Carbonyls localized

High intensity irradiation of dibenzyl ketone in hexane/isopropanol mixtures yields 1,2-diphenyl ethane and toluene as the sole products. A study of the photochemical behaviour of dibenzyl ketone adsorbed on zeolites has been carried out and the results found to be dependent upon the Sl/Al composition of the zeolite. A laser flash examination of the behaviour of dibenzyl ketone in Nafion membranes has demonstrated that a modest yield of benzyl cations is formed. A study of the physical photochemistry of the ketone (1) has shown that the excited state is a carbonyl localized singlet state. This ultimately decays to afford a naphthalene localized triplet state. Norrish Type I cleavage does occur from the singlet state to afford products derived from the radicals (2) and (3). The authors report also that the 1-naphthyl methyl radical is formed from the triplet state. A study of the modification of the photochemical behaviour of the a-alkyldibenzyl ketones (4) by complexation in... 

The positive charge m this cation cannot be shared by the two oxygens it is localized on one of them Because protonation of the carbonyl oxygen gives a more stable cation that cation is formed preferentially... 

Nickel carbonyl should be used in totally enclosed systems or under good local exhaust. Plants and laboratories where nickel carbonyl is used should make use of air-monitoring devices, alarms should be present in case of accidental leakage, and appropriate personal respiratory protective devices should be readily available for emergency uses. Monitoring of urinary nickel levels is useful to help determine the severity of exposure and identify appropriate treatment measures. Some large-scale users of nickel carbonyl maintain a supply of sodium diethyldithiocarbamate, or Antabuse, a therapeutic agent, on hand for use in case of overexposure. 

It is good practice to keep concentrations of airborne nickel in any chemical form as low as possible and certainly below the relevant standard. Local exhaust ventilation is the preferred method, particularly for powders, but personal respirator protection may be employed where necessary. In the United States, the Occupational Safety and Health Administration (OSHA) personal exposure limit (PEL) for all forms of nickel except nickel carbonyl is 1 mg/m. The ACGIH TLVs are respectively 1 mg/m for Ni metal, insoluble compounds, and fume and dust from nickel sulfide roasting, and 0.1 mg/m for soluble nickel compounds. The ACGIH is considering whether to lower the TLVs for all forms of nickel to 0.05 mg/m, based on nonmalignant respiratory effects in experimental animals. 

Another useful way to think about carbon electrophilicity is to compare the properties of the carbonyls lowest-unoccupied molecular orbital (LUMO). This is the orbital into which the nucleophile s pair of electrons will go. Examine each compound s LUMO. Which is most localized on the carbonyl group Most delocalized Next, examine the LUMOs while displaying the compounds as space-filling models. This allows you to judge the extent to which the LUMO is actually accessible to an approaching nucleophile. Which LUMO is most available Least available ... 

The antiarrhythmic activity of local anesthetics has been noted several times previously. Another such agent is prepared by first alkylating isopropylamine with sulfone 199. Reaction of the ])ioduct (200) with diethylethylenediamine and carbonyldiimidazole results in transfer of the CDI carbonyl group and formation of the urea suricainide (201) [52]. The transform in all likelihood involves stepwise replacement of the imidazole groups by the basic groups in the other reactants. 

In addition reactions to chiral carbonyl compounds, the stereochemical course taken by resonance-stabilized alkali metals or magnesium benzyl anions resembles that taken by localized carbanions of similar bulk. Thus, conditions can be delineated which lead to either the steric approach or chelation control the following serve as examples. 

P-Lactamases are enzymes that hydrolyze the P-lactam ring of P-lactamantibiotics (penicillins, cephalosporins, monobactams and carbapenems). They are the most common cause of P-lactam resistance. Most enzymes use a serine residue in the active site that attacks the P-lactam-amid carbonyl group. The covalently formed acylester is then hydrolyzed to reactivate the P-lacta-mase and liberates the inactivated antibiotic. Metallo P-lactamases use Zn(II) bound water for hydrolysis of the P-lactam bond. P-Lactamases constitute a heterogeneous group of enzymes with differences in molecular structures, in substrate preferences and in the genetic localizations of the encoding gene (Table 1). 

FIGURE 3 In an n-to-ir transition of a carbonyl group, an electron in a nonbonding orbital (one localized wholly on the oxygen atom) is excited into an antibonding TT -orbital spread over both atoms. 

Excitation of the ii-tt transition of carbonyl compounds entails transfer of an electron from an orbital essentially localized on oxygen to... 

Table XI summarizes reports of ligand fluxionality at "very mixed -metal clusters. A number of studies (c.g.. Refs. 28, 8I, 116, 213, 214) have reported that both metal-localized and global carbonyl fluxionality occur but give little mechanistic detail. The following discussion focuses on the examples for which detailed studies have been undertaken and/or those for which mechanistic speculation is available.

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