Participation Of Solvent Molecules Research Articles

Сорбция гистидина ионообменниками различной природы

С целью сравнения сорбции полифункциональных веществ на ионообменниках различной природы исследовано поглощение гистидина гидрохлорида из водных растворов анионообменником АН-251 в хлоридной ионной форме, катионообменником КБ-4П-2 в натриевой ионной форме и полиам-фолите АНКБ-2 в солевой NaCl-форме. Установлено, что в первом случае протекает необменная сорб-ция за счет ион-дипольных взаимодействий между сорбционным центром «функциональная группа - противоион» и заряженной группой аминокислоты с участием молекул воды. На катионообменнике и полиамфолите протекает ионный обмен, поскольку в системе присутствует два сорта катионов Na+ и His+, величины которого, однако, невелики. В этих же системах протекает необменное поглощение за счет ион-дипольных взаимодействий противоположно заряженных структурных элементов с участием молекул расторителя, которые, могут протекать с противоионом любой природы, возникающим в си-стеме за счет ионного обмена.Установлено, что независимо от природы сорбента количество поглощенного гистидина при-близительно одинаково из-за дополнительных взаимодействий, протекающих в системе с участием сорбционных центров (в КБ-4П-2 и АНКБ-2) и малого набухания ионита (АН-251).Проведено термодинамическое описание исследуемых систем с использованием подходов, ос-нованных на представлении о стехиометричности закрепления вещества в сорбенте. Рассчитаны вели-чины коэффициентов равновесия, исправленных коэффициентов равновесия, коэффициентов активно-сти компонентов фазы сорбента, и термодинамических констант ионообменного и необменного равно-весия, а также дифференциальные энергии Гиббса, относящиеся к ионообменнику определенного со-става. В качестве состояния сравнения для обоих случаев выбирается состояние равновесия ионооб-менника в моноионной форме с водой.Установлено, что в процессе сорбции отклонение систем от состояния сравнения незначитель-ное, что связано с малым количеством поглощенного вещества по сравнению с количеством сорбцион-ных центров, термодинамические контанты равновесия близки к единице. Совместное протекание ион-ного обмена и необменной сорбции оказывает большее влияние на состояние сорбента по сравнению с необменной сорбцией в силу изменения ионной формы сорбента с минеральной на органическую

Photoprotection or photodamage: a direct observation of nonradiative dynamics from 2-ethylhexyl 4-dimethylaminobenzoate sunscreen agent.

Apart from being an analogue of the prototype for photoinduced intramolecular charge transfer (ICT), 2-ethylhexyl 4-dimethylaminobenzoate (EHDMABA) is also one of the earliest patented and most commonly used sunscreen components. There is, however, little documented information about the photophysics and factors affecting the photophysics of this molecule. Such information is of importance for both the understanding of the ICT reaction and assessing the underlying process of photoprotection, especially in view of the "sunscreen controversy" that has arisen from the contrasting in vivo vs. in vitro photobiological results on this and related UV filters. We report herein a femtosecond broadband time-resolved fluorescence (fs-TRF), complemented by transient absorption (fs-TA) to allow a full probe of the excited state cascades for EHDMABA and two of its derivatives in solvents of varied properties. The results provide direct evidence for a nearly solvent independent inner sphere ICT reaction occurring on the sub-picosecond time scale, and an ensuing solvent dictated deactivation of the ICT state. The ICT state in the aprotic solvent acetonitrile decayed solely through the intrinsic intersystem crossing (ISC) to produce a potentially harmful triplet excited state. In the protic solvent, the solvation and formation of ICT-induced solute-solvent hydrogen (H)-bonding opened the originally inaccessible internal conversion (IC) channel of the ICT state, leading to the rapid reformation of the ground state molecule with a unitary efficiency in the aqueous solution. This H-bonding-mediated IC restrained or eliminated the intrinsic ISC, providing a mechanism at the molecular level for the benign dissipation of the electronic excitation. The precise rate of IC was observed to vary with the alkoxy substituent and its efficiency was affected by the H-bonding capacity of the solvent. The findings of this work demonstrate the pivotal role of the microenvironment and the direct participation of solvent molecules through H-bonding in drastically altering the nonradiative dynamics and promoting or inhibiting photostability and photoprotection. This may assist in developing next-generation UV filters and help in improving formulation design for the optimal efficacy of sunscreen products. The pronounced H-bonding-induced fluorescence quenching and variation in the fluorescence wavelength imply that these molecules may also serve as a sensitive fluorescence probe for the H-bonding properties of the microenvironment.

Enthalpic Interactions of HMBA with Glycine, l-Alanine, and l-Serine in Aqueous d-Mannitol Solutions at 298.15 K

The mixing enthalpies of N,N′-hexamethylenebisacetamide (HMBA) with glycine, l-alanine, and l-serine in d-mannitol solutions with different molar fractions have been determined by means of mixing-flow isothermal microcalorimetry at 298.15 K. The heterotactic enthalpic interaction coefficients (hxy, hxxy, and hxyy), in the molar fraction (x) range of d-mannitol from (0 to 0.0159), have been calculated in terms of McMillan–Mayer theory. The trends of hxy with the molar fractions of d-mannitol have been discussed based on the interactions between solute molecules under the participation of solvent molecules. The enthalpic contribution of the functional groups of solute molecules to hxy has been estimated according to the Savage–Wood additivity group (SWAG) approach. The influence of structural difference between glucose and d-mannitol on the hxy values of HMBA with glycine has been elucidated by combining the present results with our previous work.

Association Phenomena of Poly(arylene ether sulfone)s in Dimethylformamide

Three poly(arylene ether sulfone)s (two polymers and one copolymer) containing diphenylfluorene, phenolphthalein and 2,4-bis[(4-chlorophenyl)sulfonyl]-1-(phenyltio) benzene)] units were synthesized by the classical Williamson polyetherification reaction. The association phenomenon in N,N-dimethylformamide (DMF) was investigated by different methods: gel permeation chromatography (GPC), viscosity, atomic force microscopy (AFM), fluorescence spectroscopy and dynamic light scattering (DLS). Both AFM and DLS measurements evidenced the formation of aggregates with spherical or ellipsoidal shape at a concentration around 0.05 wt.%. This behavior could be explained by dipole-dipole interactions between macromolecular chains, probably with the participation of solvent molecules, and by H-bonds involving hydroxyl end groups.

Nanogravimetric studies of tungsten oxide thin films obtained by the polymeric precursor method

In this work, the intercalation/de-intercalation process of Li + ions in the tungsten oxide matrix was investigated. The reaction mechanism involved was also investigated. The WO 3 films, prepared by the polymeric precursor method, were deposited on a Pt covered quartz crystal using the dip-coating technique. The electrolyte was 0.1 mol L − 1 LiClO 4 in acetonitrile. The morphology and structure of the deposit was accomplished by scanning electron microscopy and X-ray diffraction, respectively. In the electrochemical quartz crystal nanobalance results, it was observed that the mass transport as well as the kinetic processes involved are facilitated in the films cycled at lower potential sweep-rates. The mass variation data as a function of the charge variations in the anodic and cathodic regions indicate the participation of solvent molecules (acetonitrile) during the Li + ion intercalation/de-intercalation process. This was confirmed by the development of a model of the species flux as a function of the potential.

Kinetics and mechanism of the thermal decomposition reaction of acetone cyclic diperoxide in methyl tert‐butyl ether solution

AbstractThe thermal decomposition reaction of acetone cyclic diperoxide (3,3,6,6‐tetramethyl‐1,2,4,5‐tetroxane, ACDP), in the temperature range of 130.0–166.0°C and initial concentrations range of 0.4–3.1 × 10−2 mol kg−1 has been studied in methyl t‐butyl ether solution. The thermolysis follows first‐order kinetic laws up to at least ca 60% ACDP conversion. Under the experimental conditions, the activation parameters of the initial step of the reaction (ΔH# = 33.6 ± 1.1 kcal mol−1; ΔS# = −4.1 ± 0.7 cal mol−1 K−1; ΔG# = 35.0 ± 1.1 kcal mol−1) and acetone, as the only organic product, support a stepwise reaction mechanism with the homolytic rupture of one of its peroxidic bond. Also, participation of solvent molecules in the reaction is postulated given an intermediate diradical, which further decomposes by CO bond ruptures, yielding a stoichiometric amount of acetone (2 mol per mole of ACDP decomposed). The results are compared with those obtained for the above diperoxide thermolysis in other solvents. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 302–307, 2004

Ionic Exchange Phenomena Related to the Redox Processes of Polyaniline in Nonaqueous Media

Polyaniline films in nonaqueous media, mainly propylene carbonate and acetonitrile, are studied by using simultaneous potentio‐dynamic experiments with the electrochemical quartz crystal microbalance and electrochemical impedance spectroscopy. The role of ionic transport and solvent participation during redox reactions is emphasized to obtain information about all chemical species involved in the process occurring in the conducting polymer film. The contribution of each species (neutral and ionic) depends on both the chemical nature of the ionic salt and the solvent used. Quartz crystal microbalance results have shown that both cation and anion contribute to the electroneutralization process, and the participation of solvent molecules is related mainly to changes in the oxidation state of the polymer matrix. As propylene carbonate propylene (ε = 66.1) is a more polar solvent than acetonitrile (ε = 37.3) it produces a more pronounced swelling process. © 2000 The Electrochemical Society. All rights reserved.

Solvent effects on the reactivity of solvated electrons with organic solutes in 1-butylamine–water mixtures

The values of the rate constants of the reactions of es− with the efficient scavengers nitrobenzene and acetone are ≥ 2 × 106 m3 mol−1 s−1 in the whole range of 1-butylamine–water mixtures at 298 K; the reaction rates in the mixed solvents vary approximately as the solvent fluidity. In pure butylamine at 298 K, k2(es− + nitrobenzene) = 84 × 106 m3 mol−1 s−1 and k2(es− + acetone) = 7.3 × 106 m3 mol−1 s−1. The values of the rate constants of the reactions of es− with the inefficient scavengers phenol and toluene are < 2 × 105 m3 mol−1 s−1 in the whole range of 1-butylamine–water mixtures at 298 K and have a maximum at 50 mol% water and a minimum at 99 mol% water. In pure 1-butylamine at 298 K, k2(es− + phenol) = 1.0 × 104 m3 mol−1 s−1 and k2(es− + toluene) = 0.28 × 104 m3 mol−1 s−1. The reaction rates with inefficient scavengers show strong dependence on the solvent composition and selective solvation of electron and scavenger. In the amine-rich region (0–30 mol% water), the rate constants increase with the increase of viscosity, indicating the chemical participation of solvent molecules in the reaction. In the water-rich region from 50 to 99 mol% water, the decrease of the rate constants indicates the nonhomogeneous solvation of the electrons by water and of the organic solutes by 1-butylamine. From 99 mol% to pure water the rate constant increases rapidly, which we attribute to insufficient 1-butylamine to coat the phenol or toluene molecules. The variation of the activation energies E2 for the efficient scavengers, 14–27 kJ mol−1, are similar to the variation of Eη in the mixed solvents. The values of E2 for the inefficient scavengers are from 15 to 38 kJ mol−1 for phenol and from 6 to 21 kJ mol−1 for toluene. Both k2 and E2 for the inefficient scavenger reactions show a correlation with the temperature coefficient −dEAmax/dT of the optical absorption of es− in the mixed solvents, but the reason is obscure. Key words: 1-butylamine–water solvent, solvated electron, organic solutes, reactivity, solvent effects.

Photo-induced generation of radicals from m-dinitrobenzene adsorbed on γ-Al2O3: Direct evidence for the formation of electron donor-acceptor (EDA) complexes with participation of solvent molecules

Thermal and photo-induced generation of radicals during adsorption of m-dinitrobenzene (m-DNB) on donor centers (DS) of γ-Al2O3 from various solvents has been studied. Energy characteristics of these processes are shown to be significantly dependent on the ionization potential (IP) of solvent molecules (D). A mechanism for these processes is suggested implying the formation of surface electron donor-acceptor (EDA) complexes of the type [D·NB·DS] with the participation of solvent molecules.

Dynamic NMR as a Nondestructive Method for the Determination of Rates of Dissociation. XVII. Dissociation of the N–B Bond in the Coordinated Form of 2-[2-(Dimethylaminomethyl)phenyl]-4,4-diphenyl-1,3,2-dioxaborolane and Related Compounds

Abstract The rates of dissociation of the N–B bonds in the title compounds were reexamined by the dynamic NMR technique to get further insight into the mechanism of dissociation. Entropies of activation were fairly large positive to suggest that the dissociation of a bond of ionic characters is the rate-limiting step. The rates of dissociation were generally larger in nonpolar solvents than in polar solvents, suggesting that the stabilization of the ground state by solvation was important in determining the activation energy for dissociation. Diethyl ether, tetrahydrofuran, and acetone gave larger rate constants of dissociation than those expected from their polarity. Participation of solvent molecules in the dissociation is discussed on the basis of these results. Introduction of various substituents indicated that steric effects as well as electronic effects affect the N–B bond energy.

Effect of alkyl chain length of the stationary phase on retention and selectivity in reversed-phase liquid chromatography : Participation of solvent molecules in the stationary phase

The retention behaviour of various hydrocarbons, including n-alkanes, polynuclear aromatic hydrocarbons (PAHs) and alicyclic compounds, were examined on alkylsilylated stationary phases with trimthylsilyl (C 1), octyldimethylsilyl (C 8) and octadecyldimethylsilyl (C 18) functionality. In general, a longer alkyl chain in the stationary phase provided a longer retention time. Although the retention of rigid planar aromatic hydrocarbons relative to non-polar aromatic compounds and alkanes was much larger on the C 18 phase than on the C 8 and C 1 phases in a methanolic mobile phase, the effect of chain length was much smaller in a mobile phase containing acetonitrile. A change in the organic solvent in the mobile phase from methanol to acetonitrile resulted in grossly different selectivities between aromatic hydrocarbons and the saturated hydrocarbons regardless of the rigidity or planarity in structure of the C 18 phase, in contrast with minor changes on the C 1 and C 8 phases. The increase of organic solvent content in the mobile phase resulted in much larger retention of rigid planar aromatic compounds relative to other aromatic compounds on the C 18 phase, whereas on the C 1 and C 8 phases this effect was smaller. The retention characteristics of the C 8 phase for the hydrocarbons were found to be relatively close to those of the C 1 phase. The results suggest that the effect of chain length is influenced by both the molecular structure of the solutes and the type of organic solvents used in the mobile phase.

Fragmentation of acid chlorides and azides of?-amino acids having a tertiary nitrogen atom, and some other properties of these amino acids (copper complexes)

1. Likeα-dibenzylamino acid chlorides, N.N-diethyl-, N,N-bis-p-methylbenzyl-, and N,N-bis-p-nitrobenzylalanyl chlorides can undergo fragmentation. The most essential feature in this reaction is the presence of a tertiary nitrogen atom. 2. In the reaction of nitrous acid with N,N-dibenzyl-3-phenylalanine hydrazide, N,N-dibenzylstyrylamine was isolated. 3. A method was developed for the synthesis of N,N-bis-p-nitrobenzylalanine. 4. In solution, copper complexes ofα-di(aralkyl)amino acids give two types of complexes: a) green complexes, and b) violet complexes. Such dichroism is apparently associated with change in the coordination number of copper as a result of the participation of solvent molecules in the complex.