Direct rebound mechanism

Chemical dynamics simulations of the gas phase 5 2 reactions of methyl halides have been carried out at many different levels of theory and compared with experimental measurements and predictions based on transition state theory and RRKM (Rice-Ramsperger-Kassel-Marcus) theory. Although many 5 2 reactions occur by the traditional pre-reaction complex, transition state, post-reaction complex mechanism, three additional non-statistical mechanisms were detected when the F -CH3-I reaction was analysed at an atomic level (i) a direct rebound mechanism where F attacks the backside of the carbon and CH3-F separates (bounces off) from the iodine ion, (ii) a direct stripping mechanism where F approaches CH3-I from the side and strips away the CH3 group, and (iii) an indirect reaction where the pre-reaction complex activates the C-I bond causing a CH3-I rotation and then the 5 2 reaction. The presence of these processes demonstrate that three non-statistical effects, (i) recrossing of the transition state is important, (ii) the transfer of the translational energy from the reactants into the rotational and vibrational modes of the substrate is inefficient, and (iii) there is... 

A and BC approach to centre of mass, A strips off B and then AB and C return roughly in the direction from which they came. These reactions are said to occur by a rebound mechanism and generally occur when the surface are repulsive. In such reactions the life-time of activated complex, i.e. (ABC) must be short and reaction is said to be direct or impulsive. If life-time is much, rotation may occur and the products may separate in random directions. For many such reactions, the life-time of complexes has been observed less then 5 x 10 13sec. J.C. Polanyi discussed the relationship of these reactions with shapes of PES with special attention to mass effects. 

Fig. 9.36 Direction of motion of reactants and products relative to centre of mass (rebound mechanism).

The rebound mechanism, though in a modified version, has been recently supported by theoretical calculations of KIF using the density functional theory (Yoshizawa et al., 2000). The calculations demonstrate that the transition state for the H-atom abstraction from ethane involves a linear [FeO.H...C] array a resultant radical species with a spin density of nearly one is bound to an iron-hydroxy complex, followed by recombination and release of product ethanol. According to the calculation of the reaction energy profile, the carbon radical species is not a stable reaction intermediate with a finite lifetime. The calculated KIF at 300 K is in the range of 7-13 in accord with experimental data and is predicted to be significantly dependent on temperature and substituents. It was also shown from femtosecond dynamic calculations in the FeOVCH4 system that the direct abstraction mechanism can occur in 100-200 fs. 

Thus, it is becoming increasingly evident that the rebound mechanism is the most probable mechanism of the hydroxylation. Nevertheless, direct proof of occurrence of the ferryl active intermediate is as yet incomplete. Above mentioned the masked radical rebound mechanism can not be excluded. 

Groves examined the hydroxylation of optically active mono-deuterated ethylbenzene using optically active vaulted iron-porphyrin complex 2 as the catalyst and disclosed that d/ h was 6.4 and enantiotopic selectivity in the hydrogen atom abstraction step was 84% ee (A h/ sd = d/ h x [pro-.R]/ [pro-S] = 92 8). However, this enantiotopic selectivity is not directly reflected in the enantiomeric excess (77% ee) of the product (Scheme 3) [9]. These results indicate that hydrogen atom abstraction is the rate-determining step and that the reaction is not concerted but stepwise, and are well compatible with the oxygen rebound mechanism. Discrepancy between the enantiotopic selectivity and the enantiomeric excess is rationalized... 

However, the oxidized metal complex may be able to perform a second oxidative event at the deoxyribose radical ( 2-electron oxidation mechanism). This oxidative event may be an electron transfer or an oxygen rebound mechanism (direct hydroxylation by Mn -OH species). Alternatively, hydroxyl radical can combine with a carbon-centered radical. Oxygen rebound, reaction of hydroxyl radicals, or the attack of a molecule of water at a carbon cation, give rise to an alcohol at the oxidized carbon of the deoxyribose. 

When a reaction occurs either by a stripping or a rebound mechanism the lifetime of the activated complex must be short if it were long enough for rotation to occur the products would separate in random directions and the reaction is then said to occur by an indirect or complexmode mechanism. When the lifetime of the complex is short, as in the stripping and rebound mechanisms, the expression direct or impulsive is used. The occurrence of a reaction by a complex-mode mechanism appears to be associated with the existence of a basin in the potential energy surface. 

C-H functionahzations can generally proceed through three types of mechanisms [5, 8] (i) radical rebound mechanisms, which proceed through radical intermediates (ii) direct insertions into C-H bonds and (iii) mechanisms through an organometaUic intermediate with a metal-carbon bond. These three different pathways are outlined in Scheme 23.2. 

As indicated in the sections above, C—H bond activations can proceed through mechanistically distinct manifolds (i) H-atom abstraction followed by radical functionalization (radical rebound mechanism), (ii) direct insertion into the C—H bond, and (iii) organometallic C—H activation proceeding throngh an intermediate with an M—C bond. The different mechanisms determine the types of C—H bonds that can be broken and can thus be used to achieve selectivity. 

Kinetic isotope effects on the hydroxylation of a various types of hydrocarbons and oxidative demethylation of anisole derivatives both by cytochrome P-450 and by model systems were examined, and large kinetic isotope effects in the range between 4 and 12 were observed [46,191-196]. Direct hydrogen abstract followed by rapid oxygen rebound mechanism has been proposed for the hydroxylation reactions by P-450 and their model systems. Retention of configuration in many hydroxylations mediated by P-450 indicates an extremely rapid rebound step. 

This relationship of the metastable atom deactivation mechanisms is valid for atomically pure metal surfaces and is proved true in a series of works [60, 127, 128]. Direct demonstrations of resonance ionization of metastable atoms near a metal surface are given by Roussel [129]. The author observed rebound of metastable atoms of helium in the form of ions from a nickel surface in the presence of an adsorbed layer of potassium. In case of large coverages of the target surface with potassium atoms, when the work of yield becomes less than the ionization potential of metastable atoms of helium, the signal produced by rebounded ions disappears, i.e. the process of resonance ionization becomes impossible and the de-excitation of metastable atoms starts to follow the mechanism of Auger deactivation. 

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