Microstructure and rheology of stimuli-responsive nanocolloidal systems-effect of ionic strength

The effect of ionic strength on the rheological behavior of model pH-responsive nanocolloidal systems consisting of methacrylic acid-ethyl acrylate (MAA-EA) cross-linked with diallyl phthalate (DAP) was examined. Neutralization of acid groups increases the osmotic pressure exerted by counterions trapped in the polymeric network against ions in bulk solution, which is responsible for the swelling and increase in viscosity. Swelling decreases with increasing salt concentration as a result of reduced osmotic pressure inside the microgels, which is attributed to the charge shielding effect of counterions (salt) on the negatively charged carboxylate groups. Electromotive measurements using ion-selective electrodes confirmed that not all the counterions, that is, K+, remain mobile, but a fraction of these ions can penetrate the porous microgel particles to shield the negatively charged carboxylate groups. A consequence of this is that some of the Na+ counterions inside the particles are expelled, thus regaining their translational entropy, and become mobile sodium ions in the bulk solution. We successfully developed a new scaling law that relates the swelling ratio, Q, of microgels as a function of neutralization degree, alpha, cross-linked density, Nx, molar fraction of acidic units, y, and concentration of mobile counterions, CK+ and CNa+, represented as (Nx/c0)(CK+ + CNa+Q + Q2/3 proportional, variant yNxalpha. The new scaling law no longer assumes that all the counterions are trapped inside the microgels. The proportionality reduces to the form Q proportional, variant (yalphaNx)3/2 in the absence of salt, that is, CK+ + CNa+ approximately 0. By combining the results from light scattering and rheological measurements, we are able to correlate the microstructural evolution of the colloidal systems with their bulk rheological behavior.     

Direct measurement of the energy thresholds to conformational isomerization. II. 3-indole-propionic acid and its water-containing complex

The methods of stimulated emission pumping-hole-filling spectroscopy (SEP-HFS) and population transfer spectroscopy (SEP-PTS) were used to place direct experimental bounds on the energetic barriers to conformational isomerization in 3-indole-propionic acid (IPA) and its water-containing complex. By contrast with tryptamine (Paper I), IPA has only two conformations with significant population in them. The structures of the two conformers are known from previous work [P. M. Felker, J. Phys. Chem. 96, 7844 (1992)]. The energy thresholds for A-->B and B-->A isomerizations are placed at 854 and 754 cm(-1), respectively. Lower bounds on the isomerization barrier in the two directions are determined from the last transition not observed in the SEP-PT spectra. These are placed at 800 and 644 cm(-1) for A-->B and B-->A, respectively. The combined results place bounds on the relative energies of the A and B minima, with E(B)-E(A)=46-210 cm(-1). Like the IPA monomer, the IPA-H2O complex forms two conformational isomers. Both these isomers incorporate the water molecule as a bridge between the carbonyl and OH groups of the carboxylic acid. Previous rotational coherence measurements (L. L. Connell, Ph.D. thesis, UCLA, 1991) have determined that these complexes retain the same IPA conformational structure as the monomers. SEP-PTS and SEP-HFS were carried out on the IPA-H2O complexes. It was demonstrated that it is possible to use SEP to drive conformational isomerization between the two conformational isomers of IPA-H2O. Bounds on the energy barriers to conformational isomerization are not effected greatly by the presence of the water molecule, with Ebarrier(A-->B)=771-830 cm(-1) and Ebarrier(B-->A)=583-750 cm(-1). This is a simple consequence of the fact that the barrier is an intramolecular barrier, and the water molecule is held fixed in the COOH pocket, where it interacts with the ring only peripherally during the isomerization process. Finally, changes in the SEP-PT spectral intensity in transitions near the top of the barrier to isomerization as a function of the position of SEP excitation relative to the pulsed valve exit provide some insight to the competition between vibrational relaxation and isomerization in a molecule the size of IPA.     

Application of second order BWEN perturbation theory to some simple reactions

The second order Brillouin-Wigner perturbation expansion with the Epstein-Nesbet partitioning is applied to some isomerization and insertion reactions, using the 6-31G^* basis set. BWEN2 is found to be comparable in accuracy with RSMP2 for predictions of energy barriers and isomerization energies.     

Theoretical study on the isomeric structures and the stability of silylenoid (Tsi)Cl2SiLi (Tsi = C(SiMe3)3)

The structures and isomerization of silylenoid (Tsi)Cl(2)SiLi (Tsi = C(SiMe(3))(3)) were studied by density functional theory (DFT) at the B3LYP/6-31G(d) level. Four equilibrium structures and three isomeric transition states were located. The three-membered ring and p-complex structures, 1 and 2, are the two most stable forms. Two other local minima, the sigma-complex 3 and tetrahedron structure 4, should rearrange to 1 with very low barriers, and then to the most stable isomer 2. To exploit further the stability of silylenoid (Tsi)Cl(2)SiLi, the insertion reactions of 2 and silylene (Tsi)ClSi into the HF molecule have been investigated at the B3LYP/6-31G(d) level, respectively. The results show that the insertion of 2 into HF is very similar to that of (Tsi)ClSi into HF, but the latter is more favorable. To probe the influence of the substituent Tsi on the stability of silylenoid (Tsi)Cl(2)SiLi, the isomers and insertion reaction of silylenoid CH(3)Cl(2)SiLi were investigated in a similar way of those with (Tsi)Cl(2)SiLi. The results indicate that silylenoid containing very bulky group Tsi exhibits unusual stability because of the severe steric hindrance produced by Tsi at the center to which it is attached.     

Ruthenium(II)-catalyzed hydrogenation of carbon dioxide to formic acid. Theoretical study of real catalyst, ligand effects, and solvation effects

Ruthenium-catalyzed hydrogenation of carbon dioxide to formic acid was theoretically investigated with DFT and MP4(SDQ) methods, where a real catalyst, cis-Ru(H)2(PMe3)3, was employed in calculations and compared with a model catalyst, cis-Ru(H)2(PH3)3. Significant differences between the real and model systems are observed in CO2 insertion into the Ru(II)-H bond, isomerization of a ruthenium(II) eta1-formate intermediate, and metathesis of the eta1-formate intermediate with a dihydrogen molecule. All these reactions more easily occur in the real system than in the model system. The differences are interpreted in terms that PMe3 is more donating than PH3 and the trans-influence of PMe3 is stronger than that of PH3. The rate-determining step is the CO2 insertion into the Ru(II)-H bond. Its deltaG(o++) value is 16.8 (6.8) kcal/mol, where the value without parentheses is calculated with the MP4(SDQ) method and that in parentheses is calculated with the DFT method. Because this insertion is considerably endothermic, the coordination of the dihydrogen molecule with the ruthenium(II)-eta1-formate intermediate must necessarily occur to suppress the deinsertion. This means that the reaction rate increases with increase in the pressure of dihydrogen molecule, which is consistent with the experimental results. Solvent effects were investigated with the DPCM method. The activation barrier and reaction energy of the CO2 insertion reaction moderately decrease in the order gas phase > n-heptane > THF, while the activation barrier of the metathesis considerably increases in the order gas phase < n-heptane < THF. Thus, a polar solvent should be used because the insertion reaction is the rate-determining step.     

DFT法研究离子液中EMIM+催化丁烯双键异构反应机理(II)

用密度泛函理论(DFT)在B3LYP/6-31G(d, p)的计算水平上研究了离子液中1-乙基-3-甲基咪唑阳离子(EMIM⁺)的4-H和5-H原子催化丁烯双键异构反应的可能途径，优化了反应体系的平衡态和过渡态的几何构型，分析了反应过程中键参数的变化，通过振动分析对平衡态和过渡态进行了验证. 计算结果表明, 离子液中的EMIM⁺首先通过4-H和5-H原子吸附丁烯, 进而催化丁烯的双键异构反应, EMIM⁺的4-H和5-H催化1-丁烯异构为2-丁烯的正反应活化能分别为204.2和207.3 kJ•mol^-1，逆反应活化能约为220.9和223.8 kJ•mol^-1， 反应为基元反应.     

Single molecule fluorescence fluctuations of the cyanine dyes linked covalently to DNA

The intersystem crossing and isomerization dynamics of free-Cy3, Cy3-ssDNA, free-Cy5 and Cy5-ssDNA are obtained through simple analysis of rapid on/off blinking from single molecule fluorescence intensity time-traces and the fluorescence correlation spectroscopy (FCS). The on- and off-times observed in fluorescence time traces of single cyanine dyes are due to the formation of the triplet state and isomerization, where both the interaction with DNA and long central polymethine chain of cyanine dyes increase the barriers of isomerization, leading to long off-time. The results indicate that the single molecule fluorescence fluctuation together with the resulting second autocorrelation analysis are powerful methods for determining the triplet state and isomerization dynamics, which could be the simple techniques and complementary to other spectroscopic techniques, such as fluorescence decay measurement and laser flash photolysis to study the photophysical processes of complex molecules. Supported by the National Natural Science Foundation of China (Grant Nos. 20773139, 20833008 & 20825314), and State Key Project for Fundamental Research (Grant Nos. 2006CB806000 & 2007CB815200)     

Tension trapping of carbonyl ylides facilitated by a change in polymer backbone

Epoxidized polybutadiene and epoxidized polynorbornene were subjected to pulsed ultrasound in the presence of small molecules capable of being trapped by carbonyl ylides. When epoxidized polybutadiene was sonicated, there was no observable small molecule addition to the polymer. Concurrently, no appreciable isomerization (cis to trans epoxide) was observed, indicating that the epoxide rings along the backbone are not mechanically active under the experimental conditions employed. In contrast, when epoxidized polynorbornene was subjected to the same conditions, both addition of ylide trapping reagents and net isomerization of cis to trans epoxide were observed. The results demonstrate the mechanical activity of epoxides, show that mechanophore activity is determined not only by the functional group but also the polymer backbone in which it is embedded, and facilitate a characterization of the reactivity of the ring-opened dialkyl epoxide.     

Unusual Nitrene Oxidation Product Formation by Metathesis Involving the Dioxygen O−O and Borylnitrene B−N Bonds

The reaction of dioxygen with nitrenes can have significant energy barriers, although both reactants are triplet diradicals and the formation of nitroso-O-oxides is spin-allowed. By means of matrix-isolation infrared spectroscopy in solid argon, nitrogen, and neon, and through high-level computational quantum chemistry, it is shown herein that a 3-nitreno-1,3,2-benzodioxaborole CatBN (Cat=catecholato) reacts with dioxygen under cryogenic conditions thermally at temperatures as low as 7 K to produce two distinct products, an anti-nitroso-O-oxide and a nitritoborane CatBONO. The computed barriers for the formation of nitroso-O-oxide isomers are very low. Whereas anti-nitroso-O-oxide is kinetically trapped, its bisected isomer has a very low barrier for metathesis, yielding the CatBO+NO radicals in a strongly exothermic reaction; these radicals can combine under matrix-isolation conditions to give nitritoborane CatBONO. The trapped isomer, anti-nitroso-O-oxide, can form the nitritoborane CatBONO only after photoexcitation, possibly involving isomerization to the bisected isomer of anti-nitroso-O-oxide.     

Stereoselectivity of the cyclooctene ring-opening polymerization

The stereoselectivity of the cyclooctene ring-opening polymerization in the presence of the tungsten hexachloride–tetraisobutylalumoxane (TBAO) catalyst system has been studied. The reaction conditions have been established in which the polymer chain isomerization is reduced to a minimum. Under these conditions the stereoselectivity is determined by an individual act of the monomer molecule insertion. The dependence of stereoselectivity on monomer concentration, number of active sites, and temperature has been investigated. The results obtained suggest that the cyclooctene polymerization is selective in favor of the geometrical configuration of the starting cycloolefin (the “retention” effect) by analogy with the previously reported metathesis of acyclic olefins. Considerations which support the idea that cis and trans units are reproduced by the same type of active sites are discussed. The experimental evidence indicates that in this case the stereoselectivity is determined by the difference in kinetics of two reaction paths of the monomer molecule insertion that lead to the formation of cis and trans units.     

氟代甲酰胺和N—氟代甲酰胺异构化反应路径的理论研究

采用量子化学MNDO法,计算了氟代甲酰胺和N—氟代甲酰胺的1,2—氢迁移异构化反应势能面上的反应路径。结果表明:氟代甲酰胺分子比N—氟代甲酰胺分子稳定.前者异构化为吸热反应,后者为放热反应。N—氟代甲酰胺的氢迁移势垒低于甲酰胺的氢迁移势垒.氟原子的存在使过渡态结构相对松散,有利于异构化的发生.     

Silylanions: inversion barriers and NMR chemical shifts

Alpha-substituent effects on inversion barriers and NMR chemical shifts have been studied on a set of silyl anions, [X(3-n)Y(n)Si](-) (X, Y=H, CH(3), and SiH(3)). The MP2/6-31+G* optimized structures show a pattern of increasing inversion barriers with augmenting numbers of methyl substituents. The highest barrier of 48.5 kcalmol(-1) is obtained for the (CH(3))(3)Si(-) ion. The silyl group displays the opposite effect by decreasing the inversion barrier to a minimum of 16.3 kcalmol(-1) in (SiH(3))(3)Si(-). The influence of counterions on these barriers is probed by addition of a lithium or potassium cation. In most cases, a decrease of the energy barriers with respect to the bare anions is observed. The (29)Si NMR chemical shifts calculated at the IGLO-DFT and GIAO-MP2 level of theory are also analyzed in view of the substituents and counterions.     

Different catalysis role of in‐loop and out‐of‐loop waters in assisting HNS/HSN proton transfer isomerizations: Bridging vs. surrounding effect

In this work, a density function theory (DFT) study is presented for the HNS/HSN isomerization assisted by 1–4 water molecules on the singlet state potential energy surface (PES). Two modes are considered to model the catalytic effect of these water molecules: (i) water molecule(s) participate directly in forming a proton transfer loop with HNS/HSN species, and (ii) water molecules are out of loop (referred to as out‐of‐loop waters) to assist the proton transfer. In the first mode, for the monohydration mechanism, the heat of reaction is 21.55 kcal · mol^?1 at the B3LYP/6‐311++G** level. The corresponding forward/backward barrier lowerings are obtained as 24.41/24.32 kcal · mol^?1 compared with the no‐water‐assisting isomerization barrier T (65.52/43.87 kcal · mol^?1). But when adding one water molecule on the HNS, there is another special proton‐transfer isomerization pathway with a transition state 10T′ in which the water is out of the proton transfer loop. The corresponding forward/backward barriers are 65.89/65.89 kcal · mol^?1. Clearly, this process is more difficult to follow than the R–T–P process. For the two‐water‐assisting mechanism, the heat of reaction is 19.61 kcal · mol^?1, and the forward/backward barriers are 32.27/12.66 kcal · mol^?1, decreased by 33.25/31.21 kcal · mol^?1 compared with T. For trihydration and tetrahydration, the forward/backward barriers decrease as 32.00/12.60 (30T) and 37.38/17.26 (40T) kcal · mol^?1, and the heat of reaction decreases by 19.39 and 19.23 kcal · mol^?1, compared with T, respectively. But, when four water molecules are involved in the reactant loop, the corresponding energy aspects increase compared with those of the trihydration. The forward/backward barriers are increased by 5.38 and 4.66 kcal · mol^?1 than the trihydration situation. In the second mode, the outer‐sphere water effect from the other water molecules directly H‐bonded to the loop is considered. When one to three water molecules attach to the looped water in one‐water in‐loop‐assisting proton transfer isomerization, their effects on the three energies are small, and the deviations are not more than 3 kcal · mol^?1 compared with the original monohydration‐assisting case. When adding one or two water molecules on the dihydration‐assisting mechanism, and increasing one water molecule on the trihydration, the corresponding energies also are not obviously changed. The results indicate that the forward/backward barriers for the three in‐loop water‐assisting case are the lowest, and the surrounding water molecules (out‐of‐loop) yield only a small effect. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Proton Transfer Isomerization of Pyrazole in the Ground State:σν sπ Mechanism and Water Assisting Effect

The proton transfer isomerization of pyrazole and the water assisting effect by looping 1 to 4 water molecules on the singlet state potential energy surface have been investigated by using hybrid density functional theory method (B3PW91) with a 6-311++G** basis set. Two mechanisms were proposed to explain the mono- and multi-water assisting effects, respectively. The reactants and products of all groups have been characterized on their potential energy surfaces. For the isomerization of monomolecule pyrazole, the isomerization energy barrier is 46.4 kcal·mol-1. For the monohydration assisting mechanism, the reactant complex is connected to the product complex via two saddle points. The corresponding isomerization barriers are 46.7and 23.0 kcal(mol-1, respectively. As to the multihydration assisting mechanism, the isomerization barriers are 12.0, 10.9 and 13.14 kcal(mol-1 accordingly, when the number of water molecules is 2, 3 and 4, respectively. The multihydration assisting isomerization can occur in water-dominated environments, for example, in the organism, and thereby is crucial to energy transference. The deproton and dehydrogen energies of monomolecule pyrazole and various hydrated pyrazoles were calculated and then found much bigger than the isomerization barriers of their relative complexes, suggesting the impossibility of deprotonation or dehydrogenation. The isomerization of pyrazole is a proton-coupling-electron-migration process, but two different mechanisms are noticed, viz.σ- and π-type mechanisms. The π-bond of pyrazole participates in isomerization in the π-type mechanism, whereas only σelectron takes part in isomerization in the σ-type mechanism.     

Gas-phase nitrosation of ethylene and related events in the C2H4NO+ landscape

The C2H4NO(+) system has been examined by means of quantum chemical calculations using the G2 and G3B3 approaches and tandem mass spectrometry experiments. Theoretical investigation of the C2H4NO(+) potential-energy surface includes 19 stable C2H4NO(+) structures and a large set of their possible interconnections. These computations provide insights for the understanding of the (i) addition of the nitrosonium cation NO(+) to the ethylene molecule, (ii) skeletal rearrangements evidenced in previous experimental studies on comparable systems, and (iii) experimental identification of new C2H4NO(+) structures. It is predicted from computation that gas-phase nitrosation of ethylene may produce C2H4(*)NO(+) adducts, the most stable structure of which is a pi-complex, 1, stabilized by ca. 65 kJ/mol with respect to its separated components. This complex was produced in the gas phase by a transnitrosation process involving as reactant a complex between water and NO(+) (H2O.NO(+)) and the ethylene molecule and fully characterized by collisional experiments. Among the other C 2H 4NO (+) structures predicted by theory to be protected against dissociation or isomerization by significant energy barriers, five were also experimentally identified. These finding include structures CH3CHNO(+) (5), CH 3CNOH (+) ( 8), CH3NHCO(+) (18), CH3NCOH(+) (19), and an ion/neutral complex CH2O...HCNH(+) (12).     

Ruthenium(IV)-catalyzed isomerization of the C=C bond of o-allylic substrates: a theoretical and experimental study

A general mechanism to rationalize Ru(IV) -catalyzed isomerization of the C=C bond in O-allylic substrates is proposed. Calculations supporting the proposed mechanism were performed at the MPWB1K/6-311+G(d,p)+SDD level of theory. All experimental observations in different solvents (water and THF) and under different pH conditions (neutral and basic) can be interpreted in terms of the new mechanism. Theoretical analysis of the transformation from precatalyst to catalyst led to structural identification of the active species in different media. The experimentally observed induction period is related to the magnitudes of the energy barriers computed for that process. The theoretical energy profile for the catalytic cycle requires application of relatively high temperatures, as is experimentally observed. Participation of a water molecule in the reaction coordinate is mechanistically essential when the reaction is carried out in aqueous medium. The new mechanistic proposal helped to develop a new experimental procedure for isomerization of allyl ethers to 1-propenyl ethers under neutral aqueous conditions. This process is an unique example of efficient and selective catalytic isomerization of allyl ethers in aqueous medium.     

Structure of Charged Poly(propylene imine) Dendrimers: Theoretical Investigation

A mean‐field model for charged dendrimers has been elaborated and applied to Astramol dendrimers of 5^th generation in salt‐free solution. The free energy of a dendrimer molecule was minimized with respect to the dendrimer size and to the profile of counterion distribution. The model of highly stretched freely jointed chain was used to describe the elasticity of long branches, the dissociated groups were assumed to be localized mostly on the periphery of the molecule, and the electrostatic interactions were described in the Poisson‐Boltzmann approximation. It was found that the osmotic pressure of counterions leads to moderate expansion of dendrimer molecules upon charging, and a significant fraction of counterions is localized within the dendrimer molecule under typical experimental conditions.

The schematic structure of poly(propylene imine) dendrimers for the 4^th generation.     

From pentalene to dicyclopenta[b,g]naphthalene, or the change towards delocalized structures.

Using triples-corrected coupled-cluster methods as well as other high-level theoretical approximations, the optimized parameters and isomerization barriers of the family of compounds cyclopentadiene-(benzene)x-cyclopentadiene (x = 0, 1, 2) are computed. In contrast to previous studies, s-indacene presents a localized C(2h) geometry. Also, the localized structure of pentalene is found to be the most stable, but when two benzene rings are intercalated between the five-member rings of pentalene, the resulting molecule preferably adopts a delocalized D(2h) conformation.     

2',3'-二脱氧-2',3'-二去氢鸟嘌呤核苷构象稳定性的理论研究

采用密度泛函方法在B3LYP/6-31＋G**水平上研究了2',3'-二脱氧-2',3'-二去氢鸟嘌呤核苷分子(D4G)的构象. 分别研究在气相中的孤立分子和一水合物异构体的相对稳定性和异构体之间的相互转变过程, 分析了水分子的参与对D4G异构体的相对稳定性和几何结构参数以及自然电荷的影响. 结果表明, 孤立的D4G分子在气相中存在8种稳定构象, 其中构象d4g-2是所有构象中最稳定的, 气相中D4G主要以d4g-2存在. 气相中各构象的相对稳定性为: d4g-2＞d4g-1＞d4g-5＞d4g-3＞d4g-6＞d4g-4＞d4g-8＞d4g-7. 计算得到的各构象键长和键角数据与实验值接近. 一个水分子的加入对D4G分子的构型参数有所影响, 基本不改变D4G分子各构象的稳定性顺序, 但构象转变的能垒有所提高. 氢键在分子构象中发挥了重要作用.     

2',3'-二脱氧-2',3'-二去氢鸟嘌呤核苷构象稳定性的理论研究

采用密度泛函方法在B3LYP/6-31＋G**水平上研究了2',3'-二脱氧-2',3'-二去氢鸟嘌呤核苷分子(D4G)的构象. 分别研究在气相中的孤立分子和一水合物异构体的相对稳定性和异构体之间的相互转变过程, 分析了水分子的参与对D4G异构体的相对稳定性和几何结构参数以及自然电荷的影响. 结果表明, 孤立的D4G分子在气相中存在8种稳定构象, 其中构象d4g-2是所有构象中最稳定的, 气相中D4G主要以d4g-2存在. 气相中各构象的相对稳定性为: d4g-2＞d4g-1＞d4g-5＞d4g-3＞d4g-6＞d4g-4＞d4g-8＞d4g-7. 计算得到的各构象键长和键角数据与实验值接近. 一个水分子的加入对D4G分子的构型参数有所影响, 基本不改变D4G分子各构象的稳定性顺序, 但构象转变的能垒有所提高. 氢键在分子构象中发挥了重要作用.