Dissociation, modes

The first possibility, (11), clearly is concerned with a dissociative mode of activation (d). The second, (12), mxghihsassociative led by water attack. But, this interpretation of (12) involves commitment to the proposition that no nucleophile (possibly excepting OH ) has been discovered which is better than water, i.e. the associative attack must always involve water in the first instance. The proposition... 

As the trans effect theory indicates, there should be some relationship between lability of a ligand and its role as a labilizing group in another position in a complex. In an octahedral complex reacting via a dissociative mode of activation, the transition state has five strongly bound ligands. This state will be stabilized... 

In the case of CH3CD3, various other dissociation modes exist, including C—C and C—H bond breakage, either separately or in various combinations (Freeman, 1968). A few other examples follow of dissociation of excited inorganic and organic molecules ... 

Generally speaking, the dissociation modes are established by photolysis, isotope studies, the electric field effect, and, to some extent, by special mass spectrometric methods. In addition, polymerization and isomerization studies have been helpful. 

The activation entropies determined for both these systems are large and positive, which is in general agreement with a d-activation (see Table V). Furthermore, the determination of the volume of activation of +10.6 cm3 moL1 for the anation of the [WO(OH2)(CN)4]2 complex by N3 ions (74) provides strong evidence in favor of a dissociative mode of activation for the substitution reactions in the aqua oxo complexes. 

On the basis of either interpretation, k should equal the solvolysis rate constant and if MS is an isolatable intermediate, it should be shown to react rapidly with Y. Both consequences have been realized in certain systems. Although there has been some skepticism shown towards a dissociative mode of substitution k, k in (4.98)), there is growing evidence for its existence in cases where the importance of the associatively activated pathways can be reduced. For the first step of the reaction... 

On the other hand, the hydrogen molecule dissociates on active sites in either a homolytic or heterolytic way, and the reactivity or selectivity of the dissociated hydrogen atom with olefins is strongly influenced by whether the dissociation mode is homolytic or heterloytic. 

In conformity with the dissociation modes of hydrogen, the dissociation of H2, D2, and HD molecules are described in general as follows ... 

A kinetic study of the hydroformylation has been carried out306 and the mechanism proposed by Wilkinson (Scheme 16) was extended to express both the associative and dissociative modes of alkene coordination298 and the formation of n- and iso-butyraldehydes. High-pressure IR spectroscopy using CO and either H2 or D2 has confirmed the formation of [RhH(CO)2(PPh3)2] in these reactions.307... 

In metal peroxide chemistry, the heterolytic or homolytic nature of catalytic oxidation seems to be strongly dependent on the heterolytic or homolytic dissociation mode of the peroxide intermediate, for which the triangular coordination mode of the peroxide moiety of the metal appears to be a key feature. Heterolytic oxidations require attainable coordination sites on the metal, involve strained metallacyclic reaction intermediates, and are highly selective. In contrast, homolytic oxidations involve bimolecular radical processes with no metal-substrate interactions and are less selective. In the important field of palladium oxidation chemistry, hydroperoxo... 

In indirect photofragmentation, on the other hand, a potential barrier or some other dynamical force hinders direct fragmentation of the excited complex and the lifetime amounts to at least several internal vibrational periods. The photodissociation of CH3ONO via the 51 state is a representative example. The middle part of Figure 1.11 shows the corresponding PES. Before CH30N0(5i) breaks apart it first performs several vibrations within the shallow well before a sufficient amount of energy is transferred from the N-0 vibrational bond to the O-N dissociation mode, which is necessary to surpass the small barrier. 

Resonances exist below as well as above the barrier. Their width usually increases with energy, i.e., with increasing excitation of the dissociation mode.t... 

Fig. 12.2. Schematic illustration of the absorption of an IR photon by the van der Waals complex Ar H2 and the subsequent dissociation into Ar+H2(n = 0). Voo(R) and Vn(i ) are the diagonal elements of the potential coupling matrix defined in Equation (3.6) which serve to define the zero-order radial wavefunc-tions employed in the Golden Rule expression for the dissociation rate. The assignment of the bound levels is (m,n), where m and n denote the number of quanta of excitation in the dissociation mode R and the vibrational mode of H2, respectively.

The wavefunction of the parent molecule in the electronic ground state is assumed to be a product of two harmonic oscillator wavefunctions with m and n quanta of excitation along R and r, respectively. In Figure 13.2(b) only the vibrational mode of BC is excited, n = 3, while the dissociation mode is in its lowest state, m = 0. The corresponding spectrum is smooth without any reflection structures. Conversely, the wavefunction in Figure 13.2(a) shows excitation in the dissociation mode, m = 3, while the vibrational mode of BC is unexcited. The resulting spectrum displays very clear reflection structures in the same way as in the one-dimensional case. Thus, we conclude that, in general ... 

This reaction was followed by a second slower reaction that was not characterized in the report. An analysis of the overall results showed that the influence of steric or electrostatic effects on kinetic parameters is not necessarily minor. The dynamics of binding of NO to the diaqua species is mainly tuned by modulation of electron density on the iron center by the porphyrin macrocycle. A volume profile for NO binding based on values ofAV on and A V 0ff of + 1.5 and + 9.3 cm3/mol, respectively, maybe interpreted as an interchange mechanism for the on reaction, as the Fenl-H20 bond is decidedly stabilized (see Figure 7.15a). The volume of activation for the off reaction indicates a less dissociative mode of activation compared with NO release from other porphyrins. 

---