Transition exothermic reaction

Many programs allow the user to input a weighting factor (i.e., to give a structure that is 70% of the way from reactants to products). This allows the application of the Hammond postulate that the transition structure will look more like the reactants for an exothermic reaction and more like the products for an endothermic reaction. 

In the case of a strongly exothermic reaction the final term turns into an absorbing wall, and the transition is completed whenever the distance AB reaches a certain value and the A-B bond is broken. The intra- and intermolecular coordinates Q and q are harmonic and have frequencies (Oo and oji, and reduced masses mo and mi. At fixed intermolecular displacement the tunneling probability equals... 

The Hammond Postulate implies that the transition stah of a fast exothermic reaction resembles the reactants (se( reaction energy diagram at left). This means that it wil be hard to predict the selectivity of competing exothermi( reactions both barriers may be small and similar even i one reaction is more exothermic than the other. 

Exothermic Reactions of Transition Metal Ions with Hydrocarbons. Cross sections for the formation of product ions resulting from the interaction of Ni+ with n-butane are shown in Figure 6 for a range of relative kinetic energies between 0.2 and 4 eV. In contrast to the results shown in Figure 3, several products (reactions 6-8) are formed with large cross section at low energies. These cross sections decrease with... 

In highly endothermic reaction, the transition-state (TS,) resembles product B. Case II. Highly Exothermic Reaction... 

According to Hammond s postulate in highly exothermic reaction, the transition state (TS2) resembles reactant A. 

No, the editor didn t know what this name meant either.) It means salts of the triva-lent anions of Group V, restricted in [1] to arsenides, antimonides and bismuthides and prepared by reaction of sodium pnictides with anhydrous halides of transition and lanthanide metals. This violently exothermic reaction may initiate as low as 25°C. Avoidance of hydrated halides is cautioned since these are likely to react uncontrollably on mixing. Another paper includes a similar reaction of phosphides, initiated by grinding [2], Nitrides are reported made from the thermally initiated reaction of sodium azide with metal halides, a very large sealed ampoule is counselled to contain the nitrogen [3],... 

The key to a controlled molecular weight build-up, which leads to the control of product properties such as glass transition temperature and melt viscosity, is the use of a molar excess of diisopropanolamine as a chain stopper. Thus, as a first step in the synthesis process, the cyclic anhydride is dosed slowly to an excess of amine to accommodate the exothermic reaction and prevent unwanted side reactions such as double acylation of diisopropanolamine. HPLC analysis has shown that the reaction mixture after the exothermic reaction is quite complex. Although the main component is the expected acid-diol, unreacted amine and amine salts are still present and small oligomers already formed. In the absence of any catalyst, a further increase of reaction temperature to 140-180°C leads to a rapid polycondensation. The expected amount of water is distilled (under vacuum, if required) from the hot polymer melt in approximately 2-6 h depending on the anhydride used. At the end of the synthesis the concentration of carboxylic acid groups value reaches the desired low level. 

Fig. 11. Potential energy diagram for a hypothetical exothermic reaction, showing the transition state as the saddle point.

With less hindered hydride donors, particularly NaBH4 and LiAlH4, cyclohexanones give predominantly the equatorial alcohol. The equatorial alcohol is normally the more stable of the two isomers. However, hydride reductions are exothermic reactions with low activation energies. The transition state should resemble starting ketone, so product stability should not control the stereoselectivity. One explanation of the preference for formation of the equatorial isomer involves the torsional strain that develops in formation of the axial alcohol.78... 

The relationship of the transition state (TS) to the reactants (R) and products (P) is shown by the enthalpy (energy) diagram. Fig. 3-2, for a one-step exothermic reaction A-t-B-+C + D. At equilibrium, formation of molecules with lower enthalpy is favored, i.e., CD. However, this applies only if AH of reaction predominates over T AS of reaction in determining the equilibrium state. The reaction rate is actually related to the free energy of activation, AG, where AG = AH - r A5. ... 

Equation (82) predicts that for reactions with zero free energy change a = 0.5, while for exothermic reactions, a < 0.5 and for endothermic reactions, a > 0.5. Since according to both Marcus theory and the Bell— Evans-Polanyi model early transition states are related to exothermic reactions and late transition states to endothermic reactions, a may be interpreted as a relative measure of transition state geometry. However, in our view even this interpretation should be treated with a measure of healthy scepticism. Even if one accepts Marcus theory without reservation, the a... 

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