Dissociative states

A very extreme version of surface corrugation has been found in the nonactivated dissociation reactions of Fl2 on W [, ], Pd and Rli systems. In these cases, the very strong chemisorption bond of the FI atoms gives rise to a very large energy release when the molecule dissociates. In consequence, at certain sites on the surface, the molecule accelerates rapidly downliill into the dissociation state. At the unfavourable sites, there... 

As discussed in section A3.12.2. intrinsic non-RRKM behaviour occurs when there is at least one bottleneck for transitions between the reactant molecule s vibrational states, so drat IVR is slow and a microcanonical ensemble over the reactant s phase space is not maintained during the unimolecular reaction. The above discussion of mode-specific decomposition illustrates that there are unimolecular reactions which are intrinsically non-RRKM. Many van der Waals molecules behave in this maimer [4,82]. For example, in an initial microcanonical ensemble for the ( 211 )2 van der Waals molecule both the C2H4—C2H4 intennolecular modes and C2H4 intramolecular modes are excited with equal probabilities. However, this microcanonical ensemble is not maintained as the dimer dissociates. States with energy in the intermolecular modes react more rapidly than do those with the C2H4 intramolecular modes excited [85]. 

Conformational Adjustments The conformations of protein and ligand in the free state may differ from those in the complex. The conformation in the complex may be different from the most stable conformation in solution, and/or a broader range of conformations may be sampled in solution than in the complex. In the former case, the required adjustment raises the energy, in the latter it lowers the entropy in either case this effect favors the dissociated state (although exceptional instances in which the flexibility increases as a result of complex formation seem possible). With current models based on two-body potentials (but not with force fields based on polarizable atoms, currently under development), separate intra-molecular energies of protein and ligand in the complex are, in fact, definable. However, it is impossible to assign separate entropies to the two parts of the complex. 

FIGURE 7.3 Promotion to a dissociative state results in bond cleavage. 

Trimethoxyamphetamine (TMA) causes perceptual distortions, LSD-like synesthesias, and dissociative states. At high doses, it may produce unprovoked anger, aggressive behavior, and homicidal violence (Shulgin 1978). 

The slowly dissociating state appears to reflect receptor in an inactive form. Cells can be stimulated in a pulse protocol in which the stimulus (FLPEP) is added, then at some later time, its binding to the receptor is interrupted by the addition of an antibody to fluorescein or of a receptor antagonist (tBoc-phe-leu-phe-leu-phe). 

A few seconds after the addition of the inhibitor, cell responses begin to decay (see Figure 8, Omann and Sklar, this volume). Six cell responses have been characterized in this manner (2i). Because the slowly dissociating receptor state (Figure 2) is found on cells at a time when cell responses have ceased, we suggested that this state was inactive the rapidly dissociating state could be associated with cell activation. 

Figure 5. Guanine-nucleotide-dependent receptor interconversion from LRG" to LR. Penneabilized cells (10 /mL) (no GTPyS 1 ml Mg " ) were allowed to bind FLPEP for 1 min. Data analysis begins with the addition of antibody to fluorescein (after 60 s, but not indicated). At 90 s (indicated), GTPyS is added to induce interconversion to the rapidly dissociating state. The guanine nucleotide concentrations are shown (saturated at 10" If). The solid lines are the fit to the onestep model shown in the inset, and the k for the fit is shown at the end of each solid line. Data...

Figure 1.6 Structures and electron density changes of dissociating CO on Ru(OOOl) surface (a) adsorbed CO, (b) transition state for dissociation, and (c) dissociated state.

The problem of controlling the outcome of photodissociation processes has been considered by many authors [63, 79-87]. The basic theory is derived in detail in Appendix B. Our set objective in this application is to maximize the flux of dissociation products in a chosen exit channel or final quantum state. The theory differs from that set out in Appendix A in that the final state is a continuum or dissociative state and that there is a continuous range of possible energies (i.e., quantum states) available to the system. The equations derived for this case are... 

Molecular and Dissociated States of Molecules Biphasic Systems... 

Dissociative chemisorption was considered to be either direct, when the incoming diatomic molecule has sufficient energy to surmount the barrier without being trapped into the molecular state, or indirect, when it passes via the molecular (precursor) state into the dissociated state. If the dissociated state is not immediately equilibrated with the lattice, the fragments will move across... 

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