A priori rate constant for the reaction NH2+NO → N2+H2O

Capture rates for a dipole-dipole+Morse potential, from approximate semiclassical trajectory calculations, have been combined with RRKM rates for passing through entropy bottlenecks on the potential surface for NH₂ + NO → N₂ + H₂O. The resulting overall rate constants are in good agreement with experiment. The effective lifetime of the NH₂NO collision complex is found to be of the order of 10⁻¹¹s at room temperature, which accounts for the observed lack of pressure dependence of the rate constant. Calculated rate constants increase markedly at low temperatures, though not as rapidly as some of the experimental data would indicate.     

The absolute value of the rate constant of the liquid-phase reaction SO 5 −. + Fe(II)

An analytical method for monitoring the Fe²⁺ concentration during liquid-phase sulfite oxidation catalyzed by iron ions is described. The concentration conditions under which the Fe(III)/Fe(II) ratio is uniquely related to the rate constant of the reaction SO ₅ ^?. + Fe²⁺ are found by computer analysis. The experimental and calculated Fe(III)/Fe(II) ratio data are in agreement at $k_{(SO_5^{ - \cdot } + Fe^{2 + } )} = 3.2 \times 10^6 1mol^{ - 1} s^{ - 1} $ . This is one of the three rate constant values known from the literature, which differ by a factor greater than 70.     

Quasi-classical trajectory study of the S + OH → SO + H reaction: from reaction probability to thermal rate constant

First quasi-classical trajectory calculations have been carried out for the S((3)P) + OH(X?(2)Π) → SO(X?(3)Σ(-)) + H((2)S) reaction on an ab initio global potential energy surface for the ground electronic state, X?(2)A', of HSO. Cross sections, computed for collision energies up to 1 eV, show no energy threshold and decrease with the increasing collision energy. Rate constants have been calculated in the 5-500 K temperature range. The thermal rate constant is in good agreement with approximate quantum results, while a disagreement is found at 298 K with the experimental data. Product energy distributions have also been reported at four collision energies from 0.001 to 0.5 eV. The shapes of the rovibrational and angular distributions suggest the formation of an intermediate complex that is more and more long-lived as the collision energy increases.     

Direct determination of the rate constant for the reaction CO + N2O → CO2 + N2

The rate constant for the bimolecular reaction CO + N₂O → CO₂ + N₂ was determined by comparison of calculated infrared emission profiles of CO₂ with those observed in shock-tube experiments in the temperature range 1350–2100 K for CO-N₂O-He-Ar mixtures. The rate constant was found to be k₁ = 3.2 × 10¹¹exp(−85 kJ/RT) cm³ mol⁻¹ s⁻¹.     

Measuring the rate constant of the reaction between carbon monoxide and iodine oxide at 298–363 K by the resonance fluorescence method

The rate constant of the reaction between the IO radical and carbon monoxide has been measured by the iodine atom resonance fluorescence method in the temperature range from 298 to 363 K. The reaction mainly takes place on the wall of the reactor.     

Dynamics and mode-selected reaction of NCO+H_2→HNCO+H

The reaction path of the reaction NCO+H2→HNCO + H has been traced by Fukui's theory and the ab initio method. On this basis, the dynamical properties along the reaction path, canonical variational theory (CVT) rate constants and vibrational-mode-selected rate constants have been computed. The results show that the effect of the electron correlation energy on the activation barrier is large, and tiros the correction by MP4 method is effective; the results also show that the recrossing and tunneling effects exist, and thus the corrections by the variational transition state theory (VTST) and the small curvature (SC) approximation method are also effective. In the reaction, the coupling and energy transfer between mode 8(7) and reaction path are strong, so the rate is effectively enhanced while these modes, especially H2 stretching, are vibrationally excited.     

Temperature dependence of the reaction NO + NO3 → 2NO2

The flash photolysis-visible absorption technique has been used to measure rate constants for the reaction NO + NO₃ → 2NO₂ (1) over the temperature range 224–328 K. The temperature dependence of the rate constant is given by the expression k₁(T) = (1.59 ± 0.32) × 10⁻¹¹exp(122/T) cm³ molecule⁻¹ s⁻¹ where the stated uncertainties refer to the ± 2σ limits from both random and systematic errors.     

Comparative study of the photoassociation reaction of He + X+→ HeX+ (X = H,D): Including multiphoton transitions and dissociations

Using the time-dependent quantum wave packet method, the photoassociation (PA) processes of He + H ⁺→ HeH⁺ and He + D ⁺→ HeD⁺, driven by the sin²-shaped femtosecond laser pulse in the electronic ground state, including multiphoton transitions and dissociations, are investigated for a wide range of initial collision momenta spanning from 1 to 4 a.u. (or for the collision energy roughly in the ranges of 0.009∼0.148 eV and 0.006∼0.089 eV for HeH⁺ and HeD⁺ systems, respectively). It is found that, at some collision momenta, multiphoton transitions to deeply bound states are inevitable to occur and can greatly decrease the PA probability of the target state that selected is the vibrational state v = 6. For the dissociation process, the higher-order (two- and three-photon) dissociations, measured from the target state, tend to be significant at relative high collision energies, which implies that above-threshold dissociations may also be an important loss mechanism in the PA process. In addition, it is also shown that the higher-order dissociation is much stronger for HeH⁺ systems than that for HeD⁺ systems at a given collision momentum, and could be enhanced by the strong transitions among deeply bound states.     

Temperature dependence of the relaxation rate constants of the a 1Δg and b 1Σ +g states of oxygen isotopes

The relaxation rate constants of the low-lying electronic singlet states, a ¹Δ_g and b ¹Σ⁺_g , of gaseous natural O₂ and of the isotope ¹⁸O₂ were investigated as a function of temperature from 100 to 295 K. The measured increase of the rate constants with temperature is in good agreement with a theory of electronic-to-vibrational-translational energy transfer. The significant effects of the different electronic states and of the isotope masses on the absolute values of the relaxation rate constants, which range from 1.0× 10⁻²⁰ to 3.9× 10⁻¹⁷ s⁻¹ molecule⁻¹ cm³ at 295 K, are discussed.     

Fluorescence quenching of para-substitutedstyrenes by tetramethylethylene. Correlationanalysis of the quenching coefficient KSV and thequenching rate constant k_q by the dual-parameter equation with polar and spin-delocalization substituent constants

The fluorescence quenching coefficient (Ksv) and the quenching rate constant kq of ten para-substituted styrenes (1-Ys) have been measured and correlation-analyzed by both the dual-parameter equation (Eq. 1) with (ρxσx+ρ'σ') and the single-parameter equation (Eq. 2) with ρxσx. Ex-cellent results have been obtained for the correlation of KSV against (ρxσmb+ρ'σ'JJ) or (ρxσ+ +ρ'σ'JJ). Our results suggest that, possibly, there might be no need to use excited-state substituent constant for the fluorecence quenching process of excited states of styrenes.     

Computer investigations on the asymptotic behavior of the rate coefficient for the annihilation reaction A + A → product and the trapping reaction in three dimensions

We have performed intensive computer simulations of the irreversible annihilation reaction: A + A → C + C and of the trapping reaction: A + B → C + B for a variety of three-dimensional fluids composed of identical spherical particles. We have found a significant difference in the asymptotic behavior of the rate coefficients for these reactions. Both the rate coefficients converge to the same value with time t going to infinity but the convergence rate is different: the O(t(-1/2)) term for the annihilation reaction is higher than the corresponding term for the trapping reaction. The simulation results suggest that ratio of the terms is a universal quantity with the value equal to 2 or slightly above. A model for the annihilation reaction based on the superposition approximation predicts the difference in the O(t(-1/2)) terms, but overestimates the value for the annihilation reaction by about 30%. We have also performed simulations for the dimerization process: A + A → E, where E stands for a dimer. The dimerization decreases the reaction rate due to the decrease in the diffusion constant for A. The effect is successfully predicted by a simple model.     

Effect of hydrostatic pressure,temperature, and solvent on the rate of the Diels–Alder reaction between 9,10-anthracenedimethanol and maleic anhydride

The rate of the reaction between 9,10-anthracenedimethanol and maleic anhydride in 1,4-dioxane, acetonitrile, trichloromethane, and toluene is studied at 25, 35, 45°C in the pressure range of 1–1772 bar. The rate constants, enthalpies, entropies and activation volumes are determined. It is shown that the rate of reaction with 9,10-anthracenedimethanol is approximately one order of magnitude higher than with 9-anthracenemethanol.     

Study of the copper(II)-Aztreonam™ system by potentiometry and spectrophotometry,and structural characterization by 13C NMR relaxation

Potentiometric and spectrophotometric results obtained for the Cu(II)-Aztreonam system give a clear indication of the cheating properties of this antibiotic toward the Cu²⁺ ion, coordinated already at pH ≌ 2. Only one single complex of 1:1 stoichiometry is formed. Measurements of ¹³C relaxation rates allow us to attribute a coordination scheme via the carboxylic and aminic groups.     

State-to-state quantum dynamics of the N(4S) + OH(X2Π) → H(2S) + NO(X 2Π) reaction

Quantum state-to-state dynamics for the N((4)S) + OH(X(?2)Π) → H((2)S) + NO(X(?2)Π) reaction is reported on an accurate ab initio potential energy surface of the lowest triplet electronic state (a(3)A(")) of HNO∕HON. It was found that the reaction is dominated by long-lived resonances supported by the HNO and HON wells. Significant non-reactive scattering was observed, indicating substantial deviations from the statistical limit. Due to the large exothermicity of the reaction, the NO product has hot internal state distributions: its rotational state distribution is inverted and peaks near the highest accessible rotational level; and its vibrational state distribution extends to υ = 10 and decays monotonically with the vibrational quantum number. In particular, the predicted product vibrational distribution is in reasonably good agreement with experiment. The calculated differential cross section is dominated by scattering in both the forward and backward directions, consistent with the formation of reaction intermediates.     

Picosecond and nanosecond kinetic spectroscopic investigations of the relaxation and the solute—solvent reaction of electronically excited 3,5-dinitroanisole

We investigated the relaxation of electronically excited nitro aromatic compounds, in particular 3,5-dinitroanisole (3,5-DINA), by means of picosecond and nanosecond laser kinetic spectroscopy. Complete neglect of differential overlap/spectroscopic—configuration interaction calculations were performed to characterize their excited states. The gas phase UV spectrum of 3,5-DINA and the electron spin resonance spectrum of the anion serve the same purpose. It is shown by means of IR spectroscopy that 3,5-DINA in its ground state does not form hydrogen bonds. The envelope of the first absorption band of 3,5-DINA in liquid solutions covers a weak nπ* and a strong ππ* transition. In non-hydrogen-bonding solvents, excitation in this band populates a primary excited state S_i which decays within 10 ps to S₀ and to T₀. The state T₀ (in CH₃CN) is converted within 780 ps to S₀. In hydrogen bonding solvents the decay is quite different. The conversion T₀ → S₀ and probably S_i → S₀ is much slower, while the intersystem crossing S_i → T₀ remains very fast. We conclude that in aprotic solvents both S_i and T₀ are nπ* states in which the excitation is localized on the NO₂ groups, which are consequently distorted to a large extent relative to the ground state. The distortion is thought to cause strong phonon coupling in the radiationless transitions S_i → S₀ and T₀ → S₀. Local vibrations in the NO₂ group, which modulate the overlap of lone pair orbitals on adjacent oxygen atoms, are efficient promoting modes in the radiationless decay and they make the rate constant much larger than in other cases involving local excitation of small chromophores, e.g. ketones. The primary state populated by excitation with our pulsed lasers is a ππ^* chare transfer state. When 3,5-DINA is brought into this state in hydrogen bonding solvents, it forms a (single) hydrogen bond before any other types of relaxation. Thus S_i and T₀ are both hydrogen-bonded delocalized ππ^* states. The known efficient bimolecular photo-induced reactions of 3,5-DINA involve the formation of the hydrogen bond as the first step. A reinterpretation is given of the variation in the triplet lifetime of 3,5-DINA over three orders of magnitude with solvent composition. The thermal dissociation of the hydrogen bond to the NO₂ group is the rate-determining step in the decay of 3,5-DINA in hydrogen bonding solvents. Both the activation energy and the activation entropy of this reaction depend on the solvent structure.     

Optical emission of GeF+4(D̃) produced by ion impact ionization of GeF4

Emission spectra produced by 1–20 keV He⁺, Ne⁺ and H⁺ impact on GeF₄ have been obtained and relative emission cross sections determined. By comparing the energy dependence of the ion impact data with that for charge-transfer ionization of SiF₄ leading to SiF⁺₄(D̃) and by reference to PE data of GeF₄, the 255, 290 and 420 nm emissions are attributed to D̃ → Ã, B̃ and C̃ transitions, respectively of GeF⁺₄. We found the D̃ → C̃ emission of SeF⁺₄ in the spectral region 510 to 730 nm.     

Conformational analyses of α and β (1–6) mannodisaccharides by deuterium substitution effect on relaxation rate and NOE

The study of ¹H differential relaxation rate and NOE using chirally deuterated sugars, revealed the preferred conformation of each α and β (1–6) linkage of oligomannoses in solution.     

Are insertion compounds of CH2CHF and the rare gases stable? A computational study

Ab initio calculations, using second-order M?ller-Plesset perturbation theory with a 6-311+ +G(2d,2p) basis set, predict the stability of two novel compounds of monofluoroethene, CH2CHF, with the rare-gas atoms Ar and Kr. The dissociation energies to the lowest-energy fragmentation products, CH2CHF + Rg (Rg = Ar,Kr), were computed to be -528 and -449 kJ mol(-1), respectively, at the coupled-cluster singles, doubles, and triples level of theory. Possible transition states (at second-order M?ller-Plesset theory) via a C-Rg-F bending mode for these fragmentation reactions were also located with barrier heights of about 76 and 106 kJ mol(-1), for the Ar- and Kr-containing species, respectively. However, the Ar-containing species may not exist at all as it is less stable than the fragments CH2CH + F + Ar at the higher level of theory and may possibly dissociate via this route.