Quantum effects in molecular dynamics and fracture kinetics of polymers

Molecular dynamics in crystallites of a number of polymers, such as polyethylene, nylon-6, poly-(vinyl alcohol), was studied at temperatures ranging from 4.2 to 400 K using the X-ray diffraction technique. Thermal expansion of the lattice was measured by the temperature-induced displacements of diffraction maxima. The thermal expansion coefficient was found to grow from 0 to ∼10⁻⁴ K⁻¹, this fact being the evidence of the quantum statistics in the vibrational dynamics of straightened molecules over the whole temperature range. Characteristic temperatures of vibrations were estimated. The kinetics of fracture was studied using highly - oriented nylon-6 and KEVLAR-49 fibers. Tensile strength (σ_r) was measured as a function of loading rate σ and temperature (T) in the range from 4.2 to 400–600 K. The observed specific features of the σ_r(σ) and σ_r(T) curves at low temperatures correspond to tunnel scission of stressed chain molecules.     

Some symmetry-induced isotope effects in the kinetics of recombination reactions

Symmetry-induced isotope effects in recombination and collision-induced dissociation reactions are discussed. Progress on understanding the anomalous isotope effects in ozone is reviewed. Then, calculations are performed for the simpler reaction xNe+yNe+H<-->xNeyNe+H, where x and y label either identical or different isotopes. The atomic masses in the model are chosen so that symmetry is the only difference between the systems. Starting from a single potential energy surface, the properties of the bound, quasibound, and continuum states of the neon dimer are calculated. Then, the vibration rotation infinite order sudden approximation is used to calculate cross sections for all possible inelastic and dissociative processes. A rate constant matrix that exactly satisfies detailed balance is constructed. It allows recombination to occur both via direct three-body collisions and via tunneling into the quasibound states of the energy transfer mechanism. The eigenvalue rate coefficients are determined. Significant isotope effects are clearly found, and their behavior depends on the pressure, temperature, and mechanism of the reaction. Both spin statistics and symmetry breaking produce isotope effects. Under most conditions the breaking of symmetry enhances the rates, but a wide spectrum of effects is observed; they range from isotope effects with a normal mass dependence to huge, mass-independent isotope effects to cancellation and even to reversal of the isotope effects. This is the first calculation of symmetry-induced isotope effects in recombination rates from first principles. The relevance of the present effects to ozone recombination is discussed.     

Quantum effects on hydrogen isotope adsorption on single-wall carbon nanohorns

H(2) and D(2) adsorption on single-wall carbon nanohorns (SWNHs) have been measured at 77 K, and the experimental data were compared with grand canonical Monte Carlo simulations for adsorption of these hydrogen isotopes on a model SWNH. Quantum effects were included in the simulations through the Feynman-Hibbs effective potential. The simulation predictions show good agreement with the experimental results and suggest that the hydrogen isotope adsorption at 77 K can be successfully explained with the use of the effective potential. According to the simulations, the hydrogen isotopes are preferentially adsorbed in the cone part of the SWNH with a strong potential field, and quantum effects cause the density of adsorbed H(2) inside the SWNH to be 8-26% smaller than that of D(2). The difference between H(2) and D(2) adsorption increases as pressure decreases because the quantum spreading of H(2), which is wider than that of D(2), is fairly effective at the narrow conical part of the SWNH model. These facts indicate that quantum effects on hydrogen adsorption depend on pore structures and are very important even at 77 K.     

Evaluation of boundary-layer effects in shock-tube studies of chemical kinetics

Particle times of flight in incident shock flow were determined experimentally by marking several positions of the test gas (mainly Ar) in the shock tube with an infrared emitting gas (NO or CO₂). From the local particle velocity, derived from the particle flight times, temperature and pressure changes behind the shock front were evaluated. Several experimental data were found to be correctly described by Mirels's formulations when used properly. The limitations of the formulations are discussed. It is found to be advisable to evaluate boundary-layer effects on shock-tube flow by experiments, rather than theory, in carrying out chemical kinetics studies.     

Solvent isotope effects upon the kinetics of some simple electrode reactions

The effects of replacing H₂O with D₂O solvent upon the electrochemical kinetics of simple transition-metal redox couples containing aquo, ammine or ethylenediamine ligands have been investigated at mercury electrodes as a means of exploring the possible contribution of ligand-aqueous solvent interactions to the activation barrier to outer-sphere electron transfer. The general interpretation of solvent isotope effects upon electrode kinetics is discussed; it is concluded that double-layer corrected isotopic rate ratios (k^H/k^D)^E determined at a constant electrode potential vs. an aqueous reference electrode, as well as those determined at the respective standard potentials in H₂O and D₂O (k_S^H/k_S^D), have particular significance since the solvent liquid-junction potential can be arranged to be essentially zero. For aquo redox couples, values of (k_S^H/k_S^D) were observed that are substantially greater than unity and appear to be at least partly due to a greater solvent-reorganization barrier in D₂O arising from ligand-solvent hydrogen bonding. For ammine and ethylenediamine complexes values of (k^H/k^D)^E substantially greater than, and smaller than, unity were observed upon the separate deuteration of the ligands and the surrounding solvent respectively. Comparison of isotope rate ratios for corresponding electrochemical and homogeneous outer-sphere reactions involving cationic ammine and aquo complexes yields values of (k^H/k^D) for the former processes that are typically markedly larger than those predicted by the Marcus model from the homogeneous rate ratios. These discrepancies appear to arise from differences in the solvent environments in the transition states for electrochemical and homogeneous reactions.     

Deuterium isotope effects on 13C chemical shifts of nitromalonamide

The OH chemical shift of the enol form of nitromalonamide is found at 18.9 ppm both in DMSO-d(6) and in DMF-d(7) indicating a very strong hydrogen bond. The OH chemical shift is insensitive to temperature changes. Contrary to the large OH chemical shift, a small two-bond deuterium isotope effect of 0.135 ppm due to deuteration at the OH position is found at the enolic carbon. This is confirmed by density functional theory calculations. The observed effects are interpreted as due to an equilibrium between identical enolic forms. These show a strong OH...O hydrogen bond as well as a NH...O-N=O hydrogen bond.     

Protein folding kinetics: barrier effects in chemical and thermal denaturation experiments

Recent experimental work on fast protein folding brings about an intriguing paradox. Microsecond-folding proteins are supposed to fold near or at the folding speed limit (downhill folding), but yet their folding behavior seems to comply with classical two-state analyses, which imply the crossing of high free energy barriers. However, close inspection of chemical and thermal denaturation kinetic experiments in fast-folding proteins reveals systematic deviations from two-state behavior. Using a simple one-dimensional free energy surface approach we find that such deviations are indeed diagnostic of marginal folding barriers. Furthermore, the quantitative analysis of available fast-kinetic data indicates that many microsecond-folding proteins fold downhill in native conditions. All of these proteins are then promising candidates for an atom-by-atom analysis of protein folding using nuclear magnetic resonance.1 We also find that the diffusion coefficient for protein folding is strongly temperature dependent, corresponding to an activation energy of approximately 1 kJ.mol-1 per protein residue. As a consequence, the folding speed limit at room temperature is about an order of magnitude slower than the approximately 1 micros estimates from high-temperature T-jump experiments. Our analysis is quantitatively consistent with the available thermodynamic and kinetic data on slow two-state folding proteins and provides a straightforward explanation for the apparent fast-folding paradox.     

Comparative study of isotope and chemical effects on the exciton states in LiH crystals

Scientific interest, technological promise and increased availability of highly enriched isotope have led to a sharp rise in the number of experimental and theoretical studies with isotopically controlled crystals. Isotope pure compounds are really the material of future mankind. LiH has a giant isotope effect. Therefore, this review in the first step is devoted to some peculiarities of exciton states in isotope pure and mixed crystals of LiH. Excitons are the energetically lowest excitations of the electronic system in an ideal, crystallized insulator (semiconductor) at zero temperature. It is a collective excitation which has the full translational symmetry of the crystal lattice. For the first time a systematic analysis of experimental results is presented of isotopic and chemical effects on the exciton states observed in LiH_xD_1−x crystals of various isotopic (and chemical) composition (0≤x≤1) using low temperature optical and luminescence spectroscopy. LiH (LiD) is an direct band-gap material with an energy gap 4.992 (5.095) eV at low temperature. Substituting a light isotope with a heavy one (or H→F) increases the interband transition energy (E_g) and the binding energy (E_b) of the Wannier–Mott exciton as well as the magnitude of the longitudinal–transverse splitting. The nonlinear variation of E_g, E_b with the isotope (or F) concentration is due to the compositional disordering of the crystal lattice and is consist with the concentration dependence of line half-width in exciton reflection and luminescence spectra. The free exciton luminescence spectrum of the LiH (LiH_xD_1−x, LiH_xF_1−x; 0≤x≤1) crystals under optical (X-ray) excitations consists of a narrow zero-phonon line and its more wider 5LO replicas. At 100 % substitution of hydrogen by deuterium the energy shift of the maximum of zero-phonon line is the following: ΔE=E_n=1s(LiD)−E_n=1s(LiH)=95 meV. The shift of the emission line maximum of 2LO replica overlaps the energetical interval of ≤200meV. The nonlinear dependence of the free exciton luminescence (especially LiH_xF_1−x (LiD_xF_1−x)) intensity on the excitation density allows to consider these crystals as potential solid state lasers in the UV part of spectrum. It is shown that potential fluctuation due to compositional disorder of alloy have a strong effect on both the exciton broadening and the band-gap energy shift. The review closes with a brief discussion of the present and future applications of these crystals.     

Quantum path integral simulation of isotope effects in the melting temperature of ice Ih

The isotope effect in the melting temperature of ice Ih has been studied by free energy calculations within the path integral formulation of statistical mechanics. Free energy differences between isotopes are related to the dependence of their kinetic energy on the isotope mass. The water simulations were performed by using the q-TIP4P/F model, a point charge empirical potential that includes molecular flexibility and anharmonicity in the OH stretch of the water molecule. The reported melting temperature at ambient pressure of this model (T=251?K) increases by 6.5±0.5 and 8.2±0.5?K upon isotopic substitution of hydrogen by deuterium and tritium, respectively. These temperature shifts are larger than the experimental ones (3.8 and 4.5 K, respectively). In the classical limit, the melting temperature is nearly the same as that for tritiated ice. This unexpected behavior is rationalized by the coupling between intermolecular interactions and molecular flexibility. This coupling makes the kinetic energy of the OH stretching modes larger in the liquid than in the solid phase. However, the opposite behavior is found for intramolecular modes, which display larger kinetic energy in ice than in liquid water.     

Quantum chemical investigation of lanthanum doping effects in tetragonal and rhombohedral PZT materials

Structural and electronic properties of lead–zirconate–titanate (PZT) materials doped with a lanthanum (La) impurity are studied using a quantum‐chemical approach based on the Hartree–Fock theory. Performed geometry optimization in the defective crystals shows that the atomic movements are predominantly outward with respect to the impurity position. It is found that the La impurity enhances a covalent character in the chemical bonding between the Ti and O atoms, as well as the Zr and O atoms situated in the neighborhood of the defect despite the fact that the La‐O interaction remains purely ionic. The occurrence of local energy levels within the band gap of the material is analyzed in light of the available experimental data on La concentration influence upon dielectric and piezoelectric properties in these crystals. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Conformational and long-range low field steric deuterium isotope effects on carbon chemical shifts

Comparison of the room and low temperature (203 K) ¹³C NMR spectra of 3-cis-bicyclo[4.4.0]decanone and its 2,2,4,4-tetradeuterio isotopomer provides information about the deuterium isotope effect on the conformational equilibrium in this compound. Four-bond low field deuterium isotope effects on some carbon chemical shifts are observed. These effects can be used for unambiguous assignment of the ¹³C lines to carbon atoms in alicyclic compounds.     

Uncoupled forms of tyrosine hydroxylase unmask kinetic isotope effects on chemical steps

Tyrosine hydroxylase (TyrH) catalyzes the hydroxylation of tyrosine to dihydroxyphenylalanine. In the proposed mechanism, a ferryl-oxo species attacks the aromatic ring of tyrosine, forming a cationic intermediate. However, no significant isotope effect is found for wild-type TyrH when 3,5-2H2-tyrosine is used as a substrate. The isotope effect has now been determined with 3,5-2H2-tyrosine using mutant forms of TyrH in which the oxidation of the pterin is uncoupled from hydroxylation of the amino acid. Three mutant enzymes exhibit significant inverse deuterium isotope effects and inverse solvent isotope effects. A proton inventory for the E326A enzyme is consistent with a normal solvent isotope effect of 2.4 on an unproductive step. The results support the proposed mechanism and demonstrate the utility of using mutant proteins with branched pathways to reveal isotope effects which are masked in the wild-type enzyme.     

The kinetics of isotope exchange reactions: Use of initial rates to measure isotope effects on carbon acid ionization

Multistep hydrogen isotope exchange reactions, such as the íonization of a carbon acid via a carbanion intermediate in a protic solvent, when conducted using an isotopic tracer to monitor the exchange, have the unusual feature that their rate-determining steps always refer to the transfer of the tracer isotope and never to the isotope present in macroscopic amounts. This property of these reactions is discussed and rationalized using a free energy versus reaction coordinate diagram. It is further shown that this property does not invalidate a commonly used method of measuring kinetic isotope effects on carbon acid ionization in which rates of incorporation of tritium tracers into RH and RD substrates are compared, despite the fact that tritium transfer is rate determining in both exchanges, but it is valid only if initial rate measurements are used. When the comparison is made in a protio solvent, e.g., H₂O, the portion of the initial reaction which may be used depends strongly on the magnitude of the isotope effect. It ranges from less than 1% tritium incorporation for large isotope effects to 10% or more for isotope effects near unity. On the other hand, when a deuterated solvent, e.g., D₂O, is used, the range of validity of the method for large isotope effects is extended dramatically.     

Integral invariants in chemical kinetics

Integral invariants of classical mechanical systems are used for the mathematical treatment of equilibrium systems of chemical reaction kinetics. Some conserved quantities and Hamilton equations in chemistry are shown.

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Mean-field cooperativity in chemical kinetics

We consider cooperative reactions and we study the effects of the interaction strength among the system components on the reaction rate, hence realizing a connection between microscopic and macroscopic observables. Our approach is based on statistical mechanics models and it is developed analytically via mean-field techniques. First of all, we show that, when the coupling strength is set positive, a cooperative behavior naturally emerges from the model; in particular, by means of various cooperative measures previously introduced, we highlight how the degree of cooperativity depends on the interaction strength among components. Furthermore, we introduce a criterion to discriminate between weak and strong cooperativity, based on a measure of “susceptibility.” We also properly extend the model in order to account for multiple attachments phenomena: this is realized by incorporating within the model p-body interactions, whose non-trivial cooperative capability is investigated too.     

Quantum chemical and theoretical kinetics study of the O(3P) + CS2 reaction

The triplet potential energy surface of the O((3)P) + CS(2) reaction is investigated by using various quantum chemical methods including CCSD(T), QCISD(T), CCSD, QCISD, G3B3, MPWB1K, BB1K, MP2, and B3LYP. The thermal rate coefficients for the formation of three major products, CS + SO ((3)Σ(-)), OCS + S ((3)P) and CO + S(2) ((3)Σ(-)(g)) were computed by using transition state and RRKM statistical rate theories over the temperature range of 200-2000 K. The computed k(SO + CS) by using high-level quantum chemical methods is in accordance with the available experimental data. The calculated rate coefficients for the formation of OCS + S ((3)P) and CO + S(2) ((3)Σ(-)(g)) are much lower than k(SO + CS); hence, it is predicted that these two product channels do not contribute significantly to the overall rate coefficient.     

Hydrogen/deuterium isotope effects on the excited-state proton transfer kinetics of 3-hydroxyflavone

The static and transient fluorescence spectroscopy of 3-hydroxyflavone, 3HF, has been investigated as a function of deuteration of the 3-hydroxy group and the solvent. Small H/D isotope effects are observed for at least two of the simple rate constants for the excited-state dynamics. The isotope effects are not significantly temperature dependent. The results suggest that the proton transfer process is not a simple barrier crossing reaction.     

Pre-equilibrium approximation in chemical and photophysical kinetics

For most mechanisms of chemical reactions and molecular photophysical processes the time evolution of the concentration of the intervening species cannot be obtained analytically. The pre-equilibrium approximation is one of several useful approximation methods that allow the derivation of explicit solutions and simplify numerical solutions. In this work, a general view of the pre-equilibrium approximation is presented, along with the respective analytical solution. It is also shown that the kinetic behavior of systems subject to pre-equilibration can be obtained by the application of perturbation theory. Several photophysical systems are discussed, including excimer formation, thermally activated delayed fluorescence, and external-heavy atom quenching of luminescence.     

Quantum chemical methods and their applications to chemical reactions

This review is concerned with the impact of quantum chemistry on chemical reactions. Starting from the mid-sixties it focusses on those developments which have enabled us to predict essential features of simple chemical reactions. Thus model theories and computational methods are presented which provide the tools for these predictions. Then procedures to characterize potential surfaces and search methods for reaction paths are described. It is also attempted to relate these features to the terminology of the experimentalist. Finally a systematic survey of the main types of reaction (rearrangement, addition, elimination, substitution) is given.