Substitution Studies of Second- and

Substitution Studies of Second- and Third-Row Transition Metal Oxo Complexes... 

SUBSTITUTION STUDIES OF SECOND- AND THIRD-ROW TRANSITION METAL OXO COMPLEXES... 

Substitution Studies of Second- and Third-Row Transition Metal Oxo Complexes Amdreas Roodt, Amira Abou-Hamdan, Hendrik P. Engelbrecht, and Andre E. Merbach... 

It is well known that the electrical activity of many deep-level defects disappears when the crystal is exposed to atomic hydrogen (see Pearton et al., 1987 and Chapter 5 of this Volume). This has been attributed to complex formation with the hydrogen, and it is very common for transition-metal impurities. Unfortunately, very little theoretical work has been reported for these systems. The deactivation of second- and third-period deep-level impurities is better understood theoretically. The substitutional oxygen defect in silicon ( A center Watkins and Corbett, 1961 Corbett et al., 1961) can be deactivated by exposure to hydrogen. Recently, a theoretical study of the deactivation of substitutional sulfur through the formation of a hydrogen-sulfur pair has been reported (Yapsir etal., 1988). 

This reaction takes place in a matter of seconds and produces significantly more fluorescence than the diacetylmonoxime fluorophors. Excess reagent is quickly hydrolyzed to form nonfluorescent water soluble products. Secondary, tertiary, and aromatic amines did not react with fluram to produce any measureable fluorescence. The reaction did not occur when ammonia and ammonium salts were tested for fluorescence. Mass spectrometry of an actual field sample confirmed that the substitution product is the fluorescent species that is shown above. Further mass spectra studies indicated that dimethyl-urea is not produced during this reaction. This was later confirmed by introducing known quantities of the urea and little or no fluorescence was noted. These tests indicate that Fluram does react with the primary amine intermediate on the adsorbent according to the above equation, and that monomethyl amine and other primary aliphatic amines would interfere. 

The clearest evidence for characterization of the second step as bi-molecular nucleophilic substitution with valency expansion of phosphorus, symbolized as SN2 (VE), or more briefly as VE2, is provided by the stereochemical studies of Gerrard and Green (107). Although less studied, the apparent unimolecular valency expansion (VEl) of the alkoxyphosphonium intermediate deriving from tertiary alcohols, touched upon in a previous section (cf. Section II-A-1), represents the other extreme of this mechanistic type. 

Two studies of the aquation and NCS" for Cl ion substitution reactions of a-and jff-cw-[Co(trien)Cl2]+ ions have appeared. The reactions were studied at 25— 26 C and pH 4, and two well resolved steps were evident in each reaction, the first step being ca. 15 times faster than the second step. The -cis-isomer reacts ca. 4 times as fast as the a-c/5-isomer. 

The quinone (37) has been synthesized and the circular dichroism, attributable to the isotopic substitution, measured. This represents only the second case of optical activity attributable to isotopic substitution other than by deuterium. Further analysis of the circular dichroism of a-diketones is discussed. The use of magnetic circular dichroism in the study of ketones and conjugated dienes has been fully discussed. 

Ultraviolet photoelectron spectroscopy allows the determination of ionization potentials. For thiazole the first experimental measurement using this technique was preformed by Salmona et al. (189) who later studied various alkyl and functional derivatives in the 2-position (190,191). Substitution of an hydrogen atom by an alkyl group destabilizes the first ionization potential, the perturbation being constant for tso-propyl and heavier substituents. Introduction in the 2-position of an amino group strongly destabilizes the first band and only slightly the second. 

In the second, which belongs to a systematic study of the transmission of substituent effects in heterocyclic systems, Noyce and Forsyth (384-386) showed that for thiazole, as for other simple heterocyclic systems, the rate of solvolysis of substituted hetero-arylethyl chlorides in 80% ethanol could be correlated with a constants of the substituent X only when there is mutual conjugation between X and the reaction center. In the case of thiazole this situation corresponds to l-(2-X-5-thiazolyl)ethyl chlorides (262) and l-(5-X-2-thiazolyl)ethyl chlorides (263). 

The first mechanistic studies of silanol polycondensation on the monomer level were performed in the 1950s (73—75). The condensation of dimethyl sil oxanediol in dioxane exhibits second-order kinetics with respect to diol and first-order kinetics with respect to acid. The proposed mechanism involves the protonation of the silanol group and subsequent nucleophilic substitution at the siHcone (eqs. 10 and 11). 

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