Elementary processes channels

C8-product channel. In the account given here, the focus is on the most feasible of the several stereochemical pathways for individual elementary processes.29a... 

Fig. 16.2. The elementary processes at a chemical synapse, a) In the resting state, the nenrotrans-mitter is stored in vesicles in the presynaptic cell, b) An arriving action potential leads to influx of Ca into the presynaptic cell. Consequently, the vesicles fuse with the presynaptic membrane and the neurotransmitter is released into the synaptic cleft, c) The neurotransmitter diffuses across the synaptic cleft and binds to receptors at the surface of the postsynaptic cell. Ion channel and receptor form a structural unit. The ion channel opens and there is an influx of Na ions into the postsynaptic cell. Recychng takes place in the presynaptic cell and the vesicles are reloaded with neurotransmitter.

In general, more than one product asymptotic region can be reached from a single reactant asymptote, i.e., certain reactants may undergo several elementary processes, the so-called channels. Within a channel, processes leading from specific reactant states to specific product states can be distinguished (state-to-state elementary processes). 

The values of coordinates and velocities (or momenta) at some point in the reactant and product asymptote, respectively, can be used to define the boundary conditions for solving the equations of motion describing an elementary process (18,43). The coordinates Qk = Q, Q2 = Qi and velocities Vl = V, V2 = VJ2j at the end points refer to both the internal and translational degrees of freedom the choice of the end points thus determines the channel as well as the internal (quantized) and translational states of reactants and products. 

Figure 6-6. Reaction path profile (potential energy curve) for the elementary process of NO synthesis O + N2 — NO + N, showing adiabatic and non-adiabatic reaction channels.

Fig. 1. Snapshot configuration of adsorbate on geometrically inhomogeneous substrate and elementary processes at interphase boundary S - direct scattering, including reactive one A - eidsorption channel D - desorption channel M - migration channel wave line is the sketch of the potential relief for LG model dSs, dSa - filled areas in layers 3 and 2 of total area dS that are open to the gas phase T, Ts - gas and surface temperatures, respectively.

The probability of branching of the elementary process to the a channel Pet) depends on many initial conditions, including the relative velocity v, quantum states of reactants a, quantum states of products cf, and impact parameter b. For the random choice of die initial conditions, taking into account the corresponding distribution functions, it is expressed by a rather sinq)le formula... 

The oscillations observed with artificial membranes, such as thick liquid membranes, lipid-doped filter, or bilayer lipid membranes indicate that the oscillation can occur even in the absence of the channel protein. The oscillations at artificial membranes are expected to provide fundamental information useful in elucidating the oscillation processes in living membrane systems. Since the oscillations may be attributed to the coupling occurring among interfacial charge transfer, interfacial adsorption, mass transfer, and chemical reactions, the processes are presumed to be simpler than the oscillation in biomembranes. Even in artificial oscillation systems, elementary reactions for the oscillation which have been verified experimentally are very few. 

Overall, steric and electronic factors, which are seen to be small, are found to work in opposite directions and, to some degree, cancel each other out. Consequently, the intrinsic free activation barriers and reaction free energies (AG nt, AG nt), respectively, span a small range for catalysts I-IV and differ by less than l.Okcalmol-1. Thus, oxidative coupling represents the one process (beside allylic isomerization, cf. Section 5.3) among all the critical elementary steps of the C8-cyclodimer channel, that is least influenced by electronic and steric factors. 

The allylic conversion processes clearly represent the most feasible ones among all the critical elementary steps along the Cg-channel, involving the [NiII(octadienediyl)L] complex. Consequently, the several forms as well as the different stereoisomers are in a dynamic, pre-established equilibrium, that can likely be assumed as always being attained. 

Important classes of chemical reactions in the ground electronic state have equal parity for the in- and out-going channels, e.g., proton transfer and hydride transfer [47, 48], To achieve finite rates, such processes require accessible electronic states with correct parity that play the role of transition structures. These latter acquire here the quality of true molecular species which, due to quantum mechanical couplings with asymptotic channel systems, will be endowed with finite life times. The elementary interconversion step in a chemical reaction is not a nuclear rearrangement associated with a smooth change in electronic structure, it is aFranck-Condon electronic process with timescales in the (sub)femto-second range characteristic of femtochemistry [49],... 

With the cylindrical cryptands, each macrocycle may bind one cation so that both mono- and dinuclear cryptates may be formed. Although the 12-membered (N202) macrocycles of ligand 5 are too small to bind two cations within each of the macrocycles, variable temperature 13C-NMR measurements have revealed intramolecular cation exchange between identical sites at the top and bottom of this cryptand, for Ca2+, Sr2+, and Ba2+. Cation jump between the two sites is fast with respect to intermolecular cation exchange, modeling the elementary jump processes of cations between binding sites in membrane channels (91). 

Sections 9.2 and 9.3 discussed the elementary steps that make up the important and widely used screw extrusion process. The common and outstanding feature of all the elementary steps taking place in the screw channel is that they are induced by drag brought about by a single moving surface—that of the barrel. Solids are conveyed and compressed by the... 

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