Abstraction reactions, energies

Abstraction reactions, energies vs. experimental log (relative rates), 301,303/... 

FIGURE 1.7. The potential energy surface of the CH4 + C1 supersystem for the collinear hydrogen abstraction reaction CH4 + Cl—> CH3 + HC1. The counter lines are given in spaces of 10 kcal/mol and the coordinates in angstroms. 

Abstraction reactions, see Hydrogen abstraction reactions Activation energy, see Free energy, of activation... 

Hydrogen abstraction reactions potential surfaces for, 25-26,26,41 resonance structures for, 24 Hydrogen atom, 2 Hydrogen bonds, 169,184 Hydrogen fluoride, 19-20, 20,22-23 Hydrogen molecules, 15-18 energy of, 11,16,17 Hamiltonian for, 4,15-16 induced dipoles, 75,125 lithium ion effect on, 12... 

We also observe other effects of translational energy in methane which are similar to those found in hydrogen. There is an abstraction reaction for parent ions which also produces a hydronated methane. In studying the reaction... 

Similarly, energy-transfer processes, together with electron transfer and hydrogen abstraction reactions could be induced in poly(organophosphazenes) in an intramolecular way by preparing POPs geminally substituted at the same phosphorus with two different substituent groups. 

Abstract Reaction paths on potential energy surfaces obtained from QM/MM calculations of en-... 

To illustrate the first point concerning a spectator bond for the abstraction reaction, Fig. 17 shows the total reaction probability for the abstraction reaction as a function of the translational energy for total angular momentum J = 0 on the YZCL2 PES with the H20 reactant in the ground rovibrational state [the (00)(0) state in the local mode notation], where the uncleaved bond OHb is treated in various ways. Using a limited number of one or five vibrational basis functions, VBF(OHb) = 1 or 5, means that the OHb bond is unreactive, a spectator. The abstraction reaction probability... 

Fig. 16. Potential energy contours for the H + D2O system as a function of the OH and one OD bond length. In each panel, the energy has been minimized with respect to the remaining degrees-of-freedom in the vicinity of the minimum energy paths. In (a) the saddle point for the abstraction reaction, and in (b) the shallow < >, minimum for the exchange reaction are marked with X.

Fig. 17. Total reaction probability for the H + H2O(00)(0) — H2 + OH abstraction reaction as a function of the translational energy on the YZCL2 PES for J = 0.

Thus for the photoreduction of benzophenone in the presence of benzhydrol (RH = < 2CHOH, C—H x 78 kcal/mole) the overall reaction is exothermic by 26 kcal/mole. For the photoreduction of acetophenone the value calculated in this way is —31 kcal/mole, while for fluorenone the abstraction is still exothermic by 10 kcal/mole. Clearly, the energetics of the abstraction reaction is not the reason why benzophenone and acetophenone photoreduce but fluorenone does not, since the values for the overall reaction would indicate that all three compounds should react. However, this conclusion is not necessarily valid if large activation energies are involved. ... 

Possibly for those compounds that do not photoreduce the triplet does not have enough energy to abstract a proton to form the ketyl radical. To explore this possibility, one must look at the overall energetics of the abstraction reaction. 

The quantum yields for oxetane formation have not been determined in every case, and only a few relative rate constants are known. The reactivities of singlet and triplet states of alkyl ketones are very nearly equal in attack on electron rich olefins. 72> However, acetone singlets are about an order of magnitude more reactive in nucleophilic attack on electron-deficient olefins. 61 > Oxetane formation is competitive with a-cleavage, hydrogen abstraction and energy-transfer reactions 60 64> so the absolute rates must be reasonably high. Aryl aldehydes and ketones add to olefins with lower quantum yields, 66> and 3n-n states are particularly unreactive. 76>... 

Gaffney, J.S., Levine, S.Z. (1979) Predicting gas phase organic molecule reaction rates using linear free-energy correlations. I. 0(3P) and OH addition and abstraction reactions. Int. J. Chem. Kinet. 11, 1197-1209. 

It has been proposed that this reaction intermediate could decompose to produce HCN and CH3 [55], Chemiluminescence from alkanes can be greatly enhanced by addition of HC1. The proposed explanation is that energy transfer from active nitrogen dissociates HC1 to produce chlorine atoms, which have rapid hydrogen-atom abstraction reactions with alkanes,... 

In abstraction reactions, atoms and some radicals have one more advantage over molecules. When two reacting species form the TS, the fragments of these species arranged near the reaction center are repulsed. The repulsion energy depends on the configuration of the TS. In the reaction of an X atom abstraction from a RX molecule, the minimum repulsion... 

The IPM as a semiempirical model of an elementary bimolecular reaction appeared to be very useful and efficient in the analysis and calculation of the activation energies for a wide variety of radical abstraction and addition reactions [108-113]. As a result, it became possible to classify diverse radical abstraction reactions and to differentiate in each class the groups of isotypical reactions. Later this conception was applied to the calculations of activation energies and rate constants of bimolecular reactions of chain generation [114]. In the IPM, the radical abstraction reaction, for example,... 

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