Structure of the H-Related Complexes

The microscopic structure of the complexes obtained after the neutralization is one of the fundamental problems. Electrical measurements are of relatively poor help in solving this problem and the spectroscopic techniques appeared to be the most efficient in determining these microscopic structures. There is no universal type of complexing, and in the following we shall describe the various types of complexes whose structure has been established at present. 

In fact, only the passivation of silicon in GaAs has been studied with sufficient details. 

The most documented case is the Be-H complex in GaAs, which is characterized by a LVM at 2037 cm-1 (Nandhra et al., 1988). Stavola et al. (1989) have studied the effect of uniaxial stress on this LVM. The use of uniaxial stress allows the orientational degeneracies to lift and therefore gives the symmetry of the center evidenced by its LVM. 

Experiments performed with the stress applied at temperature near LHeT (Stavola et al., 1989) show that the As—H bonds are aligned along trigonal axes. It does not come out from these experiments whether the hydrogen sits in a BC or AB position, but in analogy with the case of the neutralization of boron in silicon, it is assumed that the hydrogen is in BC sites as first proposed by Pankove et al. (1985). This hypothesis has been recently confirmed by the calculations of Briddon and Jones (1989) that 

Among the numerous lattice defect-hydrogen complexes that are included in Table IV, a few of them could be identified. Here again the effect of uniaxial stress on the LVM has been the technique which permitted the identification. 

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In the present work we have employed antibodies which were raised against the different subunits of H -ATPase complex in order to elucidate evolutionary relations among subunits of the same complex isolated from various sources. We investigated the structure of chloroplast H -ATPase complex by chemical cross-linking, by the help of the same antibodies. The composition of aggregates formed following cross-linking could be identified in a more accurate way. 

The following order of initiation rate constants was found by Grubbs et al. for 71a and some precatalysts containing one phosphine ligand 56d< 56k 71a<56h (cf. Scheme 15 for structures of 56d,h,k) [48b, 55]. Thus, 71a shows a rate of initiation comparable to that of 56k but three orders of magnitude higher than that of 56d. Nevertheless, 56d appears to be more reactive in RCM reactions than 71a [56]. Wakamatsu and Blechert were the first to report that the activity of precatalysts related to 71a can be dramatically enhanced by modification of the benzylidene unit [56]. For example, RCM of 75 using 1 mol% of BINOL-derived complex 71b yields the azacyclic product 76 in quantitative yield within 20 min (Eq. 10), whereas with 56d only 4% of 76 was obtained under these conditions [56]. 

Much of the microscopic information that has been obtained about defect complexes that include hydrogen has come from IR absorption and Raman techniques. For example, simply assigning a vibrational feature for a hydrogen-shallow impurity complex shows directly that the passivation of the impurity is due to complex formation and not compensation alone, either by a level associated with a possibly isolated H atom or by lattice damage introduced by the hydrogenation process. The vibrational band provides a fingerprint for an H-related complex, which allows its chemical reactions or thermal stability to be studied. Further, the vibrational characteristics provide a benchmark for theory many groups now routinely calculate vibrational frequencies for the structures they have determined. 

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