotational motion is observed through the changes in the positions of DXT diffraction spots. Graphitized carbon‐black nanocrystals in unvulcanized styrene–butadiene rubber showed an apparent discrepancy between their translational and rotational motions; this result seems to support a stress‐relaxation model for the origin of super‐diffusive particle motion that is widely observed in nanocolloidal systems. Combined measurements using these two techniques will give new insights into nanoscopic dynamics, and will be useful as a microrheology technique.     

Microsolvation of morpholine,a bidentate base – the importance of cooperativity

The structure, relative energies, and bonding in morpholine(water)_n (n = 1–4) clusters have been investigated at the Becke, three‐parameter, Lee–Yang–Parr/6–311 + G(3df,2p)//Becke, three‐parameter, Lee–Yang–Parr/6–311 + G(d,p) level of theory. Cooperative effects have been analyzed through the use of structural, energetic, and electron density indexes. Our analysis shows that these effects are crucial to trace the relative stability of the complexes formed. In all cases water molecules prefer to self‐associate forming chains in which each individual molecule behaves as a hydrogen bond (HB) donor and HB acceptor. The chain so formed behaves in turn as HB donor and HB acceptor with respect to morpholine, being the most stable arrangements those in which the NH group of morpholine behaves simultaneously as HB donor and HB acceptor. Higher in energy lie complexes in which the HB acceptor continues to be the NH group, but the HB donor is a CH group, or alternative structures in which the HB acceptor is the ether‐like oxygen of morpholine and the HB donor its NH group. Cooperativity increases with the number of solvent molecules, but there is a clear attenuation effect. Thus, whereas the additive interaction energy on going from dihydrated to trihydrated species increases by a factor of 3, this increase is about half on going from trihydrated to tetrahydrated complexes. Copyright © 2012 John Wiley & Sons, Ltd.     

On the Viscoelastic and Dielectric Behavior of Some Organic Compounds and Their Binary Mixtures

The dependence of dielectric relaxation time on the viscosity of the medium is being extensively used to draw certain quantitative conclusions regarding molecular motion and inter-molecular forces in liquids, liquid mixtures, dilute solutions, and multi-component polar solutes in dilute solution. In the absence of proper empirical or theoretical equations for the variation of dielectric relaxation time with viscosity, only the experimental investigations on different systems can give an insight. In the present study, the results of dielectric measurements carried out on pure samples of bromohexane, bromooctane and bromodecane in dilute solutions in different mixed solvents (benzene + paraffin) and on binary mixtures (1 : 1) of (bromohexane + bromodecane); (bromodecane + propyl alcohol) and (propyl alcohol + methyl alcohol) are reported. For comparison, the results of bromodecane + propyl alcohol and propyl alcohol + methyl alcohol are chosen as they form examples of mixture of non-associative + associative and associative + associative liquids, respectively. Different parameters determined using these dielectric measurements are also presented using different models. These studies indicate that the dielectric behavior at microwave frequencies favor the concept of dynamic viscosity and a single viscoelastic relaxation time for the systems under study.     

Dynamics of conformations,hydrogen bonds and translational diffusion of poly(methacrylic acid) in aqueous solution and the concentration transition in MD simulations

The dynamic behaviour of chain conformations, hydrogen bonds and translational diffusion of aqueous poly(methacrylic acid) (PMA) solution as a function of polymer volume fraction Φ_p across dilute to concentrated regimes inclusive of the pure polymer amorphous state was studied by molecular dynamics simulations. The behaviour of the relaxation time (τ) of the backbone dihedral angle auto-correlation function (ACF) reveals slower relaxation at higher level of polymer concentration and the existence of a concentration-driven relaxation transition for the aqueous polymer solution which occurs in the polymer volume fraction range, specifically 54% < Φ_p < 82% for this system. The relaxation constant τ for backbone dihedral angle exhibits a linear variation with Φ_p, indicating a first-order kinetic transition. The intermittent ACF for decay of the H-bond correlation shows that H-bonds among water molecules relax faster than those of the PMA–PMA and PMA–water type. The relaxation rate of PMA–water H-bonds shows a decrease up to Φ_p = 72% and becomes faster at Φ_p = 82% due to the confining influence of neighbouring PMA chains. PMA–water and water–water H-bond dynamics show transitions around Φ_p = 72% PMA. With increase in Φ_p PMA diffusion coefficient decreases exponentially and water diffusion coefficient decreases linearly, in agreement with experimental observations using fluorescence and nuclear magnetic resonance (NMR) spectroscopic studies.     

Evaluation of dew-water density by the Monte Carlo method

To construct a physical model of water vapor condensation with the formation of dew water, a geometrically based Monte Carlo method was developed. The hit and geometrical density functions of random space filling with identical spheres were determined. The parameters of these functions is the minimum allowable approach OO _min of spheres and their excess in comparison with the space capacity. The specificity of dew-water molecules and hydrogen bonds (HBs) between them appears at 2/3L _b < OO _min < 2L _b, where L _b is the HB length. As the approach OO _min = L _b, the random filling density does not exceed the packing density in the sphere models of I _h and I _c ices. The densities characteristic of the sphere model of water molecule packing in these ices are achievable at OO _min ≈ 0.8L _b and/or a significant HB kink, as well as at vapor supersaturation (excess of molecules over the space capacity).     

Positioning solvolyses within the SN2‐SN1 and SN3‐SN2 spectrum of reaction mechanisms

A simple linear regression (Q equation) is devised to position solvolyses within the established S_N2‐S_N1 spectrum of solvolysis mechanisms. Using 2‐adamantyl tosylate as the S_N1 model and methyl tosylate as the S_N2 model, the equation is applied to solvolyses of ethyl, allyl, secondary alkyl and a range of substituted benzyl and benzoyl tosylates. Using 1‐adamantyl chloride as the S_N1 model and methyl tosylate as the S_N2 model, the equation is applied to solvolyses of substituted benzoyl chlorides in weakly nucleophilic media. In some instances, direct correlations with methyl tosylate were employed. Grunwald–Winstein l values and kinetic solvent isotope effects are also used to locate solvolyses within the spectrum of mechanisms. Product selectivities (S) for solvolyses at 50 °C of p‐nitrobenzyl tosylate in binary mixtures of alcohol–water and of alcohol–ethanol for five alcohols (methanol, ethanol, 1‐propanol and 2‐propanol and t‐butanol) are reported and show the expected order of solvent nucleophilicity (RCH₂OH > R₂CHOH > R₃COH). The data support the original assignments establishing the N_OTs scale of solvent nucleophilicity. Copyright © 2016 John Wiley & Sons, Ltd.     

<Superscript>27</Superscript>Al NMR Study of the Structure and Dynamics of Natrolite

The temperature dependences of nuclear magnetic resonance and magic angle spinning nuclear magnetic resonance spectra of ²⁷Al nuclei in natrolite (Na₂Al₂Si₃O₁₀· 2H₂O) have been studied. The influence of water molecules and sodium ions mobility on the shape of the ²⁷Al NMR spectrum and framework dynamics have been discussed The temperature dependences of the spin–lattice relaxation times T₁ of ²⁷Al nuclei in natrolite have also been studied. It has been shown that the spin–lattice relaxation of the ²⁷Al is governed by the electric quadrupole interaction with the crystal electric field gradients modulated by translational motion of H₂O molecules in the natrolite pores. The dipolar interactions with paramagnetic impurities become significant as a relaxation mechanism of the ²⁷Al nuclei only at low temperatures (<270 K).     

Proton Diffusion andT1Relaxation in Polyacrylamide Gels: A Unified Approach Using Volume Averaging

The structure of polyacrylamide gels was studied using proton spin–lattice relaxation and PFG diffusion methods. Polyacrylamide gels, with total polymer concentrations ranging from 0.25 to 0.35 g/ml and crosslinker concentrations from 0 to 10% by weight, were studied. The data showed no effect of the crosslinker concentration on the diffusion of water molecules. The Ogston–Morris and Mackie–Meares models fit the general trends observed for water diffusion in gels. The diffusion coefficients from the volume averaging method also fit the data, and this theory was able to account for the effects of water-gel interactions that are not accounted for in the other two theories. The averaging theory also did not require the physically unrealistic assumption, required in the other two theories, that the acrylamide fibers are of similar size to water molecules. Contrary to the diffusion data,T₁relaxation measurements showed a significant effect of crosslinker concentration on the relaxation of water in gels. The model developed using the Bloch equations and the volume averaging method described the effects of water adsorption on the gel medium on both the diffusion coefficients and the relaxation measurements. In the proposed model the gel medium was assumed to consist of three phases (i.e., bulk water, uncrosslinked acrylamide fibers, and a bisacrylamide crosslinker phase). The effects of the crosslinker concentration were accounted for by introducing the proton partition coefficient,K^eq, between the bulk water and crosslinker phase. The derived relaxation equations were successful in fitting the experimental data. The partition coefficient,K^eq, decreased significantly as the crosslinker concentration increased from 5 to 10% by weight. This trend is consistent with the idea that bisacrylamide tends to form hydrophobic regions with increasing crosslinker concentration.     

Neutron scattering and compressibility measurements for the study of hydration effects on polymeric aqueous solutions

Summary The present paper reports the results of compressibility and incoherent quasi-elastic neutron scattering measurements performed on polymeric systems and on their aqueous solutions. From the compressibility data, the temperature evolution of the polymer hydration number can be derived. On the other hand, neutron data show that the translational diffusion coefficientD _T turns out to be higher in the case of ethylene glycol + water, with respect to that of pure, suggesting that the water molecules act as a ?structure breaker? of the intermolecular connectivity existing into the pure. Furthermore the dynamical properties of the H₂O molecules in the presence of poly(ethylene glycol) deeply differ from those in the bulk, and show that we are in the presence of entangled water. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Magnetic ordering in Co<Subscript><Emphasis Type="Italic">c</Emphasis></Subscript>Mg<Subscript>1−<Emphasis Type="Italic">c</Emphasis></Subscript>O solid solutions

The influence of dilution by diamagnetic ions on the magnetic ordering in single-crystal Co _c Mg_1−c O solid solutions was studied by Raman spectroscopy and magneto-optical microscopy in a wide range of temperatures (6 < T < 200 K). Far infrared absorption measurements of antiferromagnetic resonance (AFMR) were also performed for pure CoO. It was found that the domain structure and the contribution from Brillouin zone center magnons to Raman scattering and AFMR disappear well below the Néel temperature, whose value was determined from neutron diffraction and magnetic susceptibility measurements. The text was submitted by the authors in English.     

Metal-free activation of H<Subscript>2</Subscript>O<Subscript>2</Subscript> by synergic effect of ionic liquid and microwave: chemoselective oxidation of benzylic alcohols to carbonyls and unexpected formation of anthraquinone in aqueous condition

H₂O₂ mediated oxidation of alcohols in ionic liquid is revisited, wherein, ionic liquids under the influence of microwave irradiation have been found to facilitate activation of H₂O₂ without any metal catalyst in aqueous condition. The method utilizes a neutral ionic liquid [hmim]Br both as catalyst and solvent for efficient and chemoselective oxidation of benzyl alcohol derivatives on aromatic (β, γ) alcohols, cyclic and aliphatic analogues, which can be a useful synthetic approach in total synthesis of complex organic compounds/natural products. Moreover, an unexpected oxidation of 9-anthracenyl propanol, a polyaromatic benzyl alcohol, resulting in the formation of 9,10-anthraquinone by the loss of propyl side chain was observed. Plausible mechanism and further exploration of this method on various other related substrates are discussed in detail.     

Combined 1H NMR and DFT study of the solvent effects on the iron pentacarbonyl‐catalyzed photo‐assisted isomerization of allyl alcohol

The relative rates of iron pentacarbonyl‐catalyzed photo‐induced isomerization of allyl alcohol to propanal in various solvents have been measured using ¹H NMR spectroscopy. The reactions were run in linear and cyclic alkanes, alcohols, and amines in order to investigate solvent effects on the isomerization mechanism upon ultraviolet irradiation. The isomerization was efficient in nonane, hexane, cyclohexane, and benzene. The isomerization was very slow or completely suppressed in ethanol, propanol, isopropanol, triethylamine, and pyridine. Density functional theory calculations predicted thermodynamically favorable (ΔG^o < 0) formation of Fe(CO)₄–solvent compounds in the suppressing alcohol/amine solvents and unfavorable (ΔG^o > 0) formation in the hydrocarbon solvents. The strong solvent ligation likely prevents formation of Fe(CO)₃●(η²‐alkene) necessary for isomerization. Copyright © 2013 John Wiley & Sons, Ltd.