Stepwise irreversibility

Process 1 Finite stepwise irreversible expansion. From the initial conditions, we want to expand the methane to 1.1 bar. To create a pressure imbalance across the piston, we vent the air chamber. In this first example, air is removed by cycling the ball valve open and shut three times. During each cycle enough air is vented to reduce P by 0.1 bar. The vent line is so large that each drop in Pgj t is nearly instantaneous. Between each cycle the system is given time to reestablish equilibrium, as indicated by steady readings on the pressxue gauges and the position indicator. 

Figure 3.2. Sequence of PM-RAIR spectra taken on a sputtered Co(0001) surface. The starting conditions were lOOmbar of CO at 298 K 200mbar of H2 was then added and the temperature increased stepwise to 490 K. It can be seen that the absorption signal due to CO attached to defect sites is removed in an irreversible process. The lowest curve was obtained at room temperature and under vacuum conditions and the hydrogen desorbs from the Co, while the CO and hydrocarbons remain. (After Beitel et al. 1997.)...

As illustrated in Figure 104 for [NiBr(triphos)] +, these complexes display the stepwise reductions Ni(II)/Ni(I)/Ni(0). The former is chemically and electrochemically reversible, whereas the latter is essentially irreversible.155... 

In addition to the stepwise mechanism, Dautzenberg and Platteeuw proposed another platinum-catalyzed cyclization mechanism (23). This might correspond simply to a disguised stepwise aromatization where the further reaction of unsaturated intermediates is very rapid compared with their desorption. Thus, hydrogen pressure would govern the probability of desorption versus further reaction. Since the cyclization of triene is irreversible, a very low steady-state surface triene concentration must be sufficient to ensure a measurable reaction rate. 

A great deal of work has been focused on whether the ene reaction proceeds through a concerted or a stepwise mechanism. The initially proposed synchronous pathway was challenged by a biradical , zwitterionic or a perepoxide intermediate. Kinetic isotope effects in the photooxygenation of tetrasubstituted, trisubstituted and cis-disubstituted alkenes supported the irreversible formation of an intermediate perepoxide,... 

As described in Section 13.03.1 and CHEC-II(1996) <1996CHEC-II(9)67>, thiepines are thermally unstable through valence isomerization to the corresponding thianorcaradienes, followed by irreversible cheleotropic loss of sulfur. Thiepines, as unstable intermediates, have been postulated in some stepwise transformations. 

It should be recalled here that the alcoholic hydroxyl of serine does not possess a dissociation constant within the pH range, accessible to enzymic reactions. Therefore, this amino acid cannot influence the pH-activity curve. On the other hand, it is well known that DFP inhibition is initially reversible and becomes only slowly irreversible. This has been demonstrated for true ChE from electric eel by Nachmansohn and associates (46) and for plasma ChE by Mackworth and Webb (47). Similarly, a stepwise reaction with inhibitors, containing the diethyl phosphoryl moiety, has been made probable by Hobbiger (34)- Therefore, it appears possible that phosphates are first attacked by the imidazol moiety of the esteratic site, in conformity with the catalytic influence of free imidazol on phosphate hydrolysis (48). This step is followed by transfer to serine. The final product is a trialkyl phosphate XV, which is not split by imidazol (scheme F). 

Scheme 1.1 includes alternative concerted and stepwise routes for the transformation of one molecule into another for present purposes, we describe any transformation which is not stepwise as concerted. The formation of the intermediate from the reactant molecule in the stepwise route may be reversible, as shown, or irreversible. 

This is the left stepwise curve depicted in Figure 3.81(h). To the right of this step the ionization is reversible to the left it is irreversible. 

Gas chromatographic (GC) methods also provide possibilities to detect the irreversibly held amounts of a poison (51), although these techniques are less accurate than the gravimetric methods. The number of irreversibly adsorbed molecules can be calculated from the material balance for successively injected pulses of the poison and the eluted amounts. Alternatively, adsorption equilibrium can be attained at low temperature, the adsorbed amount being determined by frontal analysis (51). Desorption may then be carried out at the same temperature and the irreversibly held amount can be calculated either from the difference between adsorbed and desorbed amounts of a first cycle or from the difference of the adsorbed amounts of a first and a second adsorption (52). Desorption temperatures can then be raised stepwise after the first desorption and the dependence of the irreversibly adsorbed amounts on desorption temperature determined from the corresponding desorbed amounts. The accuracy of these GC measurements is limited because of the usually very pronounced tailing of the desorption trace for the systems of interest. 

The partial reversibility of the reaction is also an argument in favor of the stepwise mechanism, although the latter is not the only possible explanation 2 moles of ATP are utilized per mole of carbamyl-P synthesized (Reactions 11 and 8), yet only 1 mole of ATP can be synthesized from 1 mole of carbamyl-P. It has been thought that the synthesis of ATP from carbamyl-P reflected a reversal of Reaction 8, while Reaction 11 was assumed irreversible. The activation reactions of acetate and formate are perhaps identical with Reaction 11, and yet they are reversible, as indicated in Reactions 9 and 10. It may well be that the reverse reaction, Reaction 8—e.g., ATP synthesis from car-bamyl-P—is part of Reaction 11 in all cases. This would imply that while the synthesis of carbamyl-P is a two-step process, the activation of HCOs" and the formation of ATP from carbamyl-P (in essence a reversal of C02 activation) may be carried out in one step. 

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