Probing Mechanistic Detail

An understanding of the general variation in rates of substitution in octahedral complexes has been sought in terms of properties of the 

The detection of a reaction intermediate is usually not possible in coordination chemistry because lifetimes of intermediates are commonly extremely short. The simple mechanisms of reaction are commonly designated as an associative mechanism (A, with an intermediate of expanded coordination number formed) or a dissociative mechanism (D, with an intermediate of reduced coordination number formed). Intermediates of expanded coordination number are important in ligand substitution in square-planar complexes and in a few cases can actually be detected. For example, NifCNls is known from exchange reaction of Ni(CN)4 with CN (288). Even in octahedral complexes, some evidence for associative processes exists indirectly. The [RulNHsle] ion reacts with NO in acid to form [RuINHslsNO] and NH4 much more rapidly than can be explained by aquation of the hexaamine as the initial step, and a bimolecular mechanism with a 7-coordinate intermediate has been proposed (11, 226). 

Competition by Anions in Spontaneous and Base-Catalyzed Reactions of Pentaamminemetal(IIIj Complexes 

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It is quite common in organic chemistry to give ad hoc "explanations" of why an experiment has given an observed result. Sometimes, such proposals may give valuable suggestions as to further studies to probe mechanistic details. Sometimes, however, such "explanations" may be totally misleading when it is not realized that they are nothing else than speculations. 

The absence of a high-resolution structure of any transporter in the NT family greatly complicates the interpretation of structure-function studies. The recent identification of the bacterial and archaeal transporters and their potential suitability for direct structural studies raises the exciting prospect of being able to test specific structural hypotheses related to transport. In the meantime, the use of the numbering scheme introduced here in a context of predicted structural properties should facilitate communication among researchers on different members of the transporter family as the mechanistic details are being probed. 

In addition to the fact that hydrogen/deuterium exchange reactions can be helpful to probe ion structures as will be shown later, they can also reveal mechanistic details such as the site of reaction within ions. For example, the pentadienyl anion exchanges four protons rapidly, demonstrating, as shown in (12), that proton addition occurs more rapidly at the ends of the conjugated system than in the middle (Stewart et al., 1977 DePuy et al., 1978a). 

As with the n-TiCte and n-SrTiCh counterparts discussed earlier in Section 6.2 of this Chapter (see also Ref. 407), luminescence probes have proven to be very useful for unraveling the mechanistic details of the cathodic processes both at n-type (e.g., n-GaAs)556 and p type (e.g., p InP)557,558 Group III V semiconductor surfaces. Finally, these semiconductors share another trend with those discussed earlier (metal chalcogenides) in that the majority of the studies since 1990 have been directed at solid solutions (alloys of GaP and InP, GaAs and InAs etc.). These newer studies will be addressed in Section 12 of this Chapter. 

Pulse radiolysis is the radiation chemical analogue of flash photolysis. It is a fast-kinetics technique that enables transitory processes, initiated by the absorption of ionizing radiation, to be observed in time frames as short as the submicrosecond region. It permits the detection and characterization of short-lived intermediates, the determination of the kinetics of their decay, and a probing of reaction mechanisms. The technique finds use in the study of radiation effects on materials, and as a tool for the examination of mechanistic details. For inorganic systems, pulse radiolysis is used to characterize metal complexes in unusual oxidation states, to examine the kinetics and rates of ligand-labilization reactions and to elucidate the mechanism of electron transfer. 

While the Michaelis parameters are an important guide as to the integrity of the adsorbed sample, they are not of primary concern. By contrast, voltammetry reveals activity as a function of potential which can be varied and tuned to control and probe the enzyme in different ways. We will see that many enzymes display a "potential optimum , i.e. they have maximum activity at a particular potential. Many enzymes operate well in both redox directions, and the "bias can be measured precisely by running the voltammetry in the presence of both oxidised and reduced substrates. The exact form of the catalytic wave provides important mechanistic details, and this is often more easily analysed by taking the derivative of the catalytic current (dz/d ). 

Finally, the effects of mechanistic complexity must be addressed in any study of tunneling, particularly for enzyme-catalyzed reactions. There is no simple way to avoid the complications from multiple rate-limiting steps - they may appear in rapid-mix experiments, relaxation kinetics, and steady-state turnovers. There is good reason to believe, however, that with sufficient numbers of isotopic probes, many interesting mechanistic details can be resolved. 

The mechanistic details of these laccase/mediator catalyzed aerobic oxidations are still a matter of conjecture (51-54). However, experiments with a probe alcohol point towards one-electron oxidation of the mediator by the oxidized (cupric) form of the laccase followed by reaction of the oxidized mediator with the substrate, either via electron transfer (ET), e.g., with ABTS, or via hydrogen atom transfer (HAT), e.g., with N-hydroxy compounds which form N-oxy radicals (55). TEMPO and its derivatives form a unique case one-electron oxidation of TEMPO affords the oxoammonium cation which oxidizes the alcohol via a heterolytic pathway (Fig. 6), giving the carbonyl product and the hydroxylamine. The Tl copper center in fungal laccases has a redox potential of ca. 0.8 V vs. NHE. Consequently, fungal laccases can easily oxidize TEMPO to the corresponding oxoammonium cation, since the oxidation potential of the latter, which was first measured by Golubev and co-workers (55,57), is 0.75 V. This was confirmed by EPR measurements, which showed that laccase is reduced in the presence of TEMPO One equivalent of laccase could oxidize at least three equivalents of TEMPO within a few minutes under anaerobic conditions (58). 

Mechanistic details of electrocatalytic reactions can be probed through selectively modifying the surface with probe adlayers. Carbon adsorb in graphitic islands at low coverage, which restrict available surface sites for adsorption and reaction. As carbon coverage increases the amount of adsorbed CO decreases, consistent with siteblocking by carbon, while the reactivity of the bare sites to HCOOH increases. This... 

Beside the interest to probe supported size-selected clusters under different conditions and with the general challenges as introduced above, the use of cluster based materials in this work is motivated by a variety of individual scientific questions. The motivation for the particular choice of the different systems and reactions studied are briefly presented, a literature survey covering some theoretical and mechanistic details is subject of a later section (Sect. 2.1). 

Photo-induced Electron Transfer. Electron transfer is one of the most fundamental and widespread reactions in nature and has been extensively studied. In addition to the optical absorption spectroscopy widely used, TR EPR has become established as an appropriate method to study electron-transfer processes. In most of these investigations CIDEP effects are observed. The spin-polarization effects originate in the spin selectivity of chemical and physical processes involved in free-radical formation and decay, as well as in the spin-state evolution in transient paramagnetic precursors. For this reason, CIDEP constitutes a unique probe of the mechanistic details of electron-transfer processes. 

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