Low Activation Energy Process Research Articles

Fracture mechanisms of sodium silicate glasses

AbstractReactive classical molecular dynamics simulations of sodium silicate glasses, xNa2O–(100 − x)SiO2 (x = 10–30), under quasi‐static loading, were performed for the analysis of molecular scale fracture mechanisms. Mechanical properties of the sodium silicate glasses were consistent with experimentally reported values, and the amount of crack propagation varied with reported fracture toughness values. The most crack propagation occurred in NS20 systems (20‐mol% Na2O) compared with the other simulated compositions. Dissipation via two mechanisms, the first through sodium migration as a lower activation energy process and the second through structural rearrangement as a higher activation energy process, was calculated and accounted for the energy that was not stored elastically or associated with the formation of new fracture surfaces. A correlation between crack propagation and energy dissipation was identified, with systems with higher crack propagation exhibiting less energy dissipation. Sodium silicate glass compositions with lower energy dissipation also exhibited the most sodium movement and structural rearrangement within 10 Å of the crack tip during loading. Therefore, high sodium mobility near the crack tip may enable energy dissipation without requiring formation of structural defects. Therefore, the varying mobilities of the network modifiers near crack tips influence the brittleness and the crack growth rate of modified amorphous oxide systems.

A Mechanism Study of a Novel Acid-Activatable Michael-Type Fluorescent Probe for Thiols

A Michael addition is usually taken as a base-catalysed reaction. However, our synthesized 2-(quinolin-2-ylmethylene) malonic acid (QMA) as a Michael-type thiol fluorescent probe is acid-active in its sensing reaction. In this work, based on theoretic calculation and experimental study on 7-hydroxy-2-(quinolin-2-ylmethylene) malonic acid, we demonstrated that QMA as a Michael acceptor is acid-activatable, i.e., it works only in solutions at pH<7, and the lower the pH of solutions is, the higher reactivity QMA has. In alkaline solution, the malonate QMA[-2H+]2− cannot react with both RS− and RSH. In contrast, 2-(quinolin-2-ylmethylene) malonic ester (QME), the ester of QMA, reveal a contrary pH effect on its sensing reaction, that is, it can sense thiols in alkaline solutions but not in acidic solutions, like a normal base-catalysed Michael addition. The values of activation enthalpies from theoretic calculation support the above sensing behavior of two probes under different pH conditions. In acidic solutions, the protonated QMA is more highly reactive towards electrophilic attack over its other ionized states in neutral and alkaline solutions, and so can react with lowly reactive RSH. In contrast, there is a big energy barrier in the interaction of QME with RSH (acidic solutions), and the reaction of QME with the highly reactive nucleophile RS− is a low activation energy process (in alkaline solutions). Theoretic calculation reveals that the sensing reaction of QMA undergoes a 1,4-addition process with neutral thiols (RSH), and a 1,2-addition pathway for the sensing reaction of QME with RS−. Therefore, the sensing reaction of QMA is an acid-catalysed Michael addition via a 1,4-addition, and a normal base-catalysed Michael addition via a 1,2-addition.

Reversible redox reaction and water configuration on a positively charged platinum surface: first principles molecular dynamics simulation

The water dissociation reaction and water molecule configuration on a positively charged platinum (111) surface were investigated by means of first principles molecular dynamics under periodic boundary conditions. Water molecules on the Pt surface were mostly in the O-down orientation but some H-down structures were also found. OH(-) ion, generated by removing H from H(2)O in the bulk region, moved to the Pt surface, on which a positive charge is induced, by a Grotthuss-like proton-relay mechanism and adsorbed on it as OH(Pt). Hydrogen atom exchange between OH(Pt) and a near-by water molecule frequently occurred on the Pt surface and had a low activation energy of the same order as room temperature energy. When a positive charge (7 μC cm(-2)) was added to the Pt surface, H(3)O(+) and OH(Pt) were generated from 2H(2)O on the Pt. This may be coupled with an electron transfer to the Pt electrode [2H(2)O → H(3)O(+) + OH(Pt) + e(-)]. The opposite reaction was also observed on the same charged surface during a simulation of duration about 10 ps; it is a reversible redox reaction. When further positive charge (14 μC cm(-2)) was added, the reaction shifted to the right hand side completely. Thus, this one-electron transfer reaction, which is a part of the oxygen electrode reaction in fuel cells and water electrolysis, was confirmed to be a low activation energy process.

Dielectric and thermal characterization of low density ethylene/10‐undecen‐1‐ol copolymers prepared with nickel catalysts

AbstractEthylene and 10‐undecen‐1‐ol copolymers, prepared using a nickel complex as catalyst, were studied using differential scanning calorimetry (DSC), X‐ray diffraction, and dielectric relaxation spectroscopy. The behavior exhibited by copolymers containing incorporated 10‐undecen‐1‐ol amounts within 0.5 and 4.6 mol % was compared with neat polyethylene. DSC revealed that a new crystalline region with lower thickness lamellae emerges in copolymers due to the side‐chains crystallization. Nevertheless, the global crystallization degree decreases due to the loss of crystallinity that occurs in a greater extent in PE‐like regions. Dielectric relaxation spectroscopy detected two processes, a low activation energy process below −20 °C related with localized mobility increasing in intensity and deviating to higher temperatures with the increase in 10‐undecen‐1‐ol amount, and a high activation energy process ascribed to the glass transition, located at higher temperatures for the different copolymers relatively to neat polyethylene. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2802–2812, 2007

Amino–imino tautomerization reaction of the 4-aminopyrimidine/acetic acid system

The amino–imino tautomerization of the 4-aminopyrimidine (4APM)/acetic acid (AcOH) system through dual hydrogen bonding in n-hexane at room temperature was investigated using UV absorption and fluorescence spectroscopies, fluorescence time-profile measurements, and molecular orbital calculations, with those of the imino-model compound of 3-methyl-4(1H)-pyrimidinimine (3M4PMI). From the experimental results, it was confirmed that the imino-tautomer was formed in the first excited singlet state (S1) state through the double-proton transfer of the dual hydrogen-bonded complex of 4APM with AcOH. The fluorescences of the free 4APM monomer (band maximum at 353 nm), imino-tautomer (at 414 nm), and 3M4PMI monomer (at 437 nm) exhibit the single-exponential decays of 98, 73, and 19 ps time constants, respectively. The shorter decay time of the imino-tautomer fluorescence compared with the free monomer one is suggestive of the low activation energy process in the S1 state. From the result of the shortest decay time of the 3M4PMI fluorescence, it can be deduced that 3M4PMI has a non-planar structure in the S1 state. The theoretical calculations on the S0 and S1 state double-proton transfer for the 4APM/AcOH dual hydrogen-bonded system were performed with the use of formic acid (FoOH) in place of AcOH for the sake of simplicity. Only one peak of transition state was resolved in the S0 and S1 states. The energy barrier for the S1 state double-proton transfer of the 4APM/FoOH complex → 3H-4(1H)-pyrimidinimine/FoOH tautomer was estimated to be ∼2 kJ mol −1 using the CIS/6-31G(d) methods. On the other hand, the energy barrier for the S0 state reverse proton transfer of the 3H-4(1H)-pyrimidinimine/FoOH tautomer → 4APM/FoOH complex was estimated to be almost zero kJ mol −1 at B3LYP/6-31G(d) level.

POLYCHLORINATED NAPHTHALENE (PCN) AND DIBENZOFURAN (PCDF) CONGENER PATTERNS FROM PHENOL PRECURSORS IN THERMAL PROCESS: [I] A PRIORI HYPOTHESIS OF PCN AND PCDF FORMATION PATHWAYS FROM MONOCHLOROPHENOLS

The gas-phase formation of polychlorinated naphthalenes (PCNs) and dibenzofurans (PCDFs) was experimentally investigated by slow combustion of the three chlorophenols (CPs): 2-chlorophenol (2-CP), 3-chlorophenol (3-CP) and 4-chlorophenol (4-CP), in a laminar flow reactor over the range of 550 to under oxidative condition. Contrary to the a priori hypothesis, different distributions of PCN isomers were produced from each CP. To explain the distributions of polychlorinated dibenzofuran (PCDF) and PCN congeners, a pathway is proposed that builds on published mechanisms of PCDF formation from chlorinated phenols and naphthalene formation from dihydrofulvalene. This pathway involves phenoxy radical coupling at unsubstituted ortho-carbon sites followed by CO elimination to produce dichloro-9, 10-dihydrofulvalene intermediates. Naphthalene products are formed by loss of H and/or Cl atoms and rearrangement. The degree of chlorination of naphthalene and dibenzofuran products decreased as temperature increased, and, on average, the naphthalene congeners were less chlorinated than the dibenzofuran congeners. PCDF isomers were found to be weakly dependent to temperature, suggesting that phenoxy radical coupling is a low activation energy process. Different PCN isomers, on the other hand, are formed by alternative fusion routes from the same phenoxy radical coupling intermediate. PCN isomer distributions were found to be more temperature sensitive, with selectivity to particular isomers decreasing with increasing temperature.

NMR Investigation and Dynamic Behaviour of [2,2′‐Bipyridylbis(pyridine)platinum(II)]2+ and Related Cationic Complexes − Crystal Structure of [Pt(bipy)(py)2](PF6)2

AbstractDetailed 1H and 13C{1H} NMR analysis has been carried out on complexes with the general formulae [Pt(bipy)(n‐Rpy)2](PF6)2 and [Pt(4‐Phbipy)(n‐Rpy)2](PF6)2 [bipy = 2,2′‐bipyridyl; 4‐Phbipy = 4,4′‐diphenyl‐2,2′‐dipyridyl; n‐Rpy = pyridine or substituted pyridines]. The resonance assignments and X‐ray structures of these complexes are important steps on our route to: (i) interpretation of the intercalative geometry on the basis of the magnetic changes produced on interaction with DNA, (ii) rational design of new intercalators. The X‐ray structure of the parent complex [Pt(bipy)(py)2](PF6)2 (py = pyridine) shows pyridine rings quasiperpendicular to the coordination plane. In solution, because of the hindered rotation about Pt−N bonds of ortho‐ and meta‐monosubstituted pyridines, this arrangement leads to “head‐to‐head” s‐cis Cs and “head‐to‐tail” s‐trans C2 rotamers in equilibrium. NMR line shape and 13C relaxation time constant measurements were carried out in order to evaluate activation energies and anisotropic rotational motion of n‐Rpy. In some of these complexes a second dynamic process could be frozen on the 13C NMR timescale below 265 K. This lower activation energy process can reasonably be attributed to minimal bipyridyl torsion angle flipping driving a concerted molecular motion about the core of the four N‐donors. Indeed, the X‐ray structure of 1 proves distortion with a slight alternate up and down shift of the four nitrogen atoms about the coordination plane. Significantly, the fast rotation of unhindered pyridine rings does not allow NMR detection of this molecular distortion. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Comparative study of the mechanical relaxation behavior of polymethacrylates containing different bulky side groups

AbstractThe syntheses of poly(1,3‐dioxan‐5‐yl methacrylate), poly(cis‐2‐phenyl‐1,3‐dioxan‐5‐yl methacrylate), poly(trans‐2‐phenyl‐1,3‐dioxan‐5‐yl methacrylate), poly(cis‐2‐cyclohexyl‐1,3‐dioxan‐5‐yl methacrylate), and poly(trans‐2‐cyclohexyl‐1,3‐dioxan‐5‐yl methacrylate) are reported. The mechanical relaxation spectrum of the simplest polymer, poly(1,3‐dioxan‐5‐yl methacrylate), exhibits a prominent β relaxation centered at −98 °C, at 1 Hz, followed in increasing order of temperature by an ostensible glass–rubber relaxation process. In addition to the β relaxation, the loss curves of poly(trans‐2‐phenyl‐1,3‐dioxan‐5‐yl methacrylate) and poly(trans‐2‐cyclohexyl‐1,3‐dioxan‐5‐yl methacrylate) display in the glassy state a high activation energy relaxation, named the β* process, that seems to be a precursor of the glass–rubber relaxation of these polymers. The mechanical spectra of poly(trans‐2‐cyclohexyl‐1,3‐dioxan‐5‐yl methacrylate) and poly(cis‐2‐cyclohexyl‐1,3‐dioxan‐5‐yl methacrylate) exhibit a low activation energy process in the low‐temperature side of the spectra, which is absent in the other polymers. The molecular origin of the mechanical activity of these polymers in the glassy state is discussed in qualitative terms. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1154–1162, 2002

Chemical conversion of poly(carbonate) to 1,3-dimethyl-2-imidazolidinone (DMI) and bisphenol A: a practical approach to the chemical recycling of plastic wastes

A method for the chemical recycling of poly(carbonate) (PC) plastic wastes in the forms of bisphenol A (BPA) and 1,3-dimethyl-2-imidazolidinone (DMI) was investigated. Treatment with N, N′-dimethyl-1,2-diaminoethane (DMDAE) of PC pellets or PC wastes, derived from compact discs, in dioxane with/without a catalytic amount of base (e.g. Na 2CO 3 0.5 mol%) for a short period at 100 °C produced DMI and BPA, both in nearly quantitative yields. The aminolysis reaction can be carried out also in DMI, thus saving the use of conventional solvents. A sequential profile of this reaction was verified by examining two model reactions of diphenyl carbonate with N-methylbutylamine and DMDAE, respectively, both proceeded through low-activation energy processes.

Density functional studies on the Pauson--Khand reaction.

The Pauson--Khand reaction represents a one-step Co(2)(CO)(8)-catalyzed synthesis of cyclopentenone through [2 + 2 + 1] assembly of one molecule each of alkene, alkyne, and carbon monoxide. Density functional studies (B3LYP/631LAN) on the reaction pathway of the Pauson--Khand (PK) reaction reported here for the first time provides valuable information on the structures and energetics of various intermediates and transition states. The PK reaction consists of olefin insertion, CO insertion, and reductive elimination steps. The olefin insertion step was found to be an irreversible step that determines the stereo- and regiochemistry of the overall reaction. The following steps are low activation energy processes and reversible. The bond-forming events occur only on one of the two metal atoms, while the second metal atom not only acts as an anchor that fixes the metal cluster to the organic substrate but also exerts electronic influences on the reaction at the first atom.

Investigation of the low temperature ageing kinetics of glassy polycarbonate by mechanical damping spectroscopy

Polycarbonate samples were submitted to ageing at room temperature or at 8°C after quenching from above Tg or de-ageing by pre-yield tensile deformation. Loss curves were measured in the temperature range located between the β and the α transitions. These curves exhibit an α′ peak on the low temperature side of the α peak. The position of the α′ peak is clearly dependent on the history of the sample: the α′ peak located in the low temperature range after quenching or mechanical de-ageing is shifted to higher temperatures during further ageing. The time and temperature dependence of the α′ peak location has shown that the kinetics of the α′ relaxation near and below room temperature is controlled by a lower activation energy process than the annealing process prevailing near Tg and already identified. A classical formalism modified by taking into account two α′ relaxation processes prevailing either at low or higher temperatures yields a good description of the α′ loss curves of most samples aged for a long term at room temperature, but fails to describe the α′ peaks of specimens submitted to another ageing history. To model the loss curves of such specimens the relaxation time spectrum involved in the description of the α′ peak is assumed to be dependent on the history of the sample and on the current test temperature.

Lateral resolution in laser-patterned thermal processing of GaAs

Time-dependent two-dimensional calculations are employed to estimate the limits placed, by thermal diffusion, on the lateral resolution of thermally-activated excimer-laser-induced patterned processing of GaAs. Sinusoidal and rectangular fluence profiles are used for holographic or projection exposure. Despite thermal diffusion, holographic exposure can result in feature sizes well into the nanostructure regime, except for low activation energy processes conducted at low peak temperatures. In projection patterning, pattern fidelity is severely affected by thermal diffusion. In this case, reasonably rectangular process profiles can only be obtained for feature sizes as large as several microns, unless hard process thresholds and/or stopping layers are involved.

New low activation energy processes in La 2CuO 4+δ by elastic energy loss experiments

The anelastic relaxation spectrum of La 2CuO 4+δ in the as prepared condition is measured from 200 K down to 1.3 K at frequencies included between 0.7 and 9 kHz. Two new relaxation processes are found with maxima at 4 and 70 K. The process at 70 K consists of two close peaks, with activation enthalpies of 0.11 and 0.13 eV. Possible mechanisms are thermally activated hopping of the holes doped by interstitial O, possibly bound to some lattice defect or impurity, or of atoms in off-centre positions. The peak at 4 K is, to our knowledge, by far the fastest anelastic relaxation process ever reported in an oxide superconductor. It is broader than a single relaxation process, but definitely sharper than the maxima which are typical of amorphous materials. The relaxation should again involve polarons or off-centre atoms, with a rate which is dominated by tunnelling.

Reaction mechanisms for toluene pyrolysis

The rich chemistry occurring during the pyrolysis of toluene has been investigated by studying its decomposition in a single-pulse shock tube coupled with detailed chemical kinetic modeling to describe product formation. This work provides information on the initial decomposition steps of toluene and its primary radical benzyl as well as the detailed steps leading to the formation of polyaromatic hydrocarbons. Temperatures ranged from approximately 1200 to 1850 K for 1% toluene in argon, total pressures of approximately 10 atm, and residence times near 600 μ s. Pyrolysis products were collected and analyzed using gas chromatography. Profiles of hydrogen and hydrocarbons ranging from methane to pyrene were obtained. These semiquantitative data have been interpreted to help resolve many of the existing uncertainties involving the pyrolytic process. In particular, this work supports arguments for a low activation energy process for benzyl radical decomposition as evidenced by the low-temperature formation of cyclopentadiene and the radical recombination product, benzyl-cyclopentadienyl (b-cpd). Also, the data support suggestions for an important role of the methylphenyl radical, specifically in the formation of isomers of dimethyldiphenyl and as a key intermediate in the production of anthracene. Minimal evidence for rapid toluene decomposition to phenyl and methyl was found. Importantly, the data support general mechanisms proposed for ring growth sequences, although an additional important step involving ring closure and direct H 2 elimination is suggested by the data. In addition, the data indicate rapid anthracene/phenanthrene isomerization above 1600 K.

Polyfunctional phosphine ligands. Synthesis of 7-diphenylphosphino-2,4-dimethyl-1,8-naphthyridine and its co-ordination properties

The new trinucleating ligand 7-diphenylphosphino-2,4-dimethyl-1,8-naphthyridine (dpnapy) has been prepared and its co-ordination chemistry with rhodium(I) complexes investigated. Reactions with [{Rh(cod)Cl}2](cod = cycloocta-1,5-diene) lead to different products depending on the ligand to metal ratio. The P-monodentate ligand complex [Rh(cod)(dpnapy)Cl]1 is formed when a 2 : 1 ligand : dimer ratio is used. The cod region of the 1H and 13C NMR spectra of 1 is temperature dependent. The fluxional behaviour can be explained by the formation of a five-co-ordinate intermediate displaying rapid intramolecular exchange of the cod olefinic and CH2 protons. A lower activation energy process, not frozen at 215 K, is also observed. At room temperature (r.t.) the two-dimensional 1H and 31P chemical exchange spectra indicate a further slower dynamic equilibrium 1⇌2 with loss of dpnapy. The binuclear complex [{Rh(cod)Cl}2(µ-dpnapy)]2, in which dpnapy acts as bridging bidentate ligand using the phosphorus and the terminal nitrogen atoms, is the reaction product when a 1 : 1 ligand : dimer ratio is used. The 1H and 13C NMR data for complex 2 are consistent with a dimeric structure of low symmetry. Complex 2 is also involved in several dynamic processes. At r.t. the two-dimensional 1H chemical exchange spectrum shows a slow general scrambling of the cod olefinic protons and the presence of the equilibrium 2⇌1 with participation of [{Rh(cod)Cl}2]. The ligand reacts with [{Rh(CO)2Cl}2] giving as the only isolable product the metallocycle [{Rh(CO)(µ-dpnapy)Cl}2]·2CH2Cl23. The reaction proceeds through several steps involving the formation of cis-dicarbonyl intermediates as indicated by IR and NMR spectra. The structure of 3 has been determined by X-ray crystallography. It consists of two Rh(CO)Cl units, joined together by two dpnapy molecules in a head-to-tail arrangement.

The defect solid solution [formula omitted]: Evidence of Na I mobility in the tunnels of a quadruple rutile-chain structure

A new defect solid solution, the series Na 7 8 (Fe III 7 8 +x Ti IV 9 8 −2x Sb V x) O 4 , was synthesized. Its homogeneity range is rather wide: 0 <- x ≤ 0.33. The incorporation of Sb V gives rise to a progressive increase of the parameters of the orthorhombic unit cell. X-ray powder structure calculations point to a partial occupancy of the large double tunnels in a quadruple rutile-chain structure. A significant ordering of cations over the octahedral framework is observed, owing to a Ti IVSb V segregation. Electrical measurements emphasize a cationic conductivity, mainly related to a 1D motion of Na I cations. A transition from a low activation energy process— E A ≤ 0.20 eV—to a high activation energy one— E A ≈ 0.75 eV—systematically occurs at T ≈ 440°C, independent of the Sb V concentration. A possible skew motion from a half tunnel to another one is proposed as a tentative explanation of the high-temperature conductivity mechanism.

Anelastic relaxation evidence of very low activation energy processes in superconducting YBa2Cu3O7-x: magnetic chain excitations?

Relaxation rates obtained from a wide range of acoustic loss measurements are explained by introducing three thermally activated processes controlled by activation energies of 0.06, 0.14 and 0.2 eV in superconducting YBa2Cu3O7-x. The ultrasonic attenuation peak magnitudes indicate that the interactions responsible originate in at least 10% of unit cells in the bulk of the material. An analysis of spin excitations on the copper-oxygen chains is presented that leads to a series of discrete excitation energies matching those obtained experimentally.

Hydrogen exchange of the tryptophan residues in bovine, goat, guinea pig, and human .alpha.-lactalbumin

Hydrogen exchange of the individual tryptophan residues of bovine, goat, guinea pig, and human alpha-lactalbumin has been studied by both ultraviolet and NMR spectra. The assignment of the slowly exchanging imino proton resonances to the tryptophan residues (Trp26 and Trp60) was obtained by comparison of the nuclear Overhauser effect difference spectra of bovine, guinea pig, and human alpha-lactalbumin. Taking account of the thermal unfolding of each alpha-lactalbumin, the hydrogen exchange rates of the individual tryptophan residues are analyzed. The temperature dependence of the exchange rates classified their exchange mechanisms into two exchange processes: the "low activation energy process" and the "high activation energy process" which is associated directly with the global thermal unfolding of the protein. Trp26 of alpha-lactalbumin exchanges through the high activation energy process. The exchange behavior of Trp26 of guinea pig alpha-lactalbumin suggests a difference of the globally unfolded state of the protein from the other species. The exchange mechanism of Trp60 of human alpha-lactalbumin is the low activation energy process in contrast with those of the bovine and goat proteins, although their global thermodynamic properties are similar to each other. Trp104 and Trp118 of alpha-lactalbumin exchange through the low activation energy process, and the reaction rates are affected by the local structural differences around the tryptophan residues among these proteins. The results presented in this paper indicate that the hydrogen exchange rate through the low activation energy process provides the information only about the local nature of a protein while that through the high activation energy process provides the information about the global nature of a protein.

Electron impact induced fragmentations of 2‐chlorovinyl methyl sulphides

AbstractA series of 2‐chlorovinyl methyl sulphides, CH3SCR1CR2Cl (R1, R2 = H, Me, Et, t‐Bu, Ph; R1 = H, R2 = CH3) has been analysed under electron impact conditions. The most significant common fragmentations involve primary loss of Cl˙ and methyl radicals. Analysis of the phenyl‐substituted derivative suggests that loss of CH3˙ is a lower activation energy process than loss of Cl˙. The characteristic loss of CH3SCl from the same derivative has been examined with respect to the structure of the resulting fragment by means of suitable model compounds.

Complex Plane and 7Li NMR Studies of Arsenic Sulfide‐Based Lithium Glasses

The glass forming region of the system was explored as a function of lithium ion concentration. The most conductive base glass attainable was , and with this glass, up to could be added. Ionic conductivity was measured using complex plane techniques with room temperature conductivity reaching for . NMR linewidth data were fit to the Hendrickson‐Bray equation, and activation energies were obtained. All glasses showed a low activation energy process which was attributed to a local motion. Only the more conductive 30–40% glasses showed a second, higher activation energy process which correlated well with activation energies from conductivity measurements.