Determination of force fields for two conformers of nitromethane bycndo/Force method

cndo/Force method is used to evaluate the redundancy free internal valence force fields for two conformers of nitromethane. The initial force field is set up by taking the interaction and bending force constants from this method and transferring the stretching force constants from the force fields of chemically related molecules. The final force field is obtained by refining the initial force field using vibrational frequencies of isotopic speciesviz CH₃NO₂, CD₃NO₂, CH₃ ¹⁵NO₂ and CH₃N¹⁸O₂. The final force field thus obtained is reasonable on the basis of frequency fit and potential energy distribution. The barrier to internal rotation is found to be 0.048 kcal mol⁻¹.     

Addition and spin exchange rate constants by longitudinal field μSR: The Mu+NO reaction

The addition reaction Mu+NO+M→MuNO+M and the spin exchange reaction Mu(↑) +MO(↓)→Mu(↓)+NO(↑) have been measured by longitudinal field μSR at room temperature in the presence of up to 58 atm of N₂ as inert collider. The pressure dependence of the longitudinal relaxation rate due to the addition reaction (λ_c) demostrates that the system is still in the low pressure regime in this pressure range. The corresponding termolecular rate constant has been determined ask _0,Mu =(1.10±0.25)×10⁻³² cm⁶ molecules⁻² s⁻¹, almost 4 times smaller than the corresponding H atom reactionk _0,H=3.90×10⁻³² cm⁶ molecules⁻² s⁻¹ [I.M. Campbell et al., J. Chem. Soc. Faraday Trans. 1.71 (1975) 2097]. The average value of the spin exchange rate constants in the 2.5–58 atm pressure range,k _SE=(3.16±0.06)×10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, is in good agreement with previous values obtained by transverse field μSR [D.G. Fleming et al., J. Chem. Phys. 73 (1980) 2751].     

Experimental selection of molecules for isotope separation by multiple-photon dissociation in an intense IR field

Consideration is given to selectivity estimation methods during polyatomic molecules dissociation by multiple photon absorption. Taking the CH₃NO₂ molecule as an example, the possibilities of selectivity estimations made on the electron-excited dissociation products (luminescence) are shown, as well as on the measurements of the energy absorbed in a strong IR field being transformed into heat. Some advantages of the last method of the selectivity estimation are discussed. The experiments on nitrogen-isotopes separation in the mixture of CH₃ ¹⁴NO₂ and CH₃ ¹⁵NO₂ molecules are carried out when exciting thev ₇ vibration with the isotope shift of about 7 cm⁻¹ and thev ₁₃ vibration with no isotope shift in the linear absorption spectrum. The contribution of secondary chemical reactions to the separation process is discussed.     

Encapsulation of iron nanoparticles in alginate biopolymer for trichloroethylene remediation

Nanoscale zero-valent iron (NZVI) particles (10–90 nm) were encapsulated in biodegradable calcium-alginate capsules for the first time for application in environmental remediation. Encapsulation is expected to offers distinct advances over entrapment. Trichloroethylene (TCE) degradation was 89–91% in 2 h, and the reaction followed pseudo first order kinetics for encapsulated NZVI systems with an observed reaction rate constant (k _obs) of 1.92–3.23 × 10⁻² min⁻¹ and a surface normalized reaction rate constant (k _sa) of 1.02–1.72 × 10⁻³ L m⁻² min⁻¹. TCE degradation reaction rates for encapsulated and bare NZVI were similar indicating no adverse affects of encapsulation on degradation kinetics. The shelf-life of encapsulated NZVI was found to be four months with little decrease in TCE removal efficiency.     

Dynamics of liquid silane

Neutron scattering of cold neutrons from liquid silane at 137° K and 98°K is explained on the basis of a simple model. The rotational diffusion constant,D _r, and the delay time,τ ₀, after which rotational diffusion may be said to occur are derived on the basis of this model. At 137° K we getD _r=0.22×10¹³ sec⁻¹ andτ ₀=0.68×10⁻¹³ sec. At 98°KD _r (=0.06×10¹³ sec⁻¹) is down by a factor of more than three butτ ₀=(0.54×10⁻¹³ sec) shows only a small change. By comparison with data on liquid CH₄ it is concluded that the law of corresponding states is not applicable for describing rotational dynamics of CH₄ and SiH₄. Rotational motions in SiH₄ are more hindered than in CH₄ at the same reduced temperature.     

Measurement of the spin-exchange and chemical ionization rate constants in collisions of polarized metastable 23 S 1 helium atoms with 32 S 1/2 sodium atoms

Experimental data on the spin-exchange rate constants for the He(2³ S ₁)-Na(3² S _1/2) system are reported for the first time. Measurements show that the spin-exchange rate constant is C _se = (23 ± 11) × 10⁻¹⁰ cm³ s⁻¹ and the chemical ionization rate constant is C _si = (29 ± 14) × 10⁻¹⁰ cm³ s⁻¹ at a temperature of 420 K. The results are compared with the data calculated from the rate constants.     

Femtosecond photodissociation dynamics of HNO3 after exitation of the S3-state at 200 nm

The photodissociation dynamics of HNO₃ in the electronic S₃ (2 ¹ A ^′ ) state leading to the fragments OH and NO₂ was investigated in real time. HNO₃ was prepared either in a fluorescence cell at room temperature (LIF probing of OH) or rotationally cold in a molecular beam (probing of NO₂ by three-photon ionization). A 2 ¹ A ^′ lifetime of 60–80 fs could be obtained from the experimental results, indicating essentially barrierless dissociation. In addition, secondary dissociation of internally excited nascent fragments NO₂ ^* leading to products NO(X ² Π) and O(³ P) with a characteristic dissociation time of 2.3 ps was observed. This time is surprisingly long when compared with dissociation lifetimes of NO₂ from the literature, obtained after direct photoexcitation. The discrepancy is explained by differences in the preparation conditions of the dissociative state of NO₂. Received: 12 November 1999 / Published online: 13 July 2000     

Rotational excitation of CH<Subscript>4</Subscript> by He atoms

Quantum close-coupling and coupled-state approximation scattering calculations for rotational energy transfer of rotationally excited CH₄ due to collisions with He are presented for collision energies between 10⁻⁷ and 3000 cm⁻¹ using the MP4 potential of Calderoni et al. [J. Chem. Phys. 121, 8261 (2004)]. State-to-state cross sections and rate coefficients from selected initial rotational states of CH₄ in symmetries A, E, and F are studied from the ultra-cold to the thermal regime. Comparison of the cross sections with available theoretical results and experimental data show good agreement. Applications to astrophysics and cold laboratory environments are briefly addressed.     

Photoinduced clustering of Eu and the influence of the magnetic field on it in NaCl: Eu crystals

It has been demonstrated that, in NaCl crystals supersaturated with an Eu impurity (0.1 at %), ultraviolet light with a wavelength of 200–370 nm initiates spontaneous aggregation of Eu²⁺−V _k ⁻ impurity-vacancy dipoles into EuCl₂ flat precipitates lying in the (013) crystallographic plane. A constant magnetic field of 15 T accelerates the photoinduced formation of EuCl₂ nanoclusters at a temperature of 300 K.     

Elementary reactions of formyl (HCO) radical studied by laser photolysis—transient absorption spectroscopy

Several elementary reactions of formyl radical of combustion importance were studied using pulsed laser photolysis coupled to transient UV–Vis absorption spectroscopy: HCO → H + CO (1), HCO + HCO → products (2), and HCO + CH₃ → products (3). One-pass UV absorption, multi-pass UV absorption as well as cavity ring-down spectroscopy in the red spectral region were used to monitor temporal profiles of HCO radical. Reaction (1) was studied over the buffer gas (He) pressure range 0.8–100 bar and the temperature range 498–769 K. Reactions (2a), (2b), (2c), (3a) and (3b) as well as the UV absorption spectrum of HCO, were studied at 298 and 588 K, and the buffer gas (He) pressure of 1 bar. Pulsed laser photolysis (308, 320, and 193 nm) of acetaldehyde, propionaldehyde, and acetone was used to prepare mixtures of free radicals. The second-order rate constant of reaction (1) obtained from the data at 1 bar is: k₁(He) = (0.8 ± 0.4) × 10⁻¹⁰exp(−(66.0 ± 3.4) kJ mol⁻¹/RT) cm³ molecule⁻¹ s⁻¹. The HCO dissociation rate constants measured in this work are lower than those reported in the previous direct work. The difference is a factor of 2.2 at the highest temperature of the experiments and a factor of 3.5 at the low end. The experimental data indicate pressure dependence of the rate constant of dissociation of formyl radical 1, which was attributed to the early pressure fall-off expected based on the theory of isolated resonances. The UV absorption spectrum of HCO was revised. The maximum absorption cross-section of HCO is (7.3 ± 1.2) × 10⁻¹⁸ cm² molecule⁻¹ at 230 nm (temperature independent within the experimental error). The measured rate constants for reactions (2a), (2b), (2c), (3a) and (3b) are: k₂ = (3.6 ± 0.8) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ (298 K); k₃ = (9.3 ± 2.3) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹(298 and 588 K).     

Dissociation equilibrium of dinitrogen tetroxide in organic solvents: An electron paramagnetic resonance measurement

The dissociation equilibrium of N₂O₄−NO₂ has been measured in hexane, carbon tetrachloride and chloroform at different temperatures. The equilibrium constants at 298.15 K (25°C),K _m (molality basis), are 3.5·10⁻⁵ in hexane, 5.9·10⁻⁶ in carbon tetrachloride and 5.3·10⁻⁶ in chloroform. The EPR technique has been used to quantify the NO₂ radical. These data are compared with gas-phase and solution data of previous reports. The applicability of Hildebrand and Scatchard theory of solutions is also discussed and some thermodynamic properties are deduced, such as Henry’s N₂O₄ and NO₂ constants in different solvents.     

Dynamic and spectral mixing in nanosystems

In the framework of a simple spin-boson Hamiltonian we study an interplay between dynamic and spectral roots to stochastic-like behavior. The Hamiltonian describes an initial vibrational state coupled to discrete dense spectrum reservoir. The reservoir states are formed by three sequences with rationally independent periodicities 1; 1 ± δ typical for vibrational states in many nanosize systems (e.g., large molecules containing CH₂ fragment chains, or carbon nanotubes). We show that quantum evolution of the system is determined by a dimensionless parameter δΓ, where Γ is characteristic number of the reservoir states relevant for the initial vibrational level dynamics. When δΓ > 1 spectral chaos destroys recurrence cycles and the system state evolution is stochastic-like. In the opposite limit δΓ < 1 dynamics is regular up to the critical recurrence cycle k _c and for larger k > k _c dynamic mixing leads to quasi-stochastic time evolution. Our semi-quantitative analytic results are confirmed by numerical solution of the equation of motion. We anticipate that both kinds of stochastic-like behavior (namely, due to spectral mixing and recurrence cycle dynamic mixing) can be observed by femtosecond spectroscopy methods in nanosystems in the spectral window 10¹¹–10¹³ s⁻¹     

Critical point phenomena in the electrical resistivity of binary liquid systems: CS2 + CH3CN and CS2 + CH3NO2

Electrical resistance measurements are reported on the binary liquid mixtures CS₂ + CH₃CN and CS₂ + CH₃NO₂ with special reference to the critical region. Impurity conduction seems to be the dominant mechanism for charge transport. For the liquid mixture filled at the critical composition, the resistance of the system aboveT _c follows the relationR=R _c−A(T−T _c) ^b withb=0·6±0·1. BelowT _c the conductivities of the two phases obey a relation σ₂−σ₁=B(T _c−T)^β with β=0·34±0·02, the exponent of the transport coefficient being the same as the exponent of the order parameter, an equilibrium property.     

Electrical transport properties of CuWO4

The temperature dependence of the electrical conductivity, thermoelectric power and dielectric constant of the antiferromagnetic CuWO₄ have been studied in the temperature range 300–1000 K. The conductivity results can be summarised by the equations σ_I=6.31 × 10⁻³ exp (−0.29 eV/kT) ohm⁻¹ cm⁻¹ in the temperature range 300–600 K and σ_II=3.16 × 10⁵ exp (−1.48 eV/kT) ohm⁻¹ cm⁻¹ between 600 K and 1000 K. The thermoelectric power can be expressed byθ=[− 1.25 (10³/T) + 3.9] mV/K. Initially dielectric constant increases slowly but for high temperatures its increase is fast.     

Steric effects on the mechanism of reaction of nucleophilic substitution of β‐substituted alkoxyvinyl trifluoromethyl ketones with four secondary amines

The kinetics of the reaction of β‐substituted β‐alkoxyvinyl trifluoromethyl ketones R¹O‐CR²?CH‐COCF₃ ( 1a – e ) [( 1a ), R¹?C₂H₅, R²?H; ( 1b ), R¹?R²?CH₃; ( 1c ), R¹?C₂H₅, R²?C₆H₅; ( 1d ), R¹?C₂H₅, R²?V^?pNO₂C₆H₄; ( 1e ), R¹?C₂H₅, R²?C(CH₃)₃] with four aliphatic amines ( 2a – d ) [( 2a ), (C₂H₅)₂NH; ( 2b ), (i‐C₃H₇)₂NH; ( 2c ), (CH₂)₅NH; ( 2d ), O(CH₂CH₂)₂NH] was studied in two aprotic solvents, hexane and acetonitrile. The least reactive stereoisomeric form of ( 1a – d ) was the most populated ( E‐s‐Z‐o‐Z ) form, whereas in ( 1e ), the more reactive form ( Z‐s‐Z‐o‐Z ) dominated. The reactions studied proceeded via common transition state formation whose decomposition occurred by ‘uncatalyzed’ and/or ‘catalyzed’ route. Shielding of the reaction centre by bulky β‐substituents lowered abruptly both k′ (‘uncatalyzed’ rate constant) and k″ (‘catalyzed’ rate constant) of this reaction. Bulky amines reduced k″ to a greater extent than k′ as a result of an additional steric retardation to the approach of the bulky amine to its ammonium ion in the transition state. An increase in the electron‐withdrawing ability of the β‐substituent increased ‘uncatalyzed’ k′ due to the acceleration of the initial nucleophile attack (k₁) and ‘uncatalyzed’ decomposition of transition state (k₂) via promoting electrophilic assistance (through transition state 8 ). The amine basicity determined the route of the reaction: the higher amine basicity, the higher k₃/k₂ ratio (a measure of the ‘catalyzed’ route contribution as compared to the ‘uncatalyzed’ process) was. ‘Uncatalyzed’ route predominated for all reactions; however in polar acetonitrile the contribution of the ‘catalyzed’ route was significant for amines with high pK_a and small bulk. Copyright © 2007 John Wiley & Sons, Ltd.     

Chemical dynamics of muonium in liquids

The bimolecular rate constants for muonium addition to ethene (CH₂=CH₂) in hydrocarbon liquids were found to be ∼2×10¹⁰ M⁻¹s⁻¹. These rate constants change with temperature in accordance with the Arrhenius equation; but the energy barrier to reaction (E_a) in 2-methylbutane is much less than that for viscous flow. This suggests either non-classical interaction rates stemming from the quantum character for muonium, or non-Stokes-Einstein behavior.     

Effect of Viscosity on the Spin Exchange of TCNE and TEMPO Radicals with Iron(III) Acetylacetonate

The spin exchange of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and the radical anion of tetracyanoethene (TCNE), with iron acetylacetonate, Fe(acac)₃, was investigated as a function of solvent viscosity and temperature by X-band EPR spectroscopy. Acetone, methanol, ethanol, 1-propanol, 1-pentanol, 1-hexanol, 1-octanol, and chloroform were used as solvents because they provide a wide range of viscosity. The spin exchange rate constants measured in the different solvents ranged from 3 × 10⁸ to 8 × 10⁹ M⁻¹ s⁻¹, and from 7 × 10⁸ to 1 × 10¹⁰ M⁻¹ s⁻¹ for the TEMPO/Fe(acac)₃ and the TCNE/Fe(acac)₃ systems, respectively. The spin exchange rate constants, k _e, are compared with the corresponding diffusion rate constants. It was found that the k _e values correlate well with those of diffusion and hence the intermolecular spin exchange is strongly diffusion-controlled. The activation energies and pre-exponential factors are calculated from the Arrhenius plots.     

Temperature dependence of gamma-ray-induced luminescence of xylene-based liquid scintillator between 200 and 242 K

The luminescence response of xylene-based liquid scintillator (scintillation grade), xylene + 1 g/l PPO + 0.1 g/l BBOT, has been studied as a function of temperature in the range 200 to 242 K. It has been observed that under gamma-ray excitation the light output increases with the decrease of temperature. The data are well described by the Arrhenius relation,I _low−I=I ₀ exp(−E/kT), whereI is the count rate at temperatureT.I _low is a constant equal to 5600 counts per min, theI ₀ factor is 3.3 × 10⁶ counts/min,k is the Boltzmann constant. The activation energyE was found to be equal to 0.20 eV. It is typical for thermally activated diffusion controlled process.     

Temperature dependence of electric resistance and magnetoresistance of pressed nanocomposites of multilayer nanotubes with the structure of nested cones

Bulk samples of carbon multilayer nanotubes with the structure of nested cones (fishbone structure) suitable for transport measurements, were prepared by compressing under high pressure (∼25 kbar) a nanotube precursor synthesized through thermal decomposition of polyethylene catalyzed by nickel. The structure of the initial nanotube material was studied using high-resolution transmission electron microscopy. In the low-temperature range (4.2–100 K) the electric resistance of the samples changes according to the law ln R ∝ (T ₀/T)^1/3, where T ₀∼7 K. The measured magnetoresistance is quadratic in the magnetic field and linear in the reciprocal temperature. The measurements have been interpreted in terms of two-dimensional variable-range hopping conductivity. It is suggested that the space between the inside and outside walls of nanotubes acts as a two-dimensional conducting medium. Estimates suggest a high value of the density of electron states at the Fermi level of about 5×10²¹ eV⁻¹ cm⁻³. Zh. éksp. Teor. Fiz. 113, 2221–2228 (June 1998)     

NO2 sensing properties of YSZ-based sensor using NiO and Cr-doped NiO sensing electrodes at high temperature

Nickel oxide and chromium-doped nickel oxide (Ni_0.95Cr_0.03O_1−δ ) were prepared by thermal decomposition of nitrates. The obtained NiO and Ni_0.95Cr_0.03O_1−δ samples were utilized as sensing electrodes (SEs) in yttria-stabilized zirconia (YSZ)-based sensors for detection of NO₂ at 800 °C under wet condition (5 vol.% H₂O). While the mixed-potential-type planar sensor attached with NiO-SE gave rather large NO₂ sensitivity, the sensor attached with Ni_0.95Cr_0.03O_1−δ -SE exhibited fast recovery rate with an acceptable sensitivity. The Δemf (electromotive force) of the sensors varied linearly with NO₂ concentration in the examined range of 50–400 ppm on a logarithmic scale. Based on the results of measurements for polarization, complex impedance and gas phase catalysis, the fast recovery was attributable to the high rate for the anodic reaction of O₂ at the Ni_0.95Cr_0.03O_1−δ /YSZ interface, and the lower NO₂ sensitivity was caused by both the high rate for the anodic reaction of O₂ and the high degree for the gas phase conversion of NO₂ to NO.