Rotational effects in broadening factors of fall-off curves of unimolecular dissociation reactions

Strong collision fall-off curves of unimolecular dissociation and the reverse recombination reactions are calculated by using the statistical adiabatic channel/classical trajectory model (SACM/CT). This formalism properly accounts for angular momentum coupling of transitional modes with overall rotation. Calculations are made for linear molecules dissociating into linear fragments and atoms with randomly chosen properties of the transitional modes and for isotropic as well as anisotropic potentials. Analytical representations of center broadening factors as a function of molecular parameters are given. A comparison between fall-off curves from rigid activated complex RRKM theory, from the present loose activated complex SACM/CT model, and from CT calculations on an ab initio potential is made for the HO2-->H + O2 system. It is shown that, besides rotational effects, energy-dependent anharmonicities of the density of states also influence the shape of the fall-off curves in this system.     

Kinetics of the reaction of CF3 with O2 over the temperature range 233–373 K

The rate constant of the reaction CF₃ + O₂ + M → CF₃O₂ + M has been measured as a function of temperature in the fall-off region between 1 and 10 Torr, by laser photolysis and time-resolved mass spectrometry. The values of the fall-off parameters measured at room temperature are in good agreement with previous values and the temperature dependence of k₁₍₀₎ is reasonably well accounted for by the theoretical analysis proposed by Troe and co-workers. With M = N₂k₁₍₀₎ = (1.9 ± 0.2) × 10⁻²⁹ (T/298)^{(−4.7 ± 0.4)} cm⁶ molecule⁻² s⁻¹. Reliable values of k_1(∞) cannot be derived from these low pressure measurements. The rate expression proposed, k_1(∞) = (9 ± 2) X 10⁻¹² (T/298)^(0±1) cm³ molecule⁻¹ s⁻¹, is consistent with a “broadening parameter” of the form F_c = exp(−T/395) and previous values reported at room temperature.     

Velocity dependence of impact line-broadening studied by resonant doppler-free two-photon laser spectroscopy

The velocity dependence of the collison broadening of the⁴⁰Ca 4s ^{2 1} S ₀→4s5p ¹ P ₁ →4s17d ¹ D ₂ transition by neon atoms has been studied. The experiment was performed exciting one velocity group of Ca atoms within the Doppler profile of the first transition and probing this group with a counterpropagating laser beam inducing the second transition. Using thermionic detection we have measured broadening rates for relative velocities between Ca and Ne atoms which corresponds to a temperature range of about 850 K (cell temperature) to 3000 K. The measured line-width was found to have very weak temperature dependence. This is attributed to the weak velocity dependence of polarization broadening of the final level which was found to be the major contribution to the broadening of the two-photon line. For large relative velocities significant asymmetries in the measured line profiles due to velocity changing collisions had to be taken into account.     

Rate constant for the reaction of the CFCl2 radical with oxygen in the pressure range 0.2–12 Torr at 298 K

The rate constant for the reaction of CFCl₂ with oxygen is measured in the pressure range 0.2–12 Torr using pulsed-laser photolysis and time-resolved mass spectrometry. CFCl₂ radicals are generated by photolysis of CFCl₃ at 193 nm. The reaction kinetics are recorded by monitoring the build-up of the CFCl₂O₂ radical concentration. The reaction is in its fall-off region, and the parameters of the relation for the treatment of the fall-off are for M = N₂: k(0) = (5.0 ± 0.8) × 10^?30 cm⁶ molecule^?2 s^?1. k(∞) = (6.0 ± 1.0) × 10^?12 cm³ molecule^?1 s^?1. This value of k(∞) is consistent with results obtained at low pressure taking F_c = 0.6, but the uncertainty in the high-pressure limit is much higher. The results are compared to measurements performed with CH₃ and CF₃. Estimates of the relative third-body efficiencies of He and N₂ are given for CFCl₂ and CF₃.     

Analysis of the unimolecular reaction N2O5 + M ⇌ NO2 + NO3 + M

Recent experimental results on the thermal decomposition of N₂O₅ in N₂ are evaluated in terms of unimolecular rate theory. A theoretically consistent set of fall-off curves is constructed which allows to identify experimental errors or misinterpretations. Limiting rate constants k₀ = [N₂] 2.2 × 10^?3 (T/300)^?4.4 exp(?11,080/T) cm³/molec·s over the range of 220–300 K, k_∞ = 9.7 × 10¹⁴ (T/300)^+0.1 exp(?11,080/T) s^?1 over the range of 220–300 K, and broadening factors of the fall-off curve F_cent = exp(-T/250) + exp(?1050/T) over the range of 220–520 K have been derived. NO₂ + NO₃ recombination rate constants over the range of 200–300 K are k_rec,0 = [N₂] 3.7 × 10^?30 (T/300)^?4.1 cm⁶/molec²·s and k_rec,∞ = 1.6 × 10^?12 (T/300)^+0.2 cm³/molec·s.     

The pressure and temperature dependence of the rate constant for methyl radical recombination over the temperature range 296–577 K

The rate constant for methyl radical recombination has been measured over the temperature range 296–577 K and at pressures between 5 and 500 Torr using laser flash photolysis, coupled with absorption spectroscopy at 216.36 nm. Analysis of the fall-off curves gives k_∞ = (2.78 ± 0.18) × 10^?11 exp(154 ± 22 K/T) cm³ molecule^?1 s^?1 and k₀ = (6.0 ± 3.3) × 10^?29 exp(1680 ± 300 K/T) cm⁶ molecule^?2 s^?1. The quoted errors (two standard deviations) do not include the present uncertainty in the absorption cross section, which is a major source of error (± 30%).     

ClH(ClD) stretching band shape analysis in isolated 1−1 complexes of hydrochloric acid with ethers

A new theoretical approach is proposed for a quantitative representation of the v_ClH(ClD) band profiles in 1−1 complexes of ethers with hydrochloric acid in the gas phase and isolated in matrices at very low temperature. Several band shaping mechanisms are proposed. In solid matrices of gas at very low temperature, beside secondary environmental effects, the band profile is dominated by the vibrational interaction of the two stretching modees of the hydrogen bond (v_ClH(ClD) and v_Cl…O, respectively) which induces a homogeneous broadening for the v_ClH(ClD) band. In the gas phase, the same mechanism, including hot transitions, determines the shape of each subband. The band manifold is ascribed to an additional coupling wiht a low-frequency bending mode of the hydrogen bond. The rovibrational structure turns out to be only a minor band shaping factor. Born-Oppenheimer potential functions for the v_Cl…O mode are determined and a simple model is proposed to account for their particular shapes which are extremely sensitive to the isotopic substitution.     

Dielectric relaxation and effect of plasticisers on siloxane polymers

Studies of dielectric relaxation of polydimethyl siloxane have shown that there is anomalous behaviour near its melting point (≈ ?50°), probably due to the presence of “Rotator Phases”. A detailed study of polyphenyl methyl siloxane, with various polar and non-polar diluents, has been carried out. The most interesting aspect of this study is the systematic broadening of the loss curves with increasing concentration of diluents while retaining conformity with the empirical decay function π(t) = exp ? (t/τ_o)^β. The broadening of loss curves was explained in terms of a distribution of local concentration of the diluents. The range of concentration (ΔC) was calculated for various diluents. It was found that ΔC was larger for the polar diluents than for the non-polar diluents. This result shows that polar plasticiser molecules have changed considerably the environment of the segmental motion of the basic polymer. There is also a linear relationship between the loss factor maximum (?^″_m) with the number of carbon atoms in the plasticiser molecule.     

Kinetics of the gas-phase thermal decomposition of 3-bromopropene

The thermal decomposition of 3-bromopropene was investigated in the temperature range 568.2–653.2 K and pressures between 14–64 Torr in static Pyrex reaction vessel. The reaction was shown to be homogeneous gas reaction of the first order with more than 60% conversion. For the overall reaction, E _a = 153.67 ± 6.70 kJ mol^?1, and logA (A, s^?1) = 9.46 ± 0.57. Two mechanisms, dehydrohalogenation molecular elimination and C-Br bond fission are discussed, both of which account for the observed kinetics and products of decomposition. To interpret the fall-off behaviour, RRKM/ME calculations were adopted and the pressure dependent rate constants were calculated at collision efficiency of 0.25. From the pressure dependence study and RRKM calculations, it can be deduced that we are at the high-pressure limit.     

Diatomic dissociation on aJ-averaged potential

Diatomic dissociation rates are evaluated in the weak-collision low-pressure limit from a non-dimensional impulsive stochastic model on the J-averaged effective potential V(R) − 2kT In(R/R_t), and compared with the three-dimensional results of Borkovec and Berne. The agreement within 10% over a wide range of Morse parameters and temperatures indicates that the contraction to one dimension is not only instructive, but also accurate.     

Microstructural and transport properties in substituted Bi2Sr2CaCu2O8+δ-modulated compounds

X-ray powder diffraction and resistivity measurements were performed on Bi₂Sr₂CaCu₂O_8+δ ceramics substituted by Y and Zn for Ca and Cu sites, respectively. X-ray diffraction patterns show an incommensurate modulated structure along the b-axis. The structural refinements were carried out using the four-dimensional space group Bbmb(0β1)0 0 0. From the X-ray peak profiles analysis, an anisotropic line-shape broadening was observed. The use of the “Williamson and Hall” method allows distinguishing the origin of broadening as mainly due to microstrains. A large transition from a metallic to semiconductor behaviour is observed on the resistivity curves at x≈0.4 for Bi₂Sr₂Ca_1−xY_xCu₂O_8+δ and at x^′≈0.36 for Bi₂Sr₂Ca_1−xY_xCu_1.94Zn_0.06O_8+δ, which can be also correlated to the defects. Oppositely to the metallic behaviour, which satisfies the Mathiessen's rule, the semiconducting one can be modelled by a variable range hopping process.     

Kinetics of the reactions of isopropoxy radicals with NO,NO2, and O2

A fluorescence excitation spectrum of (CH₃)₂CHO (isopropoxy radical) is reported following photolysis of isopropyl nitrite at 355 nm. Rate constants for the reaction of isopropoxy with NO, NO₂, and O₂ have been measured as a function of pressure (1–50 Torr) and temperature (25–110°C) by monitoring isopropoxy radical concentrations using laser-induced fluorescence. We have obtained the following Arrhenius expressions for the reaction of isopropoxy with NO and O₂ respectively: (1.22±0.28)×10^?11 exp[(+0.62±0.14 kcal)/RT]cm²/s and (1.51±0.70)×10^?14 exp[(?0.39±0.28)kcal/RT]cm³/s where the uncertainties represent 2σ. The results with NO₂ are more complex, but indicate that reaction with NO₂ proceeds more rapidly than with NO contrary to previous reports. The pressure dependence of the thermal decomposition of the isopropoxy radical was studied at 104 and 133°C over a 300 Torr range using nitrogen as a buffer gas. The reaction is in the fall-off region over the entire range. Upper limits for the reaction of isopropoxy with acetaldehyde, isobutane, ethylene, and trimethyl ethylene are reported.We have performed the first LIF study of the isopropoxy radical. Arrhenius parameters were measured for the reaction of i-PrO with O₂, NO, NO₂, using direct radical measurement techniques. All reactions are in their high-pressure limits at a few Torr of pressure. The rate constant for the reactions of i-PrO with NO and NO₂ reactions exhibit a small negative activation energy. Studies of the i-PrO + NO₂ reaction produce data which indicate that O(³P) reacts rapidly with i-PrO. Unimolecular decomposition studies of i-PrO indicate that the reaction is in the fall-off region between 1 and 300 Torr of N₂ and the high-pressure limit is above 1 atmosphere of N₂.     

CEPA calculations of potential energy surfaces for open-shell systems.: III. Van der Waals interaction between O(3P) and He(1S)

Quantum-chemical ab initio calculations have been performed for the van der Waals interaction between helium and oxygen atoms in their respective ground states: He(¹S)+ O(³P). As long as fine-structure effects are neglected, there are two low-lying electronic states, ³Σ^? and ³Π resulting from the degeneracy of the O(³P) ground state. Both states are purely repulsive at the SCF level, after inclusion of electronic correlation by the CEPA method they exhibit shallow van der Waals (dispersion) minima at large interatomic separation: R_? = 3.61 Å, ? = 1.0 meV (³Σ^?) and R_? = 3.05 Å, ? = 2.3 meV (³Π). The analysis of the results shows the very slow convergence of the dispersion interaction with increasing basis size, while SCF repulsion and the repulsion due to the change of the intra-atomic correlation are obtained reasonably accurately with moderate basis stes. Van der Waals coefficients C₆, C₈, C₁₀, potential curves of the type HFD (i.e. Hartree-Fock plus damped dispersion) and the influence of fine-structure effects (mainly spin-orbit coupling) on the shape of the adiabatic potential curves are discussed as well.     

Mercury-rare gas van der Waals complexes: From the lightest HgHe to the heaviest HgXe

Mercury helium and mercury xenon van der Waals complexes have been observed in a supersonic free jet expansion by laser-induced fluorescence in the vicinity of the Hg(³P₁¹S₀) transition. The helium complex very weakly bonded in the ground state (D₀ = 8 ± 1 cm⁻¹) exhibited a simple rotational structure. The strongly bonded xenon complex has shown a clear example of isotopic broadening in its vibronic spectra.     

End-point construction and systematic titration error in linear titration curves-precipitation reactions

The systematic titration error which is introduced by the intersection of tangents to hyperbolic titration curves is discussed for precipitation reactions. A simple expression for the systematic titration error is derived, and S/C_x² is proposed as a measure of the sharpness of the titration curve. The effects of the conditional solubility product (S), the concentration of the unknown component (C_x), and the ranges used for the construction of the end-point, are considered. A graphical method is presented for the selection of pairs of ranges which result in small systematic titration errors. For equal values of S/C_x² and 1/KC_x, the optimum combinations of ranges are different for precipitation and complexation titrations. The differences are not large for values smaller than about 0.002. For titration curves with a reversed L-shape, the error is calculated when the end-point is constructed by the intersection of the tangent to the second branch of the curve with volume axis; in this case equal ranges result in the same titration error for equal values of S/C_x² and 1/KC_x. The systematic titration error is equal to -S/C_x² when the tangent to the curve is taken at f_a = 3.0.     

Theoretical electronic studies of aluminum monobromide and aluminum monoiodide

By using CASSCF/MRCI methods, theoretical molecular calculations have been performed for 12 electronic states for AlBr molecule and 12 electronic states for AlI molecule in the representation ^2s+1Λ (neglecting spin‐orbit effects). Calculated potential energy curves are displayed. Spectroscopic constants including the harmonic vibrational wave number ω_e, the electronic energy T_e referred to the ground state and the equilibrium internuclear distance R_e are predicted for these singlet and triplet electronic states for both AlBr and AlI molecules. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Hollow fiber-based liquid phase microextraction combined with high-performance liquid chromatography for extraction and determination of some antidepressant drugs in biological fluids

The applicability of hollow fiber-based liquid phase microextraction (HF-LPME) was evaluated for the extraction and preconcentration of three antidepressant drugs (amitriptyline, imipramine and sertraline) prior to their determination by HPLC-UV. The target drugs were extracted from 11.0 mL of aqueous solution with pH 12.0 (source phase) into an organic extracting solvent (n-dodecane) impregnated in the pores of a hollow fiber and finally back extracted into 24 μL of aqueous solution located inside the lumen of the hollow fiber and adjusted to pH 2.1 using 0.1 M of H₃PO₄ (receiving phase). The extraction was performed due to pH gradient between the inside and outside of the hollow fiber membrane. In order to obtain high extraction efficiency, the parameters affecting the HF-LPME including pH of the source and receiving phases, the type of organic phase, ionic strength and volume of the source phase, stirring rate and extraction time were studied and optimized. Under the optimized conditions, enrichment factors up to 300 were achieved and the relative standard deviation (R.S.D.%) of the method was in the range of 2-12%. The calibration curves were obtained in the range of 5-500 μg L⁻¹ with reasonable linearity (R² > 0.998) and the limits of detection (LODs) ranged between 0.5 and 0.7 μg L⁻¹ (based on S/N = 3). Finally, the applicability of the proposed method was evaluated by extraction and determination of the drugs in urine, plasma and tap water samples. The results indicated that hollow fiber microextraction method has excellent clean-up and high-preconcentration factor and can be served as a simple and sensitive method for monitoring of antidepressant drugs in the biological samples.     

On the correlation between deuteron quadrupole coupling constants,O-H and O-D stretching frequencies and hydrogen-bond distances in solid hydrates

Correlation curves are reported relating deuteron quadrupole coupling constants (e²qQ/h) to O-H and O-D stretching frequencies (v_OH and v_OD) for isotopically-dilute HDO molecules. Correlation curves are also given relating these quantities to hydrogen-bond distances.The best correlations are found to be between (e²qQ/h) and v_OH andv_OD whereas the correlations involving intermolecular geometries show considerably greater scatter.Complications caused by the effects of anharmonicity on the stretching vibrations and of vibrational averaging of the quadrupole coupling and geometrical parameters are discussed in light of the observed correlations. It is concluded that, although these effects are large, their magnitude is also strongly correlated with the stretching frequencies, quadrupole couplings and geometrical parameters.     

Stark broadening of halogen atom lines from (1 D)n p levels

We report results of a study of the Stark broadening of halogen atom lines from (¹ D)n p levels. Wall stabilized arc is used as a plasma source. Electron densities 2.2–3.2×10²² m^?3 are determined from the width ofH _α line and electron temperature 9300–10000 K from plasma composition data. The agreement with the results of simple semiclassical calculations is within the limits of the estimated errors of both experiment and theory. An explanation for the large discrepancy between theory and experiment detected for three BrI lines is offered.     

Reasons for the line broadening in the epr spectra of copper ions in Li2-2x Zn2+x (MoO4)3 lithium-zinc molybdate crystals

An EPR study of Li_2?2x Zn_2+x (MoO₄)₃ crystals activated by copper ions shows that they occupy the M2 site, one of the three possible sites of both lithium and zinc. In the EPR spectra of Cu²⁺ copper a broadening of HFS lines and a nonequidistant splitting between them, which are unusual for the orientation H‖g _zz , A _zz , are observed. In this work possible reasons for such a broadening of HFS lines from copper ions are analyzed: a distortion of the oxygen octahedron due to the introduction of copper ions, second order perturbation theory corrections, superposition of HFS from ⁶³Cu and ⁶⁵Cu isotopes, and the effect of the charge redistribution in the oxygen octahedron because the cation vacancy providing charge compensation can be located at different distances from the copper ion. It is shown that the first three reasons do not explain the features observed in the EPR spectra. In the case of cation vacancies located in the M3 site and remote at different distances from the copper ion, the charge redistribution in the oxygen octahedron of copper should occur along with the dispersion of HFS parameters and the g-factor. The studies performed in X and Q bands confirm this assumption. The width of HFS lines from copper ions in the EPR spectra measured in the Q band is three times more compared to that measured in the X band.