Heavy atom halides

M(N03)4] anions containing the unusual unidentate niirato ion are also known. Fluoro complexes are even less prevalent, the preference of these cations being for the other halides, cyanide, N- and heavy atom-donor ligands. 

On the other hand, the introduction of halide substituents at the C-2 and C-6 position decreases fluorescence quantum yields and gives a bathochromic shift of emission maxima. For example, bromine at the C-2 and C-6 position in compound 14b deteriorates fluorescence quantum yields from 0.95 (14a) to 0.45 and the emission maximum is red-shifted by 42 nm. Moreover, iodine at the C-2,6 position in compound 14d gives the similar bathochromic shift to bromine (14b, 44 nm) and more dramatic reduction in quantum yields (almost nonfluorescent, photophysical properties were interpreted as the heavy atom effect of halides on a BODIPY core skeleton. The bathochromic shift of BODIPY dyes without dramatic decrease in quantum yield was observed by the introduction of vinyl substituents at the C-2 and C-6 position. The extension of conjugation... 

The dynamics of carbon-halogen bond reductive cleavage in alkyl halides was studied by MP3 ab initio calculations, using pseudopotentials for the halogens and semidiffuse functions for the heavy atoms [104], The effect of solvent was treated by means of the ellipsoidal cavity dielectric continuum model. Both a concerted (i.e., a one-step) and a stepwise mechanism (in which an anion radical is formed at first) were... 

Another method of seeking evidence of the EIcBirr mechanism is to exam heavy-atom isotope effects in the leaving group. Of course, these should be much more significant in an E2 process because the bond is breaking in the transition state. For example, Thibblin and co-workersfound that in the base-induced elimination of an alkyl halide in which the p-carbon is unusually acidic (indene derivative, 12), moderately strong bases (triethylamine and methoxide) lead to a significant Cl/ Cl isotope effect = 1.010 1.009, where a maximum effect of... 

Structural parameters for inert gas-hydrogen halide complexes are given in Table 7. The heavy atom separation, R, in Table 7 is derived from the effective... 

Table 7. Vibrationally averaged structural parameters for rare gas, hydrogen halide complexes. R is the heavy atom separation and the y s are effective RG—X—H angles based on .

Although the heavy atom method has been successful in establishing many structures, particularly those of alkaloids since alkaloids have a propensity for crystallizing as halide salts, there had been an urgent need to develop a procedure for phase determination that was not dependent on the presence of a heavy atom in a crystal. Such a procedure, now commonly called the direct method of phase determination, has been devised. Karle and Hauptman 13) recognized that the number of unique reflections measured in an X-ray pattern is 25-50 times greater than the number of unknowns in a crystal, the unknown quantities being the three coordinates... 

When cationic micelles are employed, the counter ions are often halide ions. It is found that the fluorescence lifetime of micellised aromatic hydrocarbons is reduced by bromide ions (Hautala et al., 1973 Miller et al., 1977). Whether the quenching is due to the heavy atom effect, electron transfer, or nucleophilic attack has not been determined. 

In summary, aryl halides can undergo substitution by addition-elimination, SrnI, or elimination-addition mechanisms under basic conditions. The addition-elimination mechanism is most reasonable when the arene is electron-poor. When the arene is not electron-poor, the SrnI mechanism is most reasonable either when the nucleophile is a heavy atom or is delocalized, when light is required, or when a catalytic amount of a one-electron reducing agent is required. The elimination-addition mechanism is most reasonable when the arene is not electron-poor and when very strong base (pk b > 35) is used. 

Quantum mechanical methods have been used to calculate Sn NMR properties such as chemical shifts and coupling constants, for stannane, tetramethylstannane, methyltin halides, tin halides, and some stannyl cations. Relativistic effects were included by using the ZORA method. Each method allows the possibility of including only scalar effects or spin orbit coupling as well. " Sn chemical shifts and spin-spin couplings were calculated and compared to experimental values. A favorable correlation was shown for the chemical shifts, except for organotin species where heavy atoms are bound to tin, in which case a good correlation was obtained only at the spin-orbital level. Therefore, it is clear that relativistic effects must be considered for these heavy-element tin systems. 

In the solid state though not in solution, the halide ions in the hydrogen halide salt of ll,12-bis(diethylaminomethyl)-9,10-dihydro-9,10-ethenoanthracene are observed to cause heavy atom effects. 

Simple aromatic halides display two principal superimposing absorption bands, n,n and a,a, tailing over 250 nm. Intersystem crossing (ISC) to T, can be fast and efficient due to increased spin orbit coupling attributed to the heavy-atom effect (Section 4.8) of halogen atoms (Br, y 42- 49,i3i6 subsequent C—X bond homolysis may afford aryl radicals and... 

Studying the mechanism of the effects of heavy atoms on metabolic processes is an important task for developing methods to monitor the toxic effects of halides. Bioluminescent reactions are convenient models for such studies because of simple and prompt recording of the rate of enzymatic process. 

Earlier, we found that heavy-atom effect can also be observed in bioluminescent systems 3,4 bioluminescence inhibition coefficients were found to decrease in the series potassium halides KC1, KBr, and KI. Two mechanisms can be responsible for the change of the intensity of bioiuminescence in the presence of heavy ions the physicochemical effect of external heavy atom mentioned above, and the biochemical effect, i.e. interactions with the enzymes resulting in changes in enzymatic activity. A series of model experiments was conducted to evaluate the contribution of the physicochemical mechanism. These involved the photoexcitation of model fluorescent compounds close to bioiuminescence emitters in chemical nature and fluorescent properties - flavin mononucleotide, firefly luciferin and coelenteramide. These results are clear evidence of the smaller contribution of the physicochemical mechanism to the decrease in the bioiuminescence intensity for the three bioluminescent systems under study.4... 

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