Clustering reaction isotopic studies

This mechanism includes the N2 formation via two parallel pathways with participation of N02 species located at the paired Co -QH sites (e.g., near acid sites) and nitrite-nitrate complexes formed on cobalt oxide clusters. The reaction turnover number (TON) over Co 4 -OH sites was ca. 20 times higher as compared with that over CoOx. The isotope studies were carried out in a wide range of feed gas compositions and temperatures [9]. This study allowed to reveal the kinetic equations for the key reaction steps and to determine their rate coefficients (Table 51.2). 

Is the paramagnetic adduct between CO and Cluster A a kinetically intermediate in acetyl-CoA synthesis Questions have been raised about whether this adduct is a catalytic intermediate in the pathway of acetyl-CoA synthesis 187, 188) (as shown in Fig. 13), or is formed in a side reaction that is not on the main catalytic pathway for acetyl-CoA synthesis 189). A variety of biochemical studies have provided strong support for the intermediacy of the [Ni-X-Fe4S4l-CO species as the precursor of the carbonyl group of acetyl-CoA during acetyl-CoA synthesis 133, 183, 185, 190). These studies have included rapid ffeeze-quench EPR, stopped flow, rapid chemical quench, and isotope exchange. 

The [NiFe] hydrogenase from D. gigas has been used as a prototype of the [NiFe] hydrogenases. The enzyme is a heterodimer (62 and 26 kDa subunits) and contains four redox active centers one nickel site, one [3Fe-4S], and two [4Fe-4S] clusters, as proven by electron paramagnetic resonance (EPR) and Mosshauer spectroscopic studies (174). The enzyme has been isolated with different isotopic enrichments [6 Ni (I = I), = Ni (I = 0), Fe (I = 0), and Fe (I = )] and studied after reaction with H and D. Isotopic substitutions are valuable tools for spectroscopic assignments and catalytic studies (165, 166, 175). 

Poluektov et al. used high-frequency time-resolved spin-polarised EPR spectroscopy of radical pairs to characterise quantitatively isotopically labelled quinine exchange in the PS I reaction centre of proteins.91 Intra-subunit interactions in the Fe-S cluster of PS I of Synechocystis sp. PCC 6803 were studied by using mutations and following the changes in stabilisation of the cluster by EPR spectroscopy.92... 

In contrast to the subsystem representation, the adiabatic basis depends on the environmental coordinates. As such, one obtains a physically intuitive description in terms of classical trajectories along Born-Oppenheimer surfaces. A variety of systems have been studied using QCL dynamics in this basis. These include the reaction rate and the kinetic isotope effect of proton transfer in a polar condensed phase solvent and a cluster [29-33], vibrational energy relaxation of a hydrogen bonded complex in a polar liquid [34], photodissociation of F2 [35], dynamical analysis of vibrational frequency shifts in a Xe fluid [36], and the spin-boson model [37,38], which is of particular importance as exact quantum results are available for comparison. 

Nonetheless, elucidation of the detailed mechanism (e.g., energetics, reaction coordinate, mechanism, potential surface) of the process is not possible with the above qualitative information to elaborate these details, a systematic variation of isotopic composition, energy in the cluster, cluster size, and even solvent, is necessary. Such a study and mechanistic model determination have been carried out (Hineman et al. 1992a) and are discussed below. 

Iron clusters exhibit facile chemisorption toward methanol, the reaction proceeding with little or no cluster-size selectivity. An interesting feature of this system is that the chemisorption rate constants are nearly identical toward various isotopic sjjecies (CH3OH, CH30D,CD3 0H). If dissociation of a C—H or O—H bond was the initial step, then this should be manifested in an observable kinetic isotope effect. Thus the initial chemisorption step most likely involves the lone-pair orbital localized on the oxygen atom. More extensive studies of the chemistry of the Fe methanol system have been explored using infrared multiple-photon dissociation spectroscopy. These results are discussed in detail in Section Vlll. 

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