Threshold reaction

Acronym Electron impact reaction Threshold energy (eV Reference ) Comment... 

Intensity of collapse, rate of the reaction, threshold/ nucleation, almost all physical properties Gas content, nucleation, collapse phase... 

NFPA now normally refers to autoignition as the Hot Flame Ignition Temperature, as a more precise definition. Subsequently the following two additional terms are being adopted by NFPA to further refine the ignition properties of materials. The lowest temperatures at which cool flame ignitions are observed are named the Cool Flame Reaction Threshold (CFT). The lowest flame temperatures at which an exothermic gas phase reaction is noticed are named the Preflame Reaction Threshold (RTT). 

ETEROAROMATics FURAN AND THIOPHENE. The chemical transformation of thiophene at high pressure has not been studied in detail. However, an infrared [441,445] study has placed the onset of the reaction at 16 GPa when the sample becomes yellow-orange and the C—H stretching modes involving sp carbon atoms are observed. This reaction threshold is lower than in benzene, as expected for the lower stability of thiophene. The infrared spectrum of the recovered sample differs from that of polythiophene, and the spectral characteristics indicate that it is probably amorphous. Also, the thiophene reaction is extremely sensitive to photochemical effects as reported by Shimizu and Matsunami [446]. Thiophene was observed to transform into a dark red material above 8 GPa when irradiated with 50 mW of the 514.5-nm Ar+ laser line. The reaction was not observed without irradiation. This material was hypothesized to be polythiophene because the same coloration is reported for polymeric films prepared by electrochemical methods, but no further characterization was carried out. 

The high energy primary cosmic rays produce many secondary neutrons and protons which in turn are responsible for most of the spallation reactions resulting in radionuclide production in the atmosphere. The formation of these radionuclides occurs at reaction thresholds of 10-40 M.e.v. Because of local ionization losses the secondary protons lose their... 

R.S. Friedman, D.G. Truhlar, Chemical reaction thresholds are resonances, Chem. Phys. Lett. 183 (1991) 539. 

On the contrary, only a few vibrational states of the transition complex may need to be considered, especially at energies just above reaction threshold. 

Figure 10. Counting channels for the FHt reaction. Conditions are as in Figure 9, except that arrows count states for CS approximation (each vibration-rotation level counts only once). Reaction threshold is at 1.65 eV.

Most of the bifurcations, which take place in the high-energy regime, are however not reproduced by the Fermi resonance Hamiltonian, essentially because they result from the superposition of the 1 2 Fermi-resonance and higher-order ones. In order to gain information on the dynamics close to the reaction threshold, one therefore has to analyze the dynamics on the PES by... 

To find the rate of reaction it is necessary firstly to multiply this expression by k2(E). This procedure gives the contribution to the reaction rate from the energy range 5E. Then the expression is integrated over all energies above the reaction threshold, giving... 

In the modified model, as illustrated in Fig. 4, the proportional depletion of the Boltzmann distribution depends on the energy. At high energies k2 E) > these energies are heavily depleted and therefore do not contribute much to the rate. On the other hand, just above the reaction threshold the rate coefficient is small, k2 E) < and the population is barely depleted at... 

Figure 32 Dissociation rates of rotationally cold NO2 as function of the mean excitation energy. Thick solid line phase space theory (PST). The two vertical dashed lines are reaction thresholds for NO2 starting in its lowest rotational level (i.e., Do) and its first excited rotational level. Reprinted, with permission of the American Institute of Physics, from Ref. 275.

The determination of the microcanonical rate coefficient k E) is the subject of active research. A number of techniques have been proposed, and include RRKM theory (discussed in more detail in Section 2.4.4) and the derivatives of this such as Flexible Transition State theory. Phase Space Theory and the Statistical Adiabatic Channel Model. All of these techniques require a detailed knowledge of the potential energy surface (PES) on which the reaction takes place, which for most reactions is not known. As a consequence much effort has been devoted to more approximate techniques which depend only on specific PES features such as reaction threshold energies. These techniques often have a number of parameters whose values are determined by calibration with experimental data. Thus the analysis of the experimental data then becomes an exercise in the optimization of these parameters so as to reproduce the experimental data as closely as possible. One such technique is based on Inverse Laplace Transforms (ILT). 

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